In the past couple of decades, houses with earthen building elements have gained significant popularity and recognition. This surge in interest can be attributed to several key factors that make clay houses an appealing choice. However, an essential characteristic is that using natural, non-toxic materials in the building process significantly reduces the risk of indoor air pollution and other health hazards, making clay homes a safer and ecofriendlier option [22]. One important parameter that characterizes the indoor air quality is ozone (O3). It readily reacts with various indoor materials and compounds in indoor air, leading to lower indoor ozone concentrations when outdoor air is the primary source. However, the byproducts of indoor ozone reactions can be irritating or harmful to building occupants. Darling et al. [23] suggest that clay-based coatings could serve as efficient passive removal materials, exhibiting relatively low emission rates of by-products that diminish quickly within a two-month period. Another study [24], assessing clay plaster as a passive removal material (PRM) for improving air quality, found that clay plaster improved the perceived air quality and considerably decreased aldehyde concentrations. However, there are relatively few data in the literature on the characteristics of clay plasters [25].

Clay houses have become increasingly popular and well-regarded, with more and more specialists showing interest in this type of construction [26,27]. The results of studies conducted on the possibility of using natural materials in construction are presented in the specialized literature, but these results, in addition to their many benefits, also reveal some drawbacks of clay that are worth investigating for potential improvements [13,28–30]. These drawbacks include a high risk of fissuring during the drying process for clay plaster due to significant axial shrinkage and a high sensitivity to water. To be used in the manufacture of construction materials, clayey soil must contain at least 15–16% clay to achieve the necessary plasticity and workability for labor [29,30]. To avoid the appearance of fissures, a linear shrinkage between 3 and 12% for soft mixtures or between 0.4 and 2% for drier mixtures is acceptable [13,28–31].

Given that there is limited research on the addition of waste materials to clay composites, and considering that in Romania, 80% of such waste is stored in vast areas set up in nature [32,33], constituting a potential environmental hazard, this study aims to valorize these wastes as additives in clay mixtures. Composites based on clay with added ash and sludge have been studied for the production of masonry elements [34,35], and optimal recipes with good results have been developed, thus continuing the study of composites based on clay with added ash and sludge for plastering. The study continues by examining clay mixtures with added ash and sludge for plastering applications.

Clay plasters are known for their eco-friendly and sustainable characteristics [3,13,22–24,36] but also have certain drawbacks [13,37]. One of the main concerns is the shrinkage during the drying process. As the water content in the plaster decreases, the material contracts, which can lead to the formation of fissures or cracks. This affects the plaster's appearance and compromises its ability to adhere properly to the substrate, potentially reducing the structural integrity of the overall construction [13,36,37]. Furthermore, the compatibility of clay plasters with various substrates can be an issue. In construction projects, a wide range of materials may be used as substrates, including wood, brick, stone, and even modern materials like concrete or oriented strand boards (OSB). To simplify the application process and increase the adoption of clay plasters, it is crucial to develop composites that can bond effectively with a diverse array of substrates. This may involve the incorporation of additional binders, fibers, and other additives that can enhance adhesion without compromising the plaster's environmental profile.

The study focused on examining the potential for developing clay compositions suitable for plastering surfaces constructed from various support materials. The research highlights the impact of different raw materials on the overall performance and characteristics of the clay composite. As a result, the originality and value of the experimental research conducted lie in the following:

principles of the Circular Economy. By finding new uses for these materials, the research aimed to reduce waste and promote sustainability. locally sourced raw materials. For preparing clay-based composite materials intended for plastering the surfaces of structures, the research focused on locally produced materials,

search aimed to reduce waste and promote sustainability.

To achieve a low-embedded energy material, it is essential to prioritize the use of

*Materials* **2023**, *16*, x FOR PEER REVIEW 4 of 23

delays in the plastering process.

#### **2. Materials and Methods** such: clay extracted directly from the soil in Valea Draganului, Cluj Napoca, Romania

**2. Materials and Methods**

To achieve a low-embedded energy material, it is essential to prioritize the use of locally sourced raw materials. For preparing clay-based composite materials intended for plastering the surfaces of structures, the research focused on locally produced materials, such: clay extracted directly from the soil in Valea Draganului, Cluj Napoca, Romania (46◦5401000 N 22◦4905300 E); fly ash generated by the Mintia power-plant, Romania (a byproduct of Romania's third largest power-plant); limestone sludge, a by-product of limestone processing industry in Hunedoara, Romania; hydraulic lime with the commercial name RÖFIX NHL5; dextrin purchased from a local producer in Cluj Napoca, Romania; and sodium chloride, NaCl, with 90% purity. (46°54′10″ N 22°49′53″ E); fly ash generated by the Mintia power-plant, Romania (a byproduct of Romanias third largest power-plant); limestone sludge, a by-product of limestone processing industry in Hunedoara, Romania; hydraulic lime with the commercial name RÖFIX NHL5; dextrin purchased from a local producer in Cluj Napoca, Romania; and sodium chloride, NaCl, with 90% purity. Clay is a fine-grained sedimentary rock composed of a blend of silica and fragments of materials such as quartz and mica [38] (X-ray diffraction (XRD) using a high-resolution Brucker D8 diffractometer). The particular clay used in this study, considered to be mostly

Clay is a fine-grained sedimentary rock composed of a blend of silica and fragments of materials such as quartz and mica [38] (X-ray diffraction (XRD) using a high-resolution Brucker D8 diffractometer). The particular clay used in this study, considered to be mostly of montmorillonite type, from the point of view of the particle size distribution, consists of approximately 38.15% clay minerals, 41.93% sand, and 19.92% silt, determined by the sieving and sedimentation method according to the Romanian standard STAS 1913-5 [39]. Based on its size distribution, it is classified as clay loam (Figure 1). It was characterized in terms of oxide composition by XRF analysis and the relevant data are provided in Table 1. From a mineralogical perspective (Figure 2), experimental tests indicated the presence of predominant minerals: quartz 66%, muscovite 32.4%, and vermiculite 1.6%. The tests were carried out in the research laboratories of the Technical University of Cluj-Napoca (XRD analysis) and the Babes,-Bolyai University of Cluj-Napoca (XRF analysis). According to the specialized literature [13,40,41], the mineralogical composition of this raw material indicates that it is appropriate for creating compositions that can be used in construction. of montmorillonite type, from the point of view of the particle size distribution, consists of approximately 38.15% clay minerals, 41.93% sand, and 19.92% silt, determined by the sieving and sedimentation method according to the Romanian standard STAS 1913-5 [39]. Based on its size distribution, it is classified as clay loam (Figure 1). It was characterized in terms of oxide composition by XRF analysis and the relevant data are provided in Table 1. From a mineralogical perspective (Figure 2), experimental tests indicated the presence of predominant minerals: quartz 66%, muscovite 32.4%, and vermiculite 1.6%. The tests were carried out in the research laboratories of the Technical University of Cluj-Napoca (XRD analysis) and the Babeș-Bolyai University of Cluj-Napoca (XRF analysis). According to the specialized literature [13,40,41], the mineralogical composition of this raw material indicates that it is appropriate for creating compositions that can be used in construction.

**Figure 1.** Characterization of the clay used as raw material—material placement in the ternary diagram.

**Figure 2.** Characterization of the clay used as raw material—mineralogical analysis, determined by XRD analysis (X-ray diffraction). **Figure 2.** Characterization of the clay used as raw material—mineralogical analysis, determined by XRD analysis (X-ray diffraction).

**Table 1.** The oxide composition of the clay, determined by XRF analysis (X-ray fluorescence). **Oxides SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 PC**  Content [%] 74.17 12.74 4.38 0.7 1.0 1.43 0.73 0.05 4.78 The fly ash underwent a comprehensive analysis to determine its properties and potential applications. Its chemical characteristics were assessed by examining the oxide composition (as shown in Table 2), revealing the percentages of various oxides present in the ash. In addition, the fly ashs physical properties, such as fineness and bulk density, The fly ash underwent a comprehensive analysis to determine its properties and potential applications. Its chemical characteristics were assessed by examining the oxide composition (as shown in Table 2), revealing the percentages of various oxides present in the ash. In addition, the fly ash's physical properties, such as fineness and bulk density, were evaluated and documented in Table 3. The fineness was determined by dry-sieving using a HELOS RODOS/L, R5 (dry dispersion in the free aerosol jet for laser diffraction), and the bulk density was determined as a ratio between mass and volume using the pycnometer method.

were evaluated and documented in Table 3. The fineness was determined by dry-sieving **Table 2.** The oxide composition of the fly ash, determined by XRF analysis (X-ray fluorescence).

**Table 2.** The oxide composition of the fly ash, determined by XRF analysis (X-ray fluorescence). **Table 3.** Physical characteristics of ash samples.

**Table 3.** Physical characteristics of ash samples.

**Fly Ash from Mintia Power Plant**

**Fineness R0.045 Apparent Density (Mg/m3)**

39.20% 1.67

The limestone sludge underwent experimental analysis to determine its properties, focusing on particle size distribution and bulk density, which are presented in Table 4. This characterization provides insights into the sludge's potential applications and performance in various settings, particularly when used as an additive in construction materials.

**Table 4.** Physical characteristics of the limestone sludge.

The hydraulic lime, introduced into the composite as a substitute for part of the clay, was characterized by experimentally determining the bulk density and compressive strength (Table 5).

**Table 5.** Physical characteristics of the hydraulic lime.

Dextrin are constituted from water-soluble amorphous carbohydrates with a small molecule, obtained from the hydrolysis reaction of starch or glycogen, with the chemical formula (C6H10O5)n. Dextrin is available as a white, pure powder, soluble in water and poorly soluble in alcohol. When mixed with water, they form a very sticky syrup, which can be used as a substitute for gum arabic and gum tragacanth. Dextrin was characterized by experimentally determining its bulk density, 800 kg/m3 .

The water used for preparation was dosed so that the sum of the liquid raw material quantities was constant in relation to the sum of the dry material quantities, specifically, 33% by mass, ensuring a consistency of the fresh mixture of 95 ± 5 mm.

#### *2.1. Preliminary Experimental Composites*

In order to identify and analyze the performance of clay compositions intended for plastering work, a series of mixtures made from three or more of the previously presented raw materials were prepared under laboratory conditions. The compositional design was based on preliminary research regarding the influence of different raw materials on the performance of clay composites and considered a series of guidelines:

preliminary experimental research examined the effects of partially substituting clay with hydraulic lime in compositions consisting of clay, fly ash, and limestone sludge. The study focused on evaluating the risk of fissuring, adhesion to clay-based support, axial shrinkage, apparent density, and mechanical resistance in four clay composites, as outlined in Table 8. In all the mixtures, sodium chloride was incorporated as an 8% mass concentration aqueous solution. The amounts of fly ash and limestone sludge were kept constant. The ratios were determined based on prior experimental studies, keeping a constant mass proportion of additive materials (limestone sludge/fly ash) to clay. In instances where clay was partially replaced by hydraulic lime, a constant mass proportion of additive materials (limestone sludge/fly ash) to the combination of (clay + hydraulic lime) was maintained.

**Table 6.** Composition of the preliminary clay composites experimentally prepared to assess the influence on mechanical resistance of adding fly ash to clay.

**Table 7.** Composition of the preliminary clay composites experimentally prepared to assess the influence on axial shrinkage of adding limestone sludge to clay.

**Table 8.** Composition of the preliminary clay composites experimentally prepared to assess the influence of the partial substitution of clay with hydraulic lime.

*2.2. Clay-Based Compositions Developed in Accordance with the Preliminary Research Findings*

Following the preliminary experimental research results, the aim was to determine the impact of dextrin on the performance of clay composites for plastering surfaces in eco-traditional buildings. As a result, five different composites were developed, as shown in Table 9.

The chosen substrates for applying clay composite plasters were selected from commonly used construction materials in eco-traditional wall buildings. These include ceramic bricks, limestone, oriented strand boards (OSB), unfired clay masonry units, and masonry elements made from a composite material consisting of clay and cereal straw.

**Table 9.** Composition of the experimental clay composites (percentage composition reported to the total mass of the mixture).

## *2.3. Experimental Testing Methods*

For preliminary clay composites PA1–PA5, PS1–PS5, PV1–PV5, and dextrin-containing clay composites S1–S5, prismatic samples of 40 × 40 × 160 mm were produced by casting in metal molds. After maintaining for 24 h in constant laboratory conditions of 23 ± 2 ◦C and 65 ± 1% relative humidity, the samples were removed from the molds and stored for 40 days for curing/drying.

For the clay composites PV1–PV5 and S1–S5, which underwent experimental testing for adherence to the substrate, the process involved preparing flat samples using different substrates. These substrates included ceramic bricks, limestone, OSB boards, unfired clay masonry units, and masonry elements made of composite material based on clay and cereal straw. A 3–5 mm thick layer of the clay composite material was applied to the previously mentioned substrates. The surfaces of the substrate were treated with bone glue aqueous solution before applying the clay composites. For this purpose, an 8% concentration bone glue solution was prepared by dissolving bone glue in warm water. The application of the clay composites was carried out only after the drying of the support surfaces treated with the bone glue solution.

All samples were stored for 40 days until reaching constant mass. The constant mass of the sample was regarded as an indication of both material maturity and dryness. The samples were maintained under constant laboratory conditions of 23 ± 2 ◦C and 65 ± 1% relative humidity and were then experimentally tested under the same environmental conditions to analyze the following parameters:

To ensure repeatability of the experimental tests, a minimum of 3 samples were prepared for each case. The reported values represent the arithmetic mean of individual values.

#### **3. Results and Discussions**

#### *3.1. Preliminary Experimental Research Results*

*Materials* **2023**, *16*, x FOR PEER REVIEW 9 of 23

The preliminary experimental research revealed aspects that were later incorporated into the design of the tested clay compositions to determine the optimal variant for creating plaster material: was carried out to determine the compressive strength as a ratio between the maximum load recorded at the time of breaking and the surface of the plates through

**Figure 3.** The influence of adding limestone slurry on the axial shrinkage of the clay material. **Figure 3.** The influence of adding limestone slurry on the axial shrinkage of the clay material.

*Materials* **2023**, *16*, x FOR PEER REVIEW 10 of 23

**Figure 4.** The influence of adding fly ash on the compressive strength of the clay material. **Figure 4.** The influence of adding fly ash on the compressive strength of the clay material.

**Figure 5.** The influence of adding fly ash on the flexural tensile strength of the clay material. **Figure 5.** The influence of adding fly ash on the flexural tensile strength of the clay material.

posite detached from the support while drying, making visual evaluation inconclusive in this instance. However, the maximum adherence to support was recorded for sive in this instance. However, the maximum adherence to support was recorded for composite PV2, in which clay was replaced by hydraulic lime at a ratio of 40% (mass percentages). Therefore, this composite is considered the most appropriate option to **Figure 6.** Influence of partial substitution of clay with hydraulic lime on the axial shrinkage of the clay material. **Figure 6.** Influence of partial substitution of clay with hydraulic lime on the axial shrinkage of the clay material.

composite PV2, in which clay was replaced by hydraulic lime at a ratio of 40% (mass percentages). Therefore, this composite is considered the most appropriate option to

**Figure 7.** Influence of partial substitution of clay with hydraulic lime on the apparent density of the

**Figure 8.** Influence of partial substitution of clay with hydraulic lime on the mechanical strengths of

further experimental investigation.

clay material.

the clay material.

further experimental investigation.

clay material.

clay material.

*Materials* **2023**, *16*, x FOR PEER REVIEW 11 of 23

**Figure 7.** Influence of partial substitution of clay with hydraulic lime on the apparent density of the clay material. **Figure 7.** Influence of partial substitution of clay with hydraulic lime on the apparent density of the clay material. **Figure 7.** Influence of partial substitution of clay with hydraulic lime on the apparent density of the clay material.

**Figure 6.** Influence of partial substitution of clay with hydraulic lime on the axial shrinkage of the

**Figure 6.** Influence of partial substitution of clay with hydraulic lime on the axial shrinkage of the

**Figure 8.** Influence of partial substitution of clay with hydraulic lime on the mechanical strengths of **Figure 8.** Influence of partial substitution of clay with hydraulic lime on the mechanical strengths of the clay material. **Figure 8.** Influence of partial substitution of clay with hydraulic lime on the mechanical strengths of the clay material.

*Materials* **2023**, *16*, x FOR PEER REVIEW 12 of 23

#### *3.2. Results of Experimental Research for Clay-Based Compositions developed following Preliminary Research Results 3.2. Results of Experimental Research for Clay-Based Compositions Developed Following Preliminary Research Results*

low the maximum allowable limit indicated by specialized literature (12%). Therefore, all

**Figure 10.** Axial shrinkage of clay mixtures after 40 days of maturation in constant laboratory con-

Considering that these mixtures are intended for plastering applications, it is important to attempt for reduced shrinkage levels. By doing so, the potential for cracking and fissuring can be minimized, ensuring a more durable and long-lasting plaster performance. Clay compositions PV3 and PV4, which contain a large amount of hydraulic lime, have the lowest axial shrinkage compared to the other samples. Moreover, analyzing the axial shrinkage recorded for clay compositions PV1–PV4 (compositions that do not contain dextrin), a decreasing trend of this indicator is observed as the amount of lime increases. Therefore, the beneficial effect of lime in reducing the axial shrinkage of the composite is confirmed. On the other hand, the introduction of dextrin into the composite matrix results in increased axial shrinkage. The values are within a range comparable to the axial shrinkage of sample PV1, which contains neither hydraulic lime nor dextrin. However, these compositions (S1–S5) possess a lower lime content compared to the com-

The experimental results obtained are presented in Figures 10–13. Experimental outcomes for clay compositions PV1–PV4 and S1–S5 (Figure 10) showed axial shrinkage be-The experimental results obtained are presented in Figures 10–13. Experimental outcomes for clay compositions PV1–PV4 and S1–S5 (Figure 10) showed axial shrinkage

ditions.

the analyzed compositions satisfy this requirement from this viewpoint.

**Figure 9.** Influence of partial substitution of clay with hydraulic lime on the adhesion of the clay material to a clay-based masonry element support. **Figure 9.** Influence of partial substitution of clay with hydraulic lime on the adhesion of the clay material to a clay-based masonry element support.

below the maximum allowable limit indicated by specialized literature (12%). Therefore, all the analyzed compositions satisfy this requirement from this viewpoint. low the maximum allowable limit indicated by specialized literature (12%). Therefore, all the analyzed compositions satisfy this requirement from this viewpoint.

**Figure 9.** Influence of partial substitution of clay with hydraulic lime on the adhesion of the clay

The experimental results obtained are presented in Figures 10–13. Experimental outcomes for clay compositions PV1–PV4 and S1–S5 (Figure 10) showed axial shrinkage be-

*3.2. Results of Experimental Research for Clay-Based Compositions developed following*

*Materials* **2023**, *16*, x FOR PEER REVIEW 12 of 23

material to a clay-based masonry element support.

*Preliminary Research Results*

However, these compositions (S1–S5) possess a lower lime content compared to the com-**Figure 11.** The apparent density of the clay composites after 40 days of maturation under constant laboratory conditions. **Figure 11.** The apparent density of the clay composites after 40 days of maturation under constant laboratory conditions. *Materials* **2023**, *16*, x FOR PEER REVIEW 14 of 23

density by 8%, 20%, and 25%, respectively, compared to the apparent density of the clay

composite prepared without hydraulic lime (PV1). However, this decreasing trend in ap-

benefit that cannot be surpassed.

parent density is not maintained in the case of clay composites prepared with dextrin (compositions S1–S5). In this case, the parameter followed shows a slight increase, even above the 1600 kg/m3 limit indicated by specialized literature as the minimum for good thermal resistance behavior. Thus, in the case of compositions with dextrin, apparent density values ranging from 1610 to 1623 kg/m3 were recorded, increasing as the amount of dextrin used was higher. Therefore, it is considered that the use of dextrin in clay composites brings slight benefits, leading to an apparent density within a range of values favor-

able for good thermal resistance behavior. **Figure 12.** Resistance to compression and flexural extension of clay composites after 40 days of maturation under constant laboratory conditions. **Figure 12.** Resistance to compression and flexural extension of clay composites after 40 days of maturation under constant laboratory conditions.

Regarding the compressive strength of the analyzed clay composites (Figure 12), two clear trends were observed: on one hand, the partial substitution of clay with hydraulic lime leads to a decrease in compressive strength by approximately 45%, 58%, and 64% (samples PV2–PV4). On the other hand, the introduction of dextrin into the clay composite

ing as the amount of dextrin used is higher. Comparing the experimental data to sample PV1, which lacks hydraulic lime, reveals a 30–33% reduction in compressive strength for composites made with partial substitution of clay with lime and added dextrin (samples S2–S5). This is seen as a minor improvement in performance when contrasted with the 45– 64% decrease observed in samples without dextrin. Still, considering that samples with dextrin also have hydraulic lime, their compressive strength, although improved, does not reach the value recorded for sample PV1, the clay composite made without partially substituting clay with lime. In this case, when comparing the experimental results of samples S1–S5 with the performance of sample P2 (the composition without dextrin identified as the most suitable for designing compositions S1–S5), an increase in compressive strength for composites with a minimum content of 4 g dextrin per 50 g (clay + hydraulic lime) was observed. Examining the compressive strengths of compositions with dextrin (S1–S5) reveals that this additive may have a minor positive impact on the composites mechanical performance. However, this beneficial effect is constrained, with a maximum

On the other hand, analyzing the experimental results in the context of the classification limits indicated by EN 998-1 [80], a specific standard for industrially produced plaster and skimming mortars based on inorganic binders, it can be said that all tested clay composites fall within the minimum limits imposed by this standard (0.4 N/mm2—minimum value for class CS I). Furthermore, all tested clay composites could be classified in the second class in terms of compressive strength, CS II, indicated by this standard, except for the clay composite PV1, which would fall into a higher class, namely CS III. This finding is particularly valuable in terms of the applicability of these composite materials. The ease

of their in situ use increases when they can be compared to cement-based plaster or skimming mortar products, which are commonly employed in the construction industry.

formance, these positive effects are relatively reduced and limited.

designing subsequent clay compositions.

In terms of flexural tensile strength, as shown in Figure 12, a pattern similar to compressive strength is observed. With the partial substitution of clay with hydraulic lime in samples PV2–PV4, the flexural tensile strength decreases by 18%, 32%, and 47%. This reduction is in comparison to the flexural tensile strength of the composite that does not contain lime (sample PV1). Upon introducing dextrin to the clay composition, there is a visible trend of reduced strength loss as the dextrin content increases. This trend continues up to a certain threshold, as seen in samples S1–S5. However, when the amount of dextrin is further increased, the mechanical performance and flexural tensile strength decline, as observed in samples S4–S5, falling below the maximum value recorded for sample S3. This behavior suggests that the flexural tensile strength can experience slight improvement with the addition of a small amount of dextrin in the clay composite, but the extent of this improvement is limited. This finding aligns with the observation made during the analysis of compressive strength performance. As a result, it can be inferred that while adding dextrin to the clay composition may provide some benefits to the mechanical per-

Analyzing the compressive strength and axial shrinkage for the clay composites S1– S5, in relation to their density, it was observed that these parameters show a progression that can be mathematically expressed through functions of density (f(density)), as shown in Figure 13, with a sufficiently good accuracy indicated by the R2 factor being greater than 0.9. Both compressive strength and axial shrinkage can be mathematically modeled using polynomial functions. In both cases, the existence of these functions allowing mathematical reproducibility with a high degree of accuracy is considered a useful tool for

**Figure 13.** The evolution of compressive strength and axial shrinkage depending on the apparent density of the clay composite. **Figure 13.** The evolution of compressive strength and axial shrinkage depending on the apparent density of the clay composite.

Identifying mathematical functions for adhesion to the support in relation to density or compressive strength of clay composites is not as straightforward. In this case, mathematical functions satisfying the R2 > 0.9 condition are polynomial functions of degree 3 or Considering that these mixtures are intended for plastering applications, it is important to attempt for reduced shrinkage levels. By doing so, the potential for cracking and fissuring can be minimized, ensuring a more durable and long-lasting plaster performance. Clay compositions PV3 and PV4, which contain a large amount of hydraulic lime, have the lowest axial shrinkage compared to the other samples. Moreover, analyzing the axial shrinkage recorded for clay compositions PV1–PV4 (compositions that do not contain dextrin), a decreasing trend of this indicator is observed as the amount of lime increases. Therefore, the beneficial effect of lime in reducing the axial shrinkage of the composite is confirmed. On the other hand, the introduction of dextrin into the composite matrix results in increased axial shrinkage. The values are within a range comparable to the axial shrinkage of sample PV1, which contains neither hydraulic lime nor dextrin. However, these compositions (S1–S5) possess a lower lime content compared to the compositions that demonstrated the best performance regarding axial shrinkage. When comparing the experimental results of samples S1–S5 to the performance of sample P2 (composition without dextrin, identified as the most suitable for designing composites S1–S5), there was a 37–44% increase in axial shrinkage. However, since these values fall within the limit specified by specialized literature (max. 12%), this increase was not considered a drawback for the development of clay composites with dextrin intended for plastering.

According to specialized literature, density values within the range of 1600 kg/m3 and 2000 kg/m3 provide the advantage of good thermal resistance for the material [2,8,13,14,19]. However, these indications from the literature mainly consider the behavior of masonry elements intended for constructing walls made of clay composites. In this case, the intended use of the analyzed clay composites is plastering. Therefore, although thermal resistance characteristics are not negligible, they are not as important a parameter in performance evaluation. Consequently, it was determined that several factors are more crucial for evaluating composite performance. These include a smooth surface free from fissuring risks, strong adherence to the supporting material, vapor permeability, and resistance to environmental agents.

The experimental results recorded for the density of the analyzed clay composites (Figure 11) indicated several interesting aspects. It can be observed that using hydraulic lime as a substitute for clay in clay composites PV2–PV4 leads to a reduction in apparent density by 8%, 20%, and 25%, respectively, compared to the apparent density of the clay composite prepared without hydraulic lime (PV1). However, this decreasing trend in apparent density is not maintained in the case of clay composites prepared with dextrin (compositions S1–S5). In this case, the parameter followed shows a slight increase, even above the 1600 kg/m3 limit indicated by specialized literature as the minimum for good thermal resistance behavior. Thus, in the case of compositions with dextrin, apparent density values ranging from 1610 to 1623 kg/m3 were recorded, increasing as the amount of dextrin used was higher. Therefore, it is considered that the use of dextrin in clay composites brings slight benefits, leading to an apparent density within a range of values favorable for good thermal resistance behavior.

Regarding the compressive strength of the analyzed clay composites (Figure 12), two clear trends were observed: on one hand, the partial substitution of clay with hydraulic lime leads to a decrease in compressive strength by approximately 45%, 58%, and 64% (samples PV2–PV4). On the other hand, the introduction of dextrin into the clay composite (samples S1–S5) slightly improves compressive strength, with the improvement increasing as the amount of dextrin used is higher. Comparing the experimental data to sample PV1, which lacks hydraulic lime, reveals a 30–33% reduction in compressive strength for composites made with partial substitution of clay with lime and added dextrin (samples S2–S5). This is seen as a minor improvement in performance when contrasted with the 45–64% decrease observed in samples without dextrin. Still, considering that samples with dextrin also have hydraulic lime, their compressive strength, although improved, does not reach the value recorded for sample PV1, the clay composite made without partially substituting clay with lime. In this case, when comparing the experimental results of samples S1–S5 with the performance of sample P2 (the composition without dextrin identified as the most suitable for designing compositions S1–S5), an increase in compressive strength for composites with a minimum content of 4 g dextrin per 50 g (clay + hydraulic lime) was observed. Examining the compressive strengths of compositions with dextrin (S1–S5) reveals that this additive may have a minor positive impact on the composite's mechanical performance. However, this beneficial effect is constrained, with a maximum benefit that cannot be surpassed.

On the other hand, analyzing the experimental results in the context of the classification limits indicated by EN 998-1 [80], a specific standard for industrially produced plaster and skimming mortars based on inorganic binders, it can be said that all tested clay composites fall within the minimum limits imposed by this standard (0.4 N/mm2—minimum value for class CS I). Furthermore, all tested clay composites could be classified in the second class in terms of compressive strength, CS II, indicated by this standard, except for the clay composite PV1, which would fall into a higher class, namely CS III. This finding is particularly valuable in terms of the applicability of these composite materials. The ease of their in situ use increases when they can be compared to cement-based plaster or skimming mortar products, which are commonly employed in the construction industry.

In terms of flexural tensile strength, as shown in Figure 12, a pattern similar to compressive strength is observed. With the partial substitution of clay with hydraulic lime in samples PV2–PV4, the flexural tensile strength decreases by 18%, 32%, and 47%. This reduction is in comparison to the flexural tensile strength of the composite that does not contain lime (sample PV1). Upon introducing dextrin to the clay composition, there is a visible trend of reduced strength loss as the dextrin content increases. This trend continues up to a certain threshold, as seen in samples S1–S5. However, when the amount of dextrin is further increased, the mechanical performance and flexural tensile strength decline, as observed in samples S4–S5, falling below the maximum value recorded for sample S3. This behavior suggests that the flexural tensile strength can experience slight improvement with the addition of a small amount of dextrin in the clay composite, but the extent of this improvement is limited. This finding aligns with the observation made during the analysis of compressive strength performance. As a result, it can be inferred that while adding dextrin to the clay composition may provide some benefits to the mechanical performance, these positive effects are relatively reduced and limited.

Analyzing the compressive strength and axial shrinkage for the clay composites S1–S5, in relation to their density, it was observed that these parameters show a progression that can be mathematically expressed through functions of density (f(density)), as shown in Figure 13, with a sufficiently good accuracy indicated by the R2 factor being greater than 0.9. Both compressive strength and axial shrinkage can be mathematically modeled using polynomial functions. In both cases, the existence of these functions allowing mathematical reproducibility with a high degree of accuracy is considered a useful tool for designing subsequent clay compositions. *Materials* **2023**, *16*, x FOR PEER REVIEW 16 of 23

Identifying mathematical functions for adhesion to the support in relation to density or compressive strength of clay composites is not as straightforward. In this case, mathematical functions satisfying the R2 > 0.9 condition are polynomial functions of degree 3 or higher, indicating reduced accuracy and challenges in modeling the phenomenon. This implies that numerous factors influence the adhesion parameter to the support, and it does not solely depend on the clay composite's characteristics, as initially anticipated. higher, indicating reduced accuracy and challenges in modeling the phenomenon. This implies that numerous factors influence the adhesion parameter to the support, and it does not solely depend on the clay composites characteristics, as initially anticipated. The key characteristics of plaster mortars involve their ability to adhere strongly to

The key characteristics of plaster mortars involve their ability to adhere strongly to substrates and resist fissuring. A good plaster mortar should maintain a solid bond with the underlying surface while remaining fissure-free, ensuring durability and a visually appealing finish. These properties are crucial for the performance and longevity of plastered surfaces in construction applications. Regarding the adhesion to the clay composite masonry element substrate, Figure 14 shows an undeniable increase in this parameter with the introduction of dextrin in the composition (S1–S5 vs. PV1–PV4). It is important to note that the compositional design of samples S1–S5 was based on composition PV2. This further highlights the advantages of adding dextrin to the mixture, as it significantly improves the adhesion of the composites to their respective substrates. substrates and resist fissuring. A good plaster mortar should maintain a solid bond with the underlying surface while remaining fissure-free, ensuring durability and a visually appealing finish. These properties are crucial for the performance and longevity of plastered surfaces in construction applications. Regarding the adhesion to the clay composite masonry element substrate, Figure 14 shows an undeniable increase in this parameter with the introduction of dextrin in the composition (S1–S5 vs. PV1–PV4). It is important to note that the compositional design of samples S1–S5 was based on composition PV2. This further highlights the advantages of adding dextrin to the mixture, as it significantly improves the adhesion of the composites to their respective substrates.

**Figure 14.** Adhesion to the support of clay mixtures after 40 days of maturation under **Figure 14.** Adhesion to the support of clay mixtures after 40 days of maturation under constant laboratory conditions.

When examining the behavior of clay composites that incorporate dextrin (S1–S5), it becomes evident that composition S3 stands out, exhibiting the best performance irrespective of the underlying support layer. Concurrently, the type of support appears to play a role in influencing the adhesion of the clay composite. Different support materials might lead to varying levels of adhesion, which could affect the overall performance and stability of the clay composite when applied. When examining the behavior of clay composites that incorporate dextrin (S1–S5), it becomes evident that composition S3 stands out, exhibiting the best performance irrespective of the underlying support layer. Concurrently, the type of support appears to play a role in influencing the adhesion of the clay composite. Different support materials might lead to varying levels of adhesion, which could affect the overall performance and stability of the clay composite when applied.

To enhance adhesion to the support, before the clay composites were applied, all support surfaces were pre-treated with an 8% aqueous solution of bone glue. Despite this treatment, the oriented strand board (OSB) surface seems to pose the greatest challenge when it comes to compatibility with clay-based plaster. This varying behavior of clay plasters in relation to different support types may be due to specific properties of the support To enhance adhesion to the support, before the clay composites were applied, all support surfaces were pre-treated with an 8% aqueous solution of bone glue. Despite this treatment, the oriented strand board (OSB) surface seems to pose the greatest challenge when it comes to compatibility with clay-based plaster. This varying behavior of clay plasters in relation to different support types may be due to specific properties of the

surface, such as water absorption capacity and surface roughness. This hypothesis can also be supported by analyzing the presence or absence of fissures that appeared at the

is provided, which can be found in Table 10. This analysis offers insights into the texture and appearance of these composites, which can clarify their suitability for different applications and their overall performance in terms of aesthetics and functionality. It can be observed that there are cases of micro-fissuring, fissuring, and cases where fissures or even

constant laboratory conditions.

**Clay**

support surface, such as water absorption capacity and surface roughness. This hypothesis can also be supported by analyzing the presence or absence of fissures that appeared at the end of the drying period on the surface of clay plasters.

A visual analysis of the surface appearance of various clay composites, after drying, is provided, which can be found in Table 10. This analysis offers insights into the texture and appearance of these composites, which can clarify their suitability for different applications and their overall performance in terms of aesthetics and functionality. It can be observed that there are cases of micro-fissuring, fissuring, and cases where fissures or even detachment from the surface are identified (composition S1 on OSB support), depending on the type of composition and the nature of the support, but there are also cases where the dried plaster surface is smooth, without traces of fissuring or other defects. Composition S3 is identified as having the best compatibility with all analyzed supports. Figure 15 shows exemplary images of the samples made by applying clay composites S1–S5 on different support layers. *Materials* **2023**, *16*, x FOR PEER REVIEW 17 of 23 detachment from the surface are identified (composition S1 on OSB support), depending on the type of composition and the nature of the support, but there are also cases where the dried plaster surface is smooth, without traces of fissuring or other defects. Composition S3 is identified as having the best compatibility with all analyzed supports. Figure 15 shows exemplary images of the samples made by applying clay composites S1–S5 on different support layers. **Table 10.** Visual analysis of the clay composite layer applied to the support, after drying.

> **Table 10.** Visual analysis of the clay composite layer applied to the support, after drying. **The Appearance of Plaster Layer after Drying**

**Figure 15.** Representative images presenting the application of clay composite S1–S5 on various support materials, illustrating their compatibility and appearance after drying. (**a**) Ceramic brick support; (**b**) limestone support; (**c**) clay support; (**d**) support made of composite material based on clay and cereal straw; (**e**) OSB support. **Figure 15.** Representative images presenting the application of clay composite S1–S5 on various support materials, illustrating their compatibility and appearance after drying. (**a**) Ceramic brick support; (**b**) limestone support; (**c**) clay support; (**d**) support made of composite material based on clay and cereal straw; (**e**) OSB support.

Examining the adherence performance of clay composites S1–S5 to various supports (as depicted in Figure 14), we find that the results are rather promising when compared with benchmarks found in the literature (Table 11). The minimum recorded value was 0.3 N/mm2 and the maximum was 0.6 N/mm2, suggesting the suitability of these composites

Examining the adherence performance of clay composites S1–S5 to various supports (as depicted in Figure 14), we find that the results are rather promising when compared with benchmarks found in the literature (Table 11). The minimum recorded value was 0.3 N/mm2 and the maximum was 0.6 N/mm2 , suggesting the suitability of these composites for both finishing and decoration purposes in construction. When cross-reference the obtained data with other studies, as represented in Table 11, it is noticed that both limestone sludge and, more notably, hydraulic lime result in reduced axial contractions. These contractions are within the 12% limit suggested by the literature. The use of hydraulic lime, and particularly fly ash, in clay composites enhances mechanical strength, both in terms of compression and bending. Regarding the density of the clay composite, it is slightly lower than average but still aligns with references from the literature. Given these data, we can conclude that our results are reasonably in line with those from other studies. However, we must take into account two important considerations:

**Table 11.** Benchmarking against previous research findings.

#### **4. Conclusions**

The experimental findings are remarkable and practical because the research focused on identifying and highlighting the impact of alternative or additive raw materials, such as clay, and by-products from other industries on the performance of clay composites.

The experimental research investigated the potential of using clay composites as plasters for various surfaces commonly found in eco-traditional construction. These surfaces included ceramic brick, limestone, OSB boards, masonry elements made of clay, and masonry elements made of composite material based on clay and cereal straw. Based on this research, the following observations can be made:

The experimental research, on the one hand, demonstrates the possibility of making clay-based plastering mortars even at an industrial level. On the other hand, it underlines the importance of a detailed analysis of the clayey raw material whose characteristics differ depending on the place of extraction. Therefore, each case must adapt and customize the recipe and the production process. These observations and the need for customized design need to be extended to fly ash, whose characteristics differ from one power plant to another.

**Author Contributions:** Conceptualization, A.H., C.P., A.A.C., G.C. and A.B.; methodology, A.H. and G.C.; formal analysis, C.P., A.A.C. and A.B.; investigation, A.H., C.P., A.A.C., G.C. and A.B.; data curation, A.H. and C.P.; writing—original draft preparation, A.H. and C.P.; writing—review and editing, C.P. and A.A.C.; project administration, A.A.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Romanian Government Ministry of Research Innovation and Digitization, project No. PN 23 35 03 01 "Integrated system of development and scientific research of constructions and vital infrastructures to extreme environmental, seismic and climatic actions and the exploitation of sustainable resources of materials and energy".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

## *Article* **Fabrication of Thermal Insulation Bricks Using** *Pleurotus florida* **Spent Mushroom**

**Sally A. Ali 1 , Marwa Kamal Fahmy 2 , Nasser Zouli 3,\*, Ahmed Abutaleb 3 , Ibrahim M. Maafa 3 , Ayman Yousef 3,4,\* and M. M. Ahmed 4**

**Abstract:** This study explores the potential for making lightweight bricks via the use of dry, pulverized spent mushroom materials (SMM) as a thermal insulator. There are five distinct replacement proportions of SMM that are used, and they range from 0% to 15% of the weight of the clay. The firing of the fabricated bricks at temperatures of 700, 800, and 900 ◦C led to the development of pores on the interior surface of the bricks as a consequence of the decomposition of SMM. The impact of SMM on the physicomechanical characteristics of fabricated bricks is assessed based on standard codes. Compressive strength, bulk density, and thermal conductivity decreased as the SMM content increased, reaching up to 8.7 MPa, 1420 kg/m3 , and 0.29 W/mK at 900 ◦C and 15% substitution percentage. However, cold water absorption, boiling water absorption, linear drying shrinkage, linear firing shrinkage, and apparent porosity increased with the increase in SMM, reaching 23.6%, 25.3%, and 36.6% at 900 ◦C and 15% substitution percentage. In the study simulation model, there was a significant improvement in energy consumption, which reached an overall reduction of 29.23% and 21.49% in Cario and Jazan cities, respectively.

**Keywords:** lightweight bricks; spent mushroom materials; bio-based materials; thermal insulation; energy efficiency buildings

## **1. Introduction**

Due to the overexploitation of numerous natural resources, there is a lack of natural resources in the world for manufacturing conventional bricks [1]. Huge amounts of raw materials are consumed in the brick industry [2,3]. To overcome this issue, several attempts have been made to incorporate different waste materials into the brick-making process, including natural fibers, textile laundry wastewater sludge, foundry sand, granite sawing waste, perlite, processed waste tea, sewage sludge, structural glass waste, fly ash, sugar cane bagasse ash, organic residue, steel dust, bottom ash, rice husk ash, silica fume, and municipal solid waste incineration fly ash [4–7]. An innovative biotechnological method for recycling lignocellulosic waste involves growing oyster mushrooms. Mushroom substrates have a limited ability to condition and fertilize soil due to their high salt, nutrient, and alkaline contents [8]. Cellulosic substrates, including cotton wastes, maize cob wastes, bean straws, crushed bagasse, molasses wastes, coffee husks, paper wastes, industrial cardboard wastes, tree sawdust, and rice straws can be used to grow mushrooms. The majority of commercially grown Agaricus mushrooms are grown on straw or hay substrates. Rice straw mushrooms (RSM) are regarded as a nutritious food (USITC 2010) [9,10]. Growing

**Citation:** Ali, S.A.; Fahmy, M.K.; Zouli, N.; Abutaleb, A.; Maafa, I.M.; Yousef, A.; Ahmed, M.M. Fabrication of Thermal Insulation Bricks Using *Pleurotus florida* Spent Mushroom. *Materials* **2023**, *16*, 4905. https:// doi.org/10.3390/ma16144905

Academic Editor: Carlos Leiva

Received: 30 May 2023 Revised: 2 July 2023 Accepted: 3 July 2023 Published: 9 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

edible mushrooms meets the dual goals of treating rice straw, which acts as a substrate for the production of food (mushrooms), and as a source of food for the mushroom-spent bedding. One of the biggest environmental problems in the mushroom production process that utilizes rice straw as soil is the vast amounts of spent mushroom material (SMM) generated as solid waste by-products—1 kg of mushrooms produces around 5 kg of solid waste [11]. SMM waste is frequently disposed in landfills [12]. About 27 billion kg of cultivated mushrooms were produced worldwide in 2012 [13]. Governments and researchers are examining the potential uses of these leftover materials as a result of careful environmental management of this by-product of mushroom production. Today, around 50% of the population lives in cities, using a lot of energy resources and emitting more than 70% of the world's carbon emissions. This is mainly because of urban development, which has been identified as the cause of issues with sustainability [14]. One approach for addressing this is using sustainable materials such as SMM in the production of construction materials. This has some beneficial effects on the environment and the economy and, in addition to optimizing the thermal performance of the urban block form, is necessary for the management of daily wastes that cause significant environmental problems [15]. Beyond the context of the surrounding, the building's shape and the windows' parameters have an impact on energy consumption [16]. Using sustainable materials with thermally isolated behavior is considered the most effective parameter for achieving sustainability. Inefficient thermal materials, which cause high energy consumption, have environmental and economic consequences during the building's life [17,18]. Hence, avoiding or even reducing as much thermal gain as possible in buildings decreases the internal temperature and improves the indoor thermal quality, thus directly decreasing the need for air conditioning, resulting in a decrease in the energy consumed in the building and the city. The main distinguishing objectives and novelty of this study are managing waste materials by producing lightweight bricks made by replacing clay with SMM materials in various ratios that meet the obligatory values of the physicomechanical characteristics assigned by standards. It was also critical for manufacturing lightweight bricks with effective thermal insulation for controlling energy usage. These bio-based, lightweight bricks can both help with trash disposal and resource conservation because they are made from rice straw waste that has undergone the mushroom biodegradation process. Thus, it makes economic and environmental sense to utilize SMM as a clay body addition. Table S1 contains the physicomechanical and thermal characteristics of bricks substituted with different waste materials.

### **2. Experimental**

### *2.1. Materials and Methodology*

Clay was obtained from Aswan, Egypt, and spent mushroom materials (SMM) was grown in the laboratory. *Pleurotus florida* spawn (culture) was obtained from the Agricultural Research Centre, Ministry of Agriculture, Giza, Egypt.

### *2.2. Preparation of Spent Mushroom Materials from Cultivated Pleurotus florida*

Spent mushroom materials (SMM) of *Pleurotus florida*—also known as the oyster mushroom or the white oyster mushroom—were prepared after cultivation (Figure 1). The SMM was prepared in the laboratory to show the process of mushroom growth. The preparation was carried out following the protocol by Stoknes et al. [19], with slight modifications as follows: To ensure that water completely soaked through the rice straws, the straws were cut into 20–30 cm lengths, soaked in water that was three times as heavy as the straw, left to soak for 24 h, and then air dried. After soaking, the rice straws were placed and sealed in sterile plastic bags. The bags were subjected to a 15-min sterilization process at 121 ◦C and then cooled at room temperature for 24 h. Each bag was filled with 50 g of *Pleurotus florida* spawn (culture) that had already been prepared and cooled; the bag was then sealed. The inoculated rice straw was incubated at 25 ◦C with light requirements of 1000–1500 (2000 lux) for not less than 30 days, until fruit body development. The first

harvest was carried out after 30–40 days, and the yield during the first harvest was 250 g. The raw materials of this edible fungus, as well as the cultivation and harvesting conditions for spent mushroom material preparation, are shown in Figure 1a–k. After autoclaving for 20 min and drying at 60 ◦C for 48 h, each piece of residual *Pleurotus florida* spent mushroom material was crushed using a pulverizer (Elaraby, Cairo, Egypt, MX 900/2). Each substrate's powder was then separated into two parts using a sieve (through a mesh with a 2-mm pore size sieve). The sieving process is performed to separate the impurities and ungrounded particles, and only the finer part was used for the experiments. first harvest was carried out after 30–40 days, and the yield during the first harvest was 250 g. The raw materials of this edible fungus, as well as the cultivation and harvesting conditions for spent mushroom material preparation, are shown in Figure 1a–k. After autoclaving for 20 min and drying at 60 °C for 48 h, each piece of residual *Pleurotus florida* spent mushroom material was crushed using a pulverizer (Elaraby, Cairo, Egypt, MX 900/2). Each substrate's powder was then separated into two parts using a sieve (through a mesh with a 2-mm pore size sieve). The sieving process is performed to separate the impurities and ungrounded particles, and only the finer part was used for the experiments.

bag was then sealed. The inoculated rice straw was incubated at 25 °C with light requirements of 1000–1500 (2000 lux) for not less than 30 days, until fruit body development. The

*Materials* **2023**, *16*, x FOR PEER REVIEW 3 of 20

**Figure 1.** Preparation of SMM from cultivated *Pleurotus florida* (**a**) chopped rice straws, (**b**) soaked rice straw, (**c**) inoculation of rice straws with 50 g *Pleurotus florida* spawn, (**d**) sealed, inoculated, and sterilized bags, (**e**) fruitful body development, (**f**) first harvest, (**g**) formation of SMM, (**h**) SMM after autoclaving, (**i**) SMM after heat drying, (**j**) A pulverizer, and (**k**) powdered spent mushroom materials in a petri dish. **Figure 1.** Preparation of SMM from cultivated *Pleurotus florida* (**a**) chopped rice straws, (**b**) soaked rice straw, (**c**) inoculation of rice straws with 50 g *Pleurotus florida* spawn, (**d**) sealed, inoculated, and sterilized bags, (**e**) fruitful body development, (**f**) first harvest, (**g**) formation of SMM, (**h**) SMM after autoclaving, (**i**) SMM after heat drying, (**j**) A pulverizer, and (**k**) powdered spent mushroom materials in a petri dish.

#### *2.3. Characterization of Clay and Spent Mushroom Materials 2.3. Characterization of Clay and Spent Mushroom Materials*

In this investigation, X-ray fluorescence (XRF) was used to assess the chemical composition of Aswan clay and SMM after they had been ground into powder using an AXIOS, Panalytical 2005 (Malvern Panalytical, Malvern, UK), and a wavelength dispersive (WD) XRF sequential spectrometer (Malvern Panalytical, Malvern, UK). By utilizing the Brukur (Billerica, MA, USA) D8 advanced computerized X-ray diffractometer apparatus with monochromatized CuK radiation operated at 40 kV and 40 mA, it was possible to establish the mineralogical composition of the raw material. The acquired SMM powder was used directly without further processing. The ASTM C136 standard sieve analytical approach was used to calculate the mean particle size [20]. In this investigation, X-ray fluorescence (XRF) was used to assess the chemical composition of Aswan clay and SMM after they had been ground into powder using an AX-IOS, Panalytical 2005 (Malvern Panalytical, Malvern, UK), and a wavelength dispersive (WD) XRF sequential spectrometer (Malvern Panalytical, Malvern, UK). By utilizing the Brukur (Billerica, MA, USA) D8 advanced computerized X-ray diffractometer apparatus with monochromatized CuK radiation operated at 40 kV and 40 mA, it was possible to establish the mineralogical composition of the raw material. The acquired SMM powder was used directly without further processing. The ASTM C136 standard sieve analytical approach was used to calculate the mean particle size [20].

#### *2.4. Preparation and Testing of Samples 2.4. Preparation and Testing of Samples*

The clay was substituted with six different percentages of SMM ranging from 0% to 15% by weight. In order to obtain a homogenous mixture, the raw components were stirred at a speed of 50 rpm for 10 min. The pastes were produced by adding 18 wt.% of water to each of the aforementioned mixtures to get the desired consistency. They were molded into steel cube molds (50 × 50 × 50 mm3 ) and compressed under 10 MPa pressure. The formed samples were left out in the open for a period of 12 h where they were exposed to direct sunlight. The samples were then de-molded and dried overnight in an oven at a temperature of 120 ◦C. In the final stage, the samples were sintered at three different temperatures: 700, 800, and 900 ◦C. The samples were kept in the furnace for a total of 4 h at each temperature. Figure 2 shows the preparation procedure for the bricks. The percentages of linear drying shrinkage for the dried samples were measured. Additionally, linear firing shrinkage, cold and boiling water absorption, apparent porosity, bulk density, compressive strength, and thermal conductivity tests were used to analyze the characteristics of fired samples based on ASTM standard codes and similar to our previous work [3]. The samples were tested in triplicate, and the average value of the parameter under investigation was computed. Figure 3 shows the photo-image of the fabricated bricks. The clay was substituted with six different percentages of SMM ranging from 0% to 15% by weight. In order to obtain a homogenous mixture, the raw components were stirred at a speed of 50 rpm for 10 min. The pastes were produced by adding 18 wt.% of water to each of the aforementioned mixtures to get the desired consistency. They were molded into steel cube molds (50 × 50 × 50 mm3) and compressed under 10 MPa pressure. The formed samples were left out in the open for a period of 12 h where they were exposed to direct sunlight. The samples were then de-molded and dried overnight in an oven at a temperature of 120 °C. In the final stage, the samples were sintered at three different temperatures: 700, 800, and 900 °C. The samples were kept in the furnace for a total of 4 h at each temperature. Figure 2 shows the preparation procedure for the bricks. The percentages of linear drying shrinkage for the dried samples were measured. Additionally, linear firing shrinkage, cold and boiling water absorption, apparent porosity, bulk density, compressive strength, and thermal conductivity tests were used to analyze the characteristics of fired samples based on ASTM standard codes and similar to our previous work [3]. The samples were tested in triplicate, and the average value of the parameter under investigation was computed. Figure 3 shows the photo-image of the fabricated bricks.

**Figure 2.** Schematic of the brick production process. **Figure 2.** Schematic of the brick production process.

**Figure 3.** Photo-image of the fabricated bricks. **Figure 3.** Photo-image of the fabricated bricks.

#### *2.5. Raw Material Characterization 2.5. Raw Material Characterization*

Figure 4 shows the XRD of the raw materials. As seen in the figure, silica and alumina were the two primary compounds present in the clay (Figure 4a), while SMM is composed of sodium aluminum silicate, davidsmithite, trinepheline, sodium aluminum silicate hydroxide, and wollastonite (Figure 4b). Table 1 displays the XRF analysis of the raw materials. The results indicate that SiO2 and Al2O3 are the major compounds in clay, with compositions of between 20% and 50% of SiO2 and between 10% and 20% of Al2O3, which were within the recommended ranges [68, 69]. Figure 4 shows the XRD of the raw materials. As seen in the figure, silica and alumina were the two primary compounds present in the clay (Figure 4a), while SMM is composed of sodium aluminum silicate, davidsmithite, trinepheline, sodium aluminum silicate hydroxide, and wollastonite (Figure 4b). Table 1 displays the XRF analysis of the raw materials. The results indicate that SiO2 and Al2O3 are the major compounds in clay, with compositions of between 20% and 50% of SiO2 and between 10% and 20% of Al2O3, which were within the recommended ranges [68, 69].

**Table 1***.* Chemical composition of raw materials*.* **Table 1.** Chemical composition of raw materials.

LOI 9.2 32 TOTAL 99.756 64.99

SiO2 and Al2O3 were present in lower quantities in SMM. However, it contained more calcium oxide (CaO) and potassium oxide (K2O) than clay. Furthermore, its high organic content is thought to be the cause of its significant loss on ignition (LOI) [21]. Kaolin dehydroxylation is considered the primary cause of the LOI and raising thermal conductivity [22]. Figure 5 displays the particle size distribution of the raw materials. This graph demonstrates the differences in their median sizes (D50), which equal 0.26 mm for clay and 0.6 mm for SMM, demonstrating that clay is finer than SMM. SiO2 and Al2O3 were present in lower quantities in SMM. However, it contained more calcium oxide (CaO) and potassium oxide (K2O) than clay. Furthermore, its high organic content is thought to be the cause of its significant loss on ignition (LOI) [21]. Kaolin dehydroxylation is considered the primary cause of the LOI and raising thermal conductivity [22]. Figure 5 displays the particle size distribution of the raw materials. This graph demonstrates the differences in their median sizes (D50), which equal 0.26 mm for clay and 0.6 mm for SMM, demonstrating that clay is finer than SMM.

**Figure 5.** Distribution of the raw materials' particle sizes. **Figure 5.** Distribution of the raw materials' particle sizes. **Figure 5.** Distribution of the raw materials' particle sizes.

#### *2.6. Simulation Procedure 2.6. Simulation Procedure 2.6. Simulation Procedure*

The study optimized the building design, aiming to achieve optimum thermal performance. First, the model was built in Grasshopper (Figure 6). Then, the thermal model was written in Grasshopper using Honeybee, ladybug, and climate studio (Grasshopper plugins). The plugins work as engines for EnergyPlus, Radiance, Daysim, and OpenStudio [11]. Different materials were identified and inserted into the model as exterior wall materials. The materials were made from Energy Plus 23.1.0 materials. Optimization began with simulation of different types of bricks in a room as a case study. The simulation included studying cooling loads and CO2 emissions. The study optimized the building design, aiming to achieve optimum thermal performance. First, the model was built in Grasshopper (Figure 6). Then, the thermal model was written in Grasshopper using Honeybee, ladybug, and climate studio (Grasshopper plugins). The plugins work as engines for EnergyPlus, Radiance, Daysim, and OpenStudio [11]. Different materials were identified and inserted into the model as exterior wall materials. The materials were made from Energy Plus 23.1.0 materials. Optimization began with simulation of different types of bricks in a room as a case study. The simulation included studying cooling loads and CO2 emissions. The study optimized the building design, aiming to achieve optimum thermal performance. First, the model was built in Grasshopper (Figure 6). Then, the thermal model was written in Grasshopper using Honeybee, ladybug, and climate studio (Grasshopper plugins). The plugins work as engines for EnergyPlus, Radiance, Daysim, and OpenStudio [11]. Different materials were identified and inserted into the model as exterior wall materials. The materials were made from Energy Plus 23.1.0 materials. Optimization began with simulation of different types of bricks in a room as a case study. The simulation included studying cooling loads and CO2 emissions.

**Figure 6.** Definition made in Grasshopper. **Figure 6.** Definition made in Grasshopper. **Figure 6.** Definition made in Grasshopper.

#### *2.7. Description of the Study Model 2.7. Description of the Study Model*  The case study consisted of one unit with dimensions of 4.0 m width, 6.0 m length,

The case study consisted of one unit with dimensions of 4.0 m width, 6.0 m length, and 3.0 m height, located in Cairo, Egypt (Figure 7a). Figure 7b shows the average temperature in Cairo, Egypt. The impact of the new wall section made of different fabricated bricks was compared with the base case. The energy saved using different fabricated bricks was calculated. The exterior walls were 0.25 m thick and contained three layers (plaster–brick– plaster). The material used was EnergyPlus Material. For the HVAC system, the Honeybee plugin acted as an engine for EnergyPlus, and the air conditioning system was set to 24 ◦C. The building operated 24 h/day as it is a residential unit. Energy loads were simulated using Honeybee as an engine for Radiance, Daysim, OpenStudio, and EnergyPlus, and energy performance was evaluated through cooling loads. The building's annual energy consumption (kWh/m2 ) was computed for easier comparison between various materials. and 3.0 m height, located in Cairo, Egypt (Figure 7a). Figure 7b shows the average temperature in Cairo, Egypt. The impact of the new wall section made of different fabricated bricks was compared with the base case. The energy saved using different fabricated bricks was calculated. The exterior walls were 0.25 m thick and contained three layers (plaster–brick–plaster). The material used was EnergyPlus Material. For the HVAC system, the Honeybee plugin acted as an engine for EnergyPlus, and the air conditioning system was set to 24 °C. The building operated 24 h/day as it is a residential unit. Energy loads were simulated using Honeybee as an engine for Radiance, Daysim, OpenStudio, and EnergyPlus, and energy performance was evaluated through cooling loads. The building's annual energy consumption (kWh/m2) was computed for easier comparison between various materials.

*Materials* **2023**, *16*, x FOR PEER REVIEW 8 of 20

**Figure 7.** Study model (**a**) and average temperature (**b**). **Figure 7.** Study model (**a**) and average temperature (**b**).

#### **3. Results and Discussion 3. Results and Discussion**

#### *3.1. Linear Shrinkage 3.1. Linear Shrinkage*

Due to water loss, the clay particles became compact as a result of shrinkage. The increase in the SMM content led to an increase in linear drying shrinkage (Figure 8a). Compared with the control sample, which displayed a linear drying shrinkage of 3.17%, the sample composed of 15% SMM had a maximum linear drying shrinkage of 4.9%. This effect can be attributed to the reduction in quartz content, which helped to increase the volumetric stability of the mixture [23]. The degree of densification during a fire is often determined in part by shrinkage. SMM particles fuse during sintering at high temperatures, resulting in more closely packed particles that improve linear firing shrinkage. Firing temperature is a crucial factor influencing the degree of shrinkage, and large shrinkage could be problematic since it might result in fissures and dimensional flaws. To reduce shrinkage, the firing temperature must be managed during the sintering process. It is worth mentioning that the mechanical performance of the bricks can be maintained after sintering if the linear firing shrinkage is less than 8%. As shown in Figure 8b, firing shrinkage increases with the increase in the amount of SMM and the firing temperature. The findings revealed that shrinkage in the fired clay brick samples ranged from 6.6% to 7.6% with the addition of 15% SMM, while firing shrinkage in the control fired clay bricks ranged from 0.29% to 1.7% without the addition of any SM; thus, shrinkage is influenced by the concentration of the added SMM. All brick samples' shrinkage after firing fell Due to water loss, the clay particles became compact as a result of shrinkage. The increase in the SMM content led to an increase in linear drying shrinkage (Figure 8a).Compared with the control sample, which displayed a linear drying shrinkage of 3.17%, the sample composed of 15% SMM had a maximum linear drying shrinkage of 4.9%. This effect can be attributed to the reduction in quartz content, which helped to increase the volumetric stability of the mixture [23]. The degree of densification during a fire is often determined in part by shrinkage. SMM particles fuse during sintering at high temperatures, resulting in more closely packed particles that improve linear firing shrinkage. Firing temperature is a crucial factor influencing the degree of shrinkage, and large shrinkage could be problematic since it might result in fissures and dimensional flaws. To reduce shrinkage, the firing temperature must be managed during the sintering process. It is worth mentioning that the mechanical performance of the bricks can be maintained after sintering if the linear firing shrinkage is less than 8%. As shown in Figure 8b, firing shrinkage increases with the increase in the amount of SMM and the firing temperature. The findings revealed that shrinkage in the fired clay brick samples ranged from 6.6% to 7.6% with the addition of 15% SMM, while firing shrinkage in the control fired clay bricks ranged from 0.29% to 1.7% without the addition of any SM; thus, shrinkage is influenced by the concentration of the added SMM. All brick samples' shrinkage after firing fell within the permitted range for commercial manufacturing, as stated in ASTM C326 [24]. However, the linear shrinkage of

all the evaluated additions was under 8%. The sample composed of 15 wt.% SMM showed 7.6% linear firing shrinkage at 900 ◦C. However, the linear shrinkage of all the evaluated additions was under 8%. The sample composed of 15 wt.% SMM showed 7.6% linear firing shrinkage at 900 °C.

within the permitted range for commercial manufacturing, as stated in ASTM C326 [24].

*Materials* **2023**, *16*, x FOR PEER REVIEW 9 of 20

**Figure 8.** Linear drying shrinkage (**a**) and firing shrinkage (**b**)**. Figure 8.** Linear drying shrinkage (**a**) and firing shrinkage (**b**).

## *3.2. Bulk Density*

The specific gravity of the raw material source, the manufacturing process, and the firing temperature can all influence the bulk density of fired clay bricks. Figure 9 shows

the influence of SMM and firing temperature on the bulk density of the fired clay bricks. The findings indicate that the maximum replacement percentage (15% SMM) produced the lowest bulk density of 1385.2, 1420.8, and 1419.99 kg/m3 when the samples were subjected to firing temperatures of 700, 800, and 900 ◦C, respectively. Under the same firing temperatures of 700, 800, and 900 ◦C, the bulk density was 1828.7, 1874.2, and 1922 kg/m3 , respectively, for the brick composed of 0% SMM. Accordingly, the bulk density reduced as SMM content increased, possibly due to the increased formation of pores that occur as a result of the degradation of carbonaceous matter in SMM during the sintering process [25]. On the other hand, bulk density increased slightly as the firing temperature increased at each percentage of SMM, possibly due to the verification or higher consolidation between body particles during the sintering process. Based on the results of the bulk density measurement, the brick can be categorized as lightweight if it has a density lower than 1680 kg/m3 in accordance with the requirements of the standard ASTM C90 [26,27]. The bricks met the lightweight standard at all SMM percentages analyzed. the influence of SMM and firing temperature on the bulk density of the fired clay bricks. The findings indicate that the maximum replacement percentage (15% SMM) produced the lowest bulk density of 1385.2, 1420.8, and 1419.99 kg/m3 when the samples were subjected to firing temperatures of 700, 800, and 900 °C, respectively. Under the same firing temperatures of 700, 800, and 900 °C, the bulk density was 1828.7, 1874.2, and 1922 kg/m3, respectively, for the brick composed of 0% SMM. Accordingly, the bulk density reduced as SMM content increased, possibly due to the increased formation of pores that occur as a result of the degradation of carbonaceous matter in SMM during the sintering process [25]. On the other hand, bulk density increased slightly as the firing temperature increased at each percentage of SMM, possibly due to the verification or higher consolidation between body particles during the sintering process. Based on the results of the bulk density measurement, the brick can be categorized as lightweight if it has a density lower than 1680 kg/m3 in accordance with the requirements of the standard ASTM C90 [26,27]. The bricks met the lightweight standard at all SMM percentages analyzed.

The specific gravity of the raw material source, the manufacturing process, and the firing temperature can all influence the bulk density of fired clay bricks. Figure 9 shows

*Materials* **2023**, *16*, x FOR PEER REVIEW 10 of 20

*3.2. Bulk Density*

**Figure 9.** Average values of the bulk density of the samples. **Figure 9.** Average values of the bulk density of the samples.

#### *3.3. Water Absorption 3.3. Water Absorption*

The findings of water absorption by fabricated bricks at 700, 800, and 900 °C, and the clay replacement percentages varying from 0% to 15% SMM are shown in Figure 10. Brick durability can be assessed by measuring its ability to absorb water or by its physical characteristics. As shown in the figure, water absorption by bricks is linearly correlated with the percentage of SMM present, indicating that SMM serves as a pore-generating agent. When clay was substituted with 15 wt.% of SMM, the rate of absorption of cold water rose to 34.7%, 29.6%, and 23.6% at 700, 800, and 900 °C, respectively, compared with 16.4%, 14.5%, and 13.9% at 0% SMM substitution (Figure 10a). On the other hand, boiling water absorption was 37.6%, 34.5%, and 25.3% when 15% SMM was used as a substitute for clay, up from 19.3%, 18.2%, and 17.2% at 700, 800, and 900 °C, respectively, when 0% SMM The findings of water absorption by fabricated bricks at 700, 800, and 900 ◦C, and the clay replacement percentages varying from 0% to 15% SMM are shown in Figure 10.Brick durability can be assessed by measuring its ability to absorb water or by its physical characteristics. As shown in the figure, water absorption by bricks is linearly correlated with the percentage of SMM present, indicating that SMM serves as a pore-generating agent. When clay was substituted with 15 wt.% of SMM, the rate of absorption of cold water rose to 34.7%, 29.6%, and 23.6% at 700, 800, and 900 ◦C, respectively, compared with 16.4%, 14.5%, and 13.9% at 0% SMM substitution (Figure 10a). On the other hand, boiling water absorption was 37.6%, 34.5%, and 25.3% when 15% SMM was used as a substitute for clay, up from 19.3%, 18.2%, and 17.2% at 700, 800, and 900 ◦C, respectively, when 0% SMM substitution was used (Figure 10b). Furthermore, the bricks demonstrated the lowest water absorption percentage and pore volume at 900 ◦C in accordance with the requirements of the standard ASTM C62 [6,28], likely due to the increase in the formation of a glassy phase at high firing temperatures. In general, water absorption increased with the increase in the

SMM in the mixtures and decreased with increasing firing temperature. Brick samples with high water absorbency had greater total porosity, improving the insulating characteristics of the clay bricks. in the SMM in the mixtures and decreased with increasing firing temperature. Brick samples with high water absorbency had greater total porosity, improving the insulating characteristics of the clay bricks.

substitution was used (Figure 10b). Furthermore, the bricks demonstrated the lowest water absorption percentage and pore volume at 900 °C in accordance with the requirements of the standard ASTM C62 [6,28], likely due to the increase in the formation of a glassy phase at high firing temperatures. In general, water absorption increased with the increase

*Materials* **2023**, *16*, x FOR PEER REVIEW 11 of 20

**Figure 10.** Cold water absorption (**a**) and boiling water absorption (**b**). **Figure 10.** Cold water absorption (**a**) and boiling water absorption (**b**).

## *3.4. Apparent Porosity*

Figure 11 illustrates the findings of apparent porosity tests conducted on brick samples burnt at temperatures of 700, 800, and 900 ◦C using SMM clay-replacement percentages varying from 0 to 15%. It was noted that the brick samples containing SMM had more

porosity than the control bricks at different temperatures. This could be due to the gases resulting from the decomposition of carbonaceous materials and the fact that SMM has a higher LOI than clay [23]. According to Sutcu and Akkurt [29], an increase in overall porosity of over 50% is effective at improving the insulating qualities of fired clay bricks. Overall, total porosity increased almost linearly as the percentage of SMM rise, demonstrating the effectiveness of SMM as a pore-forming agent. The results demonstrated an increase in porosity of 29.6, 27.9, and 24.2% when clay was substituted with 15 wt.% SMM at firing temperatures of 700, 800, and 900 ◦C, respectively. Brick samples fired at 900 ◦C exhibited the lowest porosity, likely due to the formation of the glassy phase at high temperatures. Although the formation of pores in the brick structure is the cause of decreased compressive strength, higher water absorption, and total porosity, the increased porosity has a positive impact on the performance of bricks because these pores make it easier for water vapor to move inside the brick skeleton, thereby preventing cracks from spreading, especially in humid climate zones [30]. porosity than the control bricks at different temperatures. This could be due to the gases resulting from the decomposition of carbonaceous materials and the fact that SMM has a higher LOI than clay [23]. According to Sutcu and Akkurt [29], an increase in overall porosity of over 50% is effective at improving the insulating qualities of fired clay bricks. Overall, total porosity increased almost linearly as the percentage of SMM rise, demonstrating the effectiveness of SMM as a pore-forming agent. The results demonstrated an increase in porosity of 29.6, 27.9, and 24.2% when clay was substituted with 15 wt.% SMM at firing temperatures of 700, 800, and 900 °C, respectively. Brick samples fired at 900 °C exhibited the lowest porosity, likely due to the formation of the glassy phase at high temperatures. Although the formation of pores in the brick structure is the cause of decreased compressive strength, higher water absorption, and total porosity, the increased porosity has a positive impact on the performance of bricks because these pores make it easier for water vapor to move inside the brick skeleton, thereby preventing cracks from spreading, especially in humid climate zones [30].

Figure 11 illustrates the findings of apparent porosity tests conducted on brick samples burnt at temperatures of 700, 800, and 900 °C using SMM clay-replacement percentages varying from 0 to 15%. It was noted that the brick samples containing SMM had more

*Materials* **2023**, *16*, x FOR PEER REVIEW 12 of 20

*3.4. Apparent Porosity*

**Figure 11.** Apparent porosity results. **Figure 11.** Apparent porosity results.

#### *3.5. Compressive Strength 3.5. Compressive Strength*

Figure 12 indicates the findings of compression tests conducted on fired brick specimens. The compressive strength of the samples was determined using the equation below; the burnt samples were tested by applying a perpendicular force that is consistent with ISO 9652 [31]. Figure 12 indicates the findings of compression tests conducted on fired brick specimens. The compressive strength of the samples was determined using the equation below; the burnt samples were tested by applying a perpendicular force that is consistent with ISO 9652 [31].

The variable C denotes the compressive strength of fired samples, measured in units of N/mm2 (MPa). The variable "P" denotes the maximum load, while the variable "A" represents the average cross-sectional area of the fired samples. The mean compressive strength of the specimen was determined by computing the average of the compressive

$$\mathbf{C} = \frac{\mathbf{P}}{\mathbf{A}'} \left(\frac{\mathbf{N}}{\text{mm}^2}\right) \tag{1}$$

strengths of three distinct brick specimens. Based on the findings in the figure, the addition of SMM and firing temperature had a significant impact on the compressive strength of fired clay bricks. While the addition of SMM did enhance some physical features, it had a detrimental effect on some mechanical characteristics. After sintering the fabricated bricks at 700 °C, the compressive strength of the brick specimens decreased from 10.23 MPa to 6.06 MPa for 5 SMM, 6.04 MPa for 7.5 SMM, 6 MPa for 10 SMM, 5.89 MPa for 12.5 SMM, and 5.8 MPa for 15 SMM. Due to the significant number of pores and cavities produced by the decomposition and combustion of organic matter, the integration of SMM, as anticipated, decreased the compressive strength [32]. The greater the pore size due to the decomposition of SMM, the smaller the exposed area of the compressive-resistant brick section, suggesting that this may be the cause of the drop in compressive strength. However, increasing the brick firing temperature to 800 °C increased the compressive strength to 12.9 MPa for the 0 SMM sample, 10 MPa for the 5 SMM sample, 9.5 MPa for the 7.5 SMM sample, 8.2 MPa for the 10 SMM sample, 7.6 MPa for the 12.5 SMM sample, and 7 MPa for the 15 SMM sample. In addition, increasing the brick firing temperature to 900°C substantially improved the compressive strengths of the 0 SMM, 5 SMM, 7.5 SMM, 10 SMM, 12.5 SMM, and 15 SMM samples to 18.4 MPa, 18.8 MPa, 14.6 MPa, 10.13 MPa, 9.5 MPa, and 8.6 MPa, respectively. By sealing off internal pores, raising the firing temperature significantly accelerated verification and improved the development of strength. This is consistent with other studies that found that bricks produced at higher firing temperatures have higher densities and mechanical strengths [33,34]. Many studies [19] have shown that when firing temperatures rise, the bulk density of the brick framework increases, leading to greater compressive strength in the brick. The compressive strength requirements for non-load-bearing clay bricks are set at 3.5 N/mm2 by the IS 1077-1992 standards [35]. Therefore, bricks having 15% SMM sintered at 800 or 900 °C still meet the required standards. The ASTM C62 [28] minimum suggested a compressive strength value of 8.6 MPa, which is consistent with brick samples fired at 800 and 900 °C. Furthermore, bricks containing 5% SMM and fired at 900 °C produced the highest compressive

strength of 18.9 MPa.

The variable C denotes the compressive strength of fired samples, measured in units of N/mm2 (MPa). The variable "P" denotes the maximum load, while the variable "A" represents the average cross-sectional area of the fired samples. The mean compressive strength of the specimen was determined by computing the average of the compressive strengths of three distinct brick specimens. Based on the findings in the figure, the addition of SMM and firing temperature had a significant impact on the compressive strength of fired clay bricks. While the addition of SMM did enhance some physical features, it had a detrimental effect on some mechanical characteristics. After sintering the fabricated bricks at 700 ◦C, the compressive strength of the brick specimens decreased from 10.23 MPa to 6.06 MPa for 5 SMM, 6.04 MPa for 7.5 SMM, 6 MPa for 10 SMM, 5.89 MPa for 12.5 SMM, and 5.8 MPa for 15 SMM. Due to the significant number of pores and cavities produced by the decomposition and combustion of organic matter, the integration of SMM, as anticipated, decreased the compressive strength [32]. The greater the pore size due to the decomposition of SMM, the smaller the exposed area of the compressive-resistant brick section, suggesting that this may be the cause of the drop in compressive strength. However, increasing the brick firing temperature to 800 ◦C increased the compressive strength to 12.9 MPa for the 0 SMM sample, 10 MPa for the 5 SMM sample, 9.5 MPa for the 7.5 SMM sample, 8.2 MPa for the 10 SMM sample, 7.6 MPa for the 12.5 SMM sample, and 7 MPa for the 15 SMM sample. In addition, increasing the brick firing temperature to 900 ◦C substantially improved the compressive strengths of the 0 SMM, 5 SMM, 7.5 SMM, 10 SMM, 12.5 SMM, and 15 SMM samples to 18.4 MPa, 18.8 MPa, 14.6 MPa, 10.13 MPa, 9.5 MPa, and 8.6 MPa, respectively. By sealing off internal pores, raising the firing temperature significantly accelerated verification and improved the development of strength. This is consistent with other studies that found that bricks produced at higher firing temperatures have higher densities and mechanical strengths [33,34]. Many studies [19] have shown that when firing temperatures rise, the bulk density of the brick framework increases, leading to greater compressive strength in the brick. The compressive strength requirements for non-load-bearing clay bricks are set at 3.5 N/mm2 by the IS 1077-1992 standards [35]. Therefore, bricks having 15% SMM sintered at 800 or 900 ◦C still meet the required standards. The ASTM C62 [28] minimum suggested

**Figure 12.** Compressive strength results. **Figure 12.** Compressive strength results.

a compressive strength value of 8.6 MPa, which is consistent with brick samples fired at 800 and 900 ◦C. Furthermore, bricks containing 5% SMM and fired at 900 ◦C produced the highest compressive strength of 18.9 MPa. *Materials* **2023**, *16*, x FOR PEER REVIEW 14 of 20

#### *3.6. Thermal Conductivity 3.6. Thermal Conductivity*

Figure 13 shows the variance in thermal conductivity of bricks fired at 900 ◦C as a function of the SMM content. As shown in the figure, thermal conductivity reduced as the SMM content increased. The brick-free SMM had the greatest thermal conductivity (0.77 W/mK), with the brick being denser than other formulations; on the other hand, bricks composed of 15% SMM had the lowest thermal conductivity (0.293 W/mK). As is known, heat is transferred through solid materials by free electrons and/or lattice vibration waves (phonons). In ceramic materials, thermal conduction results from phonons. The presence of pores in the structure of ceramic materials facilitates phonon scattering, which may reduce the thermal conductivity of ceramic materials [36]. The bricks fired at 900 ◦C, including 5 SMM, 10 SMM, 12.5 SMM, and 15 SMM, showed significant reductions of 0.481, 0.43, 0.37, 0.32, and 0.29 (W/mK), respectively, in thermal conductivity compared with 0.77 (W/mK) of 0 SMM. The development of pores during sintering as a result of the production of gases that arise from the decomposition of carbonate materials and the high LOI in the SMM may be a possible explanation for the decrease in heat conductivity that occurs with the increase in the proportion of SMM in the material. Figure 13 shows the variance in thermal conductivity of bricks fired at 900 °C as a function of the SMM content. As shown in the figure, thermal conductivity reduced as the SMM content increased. The brick-free SMM had the greatest thermal conductivity (0.77 W/mK), with the brick being denser than other formulations; on the other hand, bricks composed of 15% SMM had the lowest thermal conductivity (0.293 W/mK). As is known, heat is transferred through solid materials by free electrons and/or lattice vibration waves (phonons). In ceramic materials, thermal conduction results from phonons. The presence of pores in the structure of ceramic materials facilitates phonon scattering, which may reduce the thermal conductivity of ceramic materials [36]. The bricks fired at 900 °C, including 5 SMM, 10 SMM, 12.5 SMM, and 15 SMM, showed significant reductions of 0.481, 0.43, 0.37, 0.32, and 0.29 (W/mK), respectively, in thermal conductivity compared with 0.77 (W/mK) of 0 SMM. The development of pores during sintering as a result of the production of gases that arise from the decomposition of carbonate materials and the high LOI in the SMM may be a possible explanation for the decrease in heat conductivity that occurs with the increase in the proportion of SMM in the material.

**Figure 13.** Thermal conductivity results. **Figure 13.** Thermal conductivity results.

#### *3.7. SEM Microstructure Analysis 3.7. SEM Microstructure Analysis*

Figure 14 shows the microstructural characteristics of bricks that were burned at 900 °C and contained either 5% or 15% SMM. It is plain to see that the bricks that were infused with SMM produced a sizeable number of micropores, all of which were dispersed throughout the matrix of the hardened bricks. The pores that were visible took on a number of different shapes, including spheres and ovals. When the SMM percentage was increased, both the number of micropores and their diameters also increased. The average pore size increased from 2.92 to 5.71 µm when the percentage of SMM increased from 5% to 15%. This helps to explain the decrease in thermal conductivity, compressive strength, and bulk density, as well as the rise in water absorption and apparent porosity. Figure 14 shows the microstructural characteristics of bricks that were burned at 900 ◦C and contained either 5% or 15% SMM. It is plain to see that the bricks that were infused with SMM produced a sizeable number of micropores, all of which were dispersed throughout the matrix of the hardened bricks. The pores that were visible took on a number of different shapes, including spheres and ovals. When the SMM percentage was increased, both the number of micropores and their diameters also increased. The average pore size increased from 2.92 to 5.71 µm when the percentage of SMM increased from 5% to 15%. This helps to explain the decrease in thermal conductivity, compressive strength, and bulk density, as well as the rise in water absorption and apparent porosity.

**Figure 14.** SEM images of 5% (**a**) and 15% (**b**) SMM bricks fired at 900 °C. **Figure 14.** SEM images of 5% (**a**) and 15% (**b**) SMM bricks fired at 900 ◦C.

#### *3.8. Estimation of Annual Energy Consumption for Cooling 3.8. Estimation of Annual Energy Consumption for Cooling*

The effect of bricks with different SMM proportions on a case study's cooling demands was evaluated using Grasshopper plugins Rhin 7 thermal simulation software. The application starts by simulating a room as a basic case to identify the optimal SMM ratio in terms of energy efficiency. As can be seen in Figure 15, the monthly variation in energy use was largest during the summer months of April through October. In addition, traditional bricks consumed the highest energy for cooling (Figure 15a,b) in both Cairo and Jazan cities. Samples with 15 SMM bricks used substantially less energy for cooling than their standard counterparts (Figure 15c,d) in Cairo and Jazan cities. The highest monthly average for cooling energy used occurred in July, the warmest month of the year; this was particularly true in the mornings. Figure 16 shows the simulation results, which indicate that different SMM brick compositions affect cooling energy consumption. The replacement of standard bricks with a 15 SMM brick sample reduced the building's cooling needs by 129.514 kWh, which is approximately 29.23%, compared with the 0 SMM (183 kWh) bricks (Figure 16a). The total amount of energy that was utilized for cooling reduced by 134.606 kWh, which is an improvement of 26.44%, when 12.5 SMM was used. Furthermore, cooling energy consumption can be reduced by 141.898 kWh with 10 SMM, 148.611 kWh with 7.5 SMM, and 154.63 kWh with 5 SMM, representing improvement rates of 22.46%, 18.79%, and 15.50%, respectively, (Figure 16b). This significantly reduces the building's energy consumption, as space cooling accounts for 37–42% of its overall energy requirement [37,38]. The suggested manufactured brick samples were examined for thermal and energy efficiency in the city of Jazan in the Kingdom of Saudi Arabia (Figure 16a,b), which has a climate characterized by hot, arid desert conditions. The sample bricks were compared with a standard brick for the purpose of establishing a reference point. The hypothesis posits that a model identical to that implemented in Cairo should be utilized in Jazan. Using 15 SMM resulted in a reduction in energy consumption of 21.49%. Utilization of clay bricks derived from SMM exhibited promising prospects of enhancing thermal efficiency and energy conservation and reducing the carbon footprint in arid and The effect of bricks with different SMM proportions on a case study's cooling demands was evaluated using Grasshopper plugins Rhin 7 thermal simulation software. The application starts by simulating a room as a basic case to identify the optimal SMM ratio in terms of energy efficiency. As can be seen in Figure 15, the monthly variation in energy use was largest during the summer months of April through October. In addition, traditional bricks consumed the highest energy for cooling (Figure 15a,b) in both Cairo and Jazan cities. Samples with 15 SMM bricks used substantially less energy for cooling than their standard counterparts (Figure 15c,d) in Cairo and Jazan cities. The highest monthly average for cooling energy used occurred in July, the warmest month of the year; this was particularly true in the mornings. Figure 16 shows the simulation results, which indicate that different SMM brick compositions affect cooling energy consumption. The replacement of standard bricks with a 15 SMM brick sample reduced the building's cooling needs by 129.514 kWh, which is approximately 29.23%, compared with the 0 SMM (183 kWh) bricks (Figure 16a). The total amount of energy that was utilized for cooling reduced by 134.606 kWh, which is an improvement of 26.44%, when 12.5 SMM was used. Furthermore, cooling energy consumption can be reduced by 141.898 kWh with 10 SMM, 148.611 kWh with 7.5 SMM, and 154.63 kWh with 5 SMM, representing improvement rates of 22.46%, 18.79%, and 15.50%, respectively, (Figure 16b). This significantly reduces the building's energy consumption, as space cooling accounts for 37–42% of its overall energy requirement [37,38]. The suggested manufactured brick samples were examined for thermal and energy efficiency in the city of Jazan in the Kingdom of Saudi Arabia (Figure 16a,b), which has a climate characterized by hot, arid desert conditions. The sample bricks were compared with a standard brick for the purpose of establishing a reference point. The hypothesis posits that a model identical to that implemented in Cairo should be utilized in Jazan. Using 15 SMM resulted in a reduction in energy consumption of 21.49%. Utilization of clay bricks derived from SMM exhibited promising prospects of enhancing thermal efficiency and energy conservation and reducing the carbon footprint in arid and high-temperature settings.

high-temperature settings.

*Materials* **2023**, *16*, x FOR PEER REVIEW 16 of 20

**Figure 15.** Monthly required cooling load for 0 SMM in Cairo (**a**) and Jazan (**b**) and for 15 SMM in Cairo (**c**) and Jazan (**d**). **Figure 15.** Monthly required cooling load for 0 SMM in Cairo (**a**) and Jazan (**b**) and for 15 SMM in Cairo (**c**) and Jazan (**d**). **Figure 15.** Monthly required cooling load for 0 SMM in Cairo (**a**) and Jazan (**b**) and for 15 SMM in Cairo (**c**) and Jazan (**d**).

ferent percentages of SMM. *3.9. Influence of the Proposed Fabricated Bricks on CO2 Emissions*  **Figure 16.** Annual energy consumed for cooling (**a**) and percentage of energy saved (**b**) using different percentages of SMM. **Figure 16.** Annual energy consumed for cooling (**a**) and percentage of energy saved (**b**) using different percentages of SMM.

#### Embodied CO2 has become an increasingly significant contributor to the entire life-*3.9. Influence of the Proposed Fabricated Bricks on CO2 Emissions 3.9. Influence of the Proposed Fabricated Bricks on CO2 Emissions*

cycle of construction in the last few decades. This is the case despite the fact that developments in technology have led to a decrease in operational CO2 emissions. The main sources of operational CO2 emissions are HVAC and lighting; the major sources of embodied CO2 emissions are construction materials [39]. The results of this study suggest a link between CO2 emissions from building operations and the increasing prevalence of air conditioning in hot climates. The construction industry likely contributes to the rising atmospheric concentrations of embodied and operational CO2. Construction materials have been the subject of extensive studies and developments. As a result, buildings' thermal performance has improved, the amount of energy required to keep them cool has decreased, and CO2 emissions have decreased. The results of this research suggest that SMM bricks may benefit building occupants' health by decreasing their dependence on air conditioning and slowing the rate at which heat travels through walls. As a result, indoor temperatures will be more pleasant. In addition, it is possible that cutting down CO2 emissions would be possible thanks to the utilization of SMM in bricks. Figure 17 shows the Embodied CO2 has become an increasingly significant contributor to the entire lifecycle of construction in the last few decades. This is the case despite the fact that developments in technology have led to a decrease in operational CO2 emissions. The main sources of operational CO2 emissions are HVAC and lighting; the major sources of embodied CO2 emissions are construction materials [39]. The results of this study suggest a link between CO2 emissions from building operations and the increasing prevalence of air conditioning in hot climates. The construction industry likely contributes to the rising atmospheric concentrations of embodied and operational CO2. Construction materials have been the subject of extensive studies and developments. As a result, buildings' thermal performance has improved, the amount of energy required to keep them cool has decreased, and CO2 emissions have decreased. The results of this research suggest that SMM bricks may benefit building occupants' health by decreasing their dependence on air conditioning and slowing the rate at which heat travels through walls. As a result, indoor temperatures will be more pleasant. In addition, it is possible that cutting down CO2 emis-Embodied CO2 has become an increasingly significant contributor to the entire lifecycle of construction in the last few decades. This is the case despite the fact that developments in technology have led to a decrease in operational CO2 emissions. The main sources of operational CO2 emissions are HVAC and lighting; the major sources of embodied CO2 emissions are construction materials [39]. The results of this study suggest a link between CO2 emissions from building operations and the increasing prevalence of air conditioning in hot climates. The construction industry likely contributes to the rising atmospheric concentrations of embodied and operational CO2. Construction materials have been the subject of extensive studies and developments. As a result, buildings' thermal performance has improved, the amount of energy required to keep them cool has decreased, and CO2 emissions have decreased. The results of this research suggest that SMM bricks may benefit building occupants' health by decreasing their dependence on air conditioning and slowing the rate at which heat travels through walls. As a result, indoor temperatures will be more pleasant. In addition, it is possible that cutting down CO2 emissions would be possible

sions would be possible thanks to the utilization of SMM in bricks. Figure 17 shows the

thanks to the utilization of SMM in bricks. Figure 17 shows the massive amounts of CO2 emissions produced in order to provide cooling. It was demonstrated that using 5 SMM might result in an annual decrease of 2.2% in CO2 emissions. massive amounts of CO2 emissions produced in order to provide cooling. It was demonstrated that using 5 SMM might result in an annual decrease of 2.2% in CO2 emissions.

*Materials* **2023**, *16*, x FOR PEER REVIEW 17 of 20

**Figure 17.** Annual CO2 emission of the fabricated bricks. **Figure 17.** Annual CO2 emission of the fabricated bricks.

#### **4. Conclusions 4. Conclusions**

The following are inferences drawn from the study's practical findings: The following are inferences drawn from the study's practical findings:

**Supplementary Materials:** The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Table S1: Demonstrates physco-mechanical and thermal characteristics of bricks substituted with different waste material. References [40–52] are cited in the supplementary materials. **Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma16144905/s1, Table S1: Demonstrates physco-mechanical and thermal characteristics of bricks substituted with different waste material. References [40–52] are cited in the supplementary materials.

sumption were achieved.

**Author Contributions:** Methodology, S.A.A., M.K.F. and M.M.A.; Validation, N.Z. and I.M.M.; Formal analysis, A.A.; Investigation, N.Z. and A.A.; Data curation, M.K.F. and I.M.M.; Writing original draft, S.A.A., M.K.F. and M.M.A.; Writing—review & editing, A.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through project number ISP22-4.

**Institutional Review Board Statement:** Not Applicable.

**Informed Consent Statement:** Not Applicable.

**Data Availability Statement:** The data presented in this study are available from the corresponding authors upon reasonable request.

**Acknowledgments:** The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through project number ISP22-4.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

## *Review* **A Study on Sustainable Concrete with Partial Substitution of Cement with Red Mud: A Review**

**Hisham Jahangir Qureshi 1,\* , Jawad Ahmad 2 , Ali Majdi 3 , Muhammad Umair Saleem 4 , Abdulrahman Fahad Al Fuhaid 1 and Md Arifuzzaman 1**

**Abstract:** Every year, millions of tons of red mud (RDM) are created across the globe. Its storage is a major environmental issue due to its high basicity and tendency for leaching. This material is often kept in dams, necessitating previous attention to the disposal location, as well as monitoring and maintenance during its useful life. As a result, it is critical to develop an industrial solution capable of consuming large quantities of this substance. Many academics have worked for decades to create different cost-effective methods for using RMD. One of the most cost-effective methods is to use RMD in cement manufacture, which is also an effective approach for large-scale RMD recycling. This article gives an overview of the use of RMD in concrete manufacturing. Other researchers' backgrounds were considered and examined based on fresh characteristics, mechanical properties, durability, microstructure analysis, and environmental impact analysis. The results show that RMD enhanced the mechanical properties and durability of concrete while reducing its fluidity. Furthermore, by integrating 25% of RDM, the environmental consequences of cumulative energy demand (CED), global warming potential (GWP), and major criteria air pollutants (CO, NOX, Pb, and SO2 ) were minimized. In addition, the review assesses future researcher guidelines for concrete with RDM to improve performance.

**Keywords:** red mud; eco-friendly concrete; durability; slump; mechanical strength

## **1. Introduction**

To make constant strides toward sustainable growth, a massive revolution in the cement and concrete sector is required to decrease environmental pollution, particularly carbon dioxide [1]. Concrete manufacturing has increased in recent decades, and it is currently one of the top concerns of scholars who are interested in sustainable development [2–5]. The utilization of waste materials in concrete manufacturing has been shown to minimize natural resource use [6–9]. Construction uses more raw materials and energy than any other economic activity on the planet today. Concrete is a common construction material used all over the globe. Cement, being a fundamental component of concrete, is produced via an energy-intensive process. Cement manufacturing produces a lot of greenhouse gas emissions, which contribute to global warming [5,10]. The cement factories are the second-largest industrial carbon dioxide emitter, accounting for 5 to 7% of total CO2 emissions [11,12]. The use of various waste materials as cement substitutes has been researched for CO2 reduction [13–15]. In contemporary civilizations, using garbage as a secondary material in the building sector is a cost-effective and environmentally friendly means of disposing of waste [16,17]. Sustainability in the construction industry has become crucial, and numerous solutions have emerged to lessen the environmental effect of present

**Citation:** Qureshi, H.J.; Ahmad, J.; Majdi, A.; Saleem, M.U.; Al Fuhaid, A.F.; Arifuzzaman, M. A Study on Sustainable Concrete with Partial Substitution of Cement with Red Mud: A Review. *Materials* **2022**, *15*, 7761. https://doi.org/10.3390/ ma15217761

Academic Editors: Stefano Guarino and José Barroso de Aguiar

Received: 9 September 2022 Accepted: 17 October 2022 Published: 3 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

building operations [18,19]. Rising energy supply costs, decreasing CO2 ejections, and the delivery of unrefined, low-quality ingredients all pose risks to the cement business [20]. As a result, alternative suppliers must be sought instead of cement. According to research, concrete created from waste materials such as plastic waste is one of the waste disposal alternatives [21].  In addition to utilizing different waste materials cementitious materials in concrete, RMD is also a good option.

### *1.1. RMD*

Bauxite residue is an insoluble byproduct of the Bayer alumina manufacturing process. The high alkalinity of the liquid phase isolated from the RMD slurry is a major environmental concern. The challenges related to bauxite waste formation were first overlooked and ignored throughout the discovery and usage of bauxite for profit. However, when the world's population exploded in the mid-twentieth century, resources began to deplete and waste products began to accumulate on the planet's surface [31]. As a consequence, by the end of the twentieth century, the use of bauxite waste had become a worldwide issue. Every year, roughly 150 million tons of bauxite residue are generated worldwide [32]. In general, 1–2.5 tons of bauxite residue are created for every ton of alumina produced, with the amount varying depending on the kind of bauxite ore utilized [33]. The RMD collected from an alumina refinery is shown in Figure 1.

**Figure 1.** RMD [34].

Bauxite is a mineral composed of hydrated aluminium oxides and mixtures of other elements, such as iron. The aluminium mixtures in the bauxite may be found in many kinds of aluminium parts, and contaminations may influence extraction conditions. Aluminium oxides and hydroxides are amphoteric, which means they are acidic and fundamental at the same time. Al3 solubility in water is minimal, although it increases dramatically with high or low PH. Bauxite metal is warmed in a weight vessel beside a sodium hydroxide setup at a temperature of 150 to 200 ◦C in the Bayer technique. In an extraction technique, the aluminium is broken down as sodium aluminate (basically [Al(OH)4]) at these temperatures. Gibbsite is accelerated when the fluid is cooled and seeded with fine-grained aluminium hydroxide precious stones from previous extractions after partitioning the accumulation by sifting. The RMD process is seen in Figure 2 [35].

The chemical difference in every circumstance, applying RMD is a challenge. However, detecting the RMD features for each location where it is created and establishing a template as a foundation is achievable. Two variables influence the quantity of RMD produced. The first pertains to the ore quality, while the second refers to the processing conditions. The amount of RMD produced has been approximated by many writers. The reported values vary from 0.3 to 2.5 tons per ton of poorer grade bauxite treated [36]. Patents for RMD applications were created between 1964 and 2008, according to research [37]. It is expected that 33 percent of them are used in civil building projects, as shown in Figure 3. Although RMD has a variety of uses, as seen in Figure 3, its primary use is in the building industry.

**Figure 3.** RMD Application (1964 to 2008): Data source [38].

Figure 4 depicts data on bauxite residue applications research publications by companies, academic institutions, and research organizations during a 20-year period. The biggest number of research articles have been published in the field of building and construction. The largest volume of RMD consultation materials was released from 1971 to 1990, and subsequently, quickly declined, as seen in Figure 4. As a result, a concise review is needed to highlight current developments and prospective applications of RDM in the construction industry.

**Figure 4.** No. of Publication on RMD (1931 to 2018): Data Source [36].

## *1.2. Weaknesses of RMD*

RMD affects agricultural landscapes due to its strong alkalinity, hence, it should be cleaned numerous times before usage. There is a chance that the RMD lake may leak. If it is organized underground, it pollutes the ground water table. Based on the writing overview, RMD fills in as an excellent folio material and has proven to be a decent cementation material. The tiny pores of cement are diminished by RMD, and as the amount of RMD in solid increments increases, the amount of water swallowed decreases. A basic combination of 70% RMD and 30% CaO yields a product with a compressive quality of 7 MPa. Ca(OH)2, C4AH13 and C4AH11 are the hydrates framed after 4 days. Later investigations have confirmed these findings. When compared to fly powder, RMD does not impart much compressive quality, although flexural quality and resistance to porousness have been seen in RMD bond concrete. Despite the fact that RMD is less accessible than flies' fiery remnants, it is critical to use or reuse RMD since it has several harmful ecological effects [39].

#### *1.3. Chemical Composition*

Table 1 shows the chemical components of RDM while Figure 5 shows the XRD of RDM. The primary components are alumina and iron oxide, as assumed, but the relative proportions of SiO2 and Na2O are also important. In addition to aluminium hydroxide and a complicated Na5Al3CSi3O15 phase, XRD detects several of these oxides. Different researchers have reported different chemical compositions, as can be shown in Figure 5. It is likely that the chemical composition of RDM varies as the source changes. However, the accumulation of different ingredients such as silica, alumina, and iron reported by each author is higher than 50%, indicating that RDM has the potential to be employed as binding material. According to ASTM [29], the sum of the three principal oxide constituents, namely, silica, alumina, and iron, must be at least 50% for a material to be classed as pozzolanic. According to Table 1, all the RDM samples used in the various research projects may be classified as pozzolanic as per ASTM [29].

#### **Figure 5.** XRD of RDM [45].

### *1.4. Significances*

The use of RMD in the manufacturing of concrete as a cement substitute has a number of environmental and economic advantages, including minimizing soil and groundwater contamination, reducing dust pollution, conserving natural resources for cement clinker, and lowering the cost of concrete production. The purpose of this paper is to present a detailed review of current progress on the use of RMD in concrete production, as well as to clearly point out four directions for using RMD in concrete production, namely, fresh properties, mechanical properties, durability aspects, and environmental aspects. Furthermore, the review also evaluates the future researcher guideline for better performance.

#### **2. Fresh Properties**

#### *2.1. Workability*

As indicated in Figure 6, the slump flow diminishes dramatically as the quantity of RMD supplied boosts. The rise in adsorbed water generated by the porous nature and the greater specific surface area of RMD causes the reduction in workability. Although the addition of RMD may cause water between particles to relax, it also raises the packing

density, which squeezes the free water between the particles and improves workability [46]. This might be due to the porous RMD which enhanced water absorption ability [47]. RDM, which has smaller particles than cement, reduces the fluidity of concrete by absorbing more moisture in the fresh concrete condition. However, since water made up around 48 percent of the total mass of the RDM employed in this investigation, adequate moisture could be provided in the fresh concrete phase [48]. However, the larger surface area of the seawater-neutralized bauxite refinery residence is most likely to blame for the decrease in a slump. Furthermore, unlike water added to natural sand during concrete production, water in seawater-neutralized bauxite refinery reside containing mix may not be as readily available to lubricate the mix, as it may be held within the fine-particle aggregates or chemically bound with the hygroscopic seawater-neutralized bauxite refinery reside [49]. A study [42] explored the slump flow of self-compacting concrete as per standard [50] and claimed that RMD decreased the slump flow of concrete.

**Figure 6.** Slump Flow: Data Source [51].

#### *2.2. Fresh Density*

Figure 7 depicts the changes in the fresh density of concrete after 28 days in relation to RDM cement replacement. The dry density ranges from 1685 to 1789 kg/m3 , whereas the fresh density ranges from 1752 to 1844 kg/m3 . When RDM concentration in concrete is raised, the specific gravity of the concrete drops. The specific gravity of the concrete specimen having 25% RDM is around 5% lower than the reference specimen with 0% RDM, according to the findings [40]. In contrast, a study claimed that the fresh density of concrete was enhanced due to the micro filling effect of pozzolanic material which gives more dense concrete. Furthermore, due to the pozzolanic reaction, the binding properties of concrete paste also contribute to the improved density of concrete. The combined micro filling voids and pozzolanic reaction have a positive influence on the density of concrete [52]. According to research, due to improved particle packing, there was an initial rise in density and a reduction in porosity due to micro filling voids in concrete aggregates. However, at 20% RMD addition, the behaviour becomes restrained because of additional challenges in moulding and shaping samples due to lack of flowability [45].

**Figure 7.** Fresh Density: Data Source [40].

## *2.3. Setting Time*

The addition of RMD has the effect of speeding up the setting process, as shown in Figure 8. For mortars without RMD and those containing 20% waste, the end of the setting time changes from 345 to 300 min.

**Figure 8.** Setting Time [45].

The presence of aluminium and sodium hydroxides, which are known as accelerators [53], might explain this impact, in the mud, and also because of its high alkaline content. The fineness of waste particles might potentially help to retain water by competing

with cement. As all formulas have the same amount of water, the leftover free fraction, which may be coupled with cement particles, will be consumed quickly [54]. In contrast, another study suggests that the pozzolanic materials decrease the setting time of cement paste as the reaction proceeds slowly [55]. The change in behavior might be possible due to a change in chemical composition.

#### **3. Mechanical Properties**

#### *3.1. Compressive Strength (CMS)*

Figure 9 depicts the CMS of concrete with various RDM replacement ratios. It can be shown that with 10% RDM replacement, maximum CMS is obtained. According to research, samples with a low level of RDM had good CMS [56]. The characteristics of RMD cementitious content change when 20% of the RMD is replaced with cement by weight. As a result, more than 20% RMD substitution reduces the concrete solid mass's compressive quality and stiffness [57]. Increases the load-bearing capability of RMD-based concrete samples by up to 10% by increasing the RMD component [58]. The self-compacting concrete (SCC) mixes created with RMD inclusion showed equivalent CMS to the control at 28 days, according to the findings. However, the impacts of red dirt content on CMS were more significant at 56 and 90 days. At 56 days, the strength of the SCC mixture samples containing 30% and 40% RMD was 89.4 MPa and 90.1 MPa, respectively, which were 8% and 9% higher than the control sample [43]. The addition of RMD had no effect on the hydration process, but when the RMD content was more than 20% (by weight of cement), the hydration of cement paste was reduced [54]. The result shows that the CMS of 20 percent partly substituted RMD concrete is higher than that of conventional concrete. This is due to an increase in the cement's pressure quality and bond with the aggregate. As the contribution of bonding between cement and aggregate is reduced as compared to the former, increasing the amount of RMD in concrete does not result in an increase in CMS values [59]. The biggest changes were noted in the 7-day strength improvements, with 13% disparities between the 12.5 percent and 50 percent replacement levels. The major explanation is most likely owing to a high concentration of hatrurite in the cement matrix, which will be explained later. Enhanced RMD content increased CMS somewhat at 56 days, with a maximum difference of less than 4%. As a result, it is possible that RMD concentration has no effect on compressive strength, however, RMD SCC mixes in general had greater CMS than control concrete [48]. The impact of RMD on the properties of cement mortars in terms of setting time, pozzolanic activity, and mechanical strength was explored in the research. They discovered that adding RMD sped up the setting process and lowered the pozzolan reaction and that the CMS fell as the quantity of RMD increased. It was discovered that RMD may be used to partly substitute cement in non-structural mortars and concrete [60]. The compressive strength and TS of a cement mortar, in which RDM substituted up to 50% of the cement, were observed to decline when the RDM concentration was increased [48]. With an increasing amount of replacements, RMD accelerates the curing time of concrete and lowers CMS [61]. According to research, replacing 6% of RMD in concrete boosted CMS by 6%. This might be attributed to microstructure densification, as seen by the decrease in calcium hydroxide concentration after RMD replacement [62]. The strength of the concrete was reduced after 10% RMD substitution, however, it was not lower than regular concrete mixtures. CMS was lowered at 15% and 20% RMD replacement in concrete owing to inadequate cement hydration due to the presence of increased RMD content at higher replacement levels [63]. A similar justification was given by Cheng et al. [64], the large specific surface area of RMD absorbs more water in the concrete mix, resulting in a lack of water for adequate cement hydration. Table 2 shows a summary of CMS with partial substitution of RDM.

**Figure 9.** Compressive Strength: Data Source [44].

**Table 2.** *Cont.*

Fiber = F; Bauxite = B; RMD = RDM; Treated RMD = UTRM; Untreated RMD = TRM.