**4. Modified waste and recycled materials and their uses in construction materials**

In this section different studies concerning the structural modification of waste and recycling materials by using ionizing irradiation and their possibilities as reinforced materials of hydraulic and polymer concrete are shown.

For recycled PET, nonirradiated concrete follows a typical behavior for compressive strain: it increases progressively as PET particle concentration increases, but it does not happen for compressive strength or elasticity modulus. In the case of irradiated concrete, different behaviors are observed regarding nonirradiated ones. When increasing PET concentration, the compressive strength values diminish; it is more notable: the diminution of compressive strain. In general, irradiated concrete containing PET particles had similar modulus of elasticity, higher compressive strength, and lower compressive strain values compared to nonirradiated concrete.

Compressive strength and Young's modulus of concrete specimens containing waste PET particles of beverage bottles were evaluated before and after irradiation. Three different sizes of waste PET particles (0.5, 1.5, and 3.0 mm) were considered, and for each size, three different concentrations of waste PET particles were used (1.0, 2.5, and 5.0% by volume). Concrete specimens after 28 days of moist curing were irradiated at 100 kGy with gamma rays at 3 kGy/ h ratio.

In the case of irradiated concrete, different behaviors are observed regarding nonirradiated ones. When increasing PET concentration, the compressive strength values diminish; it is more notable: the diminution of compressive strain. Nevertheless, elasticity modulus has an opposite behavior to that shown for nonirradiated concrete. In terms of the particle sizes, different behaviors are observed; at the lowest sizes, compressive strength has minimal values; whereas for highest sizes, both compressive strength and modulus of elasticity have the maximal values. Such situations are similar for irradiated specimens because modulus of elasticity, higher compressive strength, and lower strain values are maximal.

Irradiation effects are caused over PET particles, as it is well known that irradiation causes chain scission and generation of free radicals, which can produce a hard material instead of a ductile. In the case of irradiated PET particles (at 150 kGy), a smooth and homogeneous surface is observed (**Figure 2**); when increasing the irradiation dose, morphological changes are produced; small particles and cracks are observed (at 400 kGy). For the highest irradiation dose, more defined cracks and particles of different sizes are observed (at 800 kGy); in general, a roughness surface is obtained (**Figure 2**).

**Figure 2.** SEM images of irradiated PET at different application doses.

tration of chemicals is observed. Such zones have different appearances such as cracks and

Tetra Pak panel boards (TPPBs) show decrease in the mass up to 200°C which is related to the evaporation of physical water. In general, thermal degradation of paper is located between 200°C and 400°C, particularly two decomposition peaks are observed. The first one at 300°C due to hemicellulose and the second at 360°C due to thermal degradation of α-cellulose. For higher temperature from 400°C to 461°C, degradation of remaining paper and LDPE is considered. After thermal process can be found two kinds of residues, char and aluminum foil

In the case of irradiated polyester resin some physicochemical properties are affected, for example, when increasing the dose a better thermal stability is obtained at low temperatures, because its glass transition temperature increases. But at high temperatures, the decomposition temperature is unaffected. After analyzing both thermal and mechanical properties a relationship is observed. Moreover, a typical behavior is observed: improvement of the compres-

**4. Modified waste and recycled materials and their uses in construction**

In this section different studies concerning the structural modification of waste and recycling materials by using ionizing irradiation and their possibilities as reinforced materials of

For recycled PET, nonirradiated concrete follows a typical behavior for compressive strain: it increases progressively as PET particle concentration increases, but it does not happen for compressive strength or elasticity modulus. In the case of irradiated concrete, different behaviors are observed regarding nonirradiated ones. When increasing PET concentration, the compressive strength values diminish; it is more notable: the diminution of compressive strain. In general, irradiated concrete containing PET particles had similar modulus of elasticity, higher compressive strength, and lower compressive strain values compared to nonirradiated

Compressive strength and Young's modulus of concrete specimens containing waste PET particles of beverage bottles were evaluated before and after irradiation. Three different sizes of waste PET particles (0.5, 1.5, and 3.0 mm) were considered, and for each size, three different concentrations of waste PET particles were used (1.0, 2.5, and 5.0% by volume). Concrete specimens after 28 days of moist curing were irradiated at 100 kGy with gamma rays at 3 kGy/

In the case of irradiated concrete, different behaviors are observed regarding nonirradiated ones. When increasing PET concentration, the compressive strength values diminish; it is more notable: the diminution of compressive strain. Nevertheless, elasticity modulus has an opposite behavior to that shown for nonirradiated concrete. In terms of the particle sizes,

sive strength depends on the increment of the irradiation dose [23].

hydraulic and polymer concrete are shown.

irregular morphological shapes.

168 Composites from Renewable and Sustainable Materials

[24].

**materials**

concrete.

h ratio.

Generally speaking, as waste PET concentration increases in the concrete specimens, a decreasing tendency on the mechanical properties is observed. Moreover, irradiated concrete specimens show higher compressive strength values, similar elasticity modulus values, but lower deformations when compared to nonirradiated specimens.

Some studies covered the effects of gamma radiation on composite materials, for example, on the mechanical properties and durability of cement concretes. Some applications include concrete as material for nuclear power reactors; for this purpose, the specimens were submitted to dosages from 227 and 470 MGy with a dose rate of 5.0 kGy/h. The results show a diminution of about 10% on the elastic and tensile properties, as well as loss of weight, caused by one or more of the following mechanisms: (a) "natural" drying (including gamma heating); (b) radiolysis-induced accelerated drying (where large gas is released); (c) radiolysis-induced carbonation; and (d) degradation of the calcium-bearing cement hydrates.

In hydraulic concrete where silica sand is partially replaced by recycled automotive tire fibers. Both tire fibers and modified concrete are irradiated at different gamma doses. Main mechanical properties are studied before and after irradiation process. These include compression and flexural strength. The mechanical properties of concrete depend on the waste tire particle sizes and their concentration. Compressive and tensile strength values decrease due to waste tire particles, because they promote stress concentration zones, as well as, generation of tensile stresses into concrete, resulting in a fast cracking and soon failure. Nevertheless, when applying gamma radiation to waste tire particles, in some cases, improvements on mechanical properties are found. Concrete with irradiated particles can be support up to 30% of tire particles, making possible to reduce the final cost of the concrete.

In the case of polymer concrete with recycled tire fibers, strength and strain results show improvements of mechanical properties according to the tire fiber concentration as well as gamma irradiation dose. In general terms, addition of recycled tire fibers as well as higher radiation doses generate greater ductility on the polymer concrete; features no common for ordinary polymer concrete.

In **Figure 3**, surface characteristics of the recycled tire particles are shown. Nonirradiated particles have different sizes; some of them show roughness on their surface and others smooth surfaces. Average size of recycled particles varies from 30 to 600 μm. In general, when recycled particles are added to concrete, a poor elastomer-matrix adherence is found, but when increasing the volume fraction of particles, mechanical interactions are augmented, therefore improvements on the mechanical properties are obtained. For irradiated tire particles, at 200 kGy, rough surfaces are created, with some small and disperse particles. According to the literature, sometimes smooth surfaces are generated after irradiation as a consequence of the cross-linking of polymer chains, while for higher dose, scissions of the polymer chains are done, which is manifested by appearances of cracks on the surfaces; as it is shown for irradiated particles at 250 kGy (**Figure 3**).

**Figure 3.** SEM images of nonirradiated and irradiated tire rubber.

In polymer concrete elaborated with polyester resin and silica sand; partial replacement of the silica sand by recycled tire fibers at concentration from 0.3 to 1.2% in volume, was done. Such concrete was submitted to gamma rays at doses from 50 to 100 kGy, and studied its mechanical properties, including compression and flexural strengths, as well as elasticity modulus. The results show noticeable improvements on the mechanical deformation, which are related with morphological and structural changes of the recycled tire fibers.

The effects of gamma irradiation on the compressive properties of polymer concretes show that the compressive strain and the elasticity modulus depend on the particle sizes used and the applied radiation dose; in particular, more resistance to crack propagation is obtained. In studies based on two parameters, use of recycled polymers and gamma radiation shows that: (a) polymer concrete with recycled high-density polyethylene (HDPE) and tire rubber particles, irradiated from 25 to 50 kGy, has significant increase on the impact strength as well as in the elongation-at-break; such improvements are attributed to the good adhesion between tire rubber particles and the polymer matrix [21]; (b) polymer concrete with waste tire rubber and styrene-butadiene-rubber (SBR) improves its tensile strength, elongation, and heat resistance up to 75 kGy [25].

properties are found. Concrete with irradiated particles can be support up to 30% of tire

In the case of polymer concrete with recycled tire fibers, strength and strain results show improvements of mechanical properties according to the tire fiber concentration as well as gamma irradiation dose. In general terms, addition of recycled tire fibers as well as higher radiation doses generate greater ductility on the polymer concrete; features no common for

In **Figure 3**, surface characteristics of the recycled tire particles are shown. Nonirradiated particles have different sizes; some of them show roughness on their surface and others smooth surfaces. Average size of recycled particles varies from 30 to 600 μm. In general, when recycled particles are added to concrete, a poor elastomer-matrix adherence is found, but when increasing the volume fraction of particles, mechanical interactions are augmented, therefore improvements on the mechanical properties are obtained. For irradiated tire particles, at 200 kGy, rough surfaces are created, with some small and disperse particles. According to the literature, sometimes smooth surfaces are generated after irradiation as a consequence of the cross-linking of polymer chains, while for higher dose, scissions of the polymer chains are done, which is manifested by appearances of cracks on the surfaces; as it is shown for irradiated

In polymer concrete elaborated with polyester resin and silica sand; partial replacement of the silica sand by recycled tire fibers at concentration from 0.3 to 1.2% in volume, was done. Such concrete was submitted to gamma rays at doses from 50 to 100 kGy, and studied its mechanical properties, including compression and flexural strengths, as well as elasticity modulus. The results show noticeable improvements on the mechanical deformation, which are related with

The effects of gamma irradiation on the compressive properties of polymer concretes show that the compressive strain and the elasticity modulus depend on the particle sizes used and the applied radiation dose; in particular, more resistance to crack propagation is obtained. In studies based on two parameters, use of recycled polymers and gamma radiation shows that: (a) polymer concrete with recycled high-density polyethylene (HDPE) and tire rubber particles, irradiated from 25 to 50 kGy, has significant increase on the impact strength as well as in the elongation-at-break; such improvements are attributed to the good adhesion between

particles, making possible to reduce the final cost of the concrete.

ordinary polymer concrete.

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particles at 250 kGy (**Figure 3**).

**Figure 3.** SEM images of nonirradiated and irradiated tire rubber.

morphological and structural changes of the recycled tire fibers.

In some experiments, waste Tetra Pak particles obtained from trash beverage bottles are used as reinforcements in polymer concrete; they partially substitute the mineral aggregates. The effects of the concentration and size of them on the compressive and flexural strength of polymer concrete are evaluated. The results show that the compressive and flexural strength as well as modulus of elasticity values decreases gradually when increasing the addition of waste particle concentration. A slight increment on the flexural strength values is observed for polymer concrete with smallest particle size. It is convenient to mention that to improve the mechanical properties of polymer concrete, gamma irradiation has been an adequate tool, because this improves the interfacial interaction between polymer concrete and Tetra Pak particles. However, improvements in compressive and flexural strength, as well as modulus of elasticity, when irradiating the concrete specimens, are observed.

Through SEM images the influence of gamma radiation on waste cellulose obtained from Tetra Pak packaging and its effect on the mechanical properties of concrete can be observed. As it is appreciated, a smooth and homogeneous surface, as well as agglomerations of particles is appreciated for polymer concrete. There are no chemical interactions between polyester resin and waste cellulose particles, and as a consequence, decrements of mechanical properties can be observed (**Figure 4**). For irradiated polymer concrete, deformation decreases which can be attributed to the stress transfer between polymer matrix and waste cellulose particles. The greater contact area between the particles and the concrete matrix, thus the greater stress transfer; moreover, rough surface and irregular distribution of the particles are observed (**Figure 4**).

**Figure 4.** SEM images of nonirradiated and irradiated polymer concrete.

The effects of the concentration of Tetra Pak` particles as mechanical reinforcements and gamma irradiation as a tool for improvement of interfacial coupling in polyester-based composite are evaluated. The main proposal is to finda material with improved ductility, that is, with more elasticity instead of a rigid property. After irradiation, the deformation increased substantially, having a maximum value at 400 kGy when compressive evaluation is done; while for flexural test, maximal deformation is obtained at 500 kGy. Such improvements are due to the cross-linking and degradation processes in both cellulose and polyester resin.

In the case of polymer concrete for improvement of the interfacial surface, gamma irradiation is a novel proposal. As it is known that in a composite material only physical interactions are present between matrix and aggregates, nevertheless, by using gamma irradiation, chemical bonds can be obtained [26]. In **Figure 5**, the irradiation process in the polyester resin causes chain scission and it also produces some cross-linking, chain relaxation, and cage breaking. As a consequence, the formation of bonds into polymer chains increases the degree of polymerization of the resin matrix. Homogenous surface is affected by gamma radiation because a higher number of chemical bonds are established and a rougher surface is observed (**Figure 5**), and for higher radiation dose, voids and small particles created from the cross-linking of the resin are observed. One can achieve good control of the dimensions and the elimination of internal stress, which cause reduction in mechanical strength [27, 28].

**Figure 5.** SEM images of irradiated polyester resin.

Other studies show different behaviors, for example: (a) molecular defects on mineral aggregates such as calcium bentonite have been observed [29]; (b) compressive strength values increased while total porosity and water absorption values decreased with increasing irradiation dose, in polymer-modified cement mortar specimens, with styrene-acrylic ester as adding polymer [30]; and (c) improvement on mechanical properties such as compressive strength and Young's modulus was observed for concrete reinforced with polypropylene fibers [31].


*3.3. Release of XOS from Substrates by Enzymatic Hydrolysis*

In order to evaluate synergistic interactions between FAEs and Xyn11 on untreated and pre-treated CC (agricultural residue), the enzymes were tested for their ability to release XOS and FA from a model arabinoxylan substrate (WAX). A 0.5% WAX substrate loading was used for a better comparison with natural substrates since they have xylan contents of between 20–40%. Figure 3 shows the production of reducing sugars after enzymatic hydrolysis of WAX (a), untreated (b), hydrothermal treated (c) and acid-treated CC (d) by single or combinations of the enzymes. The trend in the production of reducing sugars was similar in all substrates, with the Xyn11 to FAE5/6 combinations releasing higher reducing sugars than those from the reactions containing individual enzymes. As expected, FAE5 or FAE6 alone were not able to release reducing sugars, this was also indicated in the specific activity determination study (Table 2). For hydrolysis of WAX (Figure 3a), both FAEs displayed synergy with Xyn11 by improving the release of reducing sugars. It was found that Xyn11 alone released 0.89 mg/mL, while co-incubation with FAE5 or FAE6 resulted in 1.18 mg/mL (25.61% increase) and 1.22 mg/mL reducing sugars (a 28.06% increase), respectively. The hydrolysis of untreated CC with xylanase alone and a combination of Xyn11 and FAE5 or FAE6 produced reducing sugars at concentrations of 0.94 mg/mL, 1.17 mg/mL, and 1.22 mg/mL, respectively (Figure 3b). The hydrothermal pre-treated sample exhibited a similar hydrolysis pattern to untreated CC, but the amounts of reducing sugars released in the co-incubation sets were higher (Figure 3c). However, the amount of reducing sugars was significantly less for acid pre-treatment when Xyn11 was applied alone (yield of 28.80%), but a combination of Xyn11 and FAE5 or FAE6 resulted in more reducing sugars (yield of 100% for both combinations) compared to untreated CC (Figure 3d). The hydrothermally pre-treated sample exhibited yields of 60.76% for Xyn11 alone, while co-incubation with FAE5 and FAE6 resulted in yields of 98.21% and 100%, respectively. These results indicate that FAEs play an important role in the synergistic hydrolysis of WAX, and the arabinoxylan contained in untreated and pre-treated CC samples.

**Figure 3.** Release of reducing sugars during hydrolysis of 0.5% wheat arabinoxylan (WAX) (**a**), 1% untreated (**b**), hydrothermally pre-treated (**c**) and acid pre-treated CC (**d**) by individual enzymes or a combination of 66% Xyn11: 33% FAE5 or FAE6. Sub represents substrate control. Statistical analysis was conducted using *t*-test for improvement of hydrolysis with respect to reducing sugar by the enzyme combinations compared to single enzyme (Xyn11), key: \* (*p* value < 0.05).

**Table 2.** Specific activities of the enzymes assessed in this study (U/mg protein).

a Analyzed by 3,5-dinitrosalicylic acid (DNS) method for xylanase activity and b analyzed by HPLC diode array detector (DAD) for feruloyl esterase activity. "Nd" = not detected.


**3. Biomass conversion processes**

The development of conversion technologies for the utilization of biomass resources for energy is growing at a fast pace. Most developing countries find it hard to catch up because

Biomass Energy Conversion 213

typical mixture consists of 65% methane and 35% CO2 with traces of other gases. The methane producing bacteria (called methanogenic bacteria) generally require a pH range for growth of 6.4 to 7.2. The acid producing bacteria can withstand low pH. In doing their work, the acid producing bacteria lower the pH and accumulate acids and salts of organic acids. If the methane-forming organisms do not rapidly convert these products, the conditions become adverse to methane formers. This is why the first type of reactors developed for conversion of biomass wastes into methane have long retention times seeking equilibrium

Municipal wastes and livestock manures are the most suitable materials for anaerobic digestion. In the US, numerous landfill facilities now recover methane and use it for power generation. Aquatic biomass such as water hyacinth or micro-algae can be digested and may become valuable sources of energy in the future. Anaerobic digestion of organic wastes may constitute an effective device for pollution control with simultaneous energy generation and nutrient conservation. A major advantage of anaerobic digestion is that it utilizes biomass with high water contents of as high as 99%. Another advantage is the availability of conversion systems in smaller units. Also the residue has fertilizer value and can be used in crop production. The primary disadvantage of anaerobic digestion of diluted wastes is the large quantity of sludge that must be disposed of after the digestion process including the wastewater and the cost of biogas storage. In cold climates, a significant fraction of the gas produced may be used to maintain the reactor operating temperature. Otherwise,

microorganisms that thrive on lower or moderate temperatures should be used.

Fig. 2. Steps in anaerobic digestion process with energy flow represented as % chemical

Three main types of biogas facilities have been successfully developed in Asia for widespread biogas production in households and industrial use. These are the "Chinese Digester" of fixed dome type, the "Indian Gobar Gas Plant" of floating gas holder type and the rectangular commercial size biogas digesters developed in Taiwan. These are what we may call the first generation biogas reactors. Shown in Figure 3 is the common Chinese digester design. These

between acid and methane formers.

(Source: American Chemical Society)

**<IP_ADDRESS> The first generation biogas reactors**

oxygen demand (COD).

the level of technology is beyond their manpower as well as their manufacturing and technological capability. Added to this is the unavailability of local materials and parts for the fabrication of these conversion units. Figure 1 shows the different methods for converting biomass into convenient fuel. Biomass conversion into heat energy is still the most efficient process but not all of energy requirement is in the form of heat. Biomass resources need to be converted into chemical, electrical or mechanical energy in order to have widespread use. These take the form of solid fuel like charcoal, liquid fuel like ethanol or gaseous fuel like methane. These fuels can be used in a wide range of energy conversion devices to satisfy the diverse energy needs. In general, conversion technologies for biomass utilization may either be based on bio-chemical or thermo-chemical conversion processes. Each process will be described separately.

Fig. 1. Methods of using biomass for energy.

### **3.1 Bio-chemical conversion processes**

The two most important biochemical conversion processes are the anaerobic digestion and fermentation processes.