 and Ca 2+ ions presenting in the solution, which uncharged the polymer chains [30,32]. The initial absorption rate was greatest for SAP B in both distilled water and cement filtrate due to its finest particle size and resulting larger specific surface area; SAP B reached 84.8% and 81.5% of the 10-min absorptivity in distilled water and cement filtrate, respectively, only within the first minute of soaking. However, the 60-min absorptivity of SAP B was rather slightly lower than that of SAP A and SAP D having a lower cross-linking density and a greater particle size, respectively, which was consistent with the findings of previous investigations [7]. The findings led to the following conclusions: (1) the absorption rate increases with the lower particle diameter; and (2) the absorption capacity increases with decreased cross-linking density and increased particle diameter.

Desorption Kinetics
The desorption behavior of SAPs is presented in Figure 6, the y-axis of which denotes the normalized weight calculated based on the weight of water initially absorbed in SAP and the weight of remaining water during the drying process. Note that the saturated SAPs exhibited almost linear weight losses for the first 100 min, and then the rate noticeably decreased thereafter, except for SAP B. This is because the surface water in SAP can be readily emitted upon drying since it is more subjected to less active and weak van der Waals forces, while the water closer to the core is further held by strong hydrogen bonds formed between the side chains of the polymer [6,30,31]. This effect delayed the diffusion of the core water to the drying front, which consequently led to nonlinear drying curves. For SAP B, however, it should be noted that the desorption curve remained almost linear over the whole drying period unlike other SAPs. This is possibly because the water absorbed in SAP B is mostly surface water due to its smaller particle diameter, which is less hindered by hydrogen bonds compared to other types of SAP [30,31]. The desorption data also shows that SAP C (highest cross-linking) emptied in 5 h after drying initiation whereas SAP A (lowest cross-linking) reached complete drying in about 6 h. One possible The initial absorption rate was greatest for SAP B in both distilled water and cement filtrate due to its finest particle size and resulting larger specific surface area; SAP B reached 84.8% and 81.5% of the 10-min absorptivity in distilled water and cement filtrate, respectively, only within the first minute of soaking. However, the 60-min absorptivity of SAP B was rather slightly lower than that of SAP A and SAP D having a lower cross-linking density and a greater particle size, respectively, which was consistent with the findings of previous investigations [7]. The findings led to the following conclusions: (1) the absorption rate increases with the lower particle diameter; and (2) the absorption capacity increases with decreased cross-linking density and increased particle diameter.

Desorption Kinetics
The desorption behavior of SAPs is presented in Figure 6, the y-axis of which denotes the normalized weight calculated based on the weight of water initially absorbed in SAP and the weight of remaining water during the drying process. Note that the saturated SAPs exhibited almost linear weight losses for the first 100 min, and then the rate noticeably decreased thereafter, except for SAP B. This is because the surface water in SAP can be readily emitted upon drying since it is more subjected to less active and weak van der Waals forces, while the water closer to the core is further held by strong hydrogen bonds formed between the side chains of the polymer [6,30,31]. This effect delayed the diffusion of the core water to the drying front, which consequently led to nonlinear drying curves. For SAP B, however, it should be noted that the desorption curve remained almost linear over the whole drying period unlike other SAPs. This is possibly because the water absorbed in SAP B is mostly surface water due to its smaller particle diameter, which is less hindered by hydrogen bonds compared to other types of SAP [30,31]. held by strong hydrogen bonds formed between the side chains of the polymer [6,30,31]. This effect delayed the diffusion of the core water to the drying front, which consequently led to nonlinear drying curves. For SAP B, however, it should be noted that the desorption curve remained almost linear over the whole drying period unlike other SAPs. This is possibly because the water absorbed in SAP B is mostly surface water due to its smaller particle diameter, which is less hindered by hydrogen bonds compared to other types of SAP [30,31]. The desorption data also shows that SAP C (highest cross-linking) emptied in 5 h after drying initiation whereas SAP A (lowest cross-linking) reached complete drying in about 6 h. One possible reason for this behavior is that the more the cross-links between polymer chains there were, the more elastic retraction forces acted in the network structure [7], which "squeezes " out the absorbed water in SAP. In addition, even though SAP B (smallest particle size) and SAP D (largest particle size) had The desorption data also shows that SAP C (highest cross-linking) emptied in 5 h after drying initiation whereas SAP A (lowest cross-linking) reached complete drying in about 6 h. One possible reason for this behavior is that the more the cross-links between polymer chains there were, the more elastic retraction forces acted in the network structure [7], which "squeezes " out the absorbed water in SAP. In addition, even though SAP B (smallest particle size) and SAP D (largest particle size) had an identical cross-linking density, there was a significant discrepancy in drying time. The reasons for the shorter drying time of SAP B are: (1) the smaller particles have larger exposure areas and a small nucleus and thus can release more water for a given drying period [31]; and (2) smaller particles are more exposed to weaker intermolecular attraction forces (i.e., van der Waals forces) because of their larger surface zone as compared to larger ones [31]. Figure 7 depicts the relationships between the normalized absorptivity and RH measured for each SAP. Here, the maximum RH was all assumed to be 95% as an air medium cannot practically guarantee full saturation. Similarly with [6], most of the absorbed water was released above 85 to 90% RH for all the SAPs, indicating that the SAPs can provide good internal curing efficiency particularly for cement-based materials with a low w/c [36,37]. an identical cross-linking density, there was a significant discrepancy in drying time. The reasons for the shorter drying time of SAP B are: (1) the smaller particles have larger exposure areas and a small nucleus and thus can release more water for a given drying period [31]; and (2) smaller particles are more exposed to weaker intermolecular attraction forces (i.e., van der Waals forces) because of their larger surface zone as compared to larger ones [31]. Figure 7 depicts the relationships between the normalized absorptivity and RH measured for each SAP. Here, the maximum RH was all assumed to be 95% as an air medium cannot practically guarantee full saturation. Similarly with [6], most of the absorbed water was released above 85 to 90% RH for all the SAPs, indicating that the SAPs can provide good internal curing efficiency particularly for cement-based materials with a low w/c [36,37].

Sorption Isotherm
It is also interesting to note that even though SAP B, SAP C, and SAP D all showed a quite similar sorption isotherm across the measured RH levels, SAP A retained approximately seven times more water at 82% RH. It appears that smaller cross-linking density and larger particle size of SAP A contributed the greater moisture retaining capacity as the larger-sized and less cross-linked SAP's network structure provides more spaces that water molecules can be captured by stronger binding forces (i.e., hydrogen bonds) [6,7,30]. This may provide inferences that SAP A has a potential to be used as an internal curing material in applications where a prolonged curing period is required.  Figure 8 displays how the absorption capacity of SAPs was estimated under mixing conditions. The x-axis of each figure is the assumed absorptivity of SAP, and the y-axis denotes the spread diameter of fresh mortar measured immediately after mixing. The results indicate that the higher the assumed absorptivity, the greater the flow. This is because as the assumed absorptivity becomes greater, the effective water content that serves as mixing water increases. Once the variation of the flow was measured for each SAP, the absorptivity resulting in same flow with control mixture (127.5 mm) was back-calculated using linear interpolation. The logic behind this analysis method is that if the absorption capacity is overestimated or underestimated than actual, the flow would be greater or less than that of control mixture. The measured absorptivity was greatest for SAP A (12.70 g/g dry SAP) and smallest for SAP C (4.82 g/g dry SAP). The finding is in good agreement with those from previous studies [4,7] in that the absorptivity decreases as the cross-linking density increases. In addition, about a 20% difference in absorptivity was noted between SAP B and SAP D, which It is also interesting to note that even though SAP B, SAP C, and SAP D all showed a quite similar sorption isotherm across the measured RH levels, SAP A retained approximately seven times more water at 82% RH. It appears that smaller cross-linking density and larger particle size of SAP A contributed the greater moisture retaining capacity as the larger-sized and less cross-linked SAP's network structure provides more spaces that water molecules can be captured by stronger binding forces (