Effect of superabsorbent polymers and expansive additives on the shrinkage of alkali-activated slag

In this study, superabsorbent polymers (SAPs) and two expansive additives (a MgO-based expansion agent and gypsum) were studied to possibly mitigate the shrinkage of AAS (alkali-activated slag) systems. To this end, an experimental study was carried out to evaluate the effect of SAPs and the expansive additives on the setting time, compressive strength, autogenous shrinkage, and total shrinkage of AAS. SAPs prolonged the setting time of AAS and decreased the compressive strength, while the expansive additives shortened the setting time of AAS. When gypsum was used as an expansive additive, the shortest initial setting time of AAS was less than 10 min. SAPs have a significant effect in reducing the autogenous shrinkage of AAS, and a better result was achieved when used together with the MgO-based expansion agent as the SAPs promoted the consumption of MgO. The 1H low-field NMR results showed that when the SAPs and gypsum were used together, the absorption capacity of the SAPs decreased, and part of the absorbed liquid was rapidly released back into the mortar matrix. However, for the total shrinkage, the SAPs were less effective. SAPs increased the mass loss of the AAS mortar upon drying, which may increase the drying shrinkage of AAS and counteract part of the reduction in autogenous shrinkage.