Effect of a Synthetic Nano-CaO-Al2O3-SiO2-H2O Gel on the Early-Stage Shrinkage Performance of Alkali-Activated Slag Mortars

The relatively high shrinkage of the alkali-activated slag (AAS) has restricted its application as a widely-used building material. This research attempts to study the effect of a hydrothermally synthesized C-A-S-H gel, which has a similar composition to that of the main AAS product, on the shrinkage performance of the AAS. The C-A-S-H nano-particles were mixed into AAS mortars in a proportion ranging from 1 wt % to 5 wt % using two different methods, and the drying and autogenous shrinkage ratio of modified AAS mortars were measured at various ages. The effect of C-A-S-H on their microstructures was also characterized. Results obtained indicated that the addition of C-A-S-H gel to AAS mortars had reduced their drying and autogenous shrinkage, with the addition of 3 wt % reaching the maximum reduction. However, the added amount was not directly proportional to the decrease of shrinkage; the proportion of early-stage drying shrinkage of AAS mortars was greater than that of autogenous shrinkage; the dispersion method was slightly better than the dry mixing method in both shrinkage reduction. MIP results suggested that the addition of C-A-S-H gel had reduced the total porosity and the average pore size of AAS mortars, optimized their pore structure distribution, and significantly reduced the volume of mesopores (