Alkali Tolerance of Concrete Internal Curing Agent Based on Sodium Carboxymethyl Starch

Internal curing agents (ICA) based on super absorbent polymer have poor alkali tolerance and reduce the early strength of concrete. An alkali tolerate internal curing agent (CAA-ICA) was designed and prepared by using sodium carboxymethyl starch (CMS) with high hydrophilicity, acrylic acid (AA) containing anionic carboxylic group and acrylamide (AM) containing non-ionic amide group as the main raw materials. The results show that the ratio of CAA-ICA alkali absorption solution is higher than that existing ICA, which solves the low water absorption ratio of the ICA in alkali environment. The water absorption ratio of CAA-ICA in saturated Ca(OH)₂ solution is 95.8 g·g⁻¹, and the alkali tolerance coefficient is 3.4. The application of CAA-ICA in cement-based materials can increase the internal relative humidity and miniaturize the pore structure. The compressive strength of mortar increases up to 12.95% at 28 d, which provids a solution to overcome the reduction of the early strength.