Nanoscale mechanisms of the non-monotonic hydrolysis effect and Ca2+ suppression of SAP swelling.

Superabsorbent polymers (SAPs) are widely used as internal curing materials in ultra-high-performance concrete (UHPC) due to their excellent water absorption and water retention capacities. However, the molecular origin of the hydrolysis-dependent swelling behavior of SAPs and their molecular interaction mechanisms in salt ion environments remain unclear. In this study, molecular dynamics simulations were used to systematically investigate the water absorption and swelling behavior of a copolymer SAP composed of N-isopropylacrylamide (NIPAM) and methacrylic acid (MAA) in pure water and Ca(OH)2 solutions. SAP models with different prescribed NIPAM hydrolysis degrees were first constructed to clarify the effect of hydrolysis on swelling behavior in pure water. The swelling volume ratio exhibited a distinct non-monotonic dependence on hydrolysis degree, characterized by an initial decrease, a subsequent increase, and a final sharp decline at high hydrolysis degrees. Subsequently, the model with 40% hydrolysis degree was selected to further investigate its swelling behavior in Ca(OH)2 solutions with different concentrations. The results show that the swelling of SAP is gradually suppressed with increasing Ca(OH)2 concentration. This inhibitory effect is mainly attributed to the strong coordination and ionic bridging between Ca2+ and carboxylate groups, which restrict polymer chain mobility and reduce the effective contact between the polymer and water molecules. Further comparison with K+-containing systems showed that Ca2+ induced stronger swelling suppression than K+. This study reveals the nanoscale mechanisms governing the non-monotonic hydrolysis effect and Ca2+-induced swelling suppression of SAPs, providing theoretical guidance for the molecular design of SAPs for UHPC internal curing.