Effect of alkalinity and calcium concentration of pore solution on the swelling and ionic exchange of superabsorbent polymers in cement paste

Swelling kinetics of superabsorbent polymers (SAP) in fresh concrete is complex, but its understanding is crucial for optimisation in practical applications. In this study, the effect of concentration of ions common in pore solution (Na+, K+, Ca2+, Cl−, OH−, SO42−) and cyclic wetting/drying on the swelling and ionic exchange of poly(AA) and poly(AA-co-AM) were investigated. Results show that swelling is not a simple function of concentration or ionic strength. In cement paste, SAP absorbs Ca2+ and releases its counterion (Na+, K+) into pore solution. Ca2+ binds with SAP and decreases initial swelling, but the bound Ca2+ can be displaced and swelling gradually recovers. Swelling increases with increase in alkalinity, but decreases with increase in calcium concentration. The higher the degree of ion exchange, the lower the swelling of SAP. Poly(AA) is more susceptible to Ca2+ complexation and therefore achieves a lower swelling ratio and slower swelling recovery compared to poly(AA-co-AM).