The water absorption and release performance and mechanism of superabsorbent polymers in the solution and paste

Superabsorbent polymers (SAPs) are organic polymers capable of absorbing and retaining large amounts of water, with variations in their absorption and release properties impacting the mix design, microstructure, mechanical properties, and durability of cement-based materials. This study investigated the effects of SAP type (sodium polyacrylate, AA and polyacrylic acid-acrylamide copolymer, AM) and particle size on water absorption capacity, as well as the effects of environmental temperature and humidity on the release behaviour of saturated SAPs. The point-counting method was used to assess the actual absorption performance of SAPs in hardened paste, and SEM was used to observe the pore structure formed after water release. The results show that as the particle size of SAP increases, its water absorption capacity enhances, whereas its water release capacity decreases. AA absorbs water better than AM in deionized water, but the reverse is true in cement filtrate and hardened paste. AA is more sensitive to temperature, while AM shows greater sensitivity to humidity. The actual absorption performance of SAP in the paste can be obtained by using the method of assuming water absorption ratio. As the water-binder ratio increases, the absorption performance of SAP in the paste increases first and then decreases. SEM observations show that AA shrinks into a ball after the paste releases water, with a clear boundary with the surrounding matrix, while AM is tightly bonded to the paste after releasing water.