Insight into aging behavior of superabsorbent polymer in cement-based materials to release microplastic pollution

Superabsorbent polymer (SAP) is widely used as an internal curing agent for cement-based materials. It is also a polymer that can potentially age over time, and the aging products may release microplastic pollution, but this has not been paid attention to. This work investigated the aging behaviour of SAP in both cement paste and simulated cement pore solution (CPS) for 70 d by the measurement of swelling ratio, particle size distribution, infrared spectroscopy, 1 H nuclear magnetic resonance, and thermogravimetric analysis. Results showed that the SAP in cement paste was aged, its ester groups decreased, carboxylate groups changed, and the aged SAP particle became difficult to re-swell. The molecular structure of SAP aged in CPS underwent significant alterations compared to the raw SAP and that aged in deionized water, with the ester groups nearly disappearing and carboxylate increasing significantly. SAP aged in CPS fragmented from100 300 μm into smaller particle sizes ranging from 0.1 to 200 μm. Compared to the raw SAP, its mean value decreased by 38.3 %, and its swelling ratio decreased by 46.9 % when tested in CPS. The aged SAP particles were difficult to re-swell due to the formation of cross-linking between –COO - groups and cement hydration products through cations, such as Ca 2+ . The results indicate that the aging of SAP in cement-based materials is inevitable, and the aging products may release as potential microplastic pollution. Therefore, the aging behaviour of SAP cannot be ignored. Superabsorbent polymer is a widely used agent in cement-based materials and also a potential aging polymer. Its aging products may leak into environmental systems and cause microplastic pollution. This study investigated the aging behavior of SAP and proposed that the aging of SAP in cement-based materials leads to the loss of internal curing and self-healing functions, with the aging product releasing microplastic pollution.