Influence of the water absorption-release processes of superabsorbent polymer with varying amounts of extra-entrained water on the surrounding cement paste

This paper investigates the influence of the water absorption-release process of superabsorbent polymer (SAP) with varying amounts of extra-entrained water on the properties of the surrounding cement paste and the macroscopic cement paste. ¹H low-field nuclear magnetic resonance (NMR) was used to analyze the water absorption-release process of SAP. The macroscopic properties, including the void system, compressive strength, and autogenous shrinkage, were systematically examined. Furthermore, the degree of hydration, pore structure, and microscopic mechanical properties were investigated using large particle-size SAP (5 mm) to expand the internal curing zone. The results indicate the existence of an “equilibrium point” in the absorption-release process of SAP within cement paste. When exceeding the “equilibrium point”, the free water in the SAP is rapidly released, leading to an increase in the local water-cement ratio. This results in a looser pore structure and a reduction in the micro-mechanical properties of the internal curing zone, ultimately weakening the compressive strength. Conversely, when below the “equilibrium point”, SAP absorbs water, reducing the water-cement ratio, compacts the pore structure, and enhances the micro-mechanical properties of the internal curing zone. This improvement compensates for the reduction in compressive strength caused by voids. Additionally, when the extra-entrained water is below the “equilibrium point”, the more water entrained in SAP, the higher the gel and capillary water content, resulting in a greater reduction in autogenous shrinkage. However, when exceeding the “equilibrium point”, the content of gel and capillary water does not increase further, and autogenous shrinkage will not decrease any further.