Influence of type and particle size of superabsorbent polymer on early water distribution and internal curing zone properties of cement paste

The types of commercial superabsorbent polymer (SAP) mainly include ionic-SAP acrylic acid (AA) and nonionic-SAP acrylamide-acrylic acid copolymer (AM), which have different particle sizes due to the limited preparation methods. This study used ¹H low-field NMR to clarify the influence of type and particle size of SAP on the water distribution of cement paste. Backscattering electrons and nanoindentation were used to quantitatively analyze the area, porosity, and micromechanical properties of internal curing zone. Finally, void system analysis, compressive strength, and autogenous shrinking were systematically studied. The results indicate that the water absorption and retention capacity of AM in cement paste increase with the increase of particle size, and the area of internal curing zone also increased. As the area of internal curing area increases, the autogenous shrinkage of cement paste significantly decreases. The internal curing effect reduces the unhydrated cement content and increases the high-density C–S–H content of internal curing zone, thereby improving the micromechanical properties and to some extent offsetting the decrease in mechanical properties caused by voids. However, larger particle size AM forms larger void and higher air content, leading to a further decrease in strength. Ionic-SAP AA releases a large amount of water before final setting, weakening internal curing effect and reducing the area of internal curing zone. In addition, the early release of water leads to an increase in the local water-cement ratio, which increases the porosity and low-density C–S–H content of internal curing zone, thereby reducing the micromechanical properties.