Internal curing with superabsorbent polymer modified by nano SiO2: Shrinkage mitigation and microstructure refinement

The existing commercial superabsorbent polymer (SAP) products have the intrinsic defects of low internal curing efficiency and weakening strength. In this paper, a novel type of organic-inorganic composite SAP (SAP-N) modified by nano SiO2 was designed and synthesized. The absorption-desorption kinetics of SAP-N in alkali pore solution were measured to evaluate the structural stability and water retention capacity. The water transport process and distribution of SAP-N in cement pastes were studied using 1H NMR and X-ray computed tomography. In addition, the effect of SAP-N on the autogenous shrinkage, microstructure, and compressive strength of cement pastes was investigated. The research results demonstrated that the incorporation of nano SiO2 could enhance the absorption capacity of SAP in pore solution. The water retention capacity of SAP-N was increased by over 50 % compared to commercial SAP-C, despite the dissolution of a proportion of nano SiO2. SAP-N exhibited a low agglomeration degree but a high effective utilization rate of internal curing water in cement pastes, and thus mitigated effectively the self-desiccation and autogenous shrinkage. The hydration degree of cement pastes internally cured with SAP-N was significantly enhanced owing to the pozzolanic effect of nano SiO2, which enabled the in-situ repair of residual voids and reduced capillary pore size, thus improving the longterm compressive strength.