The influence of nano-silica composite superabsorbent polymer on the autogenous shrinkage of mortar

Abstract

To address the diminished shrinkage reduction capacity due to high ion sensitivity of superabsorbent polymers (SAP) within cement-based materials, this study employs solution polymerization where acrylic acid and acrylamide are copolymerized with nano-silica at varying concentrations to synthesize innovative SAPs. The investigation encompasses the structural groups, microscopic morphology, liquid absorption characteristics of these SAPs, and their effect on mortar's autogenous shrinkage. The results indicate that nano-silica can be encapsulated within polyacrylate-acrylamide network via physical adsorption and chemical grafting. The incorporation of nano-silica enhances the water absorption and retention capacities of SAPs in alkaline environments. Nano-silica composite SAPs absorb sufficient internal curing water and prolong the subsequent moisture gradient-driven water release process, continuously releasing water within matrix to maintain internal humidity, thereby further enhancing the shrinkage reduction effect on mortar. Based on Scanning Electron Microscopy (SEM) observations of SAPs morphologies within mortar and X-ray Computed Tomography (X-CT) monitoring of internal curing water distribution, it is revealed that nano-silica undergoes pozzolanic reactions, forming a tightly connected domain with matrix. This facilitates continuous water release to the surrounding through capillary action, thus maintaining capillary pore liquid levels. Consequently, it effectively mitigating shrinkage stress and deformation, leading to an enhancement in shrinkage reduction effects.