Synergy between nano SiO2-modified SAP and RHA in cement pastes: Shrinkage, microstructure, and strength

The application of a single internal curing material is incapable of effectively balancing shrinkage inhibition and strength development. This paper investigated the effect of nano SiO2-modified superabsorbent polymer (SAP-n) synergized with rice husk ash (RHA) on the shrinkage and mechanical properties of cement pastes. The water desorption process of the SAP-n/RHA composite within cement pastes was characterized using 1H NMR, isothermal calorimetry, and internal relative humidity. Moreover, the hydration kinetics and microstructure of internally cured pastes were revealed. The results demonstrated that the addition of RHA reduced the amount of water released from hybrid system before the final set, and accelerated the desorption rate of SAP afterward, effectively mitigating self-desiccation. A "three-stage" gradient water release model of SAP-n/RHA composite driven by osmotic pressure and humidity differences was proposed. The porous RHA was uniformly distributed in the matrix, especially around the SAP, contributing to internal curing at later ages while providing extra silica to repair voids and densify the pore structure. Compared to pastes containing commercial SAP, the 91-day dry shrinkage of specimens with 0.2 wt% SAP-n and 3.6 wt% RHA was reduced by 16.1 % without compromising autogenous shrinkage inhibition efficiency, and the 28-day strength was increased by 20.6 %.