Agglomeration behavior of colloidal nano-silica and its effect on pore structure, mechanical properties and shrinkage of cement mortar

Abstract

The pozzolanic activity of colloidal nano-silica (CNS) can promote cement hydration, thereby improving the performance of the cement-based materials. However, the CNS itself presents highly electronegative, upon contact with cementitious materials, the electrostatic repulsion between CNS particles is lost due to the high ionic strength and the presence of multivalent cations in cement paste, which could lead to the destabilization of CNS and the formation of CNS aggregate (CNSA). Furthermore, similar to CNS, the chemical composition of CNSA is still amorphous silica because no chemical reaction occurs during the CNS agglomeration process. In theory, CNSA has similar nano effect and chemical activity effect to CNS and it can obviously accelerate the hydration rate of cement, improve the mechanical properties and pore structure of cement-based materials. This study aims to investigate the agglomeration behavior of CNS in cement pore solution (i.e. particle size distribution, zeta potential, residue Ca2+, microstructure) and its impact on the workability, pore structure and mechanical properties of cement mortar. It is proved that CNSA was formed by the complex of CNS and Ca2+ by electrostatic adsorption. And CNS content played a significant role on obtaining stable and narrow dispersion of CNSA in cement pore solution and the optimal content was found out to be 0.4 wt% where the particle size of the CNSA was still in the nanometer scale and stable at 200 nm. Cement mortar properties were also significantly improved through addition of 0.4 wt% CNS. While the basic workability was maintained, the 28-day compressive strength and flexural strength of cement mortar were improved by 8.94 % and 8.75 % respectively, the 28-day porosity was reduced by 8.76 %.