Influence of colloidal nano silica on solidification mechanisms and hydration process of nano modified cement.

This study assesses the feasibility of using Colloidal Nano-Silica (CNS) as a modifier for the radioactive waste immobilization matrix by investigating its effects on the setting time, soundness, mechanical stability, microstructure evolution, and solidification mechanism in simplified cementitious systems. It provides highlights into the isolated role of the CNS in the evolution of the solidification process, analyzes the temporal sensitivity of the mechanical strength evolution and its dependency on the CNS dosage, and provides mechanistic insights into the role of CNS in the changes in the controlling hydration reaction. In this respect, the nano-silica was synthesized via the sol-gel method and extensively characterized to determine its physicochemical properties. The resulting amorphous material, with a particle size below 2.49 nm and approximately 20 wt.% of weakly bound and chemically adsorbed water, forms a stable colloidal solution with a zeta potential of -33.9 mV. Incorporating CNS into cementitious matrices notably altered hydration kinetics, mechanical performance. and improves the durability. CNS accelerated both initial and final setting, exhibiting a non-monotonic trend as a function of CNS content, attributed to its water adsorption capacity, most pronounced at 5 wt.%. All formulations maintained acceptable soundness, with optimal enhancement observed at 3 wt.%, beyond which minor deterioration was detected at 5 wt.%. Although compressive strength generally declined with CNS addition over the curing period, the values remained above the minimum threshold for use in radioactive waste backfilling and waste immobilization. Mathematical analysis of the behavior over a 45-day curing period revealed that compressive strength was relatively insensitive to CNS variation, particularly between 1.5% and 3% material incooperation. Temporal analysis further indicated that strength development was limited during early curing stages but became material-specific at later ages. The improved durability of the 3% CNS-supplemented materials is related to the lime-silica reaction and the formation of C-S-H of low ca/si ratio. Mechanistic insights suggest that CNS addition promotes nucleation and growth mechanisms at the expense of diffusion-driven hydration, with critical transitions observed at 3% and 5% CNS content.