Nano silica admixtures: Salt ingress and reaction in mortar

Abstract

This paper examines the influence of commercially available nano silica (NS) admixtures on salt ingress and reaction in mortar. A series of mortar mixtures at dosages ranging from 2.6 to 41.8 mL/kg were tested using isothermal calorimetry (IC), thermogravimetric analysis (TGA), low temperature differential scanning calorimetry (LTDSC), and chloride profiling. Thermodynamic modeling was used to support the experiments. IC showed that the heat of hydration was slightly higher (2-3%) than the reference mixture when the NS admixtures were used. TGA indicated up to 18% reduction in calcium hydroxide at low admixture concentrations (2.6 to 7.8 mL/kg), which is greater than the reduction that would be expected due to pozzolanic reactions alone. At higher admixture dosages (above the recommended dosages), the calcium hydroxide contents were similar to or higher than those of the reference mixture. LTDSC results showed up to a 30% reduction in calcium oxychloride (CaOXY) formation potential; however, this reduction may not be sufficient to avoid the potential damage associated with expansive CaOXY formation at concrete joints in pavements and flatwork. The NS admixtures did not affect chloride ingress profiles significantly. Thermodynamic calculations indicate that although the admixtures provide reactive silica that acts pozzolanically, the amount is relatively small compared to the admixture dosages used.