Impact of nanosilica on tricalcium aluminate hydration and its reaction with sulfate solutions

Nanosilica (NS) has the potential to enhance the performance of cement-based materials through improvements in pore structure, hydration product content, and the properties of calcium–(aluminum)–silicate–hydrate (C–(A)–S–H) gel, ultimately increasing resistance to sulfate attack. However, the underlying mechanisms of these enhancements remain incompletely understood, particularly with respect to the presence of unhydrated NS particles in blended cementitious materials under sulfate attack. C₃A (tricalcium aluminate) is one of the main components of cement and a major source of aluminum phases during sulfate attack. This study aims to investigate the effects of NS on the hydration of the C₃A–gypsum system and the subsequent reaction between the hydration products and sulfates. The investigation involves qualitative and quantitative analyses of the reaction products, and microscopic morphology, as well as tests on ion concentrations, zeta potentials, and sulfate concentrations in the reaction solution. Findings suggest that NS inhibits the formation of ettringite during the C₃A–gypsum hydration process, but does not exert a notable influence on the final hydration product content. Furthermore, residual NS particles in the C₃A–NS hydration system further impede the reaction between hydrogarnet and sulfate, thereby reducing ettringite formation. NS also impedes the dissolution of hydrogarnet, resulting in lower concentrations of Ca²⁺ and Al³⁺ ions and limited consumption of SO₄²⁻. Based on the analysis of the research results, this inhibitory effect is attributed to the adsorption of NS particles onto the hydrogarnet surface, which attracts Ca²⁺, SO₄²⁻, or CaS ion pair complexes, leading to surface ion overcharging and reduced hydrogarnet dissolution. In addition, NS particles may adsorb onto the surface of ettringite, preventing the adsorption of Ca²⁺, SO₄²⁻, or CaS ion pair complexes, thereby inhibiting the formation and growth of ettringite.