Inhibition of tricalcium aluminate hydrate–sulfate interactions by size and content-dependent silica particles

The incorporation of nanoscale or microscale silica into cementitious materials has been demonstrated to markedly enhance resistance to sulfate attack. Despite this benefit, the exact mechanisms responsible for these improvements are not yet fully understood, particularly concerning the role of unhydrated silica particles in blended cement under sulfate exposure. Tricalcium aluminate (C₃A), a key component of cement, contributes a substantial proportion of the aluminum phases that are vulnerable to sulfate attack. The present study investigated how silica particles of varying sizes influence the interactions between C₃A hydrates and sulfate solutions. By conducting qualitative and quantitative analyses of the reaction products, microstructural morphology, ion concentrations, zeta potentials, and sulfate concentrations in the solution, a new impact mechanism model was developed. These findings reveal that nanoscale silica (NS) particles in the C₃A hydrates and the sulfate system impede the reaction between hydrogarnet and sulfate, thereby reducing ettringite formation. NS mainly suppresses hydrogarnet dissolution, resulting in lower concentrations of Ca²⁺ and Al³⁺ ions and reduced consumption of SO₄²⁻ ions. This suppression is attributed to the adsorption of NS on the hydrogarnet surface, and NS attracts Ca²⁺, SO₄²⁻ ions, or CaS ion pairs, leading to surface ion overcharging and thereby reducing hydrogarnet dissolution. In addition, NS particles adhere to the surface of ettringite, preventing the adsorption of Ca²⁺, SO₄²⁻, or CaS ion pairs, thereby inhibiting ettringite formation and growth. This effect is notably dependent on the presence of NS, as microscale silica or insufficient quantities of NS lessen or eliminate the impact. These findings provide insights into the use of nanomaterials to enhance the durability of cement-based materials and establish a foundational basis for future research in this area.