New insight into the effects of silane-modified silica fume on the performance of cement pastes

Abstract

Hydrophobic-modified silica fume (HSF) with superior dispersion was synthesized by coating silane onto SF surface. Its influence on cement hydration, microstructure and properties of cement pastes was investigated. Results indicated that the silane on SF surface visibly depressed pozzolanic reaction at early ages. HSF retarded cement hydration by adsorbing on cement surface and more importantly, by interacting with Ca²⁺ in aqueous phase. For the first time, the accumulation of calcium around HSF was observed, leading to the growth of needle-like C–S–H perpendicular to HSF surface and eventually evolved into loose honeycomb-like gels. These loose gels promoted pozzolanic reaction and cement hydration at late ages. TEM and XRD results revealed that silane incorporated itself into C–S–H and altered structural order. Furthermore, adding HSF increased capillary pore size but refined the inter-hydrate pores at late ages. HSF visibly decreased the compressive strength compared with SF; however, the difference diminished at late ages.