Hydration products and hybridisation mechanisms of hydrophobic cement pastes with alkyl-organosilanes

Cement-based materials are susceptible to water infiltration and moisture-induced degradation due to their hydrophilic nature and capillary-rich structure. Integral hydrophobic modification with organosilanes is a promising solution to improve their durability, while the optimal modifiers and underlying hybridisation mechanisms remain unclear. This study investigates the water resistance, pore structure, hydration and hybridisation mechanisms of hydrophobic cement pastes incorporating organosilanes with varying alkyl chain lengths, i.e., methyl- (C1TMS), butyl- (C4TMS), octyl- (C8TMS), and dodecyl-trimethoxysilane (C12TMS), using techniques including MIP, BSEM, XRD, TG, FTIR, and NMR. Among them, 0.75 % C8TMS yields the most effective hydrophobic modification, achieving a water contact angle of 144° and a 76.5 % reduction in water sorptivity. C4TMS and C12TMS also enhance water resistance by accumulating their alkyl structures into hydrates and pore networks. However, excessively long alkyl chains of C12TMS tend to twist and aggregate in the aqueous cement paste, inhibiting hydrolysis and reducing its hydrophobic modification effectiveness compared to C8TMS. In contrast, C1TMS improves hydrophilicity. These organosilanes participate in forming hybrid hydration products through adsorbing Ca²⁺ into coupled oligomers or directly bonding with C–S–H, improving hydration of raw materials by up to 6.1 %. This study offers new insights into organic-inorganic hybridisation in cement systems and designing multifunctional cement-based materials at the molecular level.