Early-Age Properties and Reaction of Hydrophobic Portland Cement and Alkali-Activated Fly Ash–Slag Pastes with Alkyl Silanes

Cementitious materials are susceptible to water ingress due to their hydrophilicity and porous microstructure, which can cause premature destruction and compromise long-term durability. Integral hydrophobic modification using alkyl silanes is an effective strategy for enhancing water resistance, while the influence of different silanes on early-age properties (within the first 7 d) of various binder systems remains unclear. This study investigates the rheology, flowability, setting behavior, reaction kinetics, compressive strength, and hydrophobicity of ordinary Portland cement (OPC) and alkali-activated fly ash–slag (AAFS) pastes incorporating alkyl silanes of varying alkyl chain lengths, i.e., methyl-(C1TMS), butyl-(C4TMS), octyl-(C8TMS), and dodecyl-trimethoxysilane (C12TMS). In OPC, C1TMS reduced yield stress and plastic viscosity by 33.6% and 21.0%, respectively, and improved flowability by 27.6%, whereas C4TMS, C8TMS, and C12TMS showed the opposite effects. In contrast, the effect of alkyl silanes on rheology and flowability of AAFS was less pronounced. Silanes delayed setting of OPC and AAFS by 5.6–164.4%, with shorter alkyl chains causing greater retardation. C1TMS and C4TMS inhibited early-age heat release and decreased the 1-day compressive strength by 14.8–35.7% in OPC and 82.0–84.5% in AAFS, whereas longer-chain silanes had comparatively minor effects. The hydrophobic performance in both binder systems was strongly correlated with alkyl chain length. C8TMS exhibited the best hydrophobicity in OPC, achieving a water contact angle of 145° and a 75.7% reduction in water sorptivity, while C4TMS demonstrated the highest hydrophobicity in AAFS. This study provides fundamental guidance for the rational selection of alkyl silanes in OPC and AAFS systems, offering insights into the design of multifunctional water-resistant cementitious composites for marine structures, building facades, and other applications with waterproofing requirements.