Effect of partial substitution of superplasticizer by silanes in Portland cement pastes

This work evaluated the effect of partially replacing a superplasticizer by silanes on the fresh and hardened properties of Portland cement pastes. For this, pastes with water/cement ratio of 0.20 and 25 and 50% replacement of a polycarboxylate-ester-based superplasticizer by tetraethoxysilane (TEOS), 3-glycidoxypropyltrimethoxysilane (GPTMS) and aminoethylaminopropyltrimethoxysilane (AEAPTMS) were produced. The specific gravity, entrapped air content and mini-slump of the pastes were determined in the fresh state. The hydration kinetics was evaluated by isothermal calorimetry up to 120 h. Finally, the compressive and flexural strength of the pastes, as well as the ratio between them (referred to as toughness) were determined at 7 and 28 days. The results showed that 25% replacement by either the silanes increased the mini-slump and decreased the air content of the pastes in comparison to the reference. In contrast, 50% incorporation of GPTMS and AEAPTMS increased the air content and decreased the mini slump of the pastes, while 50% TEOS did not significantly affected these properties. The isothermal calorimetry indicated that TEOS reduced the induction period of the pastes, while the other silanes increased it, delaying the acceleration period start in up to 53 h in comparison to the reference. In addition, the increase in both silane content progressively increased the cumulative heat up to 120 h, reaching values 11% higher than the reference. The incorporation of TEOS progressively increased the flexural strength, while did not significantly changed the compressive strength, both at 7 and 28-day. The same trend was found for the compressive strength of the pastes containing GPTMS and AEAPTMS, while the flexural strength of these pastes decreased as the replacement level increased. Finally, 7 and 28-days toughness of the pastes increased as the content of TEOS increased, while no clear trend was found for GPTMS and AEAPTMS. •25% of superplasticizer replacement by silanes enhanced the flowability of the pastes.•25 and 50% of superplasticizer replacement by silanes significantly changed the hydration kinetics.•The superplasticizer replacement by TEOS improved the mechanical properties of the matrix.