Effect of alkoxysilane on early age hydration in portland cement pastes

Abstract

Silanes added during mortar or concrete preparation, may modify the fresh state properties, hydration kinetics and mechanical strength of the final product. The effects of silanes on cement hydration have been widely studied in the literature, however, there is some controversy about its effect at short ages. The present study was undertaken to determine the effect of a TEOS-based alkoxysilane (UCA-T), produced by ultrasound-assisted pre-hydrolysis of an oligomeric precursor, on early age cement paste hydration. The nature of the processes modifying the various stages of cement hydration kinetics in the presence of the alkoxysilane was ascertained by analysing paste composition at several ages (defined on the grounds of calorimetric curve results) using XRD, TG-DTG, FTIR and Raman spectroscopy. The calorimetric curve of pastes containing UCA-T exhibited a new early age, pre-induction period exothermal peak, indicative of UCA-T hydrolysis, C(3)A and C3S dissolution and ettringite and C-S-H gel precipitation. Portlandite, however, did not precipitate but reacts with the Si(OH)(4) sourced from UCA-T hydrolysis to generate further C-S-H gel. The induction period following on that new exothermal peak was considerably longer than the period observed in the reference cement, an effect that intensified at higher UCA-T content.