Molecular Modelling of the Mechanism of Action of Borate Retarders on Hydrating Cements at High Temperature

Abstract

We consider the class of borate additives in an attempt to understand, on the basis of molecular modelling techniques, why they are effective high temperature cement setting retarder aids. Borates are known to act as cement retarders under ambient conditions, for which the Billingham-Coveney clock reaction scheme [1] based on a gel → crystalline ettringite transformation provides the basic model. Under conditions of high temperature (above ca. 80°C) ettringite is known to be unstable, so that a modification to this mechanism is then required. However, conceptually (and mathematically) the kinetic scheme remains unchanged: we are concerned with a gel → crystalline silicate transition, which can be inhibited by suitable retarders. Thus the proposed mechanism for the high temperature retardation of hydrating cements by borates is that the latter prevent the growth of polymeric or crystalline calcium silicate hydrates such as tobermorite from the initially formed C ‒ S ‒ H gel. The mechanism which we propose in this paper, which is based on chemisorption of borate species, explains some experimentally observed phenomena relating to the retarding action of both monomeric and polymeric borates in solution. On the basis of this work, we are able to predict vibrational frequencies for the cyclic borosilicate species which is formed, observation of which would confirm the validity of the mechanism proposed here. We can also use our work to explain the effectiveness of different borates. We conclude by suggesting a novel boronate retarder which might be expected to be more potent than those currently in use.