Molecular modelling of the mechanism of action of phosphonate retarders on hydrating cements

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Abstract
We consider the implications of a kinetic scheme, recently proposed by Billingham and Coveney (J. Billingham and P. V. Coveney, J. Chem. Soc., Faraday Trans., 1993, 89, 3021), for the hydration of cement slurries from a molecular modelling perspective. We study a range of known phosphonate cement-setting retarders which display widely differing capabilities, and in the first part of this paper we rationalise the relative efficacies of these compounds. To do this, we deployed a range of molecular modelling techniques, including energy minimisation using semi-empirical quantum mechanical and empirical force-field calculations, for both isolated molecules and molecules interacting with appropriate surfaces of ettringite. Molecular dynamics has also been used to seek out optimal conformations of these molecules on the surfaces in question. The implications of this docking mechanism for the resulting ettringite crystal morphology are also considered. Our work provides an explanation for the relative order of effectiveness of these retarders, which is confirmed by experimental studies. We have also been able to identify a powerful phosphonate retarder on the basis of this work. Using the same modelling methods as a basis for molecular design, we propose some new phosphonate retarders which might be expected to act as effective retarders. These include novel phosphonates based on the hexaaza-18-crown-6 macrocycle. One of these exotic molecules has now been synthesised and found to display significant retardation capabilities. The synthesis of related compounds is currently in progress.

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