The conversion of high alumina cement

Abstract

Synopsis High alumina cement (HAC) concrete suffers from a conversion reaction in which the metastable calcium aluminate hydrate compounds change to more stable compounds. This reaction takes place at a rate which depends upon a number offactors including temperature, water/cement ratio, stress and the presence of ‘releasable’ alkalis in the aggregate. As a result of the reaction, the concrete will lose strength and the strength will reach a minimum at various times depending upon conditions of storage; thereafter there may be a small increase in strength. The strength at this minimum, S, expressed as a percentage of the one-day strength depends upon the rate, R, at which the conversion reaction takes place, and upon the water/cement ratio. An empirical relationship for laboratory-prepared specimens is S = (− 37·8 logeR + 39·5) − 100(w/c − 0·4). However, this cannot be used for unknown concretes until a reliable method of determining the original water/cement ratio of HAC concrete is available. Rapidly converted HAC concretes will have increased porosity and become vulnerable to chemical attack, but porous unconverted HAC concrete will remain unattacked.