Zinc oxide in Portland cement. Part II: hydration, strength gain and hydrate mineralogy

Abstract

Soluble zinc salts added to cement mix water are generally agreed to retard hydration. The role of zinc in clinker is, however, less well-established. Experiments are reported on clinker stimulants and real industrial clinker meals, the latter doped with 1·5 wt% ZnO. Contrary to findings in the literature, relatively little zinc goes into solid solution in the main clinker phases: much is present in the interstitial phases as a solid solution based on 6CaO·3ZnO·2Al2O3. Supplementary studies of the CZA system disclose the existence of two ternary phases: C6Z3A2 and C3ZA2. Both are cementitious in combination with Ca(OH)2 or gypsum, or with both. ZnO at 1·5 wt% clinkered in industrial meal, does not affect set times but enhances the rate of strength gain. Zinc remains relatively insoluble, with aqueous Zn concentrations of stimulants not exceeding 2·2 ppm.