Expansive Binders in the Portland Cement - Calcium Aluminate Cement - Calcium Sulfate System

The volume increase during the hardening of expansive binders in the Portland cement calcium aluminate cement – calcium sulfate system is conditioned by a careful correlation between the ettringite formation and the evolution of the hardening structure. The volume change and mechanical characteristics of this type of binders are influenced by parameters such as: the composition and fineness of the components, portland cement/expansive admixture and calcium sulfate/calcium aluminate cement ratios, the nature of the calcium sulfate source and the curing conditions. This paper presents the influence of the components dosage on the kinetics of the hydration processes and on the nature and quantity of formed hydrates, as well as on the volume change and hardening processes of expansive cement. The portland cement/expansive addition ratio used in this project was in the range of 75/25...85/15 and the calcium sulfate/calcium aluminate cement between 0.55 and 0.80. The calcium sulfate source was natural gypsum as well as anhydrite obtained by gypsum calcination for 4 h at 400°C. A good correlation was observed between the volume increase and the mechanical properties of the investigated expansive binders. The initial curing of the specimens in water (2 days) followed by 4 days of curing in humid atmosphere (R.H. 85%) determines a bigger volume increase as compared to the samples cured 6 days in humid atmosphere. The expansive binders with anhydrite content develop higher mechanical strength and permit an easier control of the expansive processes as compared to those containing gypsum.