Contribution of calcium aluminates to the water resistance of hydrated calcium sulfates

Abstract

The water sensitivity of calcium sulphate materials has been known for a long time. In a humid atmosphere, plaster elements can become veiled under the action of their own weight because a low water uptake of 3% by mass can cause a drop in compressive strength of 50%. The low water resistance of calcium sulfates limits the applications of these materials to decorative elements or interior coatings despite their strengths compared to other construction materials: good fire resistance, low CO2 emission and low production cost. The objective of our research work is to evaluate the insensitivity of calcium sulfates to water by ettringitic binders containing calcium aluminate cement (CAC). Four types of calcium sulfates have been studied: natural gypsum, hemihydrate-α, hemihydrate-β, and synthetic anhydrite. In this research, only the majority binders of calcium sulfates will be studied: [70%, 80% of Calcium sultates with 30%, 20% of CAC]. The experiments results have shown that in high-calcium sulphate binders (≥ 70%), CAC helps reduce the water sensitivity of binders regardless of the nature of calcium sulphate by decreasing the total porosity, which leads to an increase in mechanical strengths; and reducing the connectivity of the pores, which contributes to reducing the entry of water and its movements into the material and consequently, reducing the degradation of the test pieces during immersion tests in the water.