Investigation of the surface charge behaviour of ettringite: Influence of pH, calcium, and sulphate ions

Abstract

Ettringite is an important mineral that contributes to the overall performance of cementitious

materials. Knowledge of the surface charge behaviour of a solid is necessary for a mechanistic

description of surface processes such as adsorption or particle-particle interactions. The objective

of this study was to develop a model capable of reproducing ettringite surface charge as a function

of calcium, sulphate, and pH.

Ettringite was synthesised and characterised using different analytical, microscopic, and

spectroscopic techniques with the help of density functional theory. Electrophoretic mobility was

measured using laser Doppler electrophoresis in alkaline waters representative of the cementitious

environment.

The behaviour of the ettringite surface charge was shown to be quite complex as sulphate and

calcium acted in a competitive manner on the overall charge. The ζ-potential increases when the

calcium content increases, whereas it decreases when sulphate increases. This is due to the

possible adsorption of these ions at the surface, and the extent of the effect depends on the

relative concentrations of Ca and SO42􀀀 .

An electrostatic double layer model (DLM) was used to calculate the surface potential,

considering the adsorption of both calcium and sulphate, as possible ions determining the potential

(IDP), and formation of different complexes with ettringite surface functional groups

(SOH). The variations of the ζ-potential could be satisfactorily predicted under the different

chemical conditions of interest in a cementitious environment.