Study of nucleation and growth processes of ettringite in diluted conditions

Ettringite is a key compound of cementitious materials, formed during hydration that influences the rheological behavior of the cement paste. Despite its importance, ettringite formation has been poorly studied. In this work, nucleation and growth processes of ettringite were separately investigated. Ettringite nucleation was described using the Classical Nucleation Theory and an interfacial crystal-solution energy was determined. After the initial primary homogeneous nucleation, when the surface of already formed crystals was still low, the precipitation rate was shown to depend almost exclusively on the solution supersaturation. When the surface of crystals in suspension was getting bigger, a secondary nucleation responsible for an increase of ettringite needle diameter was observed. The precipitation rate became surface dependent and increased significantly. Results of this work show that nucleation of ettringite is easy as it requires few energy and that after primary nucleation precipitation occurs rapidly via secondary nucleation and growth.