Formation and expansion of ettringite crystals

The formation and expansion of ettringite crystals are discussed on the basis of principles of physico-chemistry. It is suggested that ettringite forms by a through-solution mechanism and that expansive force results from crystallization pressure as well as swelling pressure converted from the reduction of chemical potentials in the reaction of ettringite formation. Expansion depends mainly on the positions where ettringite crystals form. In a pore solution of high alkalinity, ettringite crystals form in situ and cause a large expansion. Alternately, they precipitate partially or completely in voids, thus giving rise to a little expansion or no expansion.