Ettringite formation and microstructure of rapid hardening cement

This paper reports the formation and microstructure development of ettringite during hydration of two rapid hardening cements under various handling times. The rapid hardening component of one of these cements is crystalline calcium fluoroaluminate while that of the other is an amorphous calcium aluminate. During hydration, the crystalline fluoroaluminate component forms ettringite from the very beginning. The amount of ettringite increases with time producing needle-shaped crystal of various sizes. The amorphous calcium aluminate component, on the other hand, exhibits an initial induction period after which there is a rapid formation of needle-shaped ettringite crystals of nearly uniform size.