Early hydration of large single crystals of tricalcium silicate

A reaction product believed to be an initial hydrate layer was observed to have formed on large pure C₃S single crystals after 5 minutes of hydration. This layer then increased in thickness and became covered with micrometer-sized spheres of poorly crystallized Ca(OH)₂ within 30 minutes. Subsequently, the formation of a new hydrate of acicular morphology was observed to occur on the surface of the first-formed hydrate. This transformation was accompanied by the disappearance of the first hydrate layer and the calcium hydroxide spheres. The observation of a transient initial hydrate is in accord with the view that the rate of early C₃S hydration is controlled by the formation and subsequent disappearance of a barrier layer.