Delayed ettringite formation: a microstructural and microanalytical study

Cement pastes were cured at 80°C and examined by scanning electron microscopy, X-ray microanalysis and X-ray diffraction immediately or after storage in water for various periods at 20°C. Ettringite began to form a few days after the heat treatment, as very small crystals thinly dispersed throughout the paste in close admixture with C-S-H. It subsequently recrystallized in cavities typically 5-10 μm in size formed by dissolution of clinker grains or in other ways. There were no indications that this latter process disrupted the surrounding material. The results favour the view that the expansion that can be associated with delayed ettringite formation is driven by processes occurring within the paste and not by the formation of ettringite at aggregate interfaces.