The effect of drying method on ordinary Portland cement surfaces during the early stages of hydration

During the early stages of ordinary Portland cement (OPC) hydration, ettringite, calcium hydroxide and C-S-H form on the OPC surface while nanoscale pits are formed in the exposed surface areas. In order to study these features using high resolution electron microscopy, samples must be appropriately dried to remove unbound water. The work reported here compares the effects of freeze-drying to drying by solvent exchange using four different solvents on the surface features of OPC after four hours of hydration in order to determine whether the solvent exchange process produces nanoscale changes in the surface features that can be detected using high resolution scanning electron microscopy (SEM). A quantitative image analysis of secondary electron SEM images was performed and the results analyzed using small sample statistics. They suggest that all of the solvent exchange methods damage the OPC surface and produce surface structures not seen in the freeze-dried samples. Caution is therefore warranted in the use of solvent exchange in the preparation of samples for the study of the dissolution of OPC surfaces.