Three-dimensional particle size distributions of unhydrated cement in cement paste were obtained from its two-dimensional cross sections by the stereology analysis. The mean diameter of the unhydrated cement particles was used for determining the degree of hydration of cement. The degree evaluated by the average diameters was consistent with those determined by the volume fractions of unhydrated cement and non-evaporable water contents. It was also possible to determine water/cement ratios of hardened concretes, using the hydration degree based on the average diameters and the volume fractions of unhydrated cement particles in concrete at a given age. The particle size distribution of unhydrated cement can be a characteristic feature reflecting the process of hydration of cement.
