Ultimately the strength of cement is determined by the properties, amounts and distributions of the phases it contains and by the way these are bonded together. Here an example is presented of the determination of the amount and distribution of calcium hydroxide, by BEI and quantitative image analysis, in a cement paste and cement/fly ash mixture which had previously been studied by TGA. Examination of crack paths on BEIs of polished cement surfaces gives some indication as to the importance of calcium hydroxide in determining the properties of the cement.