This thesis describes the examination of the microstructure of hardened cement paste [HCP], in particular, cracking in HCP, and the measurement of subcritical crack growth in HCP.A scanning electron microscope has been used to examine, fracture surfaces, complementary fracture surfaces, polished surfaces, and polished surfaces containing introduced cracks. This examination has provided information about fracture paths in HCP, which shows that the main hydration products [C-S-H gel and calcium hydroxide] form the majority of crack path elements. The interpenetration of these products during hydration, and the expected variations in strength of these hydration products suggest that both must be recognised as significant contributors to the mechanical properties of HCP. The examination of cracks in polished surfaces appears to be a very useful way of determining the microstructural aspects of cracking, and its possible future applications are discussed.Two methods of determining the subcritical crack growth parameter [n] were performed, the first, load-relaxation in double torsion plates yielded a value of 64, while the second, the variation in the critical stress intensity of single edge notched beams with loading rate yielded a value of 27. The much higher value determined by load relaxation is thought to be due to both, the effect of crack resistant inhcmogeneities in the microstructure which have a more marked effect on load relaxation measurements, and a susceptibility to creep in the double torsion plates which reduced the amount of crack growth. The susceptibility to creep was identified by a shift of the crack velocity-stress intensity [V-K] curves to higher stress intensities with repeated loading. The value of subcritical crack growth parameter of 27, determined by the variation of critical stress intensity with loading rate, is therefore regarded as the more valid.

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.