This thesis describes the results of an investigation carried out to compare the effects on the mechanical properties of Ordinary Portland Cement paste (OPC) when partially replaced by Pulverized Fuel Ash (OPC + 25 wt. % PFA). This was done by evaluating a number of parameters at times between 1 day and 1 year at 20 C. At comparatively early ages the OPC+PFA was found to be weaker (in flexural strength), less stiff (in three point bend), and less tough (in stress intensity factor), and vice versa at later ages. However it was also found that flexural strength and stress intensity factor displayed maxima at ~28 days. The effects of increasing the water/solids ratio and of increasing the curing temperature to 50C were also investigated. These experiments were extended to check on the validity of the above parameters in describing the fracturing process. Changing the sizes and notch depths of stress intensity factor specimens illustrated the non-applicability of linear elastic fracture mechanics (LEFM) to OPC pastes. The observation of a process zone by diffuse illumination optical microscopy (DIM), and of Pe/Pf being < 100% (a measure of non-linearity in a load/displacement curve in the stress intensity factor experiments) confirmed this view. A "tied crack" model was applied to explain this behaviour up to ~28 days. Between ~28 days and one year this model had to be modified to incorporate microcrack formation due to internal stress generation and other causes which resulted from continued hydration. The OPC+PFA displayed less microcracking, which could be ascribed to consumption of calcium hydroxide in the pozzolanic reaction and the spherical nature of the PFA. The microstructure was investigated by scanning electron microscopy which gave valuable supporting evidence to the theories discussed above. Overall, the OPC+PFA gave better mechanical properties by one year.