The application of Griffith's theory to the relationship between compressive strength and porosity of hardened cement pastes are discussed. Total porosity, compressive strength and the amount of non-evaporable water were measured. The amount of non-evaporable water showed good correlation with the porosity of the hardened cement pastes, and the composition and type of cement had little affect on this relationship. It was clearly shown that the water/cement ratio significantly affected these relationships. It was shown that there were two steps in the development of strength in the function of total porosity: in the early stage of hydration, the development of strength was controlled by maximum pore size; in the later stage, it was controlled by porosity. The effects of porosity and maximum pore size on the strength can be expressed using Griffith's equation, if it can be assumed that the elastic modulus and surface energy are exponential functions of porosity.
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