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This study assesses the kinetics of hydration of Pulverised Fuel Ash and Metakaolin cement pastes and compares how the rate of reaction affects the pore-characteristics and resistance to ionic ingress. The degrees of hydration for the different mixtures were evaluated, both as a function of the calcium hydroxide content and with respect to the chemically combined water contents. The reaction rates have been evaluated using a mathematical model (Jander model), which describes the hydration kinetics of the two materials. The results show that the reaction rate for specimens incorporating Metakaolin is several folds higher than those incorporating Pulverised fuel ash. The faster rate of reaction of the pozzolanic blends results in a faster rate of filling the pore spaces with hydration products, smaller pore volumes and reduced chloride ion diffusivity. The results from this investigation will provide engineers with a much needed uunderstanding of the kinetics of hydration and setting characteristics of these types of cement systems and help in gaining an appreciation of the early structural development, ease of placement and subsequent evolution of properties.