Early-stage curing temperature has a significant effect on high performance concrete durability and strength. Concrete cures at high temperatures when poured in large volumes due to the autogeneous heat generated by the hydration reactions of cementitious materials. The addition of supplementary cementing materials to concrete reduces the heat of hydration and extends the service life in structures by improving both long term durability and strength. This thesis investigates the effects of early-age curing temperature on the durability and strength of silica fume and fly ash modified concretes. Accelerated test methods were used to measure chloride diffusion in concretes produced with silica fume cement (T10SF) and various levels and types of fly ash replacement. The concrete mix determined to be the most durable with respect to chloride diffusion had 56% of the Type 10SF cement replaced with a moderate-level calcium content fly ash. The use of the Rapid Chloride Permeability Test, (ASTM C 1202, AASHTO T 277) has been widely criticised. Despite a limited data set, good correlation was shown in the results of this research between a two-chamber accelerated chloride migration cell test, and the RCPT.