GGBS is used extensively as a cement replacement material, reducing the carbon footprint of cement while potentially improving technical performance. However, standards consider hydration of slag composite cements only at 20 °C. This may not be applicable for use in tropical climates. This work has investigated the impact of GGBS composition and curing temperature on the hydration, microstructure and subsequent transport properties of such composite cements. Two slags, of differing compositions, were combined with a CEM I 52.5 R at 30% replacement. Paste samples were characterised by calorimetry, TGA, XRD and SEM to follow hydration and microstructural development. Mortar samples were used to follow strength development and water transport properties. All tests were carried out at temperatures of 20 and 38 °C. The higher temperature resulted in an increase in the degree of hydration of the slags, but had a deleterious impact on the microstructure. The more basic slag had higher strengths and greater degrees of hydration especially at the high temperature. The results showed that temperature had a much greater influence on the reactivity of the slags than the difference in chemical composition.