Although a great deal of information on fly ash and its use in concrete is available, very little of it contributes to an understanding of the interactions between portland cement and fly ash during the hydration and the hardening process which could provide a basis for predicting and improving the performance of fly ash in concrete. It is generally accepted that, in the pozzolanic reaction of fly ash, the C a ( O H ) 2 produced during cement hydration reacts with the silicate and aluminate phases of fly ash to produce calcium silicate and aluminate hydrates / L 39/. However, the cement hydration and the pozzolanic reactions do not proceed independently. Watersoluble alkalis, sulphates, lime and organics from the fly ash may affect the surface reactions and the nucleation and crystallization processes, especially in the early stages of cement hydration. Similarly, the pozzolanic reactions will depend on the amount of calcium, alkalis, sulphates, silicate and aluminate ions released into the liquid phase from cement and fly ash. I n view of these complexities, studies of the interactions between individual clinker minerals and fly ash, as well as those between cement and fly ash, are especially useful in developing an understanding of the phenomena resulting from several independent and inter-dependent processes.