Effect of fly ash on the microstructure of cement mortar

A microstructural study of mortars prepared with a low-alkali, low-C₃A cement and a Class F fly ash, both of Swedish origin, was carried out using the scanning electron microscopy-energy-dispersive X-ray analytical technique. Supplementary phase analyses were made by X-ray diffraction and thermogravimetry-differential thermal analysis. Normally, CH crystals in the transition zone grow with their c axis parallel (or the (0 0 1) cleavage plane perpendicular) to the aggregate surface. The encapsulation of the fly ash particles by the growing CH reduces the amount of orientated CH at the aggregate-paste interface. The growth mechanism of these crystals is discussed. The reduction of CH, most significant after 28 days of hydration, is mainly due to the reaction of CH with the fly ash glass phase. Initially, the replacement of cement by fly ash weakens the paste-aggregate interfacial zone due to reduction of contact points, and increases the local water-to-cement ratio. This, however, improves significantly when the fly ash has reacted. In order to enhance the reaction of fly ash, extra gypsum was added. The results show that gypsum can accelerate the fly ash reaction, but the products formed, and the beneficial effects of gypsum, are mainly determined by the total amount of gypsum in the paste.