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 analyt
ical technique. Supplementary phase analyses were made by X-ray diffra
ction and thermogravimetry-differential thermal analysis. Normally, CH
crystals in the transition zone grow with their c axis parallel (or t
he (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 m
echanism of these crystals is discussed. The reduction of CH, most sig
nificant 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 reducti
on of contact points, and increases the local water-to-cement ratio. T
his, 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 dete
rmined by the total amount of gypsum in the paste.