The hydration reactions that occur with CFBC materials treated via the CERCHAR hydration process and combined with PFA have been examined and compared with materials produced via the LIFAC (limestone injection into the furnace and activation of unreacted calcium) process. A wide range of chemical and physical techniques have been used to differentiate the actual speciation of cubes treated for up to 150 days. The two materials behave very differently, with FBC-derived ashes being dominated by sulphate chemistry, i.e., the formation of gypsum and ettringite. The CERCHAR-treated ashes appear to ＇＇make＇＇ portlandite available for ＇＇sulpho-pozzolanic＇＇ reactions, and this seems to be the key to understanding why these materials display superior performance in applications with PFA or cement substitution. The LIFAC materials, with much less sulphate and Fe2O3 and more Al2O3, still produces ettringite, but no gypsum, and hydrated tetracalcium aluminate appears as the major hydration product.