A study on reactivity of four different Circulating Fluidized Bed Combustion (CFBC)
fly ashes has been realized in the presence of water. Paste of each ash was prepared and
analyzed for its setting time, expansion and strength. The products of hydration, and their
evolutions over a period of time were identified by X-ray diffraction and differential thermal
analysis. The results of this study show that the reactivity of the CFBC fly ashes is strongly
related to their chemical composition, essentially to their quantity of silica, alumina, lime and
sulfate, which promote principally the formation of ettringite, gypsum and C-S-H. It is further
noted that the intensity and the proportion of these phases determine the hydration behavior of
the CFBC fly ashes.