Abstract:Through the measurements of free linear expansion rate of specimens containing cement mixed with different kinds of high f CaO materials at various curing periods, the XRD peaks of CaO, the crystal dimension and lattice distortion of f CaO in different materials, a conclusion is drawn that the hydration activity of f CaO in different materials are different, the hydration activity of f CaO in high calcium fly ash (HCFA) is larger than that of f CaO in high free oxide calcium clinker (HFOCC), and also larger than that of f CaO in rotate furnace steel slag (RFSS). Considering the difference of hydration activity among those materials, it is suggested that different modification technique should be used to realize the goal of high value, high effective usage of such high f CaO materials.