Effects of residual carbon on synthesis of 11 Å tobermorite from coal gasification ash

Coal gasification ash (CGA) is the solid waste produced by coal in the process of gasification. This can be used to produce autoclaved building materials because tobermorite, the main hydration product in such materials, can be formed from CGA and CaO. In this study, tobermorite was prepared through the hydrothermal treatment of CGA. Given that residual carbon is usually present to some degree in CGA, its effects on the formation of tobermorite were specifically studied using X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, and N2 adsorption–desorption tests. The results show that tobermorite can be prepared from a CGA/CaO mixture at 160°C without additional alkaline material. However, residual carbon can retard the transformation of calcium silicate hydrate into tobermorite and thus increase the required formation temperature while prolonging the formation time. With increasing residual carbon content, the morphology of the resulting tobermorite changes from needle-like and plate-like to irregular, which may also have a detrimental effect on the mechanical properties of the material.