Effects of residual carbon on the synthesis of 1.1 nm tobermorite from coal gasification ash

Coal gasification ash (CGA) is a solid waste produced by coal gasification. It can be used to produce autoclaved building materials because tobermorite, the main hydration product in such materials, can be formed from CGA and calcium oxide (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 nitrogen adsorption–desorption tests. The results showed that tobermorite can be prepared from a CGA/calcium oxide 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 an increase in the 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.