Formation of belite-based binder from waste materials

The present work deals with the preparation of belite-based binders using a mixture of sludge waste from mining and washing of limestone (source of CaCO3 and SiO2), and waste material from acetylene gas cylinders filler containing hydrosilicates-tobermorite and xonotlite. These wastes are of suitable oxide composition for the preparation of belite cements. The materials were mixed together in different proportions and burned at temperatures from 700 to 1300 degrees C. 3% K2O was used as dopant to stabilize the reactive belite modification. The main interest was focused on the process of belite formation related to its reactivity. During burning of waste material mixtures, limestone is decomposed providing CaO, while tobermorite and xonotlite are transformed into wollastonite. Then, belite is formed by reaction of SiO2 and CaO, as it is typical in Portland clinker, but also by reaction of wollastonite and CaO. The process of belite formation influences its hydraulic properties to a great extent. Carbonation of calcium silicates was studied as well. Phase composition of burned products was studied by X-ray powder diffraction. Hydration and carbonation products were identified by differential thermal analysis. It was confirmed that the content of rankinite has a significant effect on CO2 uptake. Carbonation rate was also positively affected by a higher wollastonite content.