Structures and Metal Leachability of Sintered Sludge-Clay Ceramsite Affected by Raw Material Basicity

To solve the disposal problem of wastewater treatment sludge (WWTS), WWTS was mixed with clay and water glass (sodium silicate) for the production of lightweight ceramsite. The effect of mass ratio of $\mathrm{CaO}/\mathrm{Si}{\mathrm{O}}_2$ (defined as basicity) on physicochemical characteristics of ceramsite and stabilization of heavy metals was investigated. It was found that the optimal basicity for making ceramsite ranges was 0.024–0.069. The main thermal changes ($\le 1,000°\mathrm{C}$) are phase transformations, which are caused by the evaporation and volatilization of water and carbonous matters and, finally, by the oxidation and decomposition of inorganic matters and minerals. Anorthite [$\mathrm{Ca}({\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_8)$] and amorphous silica ($\mathrm{Si}{\mathrm{O}}_2$) increase as the basicity increases. The formation of more multiple crystalline phases is the main reason for the decrease in the compressive strength of the ceramsite with higher basicity. Leaching contents of Cd, Cu, Cr, and Pb decrease as the basicity increases. The stable compounds of heavy metals in ceramsite are $\mathrm{PbCr}{\mathrm{O}}_4$, ${\mathrm{Cr}}_2{\mathrm{O}}_3$, $\mathrm{CdSi}{\mathrm{O}}_3$, and CuO, which prove that strong chemical bonds are formed and the leachability of these heavy metals is greatly reduced by the crystallization and chemical incorporation process. Results indicate that basicity can be used as an important parameter for controlling the properties of ceramsite.