Ceramsite obtained from water and wastewater sludge and its characteristics affected by (Fe2O3 + CaO + MgO)/(SiO2 + Al2O3)

To control and optimize the process for making ceramsite from wastewater treatment sludge (WWTS) and drinking-water treatment one (DWTS), the effect of mass ratios of (Fe2O3 + CaO + MgO)/(SiO2 + Al2O3) (defined as F/SA ratios); SiO2:Al2O3 and Fe2O3:CaO:MgO (under the condition of fixed F/SA ratio) on the characteristics of ceramsite were investigated. It was found that the optimal F/SA ratios for making ceramsite range 0.175–0.45. Na–Ca feldspars and amorphous phases increase in ceramsite as F/SA ratios increase. Ceramsite with porous surfaces, expanded structures, and complex crystalline phases can be obtained at 0.275 ≤ F/SA ≤ 0.45, which accordingly cause the decrease in compressive strength. Higher strength of ceramsite with lower porosity can be obtained at 0.175 ≤ F/SA < 0.275, and under the condition of F/SA ratio = 0.275, the raw materials can produce ceramsite with desired physical properties at 18.2:35 ≤ SiO2:Al2O3 ≤ 45:10.2 and 10:2.7:1.4 ≤ Fe2O3:CaO:MgO ≤ 5.3:6:1.6. Ceramsite with higher compressive strength and lower porosity can be obtained at SiO2:Al2O3 > 27.2:15.8 and Fe2O3:CaO:MgO > 6:3.5:1.8. Results indicate that F/SA ratios could be used as an important parameter to control the production process of ceramsite with desired physicochemical properties and resolve the disposal problems of residual sludges.