Preparation of non-sintered sewage sludge based ceramsite by alkali-thermal activation and hydration mechanism

The recycling of sewage sludge (SS) and fly ash (FA) is significant for the disposal of increasing solid waste. In this study, SS and FA were used as raw materials, and sodium hydroxide (NaOH) and calcium oxide (CaO) were used as alkali activators to prepare non-sintered ceramsite under room temperature curing, and the optimal preparation conditions for the ceramsite were determined through single-factor experiments. Alkaline-thermal activation (ATA) was used to enhance the activity of SS and FA, and the results showed that the activities of SS and FA after ATA were largely improved, and composite activation by CaO and NaOH is more effective than that by either CaO or NaOH. The formed abundant sodium silicate hydrate (N-A-S-H) gels and few hydrated calcium silicate (C-A-S-H)/hydrated silicate (C–S–H) gels can bind well with the particles inside the ceramsite to form the denser structure. In addition, the cylindrical compressive strength of the ceramsite can reach 7.43 MPa, while the apparent density, bulk density, and water absorption are 2603 kg/m3, 852 kg/m3, and 8.37%, respectively under the conditions that the ratio of SS to FA is 8: 2, 9 wt% NaOH and 6 wt% CaO and 600 °C thermal activation, which can meet the standard of ceramsite (GB/T 17431.1–2010). This study provides an effective way for the utilization of SS and FA.