A Novel Process Strategy for Efflorescence Control of Ceramsites Sintered from High-Sulfur Kiln Slag

The high sulfur content in kiln slag ceramsites leads to pronounced efflorescence, significantly limiting the application of such solid-waste-derived products in construction engineering. In this study, a sintering experiment of 50 kg ceramsites with 60% kiln slag content was conducted at 1150 °C on small-scale ceramsites sintering equipment. The research focused on discussing the mechanisms of efflorescence and putting forward the process strategy to control efflorescence in kiln slag ceramsites. The results revealed that the sulfur content (4.53%) and water absorption rate of ceramsites (8.5%) were the primary causes of efflorescence. During the sintering process, sulfur evolved on the pathway of “pyrite in raw materials→partial oxidation to SO2 and partial fixation in CaSO4→decomposition of CaSO4”. In order to reduce content of sulfur, it was proposed that not only was sulfur in pyrite oxidized enough to be removed in the low-temperature stage but, also, residual CaSO4 decomposed more to eliminate S in the high-temperature stage. This optimizing sintering process reduced the sulfur content from 2.0% to 0.72%. Furthermore, enhancing its densification contributed to avoiding efflorescence. By controlling its sintering temperature and increasing the formation of Fe2O3, the densification of ceramsites was improved by decreasing its water absorption rate from 8.5% to 1.2%, and its efflorescence was thereby enhanced. This provides both a theoretical foundation and technological support for the high-value utilization of high-sulfur solid waste.