The sintering of coal gangue ceramsites (CGCs) using belt roasting technology involves the recirculation of flue gases and variations in oxygen concentrations. This study investigates the effects of temperature and pyrolysis atmosphere on the pore structure of CGCs at three temperature levels: 600 °C, 950 °C, and 1160 °C. The results revealed that apparent porosity is primarily influenced by O₂-promoted weight loss and the densification process, while closed porosity is affected by pyrolysis reactions and crystal phase transformations. Below 950 °C, enhancing the oxidative atmosphere facilitates the preparation of porous CGCs, whereas above 950 °C, reducing the oxidative atmosphere favors the preparation of high-strength CGCs. These findings provide valuable insights for the industrial production of CGCs, offering a basis for optimizing sintering parameters to achieve the desired material properties. The latest production equipment, furnished with adjustable atmospheres (such as belt sintering roasters), can better regulate the mechanical properties of the products.

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.

The large-scale accumulation of coal gangue has caused serious environmental problems, and converting it into ceramsite is an important pathway for resource utilization. Grinding is a key step in the preparation of coal gangue ceramsite, but the effect of particle size distribution on heat release and ceramsite performance remains unclear. In this study, coal gangue with a calorific value of 699.77 kcal/kg was ground for 1, 2, 3, and 4 h, respectively, followed by pelletizing and sintering. The different ground powders and sintering ceramsites were investigated using particle size analysis, TG-DSC, and XRD, as well as pore structure and strength tests. The results show that for the Datong coal gangue raw material, grinding parameters and sintering regime adopted in this work, the powder milled for 2 h presents a left-shifted particle size distribution curve with a narrow main peak, particle refinement and a concentrated particle size profile (D50 = 8.498 μm, D90 = 23.941 μm). Combined with TG-DSC, XRD, and pore property test results, the 2 h ground powder delivers the most concentrated heat release during low-temperature combustion. This concentrated heat release is inferred to promote high-temperature mineral phase reconstruction and liquid phase formation, thereby generating a dense ceramsite structure featuring low apparent porosity, high closed porosity and excellent mechanical performance (water absorption: 2.99 ± 0.44%; compressive strength: 15.12 ± 0.43 MPa). When the grinding time is extended to 3 h, the particle size distribution broadens, and both the particle size distribution curve and the DSC curve show shoulder peaks, indicating dispersed heat release. Extending grinding time from 3 h to 4 h appears to induce fine-particle agglomeration with heat release becoming more dispersed and decreasing reaction degree, leading to an uneven temperature distribution and deteriorated ceramsite performance.