Hydrothermal curing of dewatered sludge cake: Influencing factors, phase evolution, and performance optimization

Dewatered sludge cake is rich in clay-sized particles and silicon-aluminum minerals, which are derived from engineering slurry via flocculation, sedimentation, and plate-and-frame filtration. It possesses the material properties for hydrothermal curing reactions to be used in the preparation of potentially low-carbon alternative materials. In order to determine the appropriate raw material proportions and process parameters for manufacturing building blocks using hydrothermal curing, this study systematically conducted experimental investigations into the factors influencing the hydrothermal curing of dewatered sludge cake, including the Ca/(Si+Al) ratio and alkalinity of raw material, the compaction degree of specimens, reaction temperature and time. The phase evolution process was investigated and the optimal reaction conditions were determined. The results indicated that the Ca/(Si+Al) ratio and reaction temperature are the main factors influencing the types of minerals formed through hydrothermal curing. The optimum condition was found at a Ca/(Si+Al) ratio of 0.939 and a temperature of 200–220 °C, where the most tobermorite was formed and the highest compressive strength was obtained. Compaction degree strongly affected appearance quality of the blocks, with 90% as the optimal compaction degree. Reaction time affected the process of crystal growth, with the optimal time ranging from 12 to 18 h under the investigated conditions. Mechanism analysis indicates that katoite acts as an intermediate product in the reaction process, altering the formation path of tobermorite and reducing its formation temperature threshold to approximately 180 °C. Its formation is closely related to the participation of aluminum element and high alkalinity. Tobermorite appears to be closely associated with strength development, while the reticularly arranged needle-like structure may be associated with the relatively high water absorption of the specimens. In conclusion, hydrothermal curing enabled the conversion of dewatered sludge cake into lightweight, high-strength materials with potential for interior wall applications.