Geotechnical Properties of Solidified Sludge by Mixing Cement and Calcium-Bentonite

China generates up to 4 million ton dried sewage sludge every year. How to reasonably handle this huge amount of hazardous waste has become a concern. However, sewage sludge is hard to handle due to its poor geotechnical properties, such as high water content, low compressibility, and low resistance to environmental change. The use of solidification/stabilization (S/S) into sewage sludge can improve not only its geotechnical properties but also its environmental properties (e.g. stabilization of the pollutants in the sludge). Therefore, S/S can make the waste re-usable as a construction material or pre-treat it prior to landfill disposal. In this study, ordinary Portland cement (OPC) and calcium-bentonite were used as the S/S for local sewage sludge from the Nanjing Wastewater Treatment Plant in China. Geotechnical properties of the solidified sludge were studied in terms of unconfined compressive strength (UCS) and durability after the wet and dry cycles. The test results show that in a short curing time, cement alone-S/S sludge had a low compressive strength but a significant improvement was observed when bentonite was added to replace some amount of cement. In a long curing period, cement played a major role in increasing the compressive strength as compared to bentonite. In addition, initial water content of sewage sludge influenced the UCS, which should be taken into account before S/S. The test results of the dry and wet cycles indicate that bentonite was beneficial to improve the durability of solidified sludge when the ratio of bentonite was less than that of cement. Considering both UCS (>100kPa) and dry/wet cycle (<30% relative mass loss) requirements, the ratios of sewage sludge, bentonite, and cement should be approximately 5:1:2.