Strength and microstructure properties of solidified sewage sludge with two types of cement-based binders

Solidification treatment with cementitious binder is an effective way to reduce environmental hazards of sewage sludge. Two cementitious binders, i.e., ordinary Portland cement (OPC) and sulfo-aluminate cement (SAC), were compared in this study to treat the sewage sludge. The strength of solidified sewage sludge (SSS) and changes in microscopic characteristics before and after treatment were analyzed through microscopic analysis methods. The effect of organic matter in sludge on the strength of SSS were also discussed. The results showed that the strength of SSS were lower than that of the solidified clay with no organic matter, and the filtrate extracted from the sludge can also weaken the cementation of the two cements significantly. The solidification effect of the OPC on the sludge was lower than that of the SAC evidently. The organic matter in the sewage sludge caused the surface of the soil particles to carry a large negative potential, which interfered with the hydration of the binder and reduced the amount of cementation skeleton formed by the binder hydration products. This resulted in a porous structure with low mechanical strength. The amount of early hydration product formed in the SAC-based solidified samples was higher than that of the OPC-based samples. This was favorable for filling the pores of the solidified samples and increasing their density. SAC had a better compatibility with soft soil containing high organic matter than OPC, and the which provides an effective alternative binder for dealing with sewage sludge.