Effects of Humus and Solidification Agents on the Solidification/Stabilization Process of Organic-Rich River Sludge: Characteristics of the Stabilized Sludge

River sludge usually contains a high content of organic matter, leading to its low strength or difficult solidification in its solidification/stabilization (S/S) treatment projects. This study selected river sludge with medium and high content of organic matter for the S/S treatment using modified curing agent (GCP) and cement (P.O). Effects of humus and curing agent on the S/S process of river sludge were investigated via analyzing physical properties, changes in organic matter, microstructure, and mineral compositions of the solidified sludge. The results showed that the increase rate of compressive strength of the solidified sludge was influenced by the content of organic matter and composition of the curing agent. The presence of humus inhibited the hydration reaction and reduced the increase rate of compressive strength of solidified sludge. Slag and phosphogypsum in GCP promoted the hydration reaction, significantly enhancing the compressive strength of the solidified sludge to 2242.24 KPa. The water content of the solidified sludge was influenced by the environmental conditions and curing agent, which could reflect the level of hydration reaction in the solidified sludge. The pH of the solidified sludge was directly affected by the humus in the sludge, with a decreasing trend during the S/S process. Decomposition of the humus in the sludge released H+, which reacted with OH− produced by the hydration reaction via neutralization reaction. The pH of the solidified sludge was lowered, and the hydration reaction was inhibited, hindering the decrease in the water content of the solidified sludge. Therefore, the hydration reaction has an antagonistic effect on the decomposition of the humus. Microstructure analysis (SEM) confirmed that GCP could effectively solidify the organic-rich river sludge. This study provides a theoretical basis for the S/S treatment of organic-rich river sludge.