Mechanistic study of the effect of potassium ferrate and straw fiber on the enhancement of strength in cement-based solidified municipal sludge

The high content of organic matter in sludge is the primary reason for the poor solidifying effect and excessive dosage of the cement base. In this study, potassium ferrate and straw fiber are utilized to synergistically enhance the solidifying effect of the cement and elaborate the strength mechanisms. Among them, potassium ferrate was selected to oxidize and crack the structure of organic matter in sludge and consume part of organic matter; straw fiber was used as an adsorption material to absorb some of the organic material and reduce its interference with the cement hydration reaction; the skeleton function of straw fiber in solidified sludge was used to improve the final solidified sludge strength. It is shown that the presence of these two additives significantly improved the cement solidification strength and reduced the moisture content of the solidified body. Moreover, the moisture content and strength followed an obvious linear relationship (adjusted R(2) = 0.92), with the strength increasing as the moisture content decreased. After pretreatment with potassium ferrate, the free water content in the dewatered sludge increased by 4.5%, which was conducive to the adequate hydration reaction with cement. The analysis using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and mercury intrusion porosimetry (MIP) revealed potassium ferrate synergizes with straw fibers to promote the production of hemihydrate gypsum and gismondine. However, hemihydrate gypsum, calcium carbonate, and gismondine resulted in structural swelling, which was confirmed by the microscopic morphology and pore structure analysis. However, the adverse effects due to swelling were offset by the increase in strength brought by the above crystalline substances.