Abstract: The stability and safety of the solidified sludge pile will be threatened when the engineering properties change due to freezing and thawing cycles (FTC). Laboratory FTC tests are conducted on the solidified municipal dewatered sludge in the closed system. Unconfined compressive strength and permeability coefficient of the solidified sludge are tested after each FTC. According to the results of tests, some samples subjected to cyclic freezing and thawing are selected to perform mercury intrusion porosimetry (MIP) tests and to analyze with the scanning electron microscope (SEM). The results show that the 1st FTC results in a decrease in the strength by 30% and an increase in the permeability by 80% of the the solidified sludge. Whereafter the change range gradually decreases. The strength decreases by 50% and the permeability coefficient increases by about one order of magnitude after about 6 FTC, thereafter they both almost keep constant. The variation of the permeability coefficient lags slightly behind that of the unconfined compressive strength. The pore volume and the permeability coefficient gradually increase while the strength decreases little by little with the increase of FTC. In the closed system, the total swelling volume induced by the frozen water in the solidified body is constant and the internal moisture is uniformly distributed, so the pore volume doesn’t continue increasing any more while just the pores mix together and redistribute when reaching an certain number of FTC, then each parameter becomes constant. The investigation provides parameters and guiding for safe landfilling of the solidified sludge in the cold regions.

Key words: solidified sludge, freezing and thawing cycle, compressive strength, permeability coefficient, microstructure