Changes of the shear strength and pore structure for solidified sludge after crushing and remodelling

Abstract:

To investigate the macroscopic strength changes and the underlying microscopic mechanisms of solidified sludge (SS) after crushing and remodelling, which enhances the engineering applications of crushed and remodeled solidified sludge (CRSS), a series of direct shear tests, scanning electron microscopy (SEM) analyses, and mercury intrusion porosimetry (MIP) tests were conducted on both SS and CRSS to examine the effects of curing agent content, pre-curing age, and post-curing age on the shear strength characteristics of CRSS. Furthermore, the microstructure and pore structure characteristics analyses were performed to explore the intrinsic mechanisms underlying the shear strength changes in CRSS. The results indicate that the shear strength of the initial CRSS increases with both curing agent content and pre-crushing curing age. The internal friction angle of the initial CRSS is comparable to that of SS, ranging from 25° to 50°, while cohesion decreases significantly, varying from 20 kPa to 125 kPa. The shear strength of CRSS primarily originates from the friction and interlocking between soil aggregates, where higher aggregate strength corresponds to a greater internal friction angle. As the post-curing age increases, soil aggregates and the pores between them in CRSS are cemented and filled with hydration products, resulting in a significant increase in cohesion while causing minimal changes in the internal friction angle. In SS, the pore structure is predominantly characterized by small pores with diameters ranging from 0.01 μm to 0.10 μm and medium pores from 0.1 μm to 1.0 μm. With increasing curing agent content and curing age, medium pores in SS are progressively transformed into small pores. In CRSS, the volume of small pores decreases significantly, accompanied by a notable increase in large pores with diameters ranging from 1 μm to 10 μm and fissure pores larger than 10 μm, while the volume of medium pores remains largely unchanged.