Abstract: Magnesium oxychloride cement (MOC) is innovatively introduced into sludge solidification. The influence of initial water content, molar ratio of MgO/MgCl2, curing age and MgO activity on sludge solidification efficiency and the corresponding driving mechanisms are systematically studied by unconfined compressive strength, water content of solidified matrix, scanning electron microscopy (SEM) and energy dispersive spectrum (EDS). The results show that the higher the initial water content, the lower the strength of solidified samples, and the needle-rod shaped crystals (phase 3 or phase 5) can be clearly observed in MOC solidified sludge with medium water content. The increase in molar ratio of MgO/MgCl2 leads to an improvement in the compressive strength of samples, accompanied by the transformation of hydration products from amorphous gel to phase 3, phase 5 and Mg(OH)2 crystals. The strength of solidified sludge shows a increase trend with the prolongation of curing age, and the strength prior to 28 d increases relatively faster, followed by a stable trend after 28 d. Especially, the surface of specimens with high molar ratio of MgO/MgCl2 is prone to have frost at a longer curing stage. An enhancement in MgO activity produces more reactive components and leads to higher compressive strength of MOC solidified sludge, whereas the activity degree of MgO has no significant effect on the evolution of hydration products. The obtained results can provide a theoretical support for the development of green and low-carbon MOC-based cementitious material and its application in soil reinforcement fields such as sludge solidification.

Key words: sludge, magnesium oxychloride cement (MOC), solidification, strength, micro-mechanisms