Strength properties and associated mechanisms of magnesium oxychloride cement-solidified urban river sludge

Eco-friendly magnesium oxychloride cement (MOC) is incorporated to reduce the disposal obstacles and mitigate the environmental impacts related to the urban river sludge. Three major factors including MOC content, molar ratio of MgO/MgCl2 and curing time are examined by unconfined compressive strength (UCS) tests. The microscopic effect caused by the MOC-hydration process is identified using X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) technique to reveal the intrinsic mechanisms. The results show that MOC content, molar ratio of MgO/MgCl2 and curing time strongly affects the UCS of MOC-solidified sludge. The suitable parameter values for sludge treatment by MOC turn out to be molar ratio of MgO/MgCl2 of 8-10, MOC content of 10% and curing time of 60 d. The UCS and strength retention coefficient of MOC-solidified sludge present an important decreasing trend with water immersion time due to the gradual decomposition of phase 5 - 5 Mg(OH)(2)center dot MgCl2 center dot 8H(2)O into Mg(OH)(2) phase, some soluble ions and H2O molecules. The UCS development is directly related to the formation of phase 5 and brucite, which induces a transformation of pore structure and boosts to develop a much stronger interlocking microstructure in solidified matrix. The identification of brucite with a significant volumetric expansion and phase 5 provides an insightful interpretation for the strength evolution of MOC-solidified sludge. Overall, the innovative incorporation of MOC treatment is supposed to be an efficient and sustainable approach on sludge solidification. (C) 2020 Elsevier Ltd. All rights reserved.