Strength improvement and micromechanism of inorganic/organic additive-modified magnesium oxychloride cement solidified sludge

•Phosphoric acid, urea–formaldehyde resin and citric acid are confirmed most efficient.•Chemical additives improve shear and compressive strength of MOC solidified sludge.•Cohesion and internal friction angle show first an upward and later a downward trend.•Yielding stress shows a trend of rise with curing age and fall with immersion period.•Phase 5, phase 3 and brucite are identified as major phases for strength improvement.