Magnesium oxysulfate (MOS) cement has the advantages of early strength, good fire resistance and good decorative properties, but its low mechanical strength limits its wide application in the construction industry. Research indicates that incorporating the right additives into cement can enhance its strength. This study delves into the mechanical strength, hydration reactions, water resistance, phase composition, and microstructure of MOS cement when mixed with citric acid (CA), tartaric acid (TA), oxalic acid (HAC), and ethylenediaminetetraacetic acid (EDTA). The findings revealed that the inclusion of CA, TA, HAC, and EDTA significantly bolstered both the mechanical strength and water resistance of MOS cement. Moreover, these additives promoted the development of the needlelike crystal 517 phase and inhibited the reaction of the hydration layer [Mg(H₂O)_xOH]⁺ with OH- to produce Mg(OH)₂. With different types of additives, different abilities to chelate Mg²⁺, different numbers of 517 phase crystals generated, thus the mechanical strength of MOS cements incorporated with different additives is different, CA has the strongest ability to chelate Mg²⁺ and therefore has superior mechanical strength and water resistance compared to other additives.

Keywords: Magnesium oxysulfate cement, Additive, Mechanical strength, Water resistance, Microstructure.