Effect of tartaric acid on the early hydration process and water resistance of magnesium oxychloride cement

The main type of cement used worldwide is Portland cement with an annual production of about 4 billion tons and CO2 emissions of about 8% of total global anthropogenic greenhouse gas emissions. One of magnesium binders being investigated to mitigate this negative effect is associated with the use of magnesium oxychloride (MOC) cement. Different from that of Portland cement, the main composition of MOC cement is magnesia rather than calcia. MOC cement has excellent properties, including low carbon, light weight, low thermal conductivity, and high early strength, but it is not extensively applied in practical engineering due to its poor water resistance. This study investigated the effect of tartaric acid (TA) addition on the water resistance of MOC cement, as well as its effect on the setting time, hydration, compressive strength, phase composition, pore structure, and microstructure. The results show that the hydration of MOC cement was a multi-step process. The initial precipitate was 2 Mg(OH)2·MgCl2·2H2O, which was ultimately transformed 5 Mg(OH)2·MgCl2·8H2O. TA can adsorb on the surfaces of MgO particles via a coordination effect, which increased the surface negative charge concentration of MgO. This inhibited the dissolution of MgO and the formation of 2 Mg(OH)2·MgCl2·2H2O, prolonging the setting time of MOC cement. The addition of TA decreased compressive strength and increased the total porosity of MOC cement, however, promoted the formation of amorphous 5 Mg(OH)2·MgCl2·8H2O in matrix, resulting in an improvement in the water resistance, a decrease in dissolved concentration of Cl−, and an increase in the content of gel pores of MOC cement.