Performance of High-Calcium-Content Magnesium Oxysulfate Cement Modified by Boric Acid, EDTA-2Na, and Oxalic Acid

To avoid stacking and discarding low-grade magnesite tailings with high calcium contents, this study investigated the preparation of magnesium oxysulfate (MOS) cement by using boric acid–, oxalic acid–, and ethylenediaminetetraacetic acid disodium (EDTA-2Na)–modified high-calcium light-burned magnesia powder. The X-ray diffraction, thermogravimetry–derivative thermogravimetry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma spectroscopy, and X-ray photoelectron spectroscopy results show that at the initial stage of the hydration reaction, as the content of magnesium ions is much greater than the concentration of calcium ions, the modifier has little effect on calcium ions. When a hardened sample is immersed in a water/NaCl solution, the modifier can combine with calcium ions to form stable calcium salts, especially oxalic acid. However, the curing effect of calcium ions could be better related to the performance of the product. Although the quantity of dissolved calcium ions is the lowest when oxalic acid is added, early hydration is negatively affected. More brucite is generated, which inhibits the formation of the 517 phase, and its strength is only 54.0 MPa after curing for 28 days. The compressive strengths of the samples modified with boric acid and EDTA-2Na after natural curing for 28 days are 116.9% and 98.2% of that of the control, respectively.