Effects of ethylenediamine tetra-acetic acid (EDTA) and its disodium salt derivative (EDTA-Na) on the characteristics of magnesium oxysulfate (MOS) cement

Ethylenediamine tetra-acetic acid (EDTA) and its disodium salt derivative (EDTA-Na) (0.5% weight of MgO powder) were incorporated into MgO–MgSO4–H2O ternary system to prepare magnesium oxysulfate (MOS) cement. The single effect of EDTA/EDTA-Na as well as the synergistic effect of EDTA/EDTA-Na and citric acid (CA) on the hydration mechanism, phase composition, microstructure and mechanical properties of MOS cement were investigated. The results showed that EDTA and EDTA-Na addition induced no new phase, but the magnesium-EDTA complexes formed by EDTA4− and Mg2+ ions acted as nuclei for the hydration products, including brucite, 3 Mg(OH)2·MgSO4·8H2O (318 phase) and 5 Mg(OH)2·MgSO4·7H2O (517 phase). This helped to form a more homogeneous and compact microstructure, thereby improving the durability and volume stability of MOS cement in air curing condition. Moreover, the synergistic effect of EDTA/EDTA-Na and CA preferred the formation of 517 phase to that of brucite. It further improved the water resistance of MOS cement. In this study, EDTA and EDTA-Na were found to be effective additives in improving the characteristics of MOS cement. •EDTA and EDTA-Na were introduced into MOS cement with and without CA addition.•The addition of EDTA and EDTA-Na improved the durability and volume stability of MOS cement.•The formed magnesium-EDTA complexes acted as nuclei for the hydration products.•The synergistic effect of EDTA/EDTA-Na and CA preferred the formation of 517 phase to brucite precipitation.