Study on the modification mechanism of modifiers on the properties of sawdust-magnesium oxychloride cement composite

Sawdust-magnesium oxychloride cement composite (SMOCC) consists of a magnesium oxychloride cement (MOC) matrix, sawdust, and modifiers. SMOCC is expected to become a building material to replace wood. However, poor water resistance limits its application in engineering. To improve the properties of SMOCC, citric acid (CA), tartaric acid (TA), sodium hexametaphosphate (SHMP), sodium phosphate (NaP), and ethylene–vinyl acetate copolymer (EVA) emulsion were added to SMOCC in this study. The effects of these modifiers on SMOCC setting time, compressive strength, and water resistance were studied. Assisted by X-ray diffraction (XRD) and scanning electron microscopy (SEM), the effects of modifiers on the microstructure and hydration mechanism of SMOCC were analyzed. Results show that adding the modifiers delays the setting time of SMOCC, among which CA has the best setting retardation effect. CA and TA both inhibit the continuous hydrolysis of MgO and the decomposition of 5MgO∙MgCl2·8H2O (phase 5∙1∙8) crystals in water, which results in an obvious improvement of the water resistance of SMOCC. In terms of the modification effects on SMOCC, SHMP is better than NaP at increasing the setting time, maintaining the volume stability, and improving the water resistance, the microstructure, and hydration products of SMOCC, while EVA decreases the strength of SMOCC and has no obvious effect on the improvement of the water resistance.