Effects of sodium dihydrogen phosphate on properties of magnesium phosphate cement

Magnesium phosphate cement (MPC) has attracted much attention in repair materials due to its excellent properties. However, the variability and harshness of the environment put increasing demands on the mechanical properties of materials. Modifying the crystal structures of hydration products is a simple and effective method for improving the mechanical properties. In this paper, the effects of sodium ions on the structures of the hydration products were first investigated at a high water-to-cement ratio, and then the setting time, the heat of hydration, flexural strength and compressive strength were systematically studied for cement at a low water-to-cement ratio. X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, pore structure determination, micromorphology analysis and energy spectrometry was used to investigate the effect of the SDP content on MPC. The results showed that adding SDP could change the crystal structure of the hydration products and cause the formation of more uniform struvite whiskers. The main hydration product is struvite, and sodium ions can promote the hydration reaction and increase the amount of hydration products. In addition, the sodium ions replace some of the potassium ions in struvite to form a more stable crystal structure, from potassium-containing magnesium phosphate to sodium-potassium coexisting struvite whiskers. This promotion can refine the crystal structure of the hydration product, thereby changing the pore structure and improving the mechanical properties of SDP-MPC. Typically, the 1 d flexural strength of MPC with 50 wt% SDP addition is up to 12.7 MPa, and the 1 d compressive strength is up to 78.1 MPa; MPC with 50 wt% SDP was found to have 65% higher flexural strength and 122% higher compressive strength compared to MPC without SDP addition, and the 28 d flexural strength and compressive strength also increased by 58% and 70%, respectively. The results show that incorporating sodium ions changes the crystal structure of struvite and can effectively improve the strength properties of magnesium phosphate cement.