Basic magnesium sulfate cement: Autogenous shrinkage evolution and mechanism under various chemical admixtures

Basic magnesium sulfate cement (BMSC) is a new type of whisker self-toughening cement-based material. Owing to its excellent mechanical properties, it has been applied to high-speed rail cover plates and military protective materials. However, the shrinkage deformation problem affects the application of BMSC in other fields. This article describes the effects of three chemical admixtures on the basic mechanical properties and autogenous shrinkage deformation of BMSC. Isothermal calorimetry (IC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were performed to study the influence of different chemical admixtures on the phase and microstructure of BMSC. Finally, based on the microscopic phase and structure analysis results, the autogenous shrinkage mechanism of BMSC under the action of different chemical admixtures was systematically analyzed. The results show that citric acid (CA) and amino trimethylene phosphonic acid (ATMP) can significantly improve the compressive strength of BMSC. Based on the phase analysis and microstructure analysis, ATMP and CA can induce more formation of the needle-like 5-1-7 phase. The results of shrinkage deformation show that the unmodified BMSC has obvious expansion, and the expansion rate increases with time. However, the BMSC modified by chemical admixtures shows small expansions at first, and then begins to show continuous shrinkage deformation. The expansion in the unmodified BMSC during crystal growth stems from the crystallization pressure of Mg(OH)2 as the main driving force. However, the formation of Mg(OH)2 is inhibited in the modified BMSC, so the shrinkage deformation of BMSC was mainly caused by the formation of the 5-1-7 phase.