Effect of Water Immersion on Compressive Properties of Coir Fiber Magnesium Phosphate Cement

Magnesium phosphate cement (MPC) is a new type of inorganic cementitious rapid repair material, but it has poor toughness and is easy to crack. According to our previous research, these problems can be ameliorated by adding natural coir fiber (CF) into MPC. As coir fiber magnesium phosphate cement (CF-MPC) may be used in humid or rainy areas, its water resistance is an important property in consideration. However, at present, little research has focused on this aspect to provide a good theoretical and experimental basis for the practical application of CF-MPC. In this paper, static compression test and solubility test were used to study the mechanical properties and solubility of CF-MPC under water. At the same time, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to test the changes of hydration composition and microstructure of the test specimen, so as to understand the deterioration mechanism of CF-MPC in water. The results suggested that, when compared with CF-MPC cured in air, CF-MPC cured in water is more prone to encounter oblique cracks and through cracks in the compression process. Moreover, with the extension of curing time, the compressive strength and elastic modulus of CF-MPC cured in water will continue to decrease, the concentrations of PH, K⁺, and Mg²⁺ in the curing solution will change significantly, resulting in the gradual decrease in the mass ratio of MgO and MgKPO₄·6H₂O in CF-MPC matrix, cracks and pores, and looseness in the microstructure.