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Magnesium phosphate cement (MPC), possessing the characteristics of fast setting and early strength, but has the disadvantages of poor toughness and easy cracking. Our previous studies found that these shortcomings could be improved by adding coir fiber (CF). In practical projects, chances are that CF-MPC might work in watery environment like rainy day. Therefore, it is important to understand the effect of water on the properties of CF-MPC. This paper was then committed to investigate the effect of water immersion on the bending performance of CF-MPC. A three-point bending test was employed to capture the flexural strength, flexural stiffness, and toughness of MPC with different CF contents cured in air and water respectively. The results demonstrated that CF-MPC cured in water and in air displayed a similar failure mode. Specimens’ failure mode varied from brittle failure to ductile failure with CF content increasing. Moreover, the first and second peak loads of specimen load-displacement curves gradually decreased as the water curing time increased. The flexural strength dropped, different from the performances observed in specimens cured in air. But when CF content was smaller than 3%, this flexural property deterioration of specimens in water was relieved as CF content increases.

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.

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