Development of core components for calcium-based phosphate fixatives to improve the properties of magnesium potassium phosphate cement: the opening of second fixation routes

Magnesium potassium phosphate cement (MKPC) is widely recognized as a green cementitious material with numerous advantages. However, its broader application is limited by performance degradation under soft-water erosion, which is caused by the leaching of residual phosphates from the hardened matrix. In the hydration system of MKPC, phosphate species follow only one fixation route-participation in the hydration reaction. To reduce the amount of residual phosphate, this study introduces the concept of calcium-based phosphate fixatives (CPF). The core calcium components in CPF preferentially react with phosphate ions to form insoluble calcium phosphates, thereby establishing a secondary fixation routes and enhancing phosphate consumption. Experimental results demonstrate that CPF not only promotes the hydration reaction by consuming phosphate ions and shifting the reaction equilibrium forward but also generates additional insoluble calcium salts and reduces the extent of rehydration under soft-water conditions. Furthermore, CPF optimizes the pore structure of MKPC, decreases capillary water absorption, and ensures stable strength development and dissolution resistance during soft-water exposure. The improvement in water resistance achieved by CPF provides a new strategy for designing other phosphate fixatives, contributing to the sustainable development of MKPC in applications such as hydraulic concrete.