Effects of the combination of NH4H2PO4 and KH2PO4 on the interfacial bond between magnesium phosphate cement and steel fiber

In this paper, a double-side pullout testing method was used to characterize the bond strength and pullout energy of steel fiber in magnesium phosphate cement (MPC). The effects of phosphates on flowability, setting time, hydration heat, and phase composition of MPC were evaluated as well. The results indicate that ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (PDP) show significant effects on the bond properties between MPC and steel fiber. Magnesium ammonium phosphate cement (MAPC) can generate ammonia during hydration, which is easy to form pores and voids in the interfacial zone. However, the calcu-lated total volume shrinkage produced by the hydration of MAPC is greater than that of magnesium potassium phosphate cement (MPPC), which is conducive to greater friction at the interface. There is no gas generated during the hydration of MPPC, but the elastic modulus of K-struvite is lower than that of struvite. The combined use of ADP and PDP is conducive to reducing the porosity and increasing the hardness of the interface zone and improving the bond properties of MPC and steel fibers. The hydration products of MPC paste around the steel fiber form a highly compact interfacial zone with few defects, leading to mechanical interlock and strong friction. Moreover, the etching of the surface of mild steel fiber in MPC and penetration of soluble phosphates promote the interfacial bonds.