Effect of silica fume on the performance of high-early-strength UHPC prepared with magnesium ammonium phosphate cement

Although ultra-high performance concrete (UHPC) prepared from Portland cement has high long-term strength and durability, its early strength development is still can be enhanced for shortening the prefabrication period or using it as a rapid repair material. Therefore, magnesium ammonium phosphate cement ultra-high performance concrete (MAPC-UHPC) was prepared using magnesium ammonium phosphate cement (MAPC), sand, steel fiber, and silica fume (SF) in this study. The effects of replacing MAPC with SF (5%–15%) on the fluidity, strength, and microstructure of MAPC-UHPC were studied. The results showed that the spread diameter of the MAPC-UHPC mixture was greater than 200 mm, and the setting time was between 5.5 and 8.5 min. Meanwhile, the 6-h compressive strength of MAPC-UHPC was higher than 100 MPa and could reach about 140 MPa in 28 d. When the 5%–15% binder was replaced by SF, the MAPC-UHPC fluidity and setting time increased with the increase in SF content. When 5% SF was used, the 6-h and 28-d compressive strengths of MAPC-UHPC increased by 3.91% and 3.86%, respectively, compared with the plain sample. The addition of SF promoted the formation of struvite and refined the pore structure, which also increased the compactness of the microstructure.