Synergistic high-strength and high-toughness design of magnesium phosphate cement-based composites via carbon-fiber reinforcement

High-performance magnesium phosphate cement (MPC) composites that simultaneously possess high strength, high toughness and fast setting are urgently required for rapid repair and protective works in harsh environments. Here, the synergetic influence of water-to-cement ratio (0.10–0.16), borax retarder (0.08–0.10), carbon fiber content (0–1.4 wt%) and curing age (4 h–28 d) on phase evolution, hydration kinetics, pore structure and mechanical performance was systematically quantified. Results demonstrate that carbon fiber incorporation markedly improves the flexural-to-compressive strength ratio (up to 16 %), yielding an ultra-high-strength C_f/MKPC composite with balanced toughness. Quantitative MIP analysis shows that carbon fiber reduces the total porosity by 3.5 %. Micro-scale toughening mechanisms, including fiber pull-out, fracture and crack bridging observed via SEM/EDS, account for the enhanced crack resistance. After 28 d curing, the optimum composite achieves a compressive strength of 123.1 ± 2.4 MPa and a flexural strength of 21.1 ± 0.7 MPa. These findings provide new insight into the strength-toughness integration mechanism of cementitious composites and furnish a promising candidate material for emergency repair and protective structures subjected to aggressive environments.