Performance-based selection of mineral admixture modified Magnesium Phosphate Cement (MPC) repair mortar in cold marine environments

MPC shows great promise for rapid marine infrastructure repair, but its degradation mechanisms under combined low-temperature and seawater exposure are not well understood. This study presents a comprehensive performance evaluation of four mineral admixtures, including fly ash (FA), metakaolin (MK), steel slag (SS), and silica fume (SF), across a replacement level range of 5 wt% to 30 wt%. The assessment provides a wide array of properties under multiple exposure regimes, including workability, mechanical strengths development from 4 h to 28 d under both room-temperature and low-temperature curing, resistance to chloride and sulfate attacks via solution immersion, and durability under salt-freeze thaw cycles. The resulting comprehensive dataset, comprising 37 distinct metrics, was integrated using the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) multi-criteria analysis. The results ranked the mix with 10 wt% MK highly for early-age performance, showing a 14% higher 4 h compressive strength of 40.4 MPa and a 7% greater 28 d bond strength of 5.0 MPa after salt-freeze cycles compared to the control group. The mix with 20 wt% FA showed favorable workability, with a flow diameter of 245 mm, and maintained relatively high strength after chloride exposure, primarily because its spherical particles improved particle packing and reduced connected transport paths. Microstructural analysis revealed that the reactivity of MK densifies the matrix, whereas SS and SF at dosages beyond 10 wt% introduce microstructural defects. This study provides a performance-based selection matrix, facilitating the rational selection of MPC formulations that simultaneously achieve a 30 min open time and 3 h traffic-ready strength under harsh winter marine conditions.