Introduction Magnesium phosphate cement (MPC) is a novel construction material that forms a gel material consisting primarily of insoluble phosphate salts after reaction of alkaline components with magnesia and soluble acid phosphate salts. However, the ceramic properties of MPC are significantly deficient in terms of fatigue resistance and toughness, and can lead to failure under vehicle loads. Short-cut basalt fibers can inhibit the initiation and propagation of concrete microcracks, but tend to wrap around and agglomerate during mixing. In addition, China has abundant island reef resources, and large quantities of concrete materials are required in various infrastructure construction processes. Transporting sand and fresh water from inland increases project costs, and extreme climate conditions can cause transportation inconveniences. Therefore, the use of seawater and marine sand from island reefs for road repair without damaging the environment can achieve a goal of localizing raw materials and reducing transportation burden. In this paper, effect of resin-modified basalt fiber on the workability, mechanical properties, fatigue resistance, microstructure, and engineering properties of seawater-sea sand MPC mortar was investigated.Methods Dead-burned magnesia, ammonium dihydrogen phosphate, borax retarder, highly active silica fume, and ultrafine low-calcium fly ash were used to prepare two groups of mortars with seawater sea sand or freshwater river sand. The modified basalt fiber was prepared, in which the surface of the modified fiber after coating with resin has a twist and wrinkle, which strengthens the bite force between the fiber and the mortar matrix. Different meshing structures were formed between the mortar and the end head (lengths of 23-25 mm, tensile strength of 2 900 MPa, Young modulus of 90 GPa). The basalt fiber with different volume fractions (i.e., 0%, 0.25%, 0.50%, 0.75% and 1.00%) was added to investigate the effect of fiber content on the matrix properties.The workability (flowability, setting time), mechanical properties (compressive/flexural strength), and fatigue resistance (sinusoidal reciprocal loading: 0.6, 0.7 and 0.8) of the MPC mortar were investigated in accordance with the standard for test methods of basic properties of construction mortars (JGJ/T 70—2009). The heat release, mineral phase composition, microstructures, and pore structures of the MPC mortar were investigated by a hydration thermal analysis, X-ray diffraction, field emission scanning electron microscopy, and X-ray computed tomography. The field rapid repair tests were conducted to comprehensively evaluate the structural layer characteristics of the surface layer by three-dimensional ultrasonic scanning, falling hammer bending, rebound, and coring methods.Results and discussion The hydration time of sea water-sea sand MPC mortar is short at a setting time for 30 min. The mortar can be compacted without vibration due to its workability available. However, the high dosage of modified basalt fibers has a challenge to fully wet the particle surfaces, increasing friction between particles and adversely affecting the workability of MPC mortar.The strength development of sea sand MPC mortar progresses rapidly with a compressive strength of 22 MPa after 1 h. The addition of a small amount of modified basalt fiber can slightly enhance the compressive strength of the MPC matrix. The fibers act as bridges in the fracture zone, improving the flexural deformation capacity of the MPC mortar. At different stress levels, the samples with 0.75% fiber content exhibit a fatigue life approximately one order of magnitude higher than that of mortar without fiber addition.In the hydration process of seawater/sea sand MPC mortar, a highly exothermic reaction occurs with the heat release primarily concentrated within the first hour, exhibiting a single peak. The crystalline phase content increases during the condensation and hardening process of MPC with major hydration products including struvite stone, sillimanite, and monticellite. Also, the introduction of seawater/sea sand leads to the formation of sulfate-chloride products. The hydration products of phosphate crystals form a strong bonding interface between the binder and an appropriate amount of modified basalt fibers. However, the incorporation of 1% fibers results in larger defects at the fiber-matrix interface, an increased internal porosity, and greater non-uniformity in pore distribution.Based on on-site experiments, the reliability of seawater-sea sand MPC mortar in repair projects is confirmed. The mortar pouring is convenient, the process is simple, and the surface consolidates and hardens quickly. The results obtained after 1 h of pouring indicate that although the bond between the newly poured MPC mortar and the bottom crushed stone layer is relatively weak, the overall integrity within the new surface layer is good, and the density is high. The surface rebound values and maximum deflection values are distributed evenly, and the strength at the interface between the pouring surface and the old concrete pavement is relatively low.Conclusions The setting time of the early-strength MPC mortar could be controlled within 30 min. Fibers could improve the bending deformation capacity of MPC, and the fatigue life of the samples with 0.75% fiber content was increased by approximately an order of magnitude, compared to the samples without fibers. The hydration reaction of the mortar was a highly exothermic process, accompanied by an increase in crystalline phase content. The gel composed of struvite and modified basalt fiber had a strong bonding interface, while the porosity and pore variability increased with the increase in fiber content. Besides, the newly poured MPC surface layer had a high compactness in engineering application, and the distribution rebound values and deflection values was uniform, confirming the feasibility of fiber-reinforced sea sand/sea water mixed MPC system.