Carbonation-induced strength–ductility trade-off and carbonation resistance of ultra-high-toughness magnesium phosphate cementitious composites with PE fibers and fly ash

This study investigates the mechanical evolution, CO₂ transport, and microstructural regulation of ultra-high-toughness magnesium phosphate cementitious composites (UHTMC) under carbonation. UHTMC was prepared from dead-burned magnesia, potassium dihydrogen phosphate, fly ash, quartz sand, and polyethylene (PE) fibers. Three specimen states, T-0, NT-28, and T-28, were used to distinguish the initial, age-matched non-carbonated, and accelerated-carbonated conditions. Accelerated carbonation increased the load-bearing capacity and stiffness of UHTMC but reduced the peak compressive strain and ultimate tensile strain in some mixtures. This response reflects a trade-off between pore filling by carbonation products and ductility loss caused by a stronger fiber–matrix interface. Relative to NT-28, the axial compressive strength of the T-28 fly-ash series increased by 8.35–10.22%. PE-2.0 had the lowest 56-d carbonation depth in the fiber series (4.78 mm), whereas PE-2.5 showed deeper CO₂ penetration because of fiber agglomeration and interfacial defects. FA-15 had the lowest 56-d carbonation depth in the fly-ash series (3.26 mm). Its total porosity decreased slightly from 25.17% to 24.87%, and its mean pore diameter decreased from 102.39 to 85.41 nm. MIP, SEM, XRD, TG-DTA, and FTIR jointly supported carbonate formation, with the phase evidence being consistent with the presence of nesquehonite and magnesite; carbonate-related pore filling refined the pore structure and densified the fiber–matrix interface. Overall, among the mixtures examined under the present test conditions, the mixture containing 2.0 vol% PE fibers and 15% fly ash showed the most favorable balance among load-bearing capacity, deformation capacity, and carbonation resistance.