To address the durability challenges faced by traditional concrete in marine environments, this study focuses on polyoxymethylene (POM) fiber-reinforced ultra-high-performance concrete (PFUHPC) and, for the first time, systematically investigates the inhibitory effects of POM fibers on microstructural degradation and mechanical performance deterioration of ultra-high-performance concrete under various erosive environments. The results indicated the following: (1) The mass loss rate and compressive strength degradation in PFUHPC under different erosive environments initially increased and then decreased, demonstrating that the inclusion of POM fibers delayed corrosion and significantly improved the durability and stability of the material’s performance. (2) Compared to the natural environment, after 180 days of immersion in different erosive environments (seawater immersion, wet–dry cycles in seawater, chloride salt immersion, sulfate salt immersion, and complex salt immersion), the compressive strength degradations were observed to be 4.8%, 9.7%, 6.8%, 11.7%, and 10.7%. (3) Microscopic analysis after 180 days revealed that the main corrosion products were gypsum, ettringite, and Friedel’s salt (calcium chloroaluminate). Under the environments of seawater immersion and cyclic wetting and drying, the low concentrations of chloride and sulfate ions resulted in fewer corrosion products and a denser matrix. The primary corrosion product under the chloride salt immersion was Friedel’s salt, which led to surface cracking and microporosity, while under the sulfate immersion, gypsum and ettringite were predominant, resulting in more porous and loosely bound hydration products and more severe corrosions.

Concrete structures suffering from Mg²⁺ environments may suffer severe damage, which mainly has something to do with the coupled effect among Cl⁻, SO₄²⁻, and Mg²⁺. Based on a systematic review of Web of Science and Scopus database (2000–2025), we first summarized the migration behavior, reaction paths, and interaction mechanism of Cl⁻, SO₄²⁻, and Mg²⁺ in cementitious matrices. Secondly, from the perspective of Cl⁻ cyclic adsorption–desorption breaking the passivation film of steel bars, SO₄²⁻ generating expansion products leads to crack expansion, then Mg²⁺ decalcifies C-S-H and transforms into M-S-H; we analyzed the main damage mechanisms, respectively. In addition, under the coexistence conditions of three kinds of ions, the “fixation–substitution–redissolution” process and “crack–transport” coupling positive feedback mechanism further increase the development rate of damage. Then, some anti-corrosion measures, such as mineral admixtures, functional chemical admixtures, fiber reinforcements, surface coatings, and new binder systems, are summarized, and the pros and cons of different anti-corrosion technologies are compared and evaluated. Lastly, from two aspects of simulation prediction for the coupled corrosion damage mechanism and service life prediction, respectively, we have critically evaluated the advances and problems existing in the current research on the aspects of ion migration-reaction coupled models, multi-physics coupled frameworks, phase-field methods, etc. We found that there is still much work to be conducted in three respects: deepening mechanism understanding, improving prediction precision, and strengthening the connection between laboratory test results and actual projects, so as to provide theoretical basis and technical support for the durability design and anti-corrosion strategies of concrete in complex Mg²⁺ environments.

As the demand for environmental sustainability grows in the global construction industry, traditional cement production faces significant challenges due to high energy consumption and substantial CO₂ emissions. Therefore, developing low-carbon, high-performance alternative cementitious materials has become a research focus. This paper proposes a new low-carbon cement (alkali-activated lithium slag-fly ash composite cement, ALFC) as a substitute for traditional cement. First, the alkali activation reactivity of lithium slag (LS) is enhanced through calcination and grinding, revealing the reasons behind its improved reactivity. Then, alkali-activated LS and fly ash were partially used to replace cement to prepare ALFC, and the effects of the water-to-binder ratio (W/B), LS content, and NaOH addition on the flowability and mechanical properties of ALFC were investigated. XRD, SEM/EDS, and TG/DTG analyses were conducted to examine its hydration products and microstructure, revealing the hydration mechanism. The results show that the flowability of ALFC increases with W/B but decreases with a higher LS content. When W/B is 0.325 and the LS content is 25 wt.%, flowability reaches 200 mm, meeting construction requirements. LS calcined at 700 °C for 1 h significantly enhanced ALFC’s 90-day flexural and compressive strengths by 39.73% and 58.47%, respectively. The primary hydration products of ALFC are C-S-H, N-A-S-H, and C-A-S-H gels, with their content increasing as the NaOH concentration rises. The optimal NaOH concentration and LS content for ALFC are 2 mol/L and 25 wt.%, respectively.