Analysis of the Influence of Fiber-Reinforced MPCM-Modified Concrete on Chloride Ion Penetration Resistance in Marine Environments

To address the poor anti-permeation performance of concrete under long-term exposure to high temperature and high chloride ion concentration, a composite modification scheme was proposed. Fiber-reinforced phase-change concrete incorporating microencapsulated phase-change materials (MPCM), steel fibers, and polypropylene fibers was prepared to enhance chloride ion penetration resistance and ensure mechanical stability. In the experiment, ordinary Portland cement was used as the binder material. MPCM at dosages of 4%, 7%, and 9% (by volume) were used to replace part of the fine aggregate, and 0.85% steel fibers and 0.15% polypropylene fibers were fixedly incorporated to produce 100  mm × 100  mm × 100  mm cubic specimens. Mechanical property tests, 120-day chloride ion immersion tests, and SEM microscopic characterization were conducted to systematically explore the material performance. The results show that the MPCM dosages of 4%–9% cause the compressive strength and the split tensile strength of concrete to decrease by 29.3%–56.4% and 17.1%–52.7%, respectively. The incorporation of hybrid fibers can effectively compensate for the strength loss, and the compensation effect is most obvious for the 9% MPCM group: the decrease in the compressive strength narrows from 67.2% to 56.4%, and the decrease in the split tensile strength narrows from 39.0% to 6.9%. However, the strength does not recover to the level of ordinary concrete, indicating that the fiber compensation effect is limited. In the chloride ion immersion penetration test, the chloride ion concentration reduction rate in the shallow layer (1–3  mm) of the 4% MPCM group exceeds 55.2%, while at the deep layer (30  mm), the reduction rates of the 7% and 9% MPCM groups reach over 84%. The chloride ion diffusion laws within the temperature range of 0–40℃ and the long-term penetration characteristics over 10 years were further analyzed using the finite element software. The results show that under each temperature simulation, the chloride ion diffusion depth of the phase-change concrete and the fiber-reinforced phase-change concrete is significantly lower than that of ordinary concrete. The anti-chloride ion penetration effect becomes more prominent as the temperature increases. At 0℃, the diffusion depth is more than 4.66  mm lower than that of ordinary concrete, and at 40℃, it is more than 18.9  mm lower. In the long-term simulation, the penetration depth of the phase-change concrete is all below 100 mm, with the minimum penetration depth being only 56 mm. In summary, the experimental and simulation results are consistent, indicating that MPCM can effectively inhibit chloride ion diffusion, and the fibers ensure mechanical stability, giving the concrete long-term reliable anti-chloride ion penetration performance.