In cement-based matrices, polyethylene terephthalate (PET) fibers are prone to alkaline hydrolysis, limiting their structural applicability in strain-hardening cementitious composites (SHCC). This study evaluates two mitigation strategies: (i) reducing matrix alkalinity through clinker substitution and (ii) applying enforced carbonation curing. Three limestone calcined clay cement (LC3) matrices containing 50%, 35%, and 25% clinker by weight were prepared to generate systems with decreasing alkalinity levels. PET-reinforced LC3 composites were subjected to accelerated steam-curing aging (40 °C, 100% RH) for 14, 28, 60, and 90 days to assess fiber degradation, fiber-matrix bond performance via single-fiber pull-out tests, and strain-hardening behavior via uniaxial tension tests. A subset of specimens additionally underwent enforced carbonation curing (20% CO₂, 70% RH, 24 h) before steam curing to evaluate its effectiveness in preserving fiber integrity.Results revealed a strong dependence of PET stability on matrix composition: severe degradation occurred in LC3-50, moderate in LC3-35, and minimal in LC3-25. Enforced carbonation effectively mitigated degradation across all matrices, with particularly pronounced benefits in higher-alkalinity systems (LC3-50 and LC3-35). In LC3-25 composites, enforced carbonation was unnecessary and even detrimental, impairing matrix integrity, fiber-matrix bond strength, and strain-hardening performance. These findings suggest that approaches to mitigate fiber degradation must be tailored to the matrix composition. While enforced carbonation is crucial for high-clinker SHCC, where fibers are most at risk, it can be counterproductive in low-clinker systems. Overall, this research offers valuable insights for designing durable, sustainable SHCC reinforced with PET fibers.