The degradation of polyethylene terephthalate (PET) fibers in alkaline environments limits their use in strain-hardening cementitious composites (SHCC). Prolonged PET exposure to alkaline environments has a detrimental effect on its mechanical performance, mainly due to the physicochemical transformation caused by alkaline hydrolysis. This study presents a tailored cementitious matrix design containing high amounts of limestone and calcined clay, replacing 75 wt% of Portland cement, to attain and maintain the strain-hardening response of composites incorporating PET fibers as dispersed reinforcement. Analytical and mechanical tests were carried out at different curing ages, ranging from 7 to 60 days, to study the effects of aging on virgin PET fibers, both within the matrix and outside it (in the pore solution). The results showed a pronounced degradation of the PET fibers in the test pore solution at pH 12.5, manifested by a progressive reduction in the load-bearing capacity of the individual fibers with prolonged immersion. Conversely, when the PET fibers were aged in-matrix under laboratory conditions and tested under tension, the performance of the corresponding composites showed resilience to aging, exhibiting reasonable tensile strength and remarkable strain capacities that exceeded 4 %.