Alkaline hydrolysis of polyethylene terephthalate fibers in an LC3-based cementitious extract: physical, chemical, and mechanical degradation

Polyethylene terephthalate (PET) is a widely produced and recycled polymer that is increasingly explored as a reinforcement and filler in cementitious systems because of its mechanical and environmental benefits. However, the highly alkaline conditions of cementitious systems may degrade PET, raising concerns about its long-term durability. Yet reported durability outcomes remain inconsistent, leaving PET's long-term performance in cementitious matrices unresolved. This study addresses this gap by examining the degradation of commercial virgin PET fibers submerged in a limestone calcined clay cement (LC 3 )-based alkaline solution (pH 12.8) for 7, 14, 28, and 60 days at 20°C, 40°C, and 60°C. A comprehensive approach combining physical, chemical, and mechanical analyses was employed to assess the effects of alkaline exposure on PET fibers after immersion. The findings reveal that PET fibers undergo significant degradation in highly alkaline environments, as evidenced by surface pitting, diameter reduction, and a marked decrease in tensile load-bearing capacity. Strictly under continuous immersion conditions in LC 3 -based alkaline solution, the fitted model predicts a loss of approximately 50% and 90% of their original load-bearing capacity within roughly four and thirteen months, respectively, based on degradation modeling results. Mechanistic insights suggest that degradation occurs via a surface-erosion or " peel-off " mechanism that progresses from the surface inward over time. Chemical characterization indicates that the PET polymer backbone remains largely intact within the fiber core, while degradation products, i.e. , eroded surface residues, are predominantly aromatic carboxylate salts, presumably calcium terephthalate, formed through the alkaline hydrolysis of PET in the calcium-rich LC 3 solution. Overall, the findings demonstrate that PET fibers are highly susceptible to degradation in cementitious-like alkaline environments and should be used with caution, given that long-term durability is a critical design priority.