Combined effects of carbon nanotubes and recycled polypropylene fibers on ultra-high-performance concrete incorporating calcium aluminate cement and waste glass fine aggregate

Abstract

This study investigates the combined effect of carbon nanotubes (CNTs) and surface-treated recycled polypropylene fibers (STRP fibers) on the performance of calcium aluminate cement (CAC)-based ultra-high-performance concrete (UHPC). CNTs were incorporated at dosages of 0.025%, 0.05%, and 0.075% by binder weight, while STRP fibers were added at 0.5% by volume. Mechanical properties, including compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity, were evaluated alongside sorptivity and microstructural characteristics using SEM, EDX, XRD, FTIR, and elemental mapping. The results indicated that CNT incorporation improved the investigated mechanical properties, with the most noticeable performance observed at a CNT dosage of 0.05%. At this level, compressive strength increased by up to 27.9% without fibers and 32.0% with fibers at 28 days. Splitting tensile and flexural strengths also increased by 31.1% and 30.1%, respectively, with additional improvement observed in fiber-reinforced mixtures. The modulus of elasticity exhibited moderate improvement, while reduced sorptivity values suggested improved resistance to water absorption behavior. Microstructural characterization qualitatively indicated that CNT incorporation influenced the hydration-related microstructure and matrix densification behavior, while the recycled fibers contributed to crack-bridging behavior and improved fiber–matrix interaction. The hybrid incorporation of 0.05% CNTs and 0.5% STRP fibers exhibited the best overall performance among the investigated mixtures. Overall, the results suggest that the proposed multiscale reinforcement approach may provide a promising strategy for improving the mechanical and microstructural performance of CAC-based UHPC systems containing recycled polypropylene fibers.