Recycled Carbon Fiber Reinforced One-Part Alkali-Activated Materials: Reaction Kinetics, Performance and Durability

Abstract

The increasing use of carbon fiber-reinforced polymer (CFRP) composites has generated substantial end-of-life waste, including retired wind turbine blades. Recovering recycled carbon fiber (rCF) from these wastes and incorporating it into construction materials offers a potential route for waste valorization and carbon reduction. Meanwhile, one-part alkali-activated materials (OPAAMs), which are prepared by adding water to premixed solid constituents, provide a safer and more practical alternative to conventional two-part alkali-activated systems. However, their engineering application remains limited by insufficient understanding of reaction control, matrix brittleness, functional performance, and long-term durability. This thesis first establishes the reaction and processing characteristics of the OPAAM matrix and subsequently investigates the incorporation of rCF to improve its mechanical, functional, durability, and environmental performance.

A time-resolved kinetic framework was established to describe the evolution of OPAAM from initial activation to structural stabilization. Based on isothermal calorimetry and complementary XRD, FTIR, SEM–EDS, and TG–DTG analyses, the reaction process was divided into six stages: initial dissolution, pre-gelation, induction, acceleration, deceleration, and stabilization. The framework describes the staged evolution of precursor dissolution, primary gel formation, structural rearrangement, and continued polymerization in solid sodium silicate-activated systems. Its practical relevance was evaluated using 3D printing, whose time-dependent processing behavior is sensitive to early structural evolution. Phase II acted as an adjustable transition period, while early Phase III provided a suitable balance among material transport, extrusion, and shape retention. For the representative SFCR mixture, the printable period was 90–300 min, with relatively stable printing achieved between 150 and 300 min.

The reinforcing and functional roles of rCF in OPAAM were then examined. A water pre-dispersion and high-viscosity mixing procedure enabled relatively uniform fiber distribution at low dosages. At 0.1 vol.% rCF, the flexural strength reached 11.88 MPa, representing an increase of 53.42% over the unreinforced mixture. At 0.125 vol.% rCF, the 56-day drying shrinkage was reduced by more than 26%, while the electrical percolation threshold occurred at approximately the same dosage. Above this threshold, repeatable piezoresistive responses were obtained under cyclic compression after initial conditioning, demonstrating the potential of rCF-OPAAM for self-sensing applications. The combined effects of carbon black (CB) and rCF were further investigated. CB primarily refined the pore structure through filling and densification, whereas rCF improved crack resistance through interfacial bonding and fiber bridging. With 0.8 wt.% CB and 0.4 wt.% rCF, the flexural strength reached 12.16 MPa and water absorption decreased to 10.24%. Fractal analysis further showed that the backbone fractal dimension provided a complementary indicator connecting pore-network organization with strength and water absorption.

The sulfate resistance of rCF-OPAAM was evaluated under continuous Na₂SO₄ exposure. The results revealed a trade-off mechanism. Although rCF incorporation increased the proportion of medium capillary pores and could facilitate initial sulfate ingress, fiber bridging and pull-out restricted pore coarsening, crack propagation, and the formation of connected damage pathways. After 90 days of exposure, the expansion of mixtures containing 0.2 and 0.3 vol.% rCF was approximately 40% lower than that of the unreinforced mixture. The porosity of plain SF70 increased from 13.36% to 18.99%, whereas that of the rCF-reinforced mixtures remained below 13.77%. The SF70-0.1 mixture reached a compressive strength of 71.47 MPa after 90 days, reflecting the combined effects of continued reaction, local pore filling, and reduced sulfate-induced damage. Microstructural analyses showed that gypsum and ettringite formed preferentially around cracks and pore interfaces, while rCF limited the development of localized deterioration into a connected damage network.

Finally, the environmental performance of rCF-OPAAM was evaluated through a cradle-to-gate life-cycle assessment. Strength-normalized GWP, sulfate-resistance-normalized GWP, and carbon increment return indices were used to connect environmental impacts with mechanical and durability performance. Under the adopted system boundary and inventory assumptions, the GWP of rCF was 1.29 kg CO₂-eq/kg, equivalent to 8.1% of that of virgin carbon fiber. The GWP of the SF70 matrix was 251.33 kg CO₂-eq/m³, which was 52.1–69.9% lower than those of the selected OPC reference mortars. Incorporating rCF increased mixture-level GWP by only 0.9–2.8%. After sulfate exposure, the sulfate-resistance-normalized GWP identified 0.1 vol.% rCF as the most environmentally efficient dosage among the mixtures investigated.

The findings demonstrate that low-dosage rCF can deliver substantial mechanical, functional, and durability benefits with a limited increase in environmental burden, providing a scientific basis for the development of low-carbon and multifunctional alkali-activated construction materials.

Date of Award	18 Aug 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kim Meow LIEW (Supervisor) & Lu-Wen Zhang (External Co-Supervisor)