Influence of Nanoceramic-Plated Waste Carbon Fibers on Alkali-Activated Mortar Performance

Waste carbon fibers as reinforcing elements in construction materials have recently gained increasing interest from researchers, providing outstanding strength performance and a lower environmental footprint compared to virgin fibers. Combination with cement-free binders, namely alkali-activated materials, is becoming increasingly important for sustainable development in the construction industry. This paper presents results relating to the potential use of waste carbon fibers in alkali-activated mortars. The waste carbon fiber fraction utilized in this research is difficult to integrate as reinforcement in ceramic–cementitious matrices due to its agglomerated form and chemical inertness. For this reason, a nanoceramic coating pretreatment based on nanoclay has been implemented to attempt improvements in terms of deagglomeration, dispersibility, and compatibility with alkali-activated materials. After chemical–physical and microstructural analysis on the nanoclay-plated fibers (including X-ray diffraction, IR spectroscopy, contact angle measurements, and electron microscopy) mortars were produced with four different dosages of treated and untreated waste fibers (0.25 wt.%, 0.5 wt.%, 0.75 wt.%, and 1 wt.%). Mechanical tests and fractographic investigations were then performed. The nanoclay coating interacts compatibly with the waste carbon fibers and increases their degree of hydrophilicity to improve their deagglomeration and dispersion. Compared to the samples incorporating as-received fillers, the addition of nanoclay-coated fibers improved the strength behavior of the mortars, recording a maximum increase in flexural strength of 19% for a fiber content of 0.25 wt.%. This formulation is the only one providing an improvement in mechanical behavior compared to unreinforced mortar. Indeed, as the fibrous reinforcement content increases, the effect of the nanoclay is attenuated by mitigating the improvement in mechanical performance.