Valorization of a secondary stream of recycled carbon fibres in concrete application: compatibility, performance, and compounding optimization

Downcycling synthetic fibre waste for reinforced concrete in the construction sector can provide mutual benefits for both industries due to not only alleviating the strain on the environment and socio-economic impact but also enhancing the properties of the cementitious material. Incorporating carbon fibres to develop fibre-reinforced concrete (FRC) is an attractive route in enhancing some engineering performance for better applicability of the material, including mechanical strength, post-cracking behaviour, shrinkage mitigation, and thermal resistance. In the framework of eco-sustainable design of construction materials, this work dealt with the viability of engineering cementitious mixtures with scrap carbon fibres (sCF) deriving from an industrial thermal recycling processing of waste carbon-fibre composites. Due to the agglomerate-like structure of the recycled fraction, the main criticality that emerged in the manufacturing stage was to ensure adequate dispersion of the reinforcement in the matrix. Therefore, in this work, a de-agglomeration treatment of the fibres by nanoclay slurry was developed. Nanoclay-based functionalization aimed to ensure a more homogeneous distribution of the reinforcement while providing pozzolanic activity for concrete improving its microstructural characteristics. In the present research, different contents of sCF were implemented (from 0.25 w/w% to 1 w/w%) with respect to the amount of cement binder, studying FRC mix designs with and without nanoceramic treatment. The influence of the reinforcing fibres as well as the compatibilizing effect of nanoclay were investigated by a multi-methodological experimental analysis including, rheological tests, mechanical characterization, and microstructural assessment. The graphical abstract in Figure 1 illustrates the main phases of the research activity proposed in this study.