Multiscale assessment of performance of limestone calcined clay cement (LC3) reinforced with virgin and recycled carbon fibers

The carbon fibers (CFs) recovered from end-of-life CFRP scraps via pyrolysis have recently aroused ascending interest due to their highly retained fiber strength and lowered carbon footprint. However, the utilization of recycled CFs (rCFs) within cementitious matrices, as well as the exploration of their interfacial interactions, remains notably limited. In this study, we have first time incorporated rCFs into sustainable Limestone Calcined Clay Cement (LC3) composite and comprehensively investigated the microstructural changes and physical characteristics of LC3-blended mixtures. The incorporation of LC3 results in a reduction in setting times and the flowability of fresh mortar, accompanied by a decrease in cumulative heat release. Analytical analysis indicated the generation of a substantial quantity of ettringite crystallites and highly polymerized C-A-S-H gel in LC3 mixtures, which benefited the enhancement of flexural strengths but increased the micropores in the matrices. Compromised compressive strengths, with reductions ranging from 5% to 46%, were observed upon the incorporation of LC3, aligning closely with the porosity findings. Measurements of autogenous shrinkage revealed an expansion tendency as LC3 proportions increased, likely attributed to bleeding and ettringite formation. A thorough investigation into the properties of CFs after undergoing thermal recycling was undertaken. A quantitative assessment of the bonding at the fiber-cement interface revealed a progressive reduction in bond strength for both virgin and recycled CFs, diminishing from 5.1 MPa and 2.5 MPa to 0.4 MPa and 0.5 MPa, respectively, along with the addition of LC3 content. Our findings have provided a comprehensive understanding of the hydration and microstructure evolution within LC3 blends, as well as the influence of CF additions on the matrices' properties.