Processing-induced degradation of conductive carbon fiber networks in multifunctional cementitious composites

ABSTRACT: Developing electrically conductive self-compacting cement composite (SCCC) with short carbon fibers (CF) poses a fundamental trade-off: achieving high fluid workability versus preserving the integrity of the conductive percolation network. This study investigates the interplay between mix design, mixing energy, CF dispersion mechanisms, and the resulting electrical and mechanical performance of SCCC. A comprehensive experimental campaign incorporating up to 3 vol% CF was conducted, systematically evaluating different dispersion treatments (ultrasonication, thermal oxidation, and superplasticizer pre-mixing) and mixing durations. Contrary to established literature predictions for cementitious mortars and pastes, achieving self-compacting properties required extended mixing times (up to 27 min), which dramatically increased the electrical resistivity of the hardened concrete from conductive levels to dielectric ranges. Advanced microstructural characterization using Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDX) revealed that the high shear stress during prolonged mixing causes severe mechanical fragmentation and shearing of the CFs, alongside the formation of dense micro-ellipsoidal fiber clusters (<1 mm in size). These phenomena degrade the high-aspect-ratio fiber network, breaking the electrical percolation threshold. Crucially, mixes containing 2 vol% CF subjected to controlled mixing times successfully bridged this gap, exhibiting confirmed self-compactability alongside high electrical conductivity (resistivity ~300–800 Ω·cm), robust compressive strength (>70 MPa), and excellent piezoresistive self-sensing (high strain sensitivity) and electro-thermal heating performance (ΔT ≈ 15 °C at 30 V AC). These findings reveal a previously unreported critical degradation mechanism for carbon fibers mixes and establish precise mixing energy boundaries for the scalable production of smart SCCC.