Thermoelectric ultra-high-performance geopolymer concrete containing nano carbon black: synergistic enhancement of mechanical, transport, and energy-harvesting performance

Ultra-high-performance geopolymer concrete (UHPGC) offers a sustainable alternative to conventional cement-based materials by combining lower carbon emissions with superior mechanical performance; however, its thermoelectric potential remains largely unexplored. This study evaluates the effects of nano carbon black (NCB, 0–0.5 wt.% binder) on the mechanical, microstructural, transport, and thermoelectric properties of slag-rice husk ash-metakaolin-based UHPGC. Flowability, calorimetry, strength, fiber pull-out, autogenous shrinkage, pore structure, electrical and thermal conductivity, Seebeck coefficient, power factor, figure of merit, and microstructure were investigated. Low NCB dosages improved matrix densification, reaction kinetics, and fiber-matrix bonding. The optimum 0.1 wt.% dosage increased compressive strength by 26%, tensile strength by 7%, and strain energy density by 26%, while reducing porosity and shrinkage. Electrical conductivity increased continuously with NCB content, whereas optimum thermoelectric behavior occurred at 0.3 wt.% through synergistic interfacial energy filtering and electron-ion transport. Higher NCB contents caused agglomeration and performance loss.