Multiscale reinforcement and precursor engineering of fly ash-metakaolin-rice husk ash geopolymer self-sensing composites incorporating nano carbon black and fibers

Sustainable multifunctional geopolymer composites can provide structural health monitoring for smart infrastructure and underground mine ground support while valorizing mining and agricultural byproducts. This study investigated fly ash (FA), metakaolin (MK), and rice husk ash (RHA), followed by nano-carbon black (NCB) and copper-coated steel fibers (CSFs). The optimum precursor blend of 60% FA, 25% MK, and 15% RHA achieved 78.54 MPa compressive strength. Adding 5% NCB increased electrical conductivity by 72.36%, while 2% CSFs increased compressive strength by 29.53% through crack bridging. Hybrid NCB–CSF reinforcement delivered the best multifunctional performance, reducing electrical resistivity to 134.65 Ω·m and achieving a gauge factor of 150, demonstrating exceptional strain sensitivity. It also reduced average pore diameter by 9.76%, indicating enhanced pore refinement. NCB primarily enhanced electrical conductivity and self-sensing sensitivity, whereas CSFs improved overall mechanical performance and toughness. The hybrid system demonstrates strong potential for sustainable, durable, multifunctional, self-sensing mine-support materials.