Performance of self-compacting geopolymer incorporating untreated volcanic mud as partial fine aggregate replacement

This study investigated the utilization of Sidoarjo Mud (LUSI), a clay-rich volcanic mudflow by-product from Indonesia, as a sustainable fine aggregate substitute in self-compacting geopolymer mortars. The effects of untreated LUSI incorporation as a partial fine aggregate replacement (0–10 wt%) on the fresh, electrical, mechanical, and microstructural properties of the geopolymer mortars were investigated. The results showed that 5 wt% LUSI achieved self-compacting performance within the EFNARC-recommended slump flow range, while both lower and higher replacement levels fell outside this range under the present mix design. Electrical resistivity progressively decreased with increasing LUSI incorporation, reaching 1432 kΩ·cm at 10% replacement, indicating changes in electrical transport associated with moisture and pore-solution characteristics within the geopolymer matrix. Significant relative strength development of 73–297% was observed between 7 and 28 days, despite a reduction in compressive strength with increasing replacement level, demonstrating ongoing geopolymer formation even in a low-calcium precursor system. Microscopic characterization revealed considerable changes in microstructure, fracture pattern, and interfacial behavior, suggesting that untreated LUSI acts primarily through physical and transport-related mechanisms. Furthermore, the presence of clay in LUSI implies a coupled rheology–transport interaction, where increased water retention changes in ionic transport pathways simultaneously affect fresh-state stability and electrical response. The results confirm the feasibility of employing volcanic mud for self-compacting geopolymer applications without prior treatment. These findings demonstrated the valorization feasibility of unprocessed LUSI in self-compacting geopolymer systems while providing new insights into the connection between clay-rich waste, rheological behavior, electrical transport, and mechanical development.