Influence of Surfactant-Assisted Graphene Nanoplatelet Dispersion on the Mechanical and Electrical Properties of Fly Ash Geopolymer Mortar

Developing electrically functional geopolymer composites requires conductive fillers that improve electrical response while maintaining mechanical performance. This study examined that formulation tradeoff using graphene nanoplatelets (GNPs) in fly ash geopolymer mortar. An aqueous screening program comprised 48 dispersions prepared with sodium dodecyl sulfate (SDS), Triton X-100, or polyvinylpyrrolidone, combining four GNP concentrations with four dispersant-to-GNP mass ratios. Conductivity, pH, and visual observations over 12 days supported selecting SDS for subsequent mortar preparation, although these measurements did not establish superior nanoscale dispersion or long-term alkaline stability. Subsequent testing evaluated 16 SDS-GNP mortar mixtures alongside four SDS-only reference mixtures. Dispersion concentrations of 0.05, 0.10, 0.50, and 1.00 wt.% corresponded to approximately 0.0043, 0.0085, 0.0426, and 0.0852 wt.% GNP relative to fly ash. Compressive strength ranged from approximately 5.5 to 16.8 MPa, with higher loading generally associated with lower strength. At the lowest SDS-to-GNP ratio, increasing dispersion concentration from 0.10% to 1.00% reduced strength from about 11.8 to 7.2 MPa, a decrease of approximately 39%. Ultrasonic pulse velocity followed a similar trend, reaching about 3.0 km/s at lower concentrations compared with 2.08 to 2.50 km/s for the 1.00% mixtures. Bulk electrical resistivity generally ranged from 60 to 180 Ω·m after ambient storage and from 5 to 29 Ω·m after water immersion, without a consistent decrease as the GNP concentration increased. Factorial analyses indicated formulation interactions, while desirability analysis favored selected low-concentration mixtures for their combined mechanical and electrical responses. Lower GNP-dispersion concentrations therefore provided a more favorable balance under the tested conditions, but incomplete SDS-dose matching prevents attribution of the observed changes to GNPs alone. These results support formulation screening without establishing conductive-network formation or validating sensing and heating applications.