Effect of graphene nanoplatelets on engineering properties of fly ash-based geopolymer concrete containing crumb rubber and its optimization using response surface methodology

The research aims to investigate the effect of graphene nanoplatelets (GNPs) on mechanical properties, workability as well as microstructure of fly ash-based geopolymer concrete containing crumb rubber (CR) through experiments and response surface methodology (RSM). A total of 20 mix designs were utilized, with 10–30% volume replacement of fine aggregate with CR and addition of 0.1–0.4% GNPs by weight of binder. All mix designs were studied for attributes including slump flow, compressive strength, stress-strain behavior, modulus of elasticity, tensile strength, flexural strength, toughness, impact resistance, water absorption, and porosity. Morphological changes due to crack formation, microstructure, and interfacial interaction between aggregate and mortar were evaluated using field emission scanning electron microscopy (FESEM). Vickers microhardness tests were carried out from the surface of aggregate towards the matrix to evaluate the interfacial transition zone. Results indicated that CR replacement reduced the mechanical properties of geopolymer concrete, while GNPs inclusion mitigated the negative effects of CR and significantly enhanced the properties of the geopolymer concrete. Results also revealed that 0.3% addition of GNPs increased the compressive strength, elastic modulus, tensile strength, and flexural toughness of the concrete containing the CR. Moreover, addition of 0.3% GNPs in specimens having 30% CR enhanced the average microhardness by 34%, which demonstrates that GNPs addition improved the matrix compactness and interfacial transition zone between CR and matrix. Multi-objective optimization with RSM was accomplished for the combined use of CR and GNPs in fly ash-based geopolymer concrete. RSM results showed that combination of 10% CR and 0.2% GNPs is the best combination for achieving the highest strength properties with the lowest percentage of GNPs.