Synergistic role of nano-Fe2O3 and activator chemistry in enhancing mechanical, durability, and microstructural properties of fly ash-waste marble powder geopolymers

The valorization of industrial by-products such as fly ash (FA) and waste marble powder (WMP) in geopolymer composites (GCs) offers a sustainable alternative to conventional cement-based systems. This study investigates the synergistic effects of nano-Fe2O3 (n-Fe2O3) incorporation and activator chemistry (Na2SiO3/NaOH, NS/NH ratios of 1.5-3.0) on the mechanical, durability, thermal, and microstructural performance of FA-WMP-based GCs. Mixtures were designed with 0-1.5 % n-Fe2O3 (by FA mass) and thermally cured at 80 degrees C for 16 h. Comprehensive tests were conducted, including compressive and flexural strength, oven-dry density, porosity, water absorption, sorptivity, sulfate resistance (5 % MgSO4 immersion), and freeze-thaw durability, complemented by high-temperature exposure (200-600 degrees C) and microstructural characterization (SEM/EDS, XRD). The results showed that both activator ratio and nanoparticle dosage governed performance. Increasing the NS/NH ratio enhanced silicate availability and promoted cross-linked N-A-S-H and C-A-S-H gel formation, while well-dispersed n-Fe2O3 provided nucleation sites, refined pore networks, and accelerated geopolymerization. The optimum synergy was achieved at NS/NH = 3.0 with 1.0 % n-Fe2O3, yielding maximum compressive (28.50 MPa) and flexural strength (2.23 MPa), similar to 74 % and similar to 27 % higher than the control, respectively. At elevated temperatures, mixes with high NS/NH ratios and moderate nanoparticle contents exhibited improved residual strength and reduced weight loss, although severe deterioration occurred at 600 degrees C. Sulfate resistance was significantly improved at NS/NH = 3.0 with 1.0 % n-Fe2O3 (only 9.93 % strength loss after 100 days), while excessive nanoparticle content (>= 1.5 %) induced agglomeration and micro cracking, reducing long-term durability. Microstructural analysis confirmed matrix densification, reduced porosity, and effective FA dissolution at optimum nanoparticle dosages, whereas clustering of n-Fe2O3 led to voids and weak interfaces. This study demonstrates that controlled integration of nano-engineering and activator optimization can produce dense, durable, and eco-efficient geopolymer composites. The findings provide practical insights for tailoring waste-based GCs with enhanced strength, thermal stability, and chemical resistance, contributing to sustainable construction materials development.