Sustainable nanotechnology in construction: mechanical and environmental performance of nano–micro silica geopolymer concrete

The construction industry is a major source of global CO₂ emissions and uses large amounts of energy for cement production. Geopolymer concrete (GPC), which replaces conventional cement with fly ash activated by sodium hydroxide and sodium silicate, offers a sustainable alternative. This study examines how partially replacing fly ash with nano- and micro-silica affects the mechanical properties of GPC. The nano-silica used possessed an average particle size of 12 nm and a high specific surface area of 200 ± 25 m²/g, while micro-silica contained approximately 85% SiO₂. Optimal ratios of alkaline to fly ash, sodium silicate to sodium hydroxide, curing duration, and temperature were established before mechanical testing. Experimental results reveal that GPC exhibits Maximum compressive strength compared to conventional cement concrete. Incorporating 7.5% micro-silica and 2% nano-silica as partial fly ash replacements markedly improves compressive and flexural strength, surpassing plain GPC and ordinary cement concrete. However, higher nano-silica content (> 2%) reduces strength. From an environmental perspective, the geopolymer concrete emits approximately 5–6 times less CO₂ than conventional cement concrete and can reduce CO₂ emissions by nearly 80% through the replacement of cement with industrial by-products such as fly ash. Additionally, the study emphasizes that only 17–20% of the approximately 100 million tons of fly ash produced annually is currently utilized, while the remainder is disposed of in landfills. Future research should examine the durability, large-scale use, and life-cycle impacts of nano–micro silica-modified GPC to speed its adoption in sustainable infrastructure.