Geopolymer is an alternative to ordinary Portland cement (OPC). However, traditional cast geopolymers require highly alkali activators, leading to pores formation and limiting its strength development. The study aimed to optimize the aluminosilicate source-toalkali activator (AS/AA) ratios, NaOH concentrations, Na2SiO3/NaOH ratios and pressing forces of fly ash-based pressed geopolymer. The optimized pressed geopolymer was compared to cast geopolymer in terms of material properties and environmental impacts. Besides, the dual effects of aging periods (1 – 28 days) and sintering temperatures (700 – 1000°C) on the performance of pressed geopolymer were evaluated. The study also assessed the acid resistance of sintered and unsintered pressed
geopolymers by immersing them in different types (HNO3 and H2SO4) and concentrations (3% and 8%) of acids. The evaluated properties of the pressed geopolymer included physical appearance, linear shrinkage, mass, density, porosity, water absorption, compressive strength, flexural strength, sorptivity and leaching behavior. Materials characterizations encompassed microstructure, elemental composition, structural evolution, composition characteristics and Si structure identification. Results indicated that the optimal configuration for pressed geopolymer involved AS/AA ratio of 5.5, NaOH concentration of 14 M, Na2SiO3/NaOH ratio of 2.0 and pressing force of 5 tons, achieving compressive and flexural strengths of 114.2 MPa and 29.9 MPa, respectively, after 28 days of aging. The pressed geopolymer significantly surpassed cast geopolymer in performance, showing increases in compressive and flexural strengths by 90% and
382%, respectively. Not only that, the pressed geopolymer demonstrated a lower declinein compressive strength of 32% after efflorescence testing, compared to a 60% reduction in cast geopolymer. Besides, pressed geopolymers reduce embodied energy (EE), embodied carbon dioxide emission (ECO2) and embodied carbon index (ECI) by 50%, 59% and 21%, respectively. The mechanical strengths of pressed geopolymer were further improved by aging for 7 days and sintering at 1000°C. Phase analysis confirmed significant nepheline formation, enhancing the ceramic-like properties of pressed geopolymer. The sintered pressed geopolymers maintained superior mechanical strength over their unsintered counterpart, especially notable after a 28-day immersion in 3% HNO3 where sintered samples showed increased compressive strength. Both sintered and unsintered samples developed gypsum after H2SO4 immersion, but only the sintered samples prevented crack formation due to their dense cross-linked structure. This study recommended sintering the 7-day pressed geopolymer at 1000°C for developing acidresistant
building materials suitable for waste encapsulation applications. These findings demonstrate the potential of pressed geopolymer as a robust, eco-friendly alternative to OPC for construction applications, aligning with the Sustainable Development Goals (SDGs) by promoting sustainable infrastructure and reducing environmental impact through lower carbon emissions and energy consumption.
