Sustainable Alkali Activators for Geopolymer Concrete: A Systematic Review of Macro- and Microstructural Properties

Abstract

Geopolymer concrete (GPC) has gained attention as a sustainable alternative to Portland cement-based concrete due to its reduced carbon emissions and favorable mechanical properties. However, its broader application is limited by the high cost and carbon footprint associated with commercial alkaline activators. To address this, recent studies have explored the use of alternative activators derived from commercial and agro-industrial wastes, aiming to improve the environmental and economic viability of GPC. This review evaluates the performance of GPC incorporating waste-derived activators, with insights from microstructural analyses such as Scanning Electron Microscope(SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA) and Nuclear Magnetic Resonance (NMR) Analysis. These activators demonstrated enhanced dissolution rates and nucleation effects, promoting the formation of reaction products like C-A-S–H, N-A-S–H, and K-A-S–H gels. For instance, dedust and rice husk ash (RHA) reduced initial setting time by 64% and 57%, respectively. Compressive strength increased by 16.4% with dedust and 4% with silica fume (SF), while flexural strength improved by 47% with olive biomass ash (OBA) and 28% with RHA. In terms of durability, paper sludge and wood biomass ash (WBA) reduced water absorption by 22% and 13%, respectively, and SF reduced drying shrinkage by 25% compared to commercial activators. Cost–benefit analysis further confirms the economic feasibility of these alternatives. While the results are promising, further research is needed to fully understand the environmental impacts and long-term performance of waste-derived activators in GPC.