Alkali activators with different cations transforming low-value volcanic ash into eco-friendly geopolymers

Geopolymers are green and sustainable cementitious materials as a possible alternative to cement due to lower energy consumption and emissions. Volcanic ash is considered a low-value material because of its worldwide distribution and huge reserves but limited applications. Beneficial to its high silica and alumina content, volcanic ash can be used as raw material to prepare geopolymers by alkali activation. However, research on volcanic ash-based geopolymers is still limited, no consensus on the optimal alkali activator is obtained, and the reaction mechanism is not yet clear. This paper aims to investigate the effect of alkali activators with different cations on the mechanical properties and microstructure of volcanic ash-based geopolymers. The flexural and compressive strength were evaluated. X-ray diffractometry, Fourier Transform Infrared Spectrometry, Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy, and Si-29 Magic Angle Spinning-Nuclear Magnetic Resonance were used to characterize the microstructural properties. The results showed that the alkali activation effect of NaOH on volcanic ash was superior to that of LiOH and KOH, especially at a concentration of 8 mol/L, with 28-d flexural and compressive flexural strengths of 3.0 MPa and 28.3 MPa, respectively. Microstructural results indicated that LiOH and KOH could induce volcanic ash to undergo geopolymerization to generate loose particles as products. In contrast, the NaOH-activated geopolymers formed dense, continuous, highly polymerized silica-aluminate gels. This paper provides a proper ratio scheme for activating the volcanic ash mined from the deposit in the present study and promoting practical applications conducive to sustainable engineering development in the future.