Alkali activators with different cations turning volcanic ash into eco-friendly geopolymers

Geopolymers are green and sustainable cementitious materials that are a possible alternative to cement due to lower energy consumption and emissions. Volcanic ash can be used as raw material to prepare geopolymers by alkali activation owing to its high silica and alumina content. However, research on volcanic-ash-based geopolymers is still limited and no consensus on the optimal alkali activator and reaction mechanism has been obtained. This study investigates the effect of alkali activators with different cations on the mechanical properties and microstructure of volcanic-ash-based geopolymers. The flexural and compressive strengths were evaluated. X-ray diffractometry, Fourier transform infrared spectrometry, scanning electron microscopy coupled with energy-dispersive spectroscopy and silicon-29 (²⁹Si) magic-angle spinning nuclear magnetic resonance were used to characterise the microstructural properties. The results showed that the alkali activation effect of sodium hydroxide (NaOH) on volcanic ash was superior to those of lithium hydroxide (LiOH) and potassium hydroxide (KOH), particularly at a concentration of 8 mol/l, with 28-day flexural and compressive strengths of 3.0 and 28.3 MPa, respectively. Microstructural results indicated that the sodium hydroxide-activated geopolymers formed dense, continuous, highly polymerised silica–aluminate gels. This paper provides a proper ratio scheme for activating the volcanic ash from similar deposits and promoting practical applications conducive to sustainable engineering development in the future.