Wood Ash Lye as a Sustainable Alkaline Activator for Halloysite Nanotube-Based Geopolymers with Improved Performance and Durability

Geopolymers are promising inorganic binders and alternative cementitious materials due to their high mechanical performance, durability, and reduced environmental impact in comparison with Portland cement. Nevertheless, the application of geopolymeric materials is limited due to the use of conventional alkaline activators, such as sodium hydroxide, which are associated with energy-intensive production and safety concerns. In this study, wood ash lye has been explored as a sustainable and low-cost alternative alkaline activator for the synthesis of geopolymers based on natural halloysite nanotubes, which represent the aluminosilicate precursors. The effects of the activator (wood ash lye or sodium hydroxide) and curing temperature (25 and 50 °C) on the structural, textural, mechanical, and durability-related properties of the geopolymeric materials have been investigated. Lye-activated geopolymers exhibit a more compact structure and reduced microporosity. Specifically, lye-activated samples cured at 50 °C exhibit superior mechanical stiffness, achieving a strain of 0.58% under 18 N compression compared to the NaOH-activated sample (2.83%), as well as a reduced moisture uptake (9.4 vs 13.0% after 72 h). Furthermore, lye activation provided enhanced thermal shielding, reducing the initial heating rate under flame exposure from 290 °C min–1 (pristine halloysite) to 35 °C min–1, while maintaining full structural integrity during water immersion testing, lower moisture uptake, and improved resistance to water immersion with respect to materials activated by sodium hydroxide. These findings demonstrate that wood ash lye is an effective alkaline activator and represents a viable strategy for biomass ash valorization and development of sustainable geopolymeric materials.