Impact of Low CO 2 Footprint-Dissolution Treatment of Silica and Potassium-Rich Biomass Ashes on the Compressive Strength of Alkali-Activated Mortars

Using almond shell biomass ash (ABA) as a potassium alkaline source and rice husk ash (RHA) as a soluble silica source to produce blast furnace slag (BFS)-based alkali-activated mortars offers a sustainable alternative to commercial activators. However, some thermal treatment is often needed to enhance ash dissolution, potentially increasing the CO 2 footprint. In this study, we evaluated how a low-CO 2 -footprint thermal treatment for dissolving ABA, as well as RHA combined with ABA, affects the strength performance of binary (ABA/BFS) and ternary (RHA/ABA/BFS) alkali-activated mortars. This thermal treatment involved mixing the biomasses with hot water (85 °C) in a thermally insulated bottle (TIB). The binary alkali-activated mortar, cured for 7 days in a thermal bath at 65 °C, achieved 58.0 MPa in compressive strength, applying 1-h dissolution of ABA in a TIB. Additionally, the previous dissolution of RHA in conjunction with ABA for ternary alkali-activated mortar, cured also for 7 days in a thermal bath at 65 °C, resulted in mortars with a higher compressive strength, achieving 64.7 MPa. With the prior biomass dissolution method, the binary and ternary alkali-activated mortars cured at room temperature (20 °C) showed compressive strengths of 54.7 and 67.0 MPa after 28 curing days, respectively. Moreover, after 135 curing days, these mortars reached a compressive strength of 61.4 and 71.9 MPa, respectively. The BFS-alkali-activated binders with ABA and ABA plus RHA cut CO 2 emissions by 86.8% and 85.7% compared to the OPC-based binder, respectively.