Feasibility of Using California-Based Almond Shells for Alkali Activation of Slag: Physicochemical Properties and Hydration Behavior of Almond Ash Binders

This study investigated the feasibility of using almond shell ash, a byproduct of California’s almond industry, as an alternative alkali source in alkali-activated binders (AABs) based on ground-granulated blast furnace slag (GGBFS). Reagent-grade potassium hydroxide (KOH) and potassium carbonate (K2CO3) were used as control alkali sources. The performance of almond shell ash from two farms in California (WD and AND) was evaluated by replacing the control alkali source at 0% (control), 10%, 50%, and 100%. The physicochemical properties of the almond shell ashes were fully characterized, and AAB mortars were evaluated for compressive strength at 3 days, 7 days, and 28 days, as well as for reaction kinetics and drying shrinkage. Both almond shell ashes were rich in alkali-based carbonates. WD ash had higher solubility and carbonates than AND ash. The ashes reacted with GGBFS, yielding strengths comparable to both controls at replacement levels of up to 50%. At a 100% replacement rate, the 7-day strength was lower than the K2CO3-control for AND by 45% and WD by 37%; however, the 28-day strength was comparable to the KOH-control and lower than the K2CO3-control by 33%–47%. Based on isothermal calorimetry, AAB ash showed significant heat release until 12 h after mixing. Thermogravimetric analysis confirmed hydrotalcite and carbonate phases in the ash-AAB. However, drying shrinkage was higher by 16%–94% than the controls at 28 days, likely due to the mesopore system of ash-alkali activated systems. These findings highlighted that the almond shell ash has potential as an alternative alkali source, advancing the production of sustainable AAB mortar.