Phlogopite Mica Under Acidic and Alkaline Conditions: Reactivity and Silicate Gel Formation

ABSTRACT Alkali‐activated materials (AAMs) produced using industrial residues offer a sustainable alternative to Portland cement, the production of which contributes nearly 8% of global CO 2 emissions. However, commercial sodium silicate, commonly used as an activator, remains costly and energy‐intensive to produce, motivating the development of alternative silicate sources. This study proposes a novel approach for producing alkali activators from phlogopite, a magnesium‐rich mining by‐product, through a combined mechanical activation and acid leaching route. In particular, the use of ultrasonic‐assisted leaching (sonication) is investigated as a non‐conventional method to enhance dissolution and improve activator composition. Phlogopite was milled and subsequently treated with citric acid using either mechanical stirring or sonication. The leached cations (e.g., K and Mg) were identified as potential secondary resources, while the resulting silicate‐rich residue was dissolved in NaOH to synthesize alkali activators. Sonication significantly enhanced the process, yielding an activator with improved silicate content (∼6 wt.% SiO 2 ) and reduced Na 2 O concentration (∼12 wt.%), compared to mechanically treated systems. Incorporating 20 wt.% sonicated phlogopite‐derived activator in blast furnace slag (BFS) resulted in a compressive strength of 40 MPa after 28 days, compared to 22 MPa for the reference, corresponding to an increase of ∼82%. Microstructural analysis revealed denser reaction products and reduced porosity, with the formation of calcium (sodium) aluminosilicate hydrates and layered double hydroxides. These results demonstrate the potential of ultrasonic‐assisted processing of phlogopite can be used to produce effective alkali activators, offering a promising pathway to reduce reliance on commercial sodium silicate and improve the sustainability of AAM systems.