Avoiding calcination: Low-carbon alkali-activated binders from mechanochemically activated illitic, carbonate-rich clays

The growing demand for low-carbon construction materials has intensified interest in alkali-activated binders (AABs) based on alternative clay precursors beyond high-purity kaolinite. In this study, mechanochemical activation is investigated as a sustainable pre-treatment strategy for an illitic, carbonate-rich clay from southern Italy, aiming to avoid energy-intensive calcination and CO2 emissions associated with carbonate decomposition. Two grinding approaches-high-energy vibro-milling and conventional ball milling-were applied and systematically compared in terms of their effects on clay structure, reactivity, and performance in alkali-activated systems. The mechanically treated clays were characterized using a multi-analytical approach including laser granulometry, XRPD, TGA/DTG, SEM-EDS, and 27Al and 29Si MAS NMR spectroscopy. High-energy milling induced extensive delamination and amorphization of clay minerals, increasing the amorphous content to 41 +/- 5 wt%, while conventional ball milling produced more limited structural disorder (25 +/- 3 wt%). These structural changes significantly influenced alkali reactivity and geopolymerization efficiency. Alkali-activated pastes synthesized from high-energy milled clay exhibited enhanced aluminosilicate network reorganization, greater Al incorporation into tetrahedral sites, and the formation of predominantly amorphous N-A-S-H to (N,C)-A-S-H gels, resulting in compressive strengths up to 16.5 MPa after 28 days, particularly under elevated curing temperatures. The binders show strength enough for certain construction and soil-stabilization uses. The findings confirm that mechanochemical activation is a viable, more sustainable alternative to thermal treatment for carbonate-rich clays, allowing them to be used as low-carbon precursors in alkali-activated binders and broadening the range of local clays suitable for sustainable construction.