The latest report from the Intergovernmental Panel on Climate Change made once again clear the urge to take immediate actions to reduce the emissions of carbon dioxide and other greenhouse gases. Among the UN Sustainable Development Goals (SDGs), Nr. 12 (“Ensure sustainable consumption and production of raw materials”) aims to improve the industrial sector and to ensure a high quality of life. Concrete is the second most used material after water and in its traditional form utilizes cement clinker, whose production contributes to 8-10% of the anthropogenic CO₂ emissions. Among the strategies to diminish the CO₂ footprint, use of supplementary cementitious materials (SCM) and alkali-activated materials (AAM) are currently considered the most efficient countermeasures.  

Within this framework, revalorization of poorly reactive sources by mechanochemical activation can contribute to the development of novel binders with decreased CO₂ footprint that can be utilized as partial or full replacement of Portland cement in concrete. Natural clays, mine tailings and air-cooled blast furnace slags (ACBFS), were activated in this study. Their applicability to be used in concretes as SCMs or/and AAMs was assessed. Natural clays are a mixture of various phases, whose compositions depends on weathering conditions. Naturally, they do not possess sufficient chemical reactivity to be utilized as SCMs. Similar properties possess mine tailings generated after extraction of precious elements, and slags produced in  blast furnaces of traditional steel plants.  

The present study aims to enhance the reactivity of these resources through mechanochemical activation (MCA) in a planetary ball mill. The process is considered a clean technology able to enhance the reactivity of crystalline materials without resorting to high processing temperatures or additional chemicals. MCA can induce amorphization, destroying the structure and breaking the bonds within the aluminosilicates and other minerals structure. The chosen parameters in the ball mill, as i.e. the filling amount, time of grinding, or speed of rotation, are strictly related to the degree of amorphization. Longer time of grinding, higher ball to processed powder (B/P) ratio, and higher grinding speeds generally increased the degree of the obtained amorphization. In such regard, an optimized process was chosen and further utilized to process all the poorly reactive resources. After MCA, the potential of clays and tailings as a SCM was investigated, while ACBFS was investigated as a precursor for alkali-activated materials. The achieved mechanical properties indicated a direct correlation between the enhanced amorphization degree of the mechanically activated clay and the increased strength values. The evaluation of SCMs was done by testing of their pozzolanic reactivity, enhanced after the mechanochemical activation. The reactivity was assessed by the strength activity index (SAI) and the Frattini test. Clays with higher content of clay minerals and tailings from the Kiruna mine deposit in Sweden showed increased pozzolanic reactivity and a great potential to be utilized as partial replacement of cement in concrete production. Furthermore, preliminary tests have shown that the alkali activation of the processed ACBFS produced solidified matrixes with considerable mechanical properties.