The crucial need to reduce the global emissions of greenhouse gases, in particular carbon dioxide, pushed the development of new technologies, such as carbon capture and storage (CCS). Among the CCS technologies, accelerated carbonation allows CO₂ permanent fixation through a mineralization process. It consists in the reaction between CO₂ and basic alkaline earth compounds and its conversion into solid inorganic carbonates.
The aim of this research is to treat two selected industrial by-products with accelerated carbonation, and evaluate their potential towards carbon mineralization. Precisely, Electric Arc Furnace slag (EAF slag) and Cement by-pass Dust (CBPD) were investigated in this study, since their alkaline composition makes them suitable for CO₂ mineralization. The process was carried out through a slurry carbonation technique, in which a wet carbon capture process was applied to a powdered material. As a future perspective, the mineralized powders could be reused as supplementary cementitious materials (SCMs) or fine aggregates, thus contributing not only to reduce the CO₂ levels, but even to decarbonize the construction industry.
EAF slag is commonly generated during the production of steel from metal scraps. Due to their origin, EAF slags tend to have a non-homogeneous composition, but usually contain three major phases of calcium: portlandite/calcite, calcium-iron silicates, and calcium-iron oxides.
CBPD is generated during the cement production, in the pre-heating stage before the rotary kiln. The by-pass is used to extract alkali, sulphates and chlorides that originated from the kiln fuel. As a result, besides basic compounds such as CaO, SiO₂, Ca(OH)₂/CaCO₃, CBPD also contains a significant amount of inorganic salts (potassium and sodium chlorides).
The accelerated slurry carbonation was performed in an open system, at room temperature and atmospheric pressure, for 1 hour. Three replicates were reproduced at the same conditions to verify the reproducibility of the tests.
Different methods for quantification of CO₂ content were compared. For thermogravimetric analysis, carried out into a simultaneous thermal analyzer (thermogravimetric-differential thermal analysis, TG-DTA), and thermal decomposition (carried out in a furnace), the carbon content was calculated as the weight loss in the temperature range 550-850 °C, commonly recognized as carbonates decomposition range. A further and almost new quantification method was here proposed, that is acid digestion, which evaluates the weight loss due to CO₂ released under carbonates dissolution by diluted HCl solution.
The composition analysis, studied through X-ray diffraction, confirmed the formation of calcium carbonates after carbonation. The efficiency of the process was then calculated in terms of CO₂ uptake, considering the content of CO₂ before and after the carbonation, based on the mass of the uncarbonated material. The study confirmed the effectiveness of both industrial wastes to provide CO₂ mineralization. Specifically, CBPD provided about 23% CO₂ uptake, highlighting the great potential of such by-product to contribute to CO₂ permanent storage.  In addition, the three quantification methods led to comparable results. Furthermore, as the process was carried out in wet conditions, separation of the carbonated powder from the liquid resulted in a drastic reduction of the chlorine content, boosting its possible employment as SCM/aggregates.