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Carbonation of industrial wastes rich in earth-alkali oxides is found to have a significant potential for CO2 sequestration. This process opens new perspectives not only for carbon dioxide mitigation, but also for the valorization and new applications of industrial waste materials from coal-burning power 2 plants. In this study, mineral carbonation of high-calcium fly ash is investigated under dry and moist conditions in a continuous flow reactor during up to 2 hours, at temperatures ranging from 160 to 290 ºC and CO2 pressures between 1 and 6 bar. A comprehensive charaterization of treated and untreated samples was carried out before and after carbonation using X-ray diffraction, X-ray fluorescence spectroscopy, thermogravimetric analysis, infrared spectroscopy and scanning electron microscopy. The maximum sequestration capacity achieved was 117.7 g CO2/kg fly ash (48.14 % carbonation efficiency) under dry conditions. Results showed that increasing the pressure and temperature enhances the process of carbonation, as well as the presence of moderate amounts of water vapor in the CO2 gas flow. Newly formed carbonates were always present in the treated samples. This study shows that about 21% of all CO2 emissions of a coal-burning power plant could potentially be sequestered as carbonates.

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Limited utilization possibilities of high-calcium fly ashes (HCFA) are a serious issue not only in Europe, but also worldwide. The properties of such waste from coal-fired power plants could be conveniently treated in order to make their compositions compliant with national regulations and allow their use in a variety of industrial applications. This work reports on an investigation of mineral carbonation of HCFA from Greece, Poland and Spain with total CaO contents between 10 and 15 wt.%. Two types of experiments, batch and continuous flow, with and without the addition of water vapor, were performed. Best carbonation efficiency obtained was 47 % of the bulk CaO content. The free lime content of the samples was found to be the controlling factor. After treatment, the amount of free lime was reduced 2 to values suitable for their utilization as supplementary cementitious materials. The addition of water to the system played also an important role in the progress of the carbonation reactions. Our results strongly suggest that a carbonation treatment of HCFA could contribute to the circular economy of such waste materials and potentially increase their utilization in the construction industry, as well as make a significant contribution to lowering of the CO 2 emissions in coal-burning industrial facilities.

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The utilization of high-calcium fly ashes (HCFA) from coal-fired power plants in the construction industry is problematic, since their high free lime contents can lead to durability problems. In this research, the carbonation of a high-CaO fly ash has been carried out using simulated flue gas and concentrated CO2, with the aim to assess the valorization potential of such materials in the construction industry. The results show that, at 7 bars total pressure, an up to 36% carbonation efficiency can be achieved in just 30 min when pure CO2 is used; a comparable result with flue gas requires about 4 h of reaction. On the other hand, experiments carried out at atmospheric pressure show significantly different carbonation efficiencies depending on the CO2 concentration of the gas used. All experiments resulted in a substantial reduction in the original free lime content, and after reaction times of 4 h (at atmospheric pressure) and pressures of 7 bars (for any reaction time >30 min), the final free lime values were low enough to comply with the requirements of European Standards for their utilization as additions in cement.

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