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At present, the production of cement contributes approximately 8% to global carbon dioxide (CO2) emissions, whereas a further increase in the future is expected. To reduce these emissions, a prominent measure of the cement industry is to use materials such as coal fly ash (CFA) as a cement substitute, but the availability of qualified ash is limited. The development of 'green cements' with lower CO2 emissions attracts attention within the cement industry and politics. Recently, a new invention for a calcium carbonate (CaCO3)-based cement intends to use CFA as an alkaline resource during a manufacturing process based on CO2 mineralization converting anthropogenic CO2 into stable calcareous materials by using calcium-bearing brines. In our systems analysis, we demonstrate that the alkalinity of CFA usable for mass production of cements is poor. The alkalinity of CFA primarily depends on the dissolution of free lime. For CO2 mineralization, the resulting alkalinity is just sufficient to precipitate the calcium supplied by the CFA itself, and the capacity of CO2 mineralization is low. Thus, CFA could only be used as a supplementary resource. Even at locations where exceptional calcium-rich ash is available, very large amounts of ash must be processed. Globally, only a few million tonnes of calcium could be extracted in this way. Because qualified CFA is already used in the construction industry, the extraction of calcium from nonqualified, more polluted CFA is associated with a risk of unwanted environmental effects and implies an elaborated pollutant control. [ABSTRACT FROM AUTHOR]