###### Abstract  
Carbonation of cementitious binders implies gradual capture of CO\({}_{2}\) and significant compensation for the abundant cement-related CO\({}_{2}\) emissions. Therefore, one should always look at the CO\({}_{2}\)-sequestration-to-emission ratio (CO\({}_{2}\)SP/EM). Here, this was done for High-Volume Fly Ash (HVFA) mortar (versus two commercial mortars). Regarding their CO\({}_{2}\) sequestration potential, effects of accelerated testing (at 1-10% CO\({}_{2}\)) on as such estimated natural carbonation degrees and rates were studied. Production related CO\({}_{2}\) emissions were evaluated using life cycle assessment with no/economic allocation for fly ash. Natural carbonation rates estimated from accelerated tests significantly underestimate actual natural carbonation rates (with 29-59% for HVFA mortar) while corresponding carbonation degrees are significantly overestimated (67-74% as opposed to the actual 58% for HVFA mortar). It is advised to stick with the more time-consuming natural tests. Even then, CO\({}_{2}\)SP/EM values can vary considerably depending on whether economic allocation coefficients (C\({}_{\text{e}}\)) were considered. This approach imposes significant portions of the CO\({}_{2}\) emissions of coal-fired electricity production onto fly ash originating from Germany, China, UK, US and Canada. C\({}_{\text{e}}\) values of \(\geq\)0.50% lower the potential CO\({}_{2}\)SP/EM values up to a point that it seems no longer environmentally worthwhile to aim at high-volume replacement of Portland cement/clinker by fly ash. materials, also their CO2 sequestration potential should be addressed in order to adequately evaluate their sustainability. A balance should be sought that minimizes production related CO2 emissions and maximizes CO2 uptake, yet without compromising the service life of concrete structures. The favorability of this balance could be assessed by calculating CO2 sequestration-to-emission ratios for binder systems with high SCM contents replacing the Portland clinker/cement and comparing these with those of traditional, more commonly used binders. This was one of the main goals of this study, in particular for a High-Volume Fly Ash (HVFA) mortar with 50% of Portland cement replaced by pozzolanic fly ash originating from coal-fired electricity production. From the viewpoint of production, HVFA mortar counts as a potentially 'green' material since the Portland cement responsible for a carbon footprint of almost 1 kg/kg [5] is reduced by half [6]. Moreover, an industrial by-product from another industry is given a useful destination implying less mere landfilling of fly ash and less exploitation of virgin resources for producing Portland cement. Quantitative assessment in that regard can be achieved by means of environmental impact calculations in Life Cycle Assessment (LCA) software. As will be seen in this study, the outcome of such calculations can be seriously affected by the considered product status for the fly ash and the related fair assignment of impacts related to the main production process, i.e., coal-fired electricity production, by means of allocation [5; 6; 7; 8; 9; 10]. Beyond the production stage, the greenness of the HVFA binder will mainly depend on the carbonation resistance in steel reinforced concrete applications. For non-steel reinforced use, the focus of this paper, the key sustainability factor is the achievable carbonation extent in the 'beyond-production' stage to assess the portion of production related CO2 emissions that could potentially be recuperated through carbonation-induced CO2 sequestration. This requires a thorough understanding of the material's carbonation behavior under its expected natural exposure conditions. However, since natural carbonation is a process that occurs slowly, one often tends to shift to accelerated laboratory testing. This then implies exposing the binder system to elevated CO2 levels.