The global construction sector increasingly champions sustainability, driving the adoption of alternative binders like blast furnace slag cement (CEM III) to mitigate the significant CO2 emissions associated with traditional Portland cement (CEM I) production. While CEM I is known for robust carbonation resistance, its substantial environmental footprint prompts this vital shift. However, a critical, often overlooked factor, i.e. the initial curing execution, profoundly impacts the long-term performance of these sustainable concretes. Inadequate curing, particularly during early age, severely compromises the quality of the concrete cover (skin-crete). This deficiency is even more pronounced in CEM III concrete compared to CEM I. This creates a detrimental paradox: the presumed environmental gains from selecting CEM III are drastically undermined if proper ‘curability’ is neglected. A concrete structure intended for a 50-year service life might prematurely degrade, leading to costly and unforeseen maintenance, repairs leading towards a higher overall environmental impact and cost, seen from a life cycle perspective. This paper highlights this critical issue through a case study of a concrete water retaining structure cast in 2010 using CEM III. Despite its relatively young age of 15 years, it shows advanced carbonation, primarily attributed to insufficient initial curing and unprotected casting in harsh winter conditions. Intensive non-destructive testing (electrical resistivity, air permeability, rebar scanning) and destructive testing (core drilling for petrographic analysis and carbonation depth determination via phenolphthalein) was performed. Furthermore, the paper will present results from Life Cycle Assessment (LCA) and Life Cycle Cost Analysis (LCCA) studies to quantify the environmental impact and economic consequences of inadequate curing. The findings highlight that the pursuit of truly sustainable concrete solutions demands an unwavering commitment to ‘curability’, recognizing it as an indispensable pillar underpinning both concrete durability and its genuine environmental integrity throughout its intended service life.