## 1 Introduction  
Concrete is a key, sustainable building material that was initiated by the modern society at the beginning of the 21 st century. Concrete helps in the development of cities, regions and countries. Its pervasive importance is because it is possible to apply in many civil engineering and buildings uses with a relative low cost. Long-term reinforced concrete structures such as bridges, pipes, highways, and buildings demonstrate the indisputable high durability of this essential material and, therefore, its sustainability.  
Cement is concrete's main constituent because is responsible for the binding of all the components, which comprise, at least, clinker and gypsum. More sustainable cements also contain different amounts of industrial wastes such as coal fly ash, ground granulated blast-furnace slag, and/or silica fume. Clinker production requires the calcination of the limestone used as raw material. This reaction produces huge amounts of carbon dioxide, which is emitted into the atmosphere. Consequently, the reduction of these carbon dioxide emissions is paramount for a better future. On the other hand, mortars and concretes suffer a well-known chemical reaction named carbonation, i.e., calcium hydroxide, where C-S-H gel, ettringite, calcium aluminates, and others react with the carbon dioxide to form calcium carbonate [1]. Thanks to this chemical reaction, it can be said that part of the carbon dioxide emitted during the calcination process is reabsorbed by cement-based materials. Consequently, hydrated cement can be considered a carbon sink (Figure 1). However, the current methodology for reporting carbon dioxide emissions referred to in the Guidelines for National Greenhouse Gas Inventories [2; 3], published by the Intergovernmental Panel on Climate Change (IPCC), does not considers the carbonation process. Therefore, carbonation is not included in national inventories. The net carbon dioxide emissions (carbon dioxide released by calcination minus carbon dioxide uptake by carbonation) should be calculated by the National Inventories of Greenhouse Gas Emissions in order to have accurate data for country reporting.  
Some climate models are not accurate because they do not account for carbon dioxide uptake due to concrete and mortar carbonation. However, they can further improve the implementation of carbon dioxide uptake via cement-based materials. The need for such an improvement is based on the possibility that it could facilitate the inclusion of supplementary methodologies, not only for the sources but also for greenhouse gas (GHG) and sink effects, especially during the material's working life phase. Thus, it is a goal of the cement industry that the IPCC recognizes a method for calculating this sink due to the carbonation of cement-based materials. That is why The European Cement Association (CEMBUREAU) has promoted the creation of the concrete CO\({}_{2}\) sink project and has agreed to propose a methodology of the calculation of CO\({}_{2}\) uptake [3]. According to the simplified methodology [3], during the service life of the cement-based applications, up to 20% of the process  
Figure 1: Life cycle of CO\({}_{2}\) in the cement industry.  
emissions that are related to the production of the cement can be absorbed. In addition, after the cement-based material's service-life, it is sometimes recycled as secondary material, such as aggregates, and there would therefore be an extra 3% of carbon dioxide uptake [3].  
The Intergovernmental Panel on Climate Change (IPCC) recognizes the physicochemical process of carbonation for carbon removal; nevertheless, it is currently not considered in the countries' GHG emissions inventories. Consequently, the cement industry is working on developing an adequate methodology in order to allow a country to calculate the CO\({}_{2}\) removal due to concrete and mortar carbonation.