# Carbon Dioxide Uptake by Cement-Based Materials: A Spanish Case Study
## 4 Carbon Dioxide Uptake by Concrete  
Most of the carbon dioxide emissions from the Spanish Portland cement industry come from the calcination of limestone (60-64%) rather than from fuel combustion. On the other hand, part of these emissions is uptake by the mortar and concrete due to their carbonation during their exposition to the environment. The chemistry of carbonation has been reviewed in many papers [10; 11; 12]. In summary, carbonation is produced because carbon dioxide is a gas present in the atmosphere that penetrates throughout the capillary pore network of concretes and mortars, where it is dissolved in the pore solution and reacts with calcium ions [10] and other alkaline species [11; 12] produced by the Portland cement paste. Calcium carbonate has a very low solubility; then, it precipitates. With regard to the pozzolanic constituents present in the cement as well as ground granulated blast-furnace slag, they decrease the amount of calcium hydroxide in the binder due to the lower content in clinker when they are used, but they increase the amount of C-S-H gel. In addition, they lower the Ca/Si ratio of the C-S-H gel [13]. Consequently, the amount of calcium hydroxide that is able to be carbonated and the composition of the C-S-H gel depend on the type and amount of the cement constituents (fly ash, natural pozzolana, ground granulated blast-furnace slag, and so on) [14]. Additionally, blast-furnace slag has some components that contribute to the carbon dioxide uptake by the Portland cement paste  
Figure 8: Direct CO\({}_{2}\) emissions due to cement production (1970–2050) [9].  
present in mortars and concretes [15]. Therefore, the carbonation increases in cement-based materials with ground granulated blast-furnace slag [16; 17] in relation to their clinker content.  
Some studies have been performed to determine the carbon dioxide uptake in order to ensure a better carbon dioxide emissions balance. For instance, a Swiss study estimated that about 9.4-10.1% of the calcination and fuel carbon dioxide emissions of the Portland cement utilized in 2010 [18] were uptake during the concrete structures' service life. This means that about 6% of the carbon dioxide emissions coming from the calcination were uptake (considering that the ratio of the carbon dioxide emission from calcination and fuel is 60:40).  
A worldwide study has estimated a carbon dioxide uptake of 43% of the CO\({}_{2}\) emissions from the production of Portland cement by concretes and mortars from 1930 to 2013 [19]. This study considers the service life period, end-of-life stage, and secondary use after demolition. In addition, it assumes a degree of carbonation of about 80% in the carbonated area. Other papers suggested a full carbon dioxide uptake (100% uptake) of the one emitted by calcination [20; 21].  
However, more conservative studies proposed lower carbon dioxide uptake levels [22; 23; 24]. A previous Spanish project reported much lower carbon dioxide uptake values (3-4%) as a consequence of the lower surface/volume ratio considered (S/V = 3) in the concrete elements for a degree of carbonation measured at one year of exposure [25]. These results were updated to 10.8-11.2% of the carbon dioxide produced annually during the calcination process, considering the degree of carbonation measured after four years of exposure [15].  
The present paper covers some background of carbon dioxide uptake and also includes the results of the simplest method proposed by the \"recarbonation project\" sponsored by The Portland Cement Association (PCA), The European Cement Association (CEMBUREAU), The Cement Sustainability Initiative (CSI), and Cementa in 2018 [26]. This methodology is applied to the cement-based materials used in Spain first during their service life, and secondly, after the end-of-life and secondary usage stages.