# Carbon Dioxide Uptake by Cement-Based Materials: A Spanish Case Study
## 6 Results and Discussion  
### Maximum Amount of Carbon Dioxide that Can Be Absorbed  
The maximum amount of carbon dioxide that can be absorbed according to EN [27] is calculated by applying Equation (3), and the result is shown in Table 2.  
\[\text{CO}_{2,\text{max}}=\text{(CaO}_{\text{reactive}}/100)\times\text{ binder}\times\text{(M}_{\text{CO2}}/\text{M}_{\text{CaO}}) \tag{3}\]where CO\({}_{2,\rm max}\): maximum amount of carbon dioxide that can be absorbed, kg CO\({}_{2}\)/kg cement; CaO\({}_{\rm reactive}\): kg CaO/kg binder \(\times\) 100, and this value is taken as 65% (average value of the Spanish clinkers); binder: amount of cement (kg); M\({}_{\rm CO2}\): 44.0 g/mol; M\({}_{\rm CaO}\): 56.1 g/mol. However, it does not consider the possible carbonation of other Portland cement components [11, 12], such as the alkaline compounds forming alkaline carbonates Equation (4). \[\rm CO_{2,max}=(M_{CO2}/M_{CaO})\times(CaO-0.7\times(SO_{3})_{CaSO4})+(M_{CO2}/ M_{Na2O})\times Na_{2}O+(M_{CO2}/M_{K2O})\times K_{2}O \tag{4}\]  
where CaO: kg CaO/kg Portland cement \(\times\) 100; 0.7 \(\times\) (SO\({}_{3}\))\({}_{\rm CaSO4}\): kg CaO released from CaSO\({}_{4}\)/kg Portland cement \(\times\) 100; Na\({}_{2}\)O: kg Na\({}_{2}\)O/kg Portland cement \(\times\) 100; K\({}_{2}\)O: kg K\({}_{2}\)O/kg Portland cement \(\times\) 100; M\({}_{\rm CO2}\): 44 g/mol; M\({}_{\rm CaO}\): 56 g/mol; M\({}_{\rm Na2O}\): 62 g/mol; and M\({}_{\rm K2O}\): 94 g/mol. If we apply Equation (3) to a CEM I with 95% clinker that contains 65% CaO reagent, it holds that the maximum amount of carbon dioxide that can be absorbed, according to EN, is equal to CO\({}_{2,\rm max}\) = (65/100) \(\times\) 0.95 \(\times\) (44/56) = 0.49 kg CO\({}_{2}\)/kg CEM I. This value only applies to cement with 95% clinker (CEM I) [28]. As far as other cements are concerned, this value will drop in proportion to the clinker content. Table 2 has been prepared in order to be able to rapidly display the maximum amount of carbon dioxide that the different can EN 197-1 cements could be absorb in theoretical and unreal circumstances. In fact, to calculate the maximum amount of carbon dioxide that can be absorbed for a cement, it could be considered that 100% of the Ca (OH)\({}_{2}\) of the portlandite (CH) and of the aluminates (AF\({}_{\rm t}\), AF\({}_{\rm m}\)) is carbonated, but only 50% of the C-S-H gel might be carbonated. Considering that in a cement paste with 65.3% of CaO, 31.4% of such CaO is bound to C-S-H gel and the rest is bound to portlandite, Ca(OH)\({}_{2}\), AF\({}_{\rm m}\) and AF\({}_{\rm t}\) phases, it would be necessary to take into account that only a maximum of 76% of the CaO in the Portland cement clinker could be carbonated Equation (5). \[\rm CaO\ (carbonated)=100\times(33.9/65.3)+50\times(31.4/65.3)=76\% \tag{5}\]  
Fraction of the cement able to be carbonated = 0.76 \(\times\) C \(\times\) CaO \(\times\) (M\({}_{\rm CO2}\)/M\({}_{\rm CaO}\)) (6)  
where C: clinker content in the cement, kg clinker/kg Portland cement \(\times\) 100; CaO: CaO content in the Portland cement clinker (\(\sim\)0.65); M\({}_{\rm CO2}\): 44 g/mol; and M\({}_{\rm CaO}\): 56 g/mol.