# Design Procedures for Sustainable Structural Concretes Using Wastes and Industrial By-Products
## 4 Discussion
### Physical Properties
#### 4.3.4 Resistance to Carbonation  
Figure 21 shows the average results of the accelerated carbonation test of concrete samples REF2, EAFS8D1 and FRC_EAFS for up to 186 days of exposure. The depth of carbonation can be expressed as a function of the square root of the exposure time, typically resulting in a linear trend of evolution. The slope of the linear trend corresponds to the carbonation coefficient, \(K_{c}\), which, as shown in Figure 21, were 2.86, 2.61 and 2.41 mm/day\({}^{0.5}\) for the REF2, EAFS8D1 and FRC_EAFS mixtures, respectively. This means that the carbonation coefficient of the REF2 mixture was 9.4% higher when compared to the carbonation coefficient of the EAFS8D1 mixture, and the carbonation coefficient of the EAFS8D1 mixtures was 8.6% higher when compared to the carbonation coefficient of FRC_EAFS. This indicates that the REF2 mixture was more susceptible to carbonation than the EAFS8D1 mixture, and the EAFS8D1 mixture was more susceptible to carbonation than FRC_EAFS.  
As shown in Table 20, the variation of the average carbonation resistance, \(R_{\mathrm{c65m}}\), from REF2 to EAFS8D1 was 25.1% smaller, which corroborates the observations made at the surface of REF2 mixture specimens that showed a greater progress depth of the carbonation front (Table 18). The same conclusion can be drawn for the EAFS8D1 mixture, which has an  
Figure 21: Results of carbonation depth along exposed time for REF2, EAFS8D1 and FRC_EAFS mixtures.  
Figure 20: (a) results obtained for air permeability coefficient, \(K_{G}\), for EAFS8D1 mixture; (b) results obtained for water permeability coefficient, \(K_{W}\), for EAFS8D1 mixture.  
average carbonon resistance, \(R_{\rm{65}m}\), 3% smaller than FRC_EAFS mixture; however, this difference is almost insignificant. Thus, somewhat contrasting to what was expected based on the previous results of water absorption at atmospheric pressure and by capillarity, the EAFS8D1 and FRC_EAFS mixtures show higher carbonation resistance than the reference mixture REF2 does in the uncracked state. This can be explained due to the changing of calcium hydroxide (Ca(OH)\({}_{2}\)) into calcium carbonate (CaCO\({}_{3}\)) by the uptake of CO\({}_{2}\). The expansion of the solid volume inside the concrete caused by the portlandite (Ca(OH)\({}_{2}\)) carbonation partially filled the free pores [70; 71; 72]. Since in the case of the EAFS8D1 and FRC_EAFS mixtures the porosity and sorptivity were greater than REF2, the reaction products resulting from the accelerated carbonatation test in the EAFS8D1 and FRC_EAFS mixtures were also greater. In this way, the EAFS8D1 and FRC_EAFS mixtures could exhibit an improved behavior to potential carbonation resistance relative to the REF2 mixture. That process can be confirmed by performing water absorption and sorptivity tests after submitting both mixtures to accelerated carbonation test, where porosity and sorptivity should be lower than the results obtained previously in the uncarbonated state. This porosity and sorptivity reduction should be more significant in the case of the EAFS8D1 and FRC_EAFS mixtures. Another reason for the EAFS mixtures presenting an enhanced carbonatation resistance behavior in relation to the reference mixture REF2 is the released of Ca\({}^{+}\) ions from the EAFS' Ca-bearing phases, which reacted with CO\({}_{2}\) forced into the microstructure. This reaction resulted in the precipitation of CaCO\({}_{3}\), which significantly densified the microstructure, and as explained above, partially filled the free pores [73].