## 4 \({}^{29}\)Si MAS NMR Analysis
### Relationship between Carbonation and \({}^{29}\)Si MAS NMR Results  
The CH (Ca(OH)\({}_{2}\)) generated by the hydration process keeps the concrete's pH high. Adding a pozzolanic material, such as fly ash, silica fume, or nanosilica, causes a pozzolanic reaction that consumes CH. Thus, the concrete's pH is reduced by the pozzolanic reaction, which can accelerate the carbonation. However, at the same time, the pozzolanic reaction generates a C-S-H gel that can reduce the permeability of the concrete and increase its carbonation resistance. As a result, the conversion fraction (or degree of reaction) of the pozzolanic material is very important for carbonation resistance. If the pozzolanic reaction is very fast, the permeability to CO\({}_{2}\) gas can be dramatically reduced, and the carbonation resistance can be enhanced, even if pH is somewhat lower than OPC.  
From the results discussed in Section 4.2, the conversion fractions of the SF35 and NS20 specimens were 83.1% and 97.4%, respectively, much higher than that of the FA15 (36.3%). From the accelerated carbonation test, the average carbonation depths were 7.34, 1.55, and 1.34 mm for the FA15, SF35, and NS20 specimens, respectively.  
Figure 11 shows the relationship between the conversion fraction and the inverse of the average carbonation depth obtained from this study, which demonstrates a strong linear relationship (R  
=0.9951). Thus, it can be concluded that the carbonation depth of the pozzolan blended concrete is inversely proportional to the conversion fraction of the pozzolan materials.  
There are several sources of calcium during carbonation. Major sources of calcium include the C\({}_{3}\)S and C\({}_{2}\)S remaining in the cement matrix. This residual C\({}_{3}\)S and C\({}_{2}\)S can be detected by the intensity of Q\({}^{0}\) in the \({}^{29}\)Si MAS NMR results.  
Table 5 shows the change (or difference) in the Q\({}^{n}\) intensity after carbonation for each pozzolan blended specimen. In Table 5, a minus sign represents a reduction after the carbonation. In Table 5, it can be seen that the Q\({}^{0}\) of all the specimens decreased after carbonation. This might be attributed to the fact that the calcium in the remaining C\({}_{3}\)S and C\({}_{2}\)S were used during the carbonation. The maximum difference was observed in the concrete with fly ash, which showed the poorest carbonation resistance. When the calcium in the C\({}_{3}\)S and C\({}_{2}\)S are used for carbonation, the silicate is separated. This separated silicate may be observed in high-order Q\({}^{n}\), such as Q\({}^{4}\), and the intensities of Q\({}^{3}\) and Q\({}^{4}\) were generally increased after carbonation, as shown in Table 5.  
In addition, the C-S-H gel can be attacked by acidic fluid and will then generate silica gel as [27].  
\[3\text{CaO}\cdot 2\text{SiO}_{2}\cdot 3\text{H}_{2}\text{O}\ +2\text{H}^{+}\ \to\ 3 \text{Ca}^{2+}\ +\ 2(\text{SiO}_{2}\cdot\text{nH}_{2}\text{O})+6\text{H}_{2}\text{O} \tag{7}\]  
As Equation (7) shows, the silicate is separated from the C-S-H gel. Thus, as in the case of the reduction of C\({}_{3}\)S and C\({}_{2}\)S, the low-order Q\({}^{n}\) can be decreased with increasing high-order Q\({}^{n}\).