# Ultrasonic Non-Destructive Testing of Accelerated Carbonation Cured-Eco-Bricks
## 4 Discussion of Results
### Microanalysis (SEM and EDS)  
The samples prepared with 0% and 5% TRC, compressed using 1.9 MPa, and carbonated for 72 h were analyzed using SEM, as shown in Figure 10a,b and Figure 11a,b, respectively. The WC samples prepared with 0 and 5% TRC were observed to have significant formations of calcium carbonate (C-C) and calcium silicate hydrates (C-S-H). This was due to the carbonation and post-carbonation hydration. For the samples that were not subjected to post-carbonation hydration, it was observed that there were more formations of C-C than C-S-H. This was mainly because the samples were only exposed to carbonation. Furthermore, the micrograph of the WC samples showed a conspicuous development of ettrinite (AFt) needles in the form of needle-like crystals. These needles indicated that the post-carbonation hydration caused the samples to move into a crystalline phase, which was part of the hydration process required to replace the water lost during the preconditioning and the carbonation stages. The ettrinite formation contributed to the additional strength recorded in the WC samples. For the NWC samples, AFt needles were not observed.  
Figure 10: (a) SEM of 0% TRC sample (WC). (b) SEM of 0% TRC sample (NWC).  
Additionally, honeycomb C-S-H was observed in both WC and NWC samples. Figure 12 shows enlarged images of the honeycomb formation. [PERSON] et al. [71] reported that this type of C-S-H keeps a low density, owing to its elevated porosity. This is mainly because OPC was completely replaced LKD-GBFS, which disrupted the C-S-H from maturing.  
The elemental analyses (EDS) of the samples (WC and NWC) are shown in Figure 13a,b and Figure 14a,b, respectively. The EDS revealed that the CC formed due to carbonation in the NWC samples was covered by honeycomb C-S-H, as evidenced by the high silicate (Si) content, in the carbonated samples. Furthermore, the WC samples had elevated the calcium (Ca) content due to the continuous hydration reactions when compared to the NWC samples. This was largely due to the different chemical processes that occurred during the carbonation and hydration processes. In addition, the post-carbonation hydration brought about an increased number of other elements that contributed to the higher strengths recorded for WC samples, as shown in Tables 4 and 5. Hence, the importance of post-carbonation hydration cannot be overemphasized and is recommended for samples exposed to ACC.  
Figure 11: (**a**) SEM of 5% TRC sample (WC). (**b**) SEM of 5% TRC sample (NWC).  
Figure 12: Honeycomb C-S-H (enlarged image).  
Figure 13: (a) EDS of 0% TRC sample (WC). (b) EDS of 0% TRC sample (NWC).  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{2}{c}{**WC Samples**} & \multicolumn{2}{c}{**NWC Samples**} \\  & **Wt \%** & **Wt \% Sigma** & **Wt \%** & **Wt \% Sigma** \\ \hline C & 13.83 & 0.16 & 19.66 & 0.17 \\ O & 45.17 & 0.14 & 50.45 & 0.13 \\ Na & 0.15 & 0.02 & 0.07 & 0.01 \\ Mg & 1.46 & 0.02 & 0.41 & 0.01 \\ Al & 3.78 & 0.03 & 1.00 & 0.01 \\ Si & 7.83 & 0.04 & 10.96 & 0.04 \\ P & 0.10 & 0.01 & 0.40 & 0.01 \\ S & 0.59 & 0.02 & 0.12 & 0.01 \\ K & 0.55 & 0.02 & 15.94 & 0.05 \\ Ca & 25.76 & 0.09 & 0.06 & 0.01 \\ Ti & 0.19 & 0.02 & 0.10 & 0.02 \\ Fe & 0.59 & 0.04 & 0.82 & 0.03 \\ Total & 100 & & 100 & \\ \hline \end{tabular}
\end{table}
Table 4: Elemental composition of 0% TRC (WC and NWC) samples.  
Figure 14: (**a**) EDS of 5% sample (WC). (**b**) EDS of 5% sample (NWC).