## 3 Results
### CaCO\({}_{3}\) Content and Carbonation Degree  
The thermal decomposition curves of all HVFA mortar samples did not show a distinct mass loss in the 400-500 \({}^{\circ}\)C temperature range. This indicates that there was no Ca(OH)\({}_{2}\) present in those samples. Clearly, the remaining fraction of Ca(OH)\({}_{2}\) that was not consumed in the pozzolanic hydration reaction of the fly ash, carbonated fully for all three exposure conditions under investigation (natural vs. slightly accelerated vs. highly accelerated carbonation). Contrarily, all these curves were characterized by a very pronounced continuous mass loss within the temperature range for CaCO\({}_{3}\) in all its common mineralogical phases (550-680 \({}^{\circ}\)C: the amorphous phase; 680-780 \({}^{\circ}\)C: the metastable phase, i.e., vaterite and aragonite; 780-990 \({}^{\circ}\)C, the stable phase, i.e., well-crystallized calcite [32]). Figure 1a shows the CaCO\({}_{3}\) content of the HVFA mortar (in % m/m relative to the binder) after 17 weeks of carbonation at 0.04%, 1% and 10% CO\({}_{2}\). A most striking observation for these results was the significant increase in CaCO\({}_{3}\) content with the applied CO\({}_{2}\) concentration during carbonation. The corresponding carbonation degrees a\({}_{\text{c}}\) (%), that directly depend on this CaCO\({}_{3}\) content, evolved in a very similar way (Figure 1b). The fact that samples carbonated at 10% and 1% CO\({}_{2}\) reached the state of full carbonation already much longer than the naturally carbonated samples after 17 weeks, could perhaps have played a role. Nonetheless, TGA analysis performed on the samples subject to accelerated carbonation after already 4 weeks of exposure could not confirm this. CaCO\({}_{3}\) contents and carbonation degrees for carbonation at 10% CO\({}_{2}\) equaled 41.8% m/m and 71%. For the 1% CO\({}_{2}\) exposure condition, these values amounted to 39.7% m/m and 67%. These values are very similar to what was recorded after 17 weeks of exposure. Carbonating the samples longer than needed to reach full carbonation (based on the colorimetric assessment method) can thus not be identified as a key influencing factor. A significant increase in total CaCO\({}_{3}\) content and carbonation degree a\({}_{\text{c}}\) with an increasing CO\({}_{2}\) concentration could also be confirmed for the mortar consisting of the commercially available fly ash based composite cement CEM II/B-V 42.5 N. Exposing the mortar for 44 weeks to 1% CO\({}_{2}\) instead of 0.04% CO\({}_{2}\) resulted in a rise in carbonation induced CaCO\({}_{3}\) content from 57.7% m/m to 63.9% m/m (Figure 1c). Consequently, the carbonation degree a\({}_{\text{c}}\) rose from 66 to 74% m/m (Figure 1d). The total exposure period for this mortar mix was more than double the exposure period adopted for the HVFA mix. This makes it difficult to make direct comparisons between the two. Nonetheless, the obviously higher CaCO\({}_{3}\) contents for the CEM II/B-V 42.5 N mix are most likely related to their much higher carbonatable material content (see Section 2.5: 49.28% CaO vs. 33.02% CaO). For this mix it could be observed that the carbonation degrees calculated were exceeding