# Gypsum-Dependent Effect of NaCl on Strength Enhancement of CaO-Activated Slag Binders
## 3 Results and Discussion
### Thermogravimetric (TG) Analysis  
The TG plots and their differential thermogravimetry (DTG) curves are shown with the identified decomposed phases in Figure 4. In this study, the results of XRD and TG tests showed complexity and inconsistency at all days as follows: (1) The XRD peaks of hydrocalumite in Ca-N1-G0 were much greater than those in Ca-N1-G5, while their DTG peaks were very similar at around 250-400 \({}^{\circ}\)C; (2) the XRD peaks of ettrinite exhibited similar intensities in Ca-N0-G5 and Ca-N1-G5, while the DTG peak of ettrinite near 80 \({}^{\circ}\)C in Ca-N0-G5 was smaller than that of Ca-N1-G5. These discrepancies were confirmed, as the XRD and TG were measured twice for two identical samples of each mixture at all days. In addition, when comparing the results of the DTG of TG plots between 7 and 28 days, the weight losses at 7 days were generally higher than those at 28 days. This is abnormal, because as the curing process progresses more weight accumulates for reaction products, which are generally thermally decomposable. However, as stated earlier, the TG/DTG was conducted for duplicate samples of each mixture at each day of curing, and their results showed consistency between duplicate samples, indicating that this discrepancy in the TG/DTG data between 7 and 28 days was more likely due to a change in the sample preparation process between these days that went unnoticed. For instance, ettrinite is highly sensitive to any change in drying conditions during preparation [48; 49; 50]. Nonetheless, as the detecting resolution of XRD appeared relatively lower than TG in this study, the analysis of TG/DTG was mainly used to examine reaction product quantities for each day of curing and the results showed adequate consistency between the samples with the same curing days. The results of phase identification in DTG were fairly consistent with the results of XRD at each day of curing. Dehydration of C-S-H [51; 52] and ettrinite [7; 53; 52] were identified at 0-200 \({}^{\circ}\)C in DTG. [PERSON] et al. [53] reported that ettrinite was completely removed at 60 \({}^{\circ}\)C and the dehydration of C-S-H started from 100 \({}^{\circ}\)C in the Ca(OH)\({}_{2}\)-activated GGBFS system, which is a similar binder system to that of this study. Thus, although the decomposition temperature ranges of ettrinigite and C-S-H were closely spaced, they are sufficiently distinguishable for reliable interpretation. The decomposition of hydrocalumite was identified in the following three temperature ranges: 25-250 \({}^{\circ}\)C, 250-400 \({}^{\circ}\)C, and around 800 \({}^{\circ}\)C [40,54]. Dehydration of Ca(OH)\({}_{2}\)[53,55], was identified at 400-450 \({}^{\circ}\)C.  
In the samples with NaCl and/or gypsum, several weight losses were observed, overlapped with the weight loss of hydrocalumite near 800 \({}^{\circ}\)C, in the range of 600-900 \({}^{\circ}\)C. Given that no other  
Figure 4: Thermogravimetry (TG) and differential thermogravimetry (DTG) results of hardened samples at (**a**) 7 and (**b**) 28 days. crystalline phase besides ettringite or hydrocalumite was identified in the XRD analysis of these samples, these additional weight losses were likely related to unknown types of amorphous phases. In the 30-140 \({}^{\circ}\)C range in the TG test, when NaCl was used with gypsum, the use of NaCl (Ca-N1-G5) observably increased the weight loss of ettringite when compared to that of the sample with only gypsum (Ca-N0-G5) at all curing days. In general, ettringite mostly forms and contributes to strength within three days of curing in cementitious binders with sulfate sources [8; 12; 37; 56]. The presence of NaCl also increases the solubility of gypsum [24]. Thus, Figure 2 shows that because the most strength improvement using NaCl with gypsum was obtained before 7 days, the use of NaCl likely boosted the formation of ettringite by notably increasing the concentrations of calcium and sulfate ions in the early days (i.e., before three days), which resulted in improving early strengths.