# Gypsum-Dependent Effect of NaCl on Strength Enhancement of CaO-Activated Slag Binders
## 3 Results and Discussion
### Powder X-Ray Diffraction (XRD)  
The XRD patterns of hardened samples are presented in Figure 3, along with the reference patterns of identified phases. Table 4 shows the list of reaction products generated in each sample.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample Label**} & \multicolumn{5}{c}{**Reaction Products**} \\ \cline{2-5}  & **C-S-H** & **Hydrotalcite** & **Ca(OH)2** & **Ettringite** & **Hydrocalumite** \\ \hline Ca-N0-G0 (Control) & O & O & O & X & X \\ Ca-N1-G0 & O & X & O & X & O \\ Ca-N0-G5 & O & X & O & O & X \\ Ca-N1-G5 & O & X & O & O & O \\ \hline \hline \end{tabular}
\end{table}
Table 4: Identified reaction products of hardened paste samples (O: Formation, X: No formation).  
Figure 3: X-ray diffraction (XRD) patterns of hardened pastes at (a) 7 and (b) 28 days. Note that the reference XRD pattern for calcium silicate hydrate (C-S-H) was taken from a 22-year-old \(\beta\)-C\({}_{2}\)S paste from an earlier study after removing the peaks of Ca(OH)2 [35]. The numbers in parentheses indicate the International Center for Diffraction Data (ICDD) and Inorganic Crystal Structure Database (ICSD) reference numbers for the identified phases.  
All hardened samples produced calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)\({}_{2}\)).  
In the sample that did not have NaCl or gypsum (Ca-N0-G0), weak peaks of hydrotalcite (Mg\({}_{6}\)Al\({}_{2}\)CO\({}_{3}\)(OH)\({}_{16}\)\(\cdot\)4H\({}_{2}\)O) were identified. Hydrotalcite has often been found in alkali- or CaO-activated GGBFS pastes and in blends of Portland cement with GGBFS [2; 6; 12; 13; 36]. Hydrotalcite belongs to layered double hydroxides (LDHs), which consist of a family of minerals with the general formula of \(\rm[M^{II}1_{-x}M^{III}_{x}(OH)_{2}]^{x+}[A^{n-}{}_{x/n}]^{x-}\cdot yH_{2}O\), where \(\rm A^{n-}=n\)-valent anion, \(\rm M^{II}=\) divalent cations, and \(\rm M^{III}=\) trivalent cations, and they are closely related to brucite (Mg(OH)\({}_{2}\)) in their basic structure.  
When NaCl and/or gypsum were incorporated in this study, hydrotalcite was mostly removed, and the use of NaCl (Ca-N1-G0) yielded hydrocalumite ((Ca\({}_{2}\)Al(OH)\({}_{6}\)(Cl\({}_{1-x}\)(OH)\({}_{x}\))\(\cdot\)3(H\({}_{2}\)O), \(x=0\)-1, Al\({}_{2}\)O\({}_{3}\)-Fe\({}_{2}\)O\({}_{3}\)-mono (AFm) [7], phase)), while the incorporation of gypsum (Ca-N0-G5) produced ettrinite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\)\(\cdot\)26H\({}_{2}\)O), which is known as a strength enhancing material by filling pores [12; 37]. These reaction products have often been observed in earlier studies of construction binders containing chloride [13; 38; 39; 40] or sulfate sources [12; 41; 42].  
The use of CaCl\({}_{2}\) has been reported to produce hydrocalumite and result in strength improvement of fly ash and lime-slag blend binders due to pore size refinement [3; 40; 43]. Although the use of NaCl in this study also generated hydrocalumite, it was not effective in increasing strength when added without gypsum.  
When NaCl and gypsum were present together (Ca-N1-G5), hydrocalumite and ettrinite were simultaneously formed. However, no other types of new reaction products were identified, contrary to the results of cases with only NaCl or gypsum added. Previous studies have reported that hydrotalcite has a significant capacity for chloride binding [36; 44; 45; 46] and ion-exchange for SO\({}_{4}\)\({}^{2-}\)[47], indicating that the removal of hydrotalcite was likely due to the presence of NaCl and/or gypsum in the corresponding samples.