# Gypsum-Dependent Effect of NaCl on Strength Enhancement of CaO-Activated Slag Binders
## 2 Materials and Methods  
A commercial GGBFS with no gypsum was purchased in the Republic of Korea. The chemical composition of the GGBFS is presented in Table 1, and was obtained using X-ray florescence (XRF). The particle size distributions of the GGBFS were measured using a laser diffraction particle size analyzer (HELOS (HI199), and RODOS, Sympatec, Clausthal-Zellerfeld, Germany), as shown in Figure 1.  
The mixture proportions of paste samples are shown in Table 2. The following chemicals used in this study were of analytical grades: CaO (Daejung Chemicals, Siheung, Korea), NaCl (Duksan Chemicals, Ansan, Korea), and gypsum (CaSO\({}_{4}\)-2H\({}_{2}\)O, Sigma Aldrich, St. Louis, MO, US). CaO was used as the main activator for the GGBFS to achieve a binding property, while NaCl and gypsum were \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Formula** & **CaO** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & MgO & **SO\({}_{3}\)** & **TiO\({}_{2}\)** & **K\({}_{2}\)O** & **Fe\({}_{2}\)O\({}_{3}\)** & **Na\({}_{2}\)O** & **MnO** \\ \hline
**Oxide content** & 44.3 & 34.3 & 14.3 & 3.3 & 1.4 & 0.7 & 0.6 & 0.5 & 0.3 & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of ground-granulated blast furnace slag (GGBFS) (wt %). Figure 1: Particle size distribution of ground-granulated blast furnace slag (GGBFS). incorporated as replacements for the GGBFS. The water-to-binder weight ratio (wt %/wt %) was set at 0.35 for all mixture samples. The solubilities of gypsum and Ca(OH)\({}_{2}\), which is a hydrated form of CaO, are 15.13 and 19.76 mmol/kg\(\cdot\)H\({}_{2}\)O, respectively, in pure water at 25 \({}^{\circ}\)C; when 1 g of NaCl is present in 35 g of water at 25 \({}^{\circ}\)C, the solubilities of gypsum and Ca(OH)\({}_{2}\) increase to 15.13-39.44 and 27.18-29.11 mmol/kg\(\cdot\)H\({}_{2}\)O, respectively [24; 25]. Thus, as in this study, the addition of NaCl is likely to make gypsum and CaO more soluble. All binder components (GGBFS, CaO, NaCl, and gypsum) were dry-mixed and then mixed further with water. For compressive strength testing, ASTM C 109 [26] requires the use of two or three identical samples; however, in this study, six identical paste samples were prepared for each proportion of the mixture using 5 \(\times\) 5 \(\times\) 5 cm cubic molds to ensure the reliability of the strength testing results. For MIP testing, the pastes were cast in cylindrical samples (\(\phi\) 2.54 \(\times\) 2.54 cm). All cast paste samples were cured in a humidity chamber at 23 \({}^{\circ}\)C with a relative humidity above 99% for all curing periods. For the TG and XRD tests, fractured samples were collected after compressive strength tests and finely ground. The MIP test may cause significant deviations from actual pore structures due to microstructural damage by non-wetting liquid (i.e., mercury) [27], and/or the ink-bottle effect [28]. However, despite the drawbacks, the MIP test is still widely accepted for evaluating microstructural changes of hardened cementitious pastes due to compositional changes, similar to those in this study, when samples are prepared with proper sizes and drying conditions are satisfied [29; 30; 31]. For the MIP test, cylindrical samples were cut into cubic samples with dimensions of 5 \(\times\) 5 \(\times\) 5 mm. All prepared samples were immersed in isopropanol for 24 h to prevent any further reaction and then dried out in a vacuum desiccator before instrumental testing. To consider the effect of (1) NaCl content, (2) gypsum content, and (3) gypsum-NaCl interaction on strength, \(p\)-values were calculated through the F-test of the analysis of variance (ANOVA) [32]. Note that the \(p\)-value is a significance level of the null hypothesis. First, it is assumed that a chosen factor is not influential. When a calculated \(p\)-value for the chosen factor is lower than 0.05, the null hypothesis is considered false at a 95% confidence level.