# Hydration of Calcium Sulfoaluminate-Based Binder Incorporating Red Mud and Silica Fume
## 3 Results and Discussion  
### Compressive Strength Development  
The compressive strengths of the paste mixes are shown in Figure 2. The 1-day and 28-day strength values from Figure 2 are tabulated in Table 3. As can be seen in Figure 2, the compressive strength of all paste mixes generally increased with curing time. When the CSA replacement level was increased up to 30%, the compressive strength also increased for each group. However, when the CSA content was 40%, the compressive strength value tended to be similar to or slightly higher than the 30% content, and the content of 50% CSA showed lower strength than the 40% content.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \multirow{2}{*}{**Sample ID**} & **CSA** & **RM** & **SF** & \multicolumn{4}{c}{**Testing**} \\ \cline{3-10}  & & **(wt.\%)** & **(wt.\%)** & **(wt.\%)** & **C** & **D** & **X** & **M** & **S** \\ \hline \multirow{4}{*}{I} & C10R90S0 & 10 & 90 & 0 & \(\surd\) & \(\surd\) & \(\surd\) & & \\  & C20R80S0 & 20 & 80 & 0 & \(\surd\) & \(\surd\) & & & \\  & C30R70S0 & 30 & 70 & 0 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\  & C40R60S0 & 40 & 60 & 0 & \(\surd\) & \(\surd\) & & & \\  & C50R50S0 & 50 & 50 & 0 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\ \hline \multirow{4}{*}{II} & C10R85S5 & 10 & 85 & 5 & \(\surd\) & \(\surd\) & \(\surd\) & & \(\surd\) \\  & C20R75S5 & 20 & 75 & 5 & \(\surd\) & \(\surd\) & & & \\  & C30R65S5 & 30 & 65 & 5 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\  & C40R55S5 & 40 & 55 & 5 & \(\surd\) & \(\surd\) & & & \\  & C50R45S5 & 50 & 45 & 5 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) \\ \hline \multirow{4}{*}{III} & C10R80S10 & 10 & 80 & 10 & \(\surd\) & \(\surd\) & \(\surd\) & & \\  & C20R70S10 & 20 & 70 & 10 & \(\surd\) & \(\surd\) & & \\ \cline{1-1}  & C30R60S10 & 30 & 60 & 10 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\ \cline{1-1}  & C40R50S10 & 40 & 50 & 10 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\ \cline{1-1}  & C50R40S10 & 50 & 10 & 10 & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & \\ \hline \hline \end{tabular} Abbreviations. CSA: calcium sulfoaluminate expansive agent; RM: red mud; SF: silica fume; C: compressive strength;  
D: drying shrinkage; X: XRD; M: MIP; S: SEM; \(\surd\): test conducted.  
\end{table}
Table 2: Mixture proportions of samples.  
Figure 2: Compressive strength developments on paste samples with various mixes: (**a**) Group I (10–50% CSA + 90–50% RM + 0% SF), (**b**) Group II (10–50% CSA + 85–45% RM + 5% SF), and (**c**) Group III (10–50% CSA + 80–40% RM + 10% SF). The numbers above each column indicate compressive strength testing result in MPa.  
Earlier studies have reported that the incorporation of SF is effective in enhancing compressive strength due to its fine particles as a filler and its pozzolanic reactions [26; 27]. In this study, for the same CSA replacement level, the 10% CSA content samples (i.e., C10R90S0, C10R85S5, and C10R80S10) showed no significant strength change due to the addition of SF. The C10R90S0, C10R85S5, and C10R80S10 pastes developed 3.9, 4.0, and 4.2 MPa compressive strength at an early age, respectively. After 28 days of hydration, the strengths of the pastes were 6.2, 6.7, and 6.9 MPa, respectively (Table 3). This shows that the hydration reaction of mixtures was insufficient and that hydration products were not properly formed under the above mixture conditions. [PERSON] et al. [28] reported that when RM content is higher, the compressive strength of a sample could decrease. They stated that since RM particles can absorb a little water, the required amount of water for hydration reaction might be insufficient.  
In the case of other CSA replacement levels, the compressive strength was affected by SF addition. The sample with SF addition yields a lower or higher early-age strength but higher later-age strength than that without SF. With respect to the amount of SF addition, 5% addition gives better strength development than 10% addition.