# MSWI Bottom Ash Application to Resist Sulfate Attack on Concrete
## 3 Results and Discussion  
### Strength Properties of Mortar and Concrete  
The flexural and compressive strengths of the mortar samples with various contents of BA at 28 days of curing are shown in Table 6. Both the compressive and flexural strengths of the mortar decreased with increasing BA content, which is similar to the results of previous research [30; 31; 32]. The compressive strength of mortar showed a large decrease at the addition of BA from 10% to 20%. There was not a significant decrease in compressive strength at the addition of BA from 20% to 25%. Beyond 25%, the compressive strength decreased significantly again. Similar to the compressive strength, with increasing BA, the flexural strength also showed a rapid reduction first and then a gentle reduction, followed by another significant reduction.  
Low hydration activity of BA was obtained, and the activity index was only 43% with the addition of 30% BA. The hydration activity of BA was apparently smaller in comparison with the records in the literature [33]. In the literature, a low W/C value of 0.38 was used. However, a similar industrial mineral admixture, electric arc furnace dust, has high hydration activity with W/C values from 0.35 to 0.7 [34]. Hence, the W/C of 0.5 used in this study is not the main cause of the low hydration activity. The samples in previous research were prepared by melting the MSWI fly ash at a high temperature and then water-quenching [33]. In this study, the BA was prepared by artificially removing the impurities. In addition, the particle size of the BA sample was controlled under 180 \(\upmu\)m, making it hard to densify the microscopic structure of the mortar. In addition, our BA sample had a higher content of SiO\({}_{2}\) and a lower amount of CaO in comparison with the finer samples [35], and CaO may participate in the cement hydration process.  
Figure 7 shows the compressive strength values of the concrete samples with different contents of BA at 28 days of curing. The compressive strength of the concrete decreased gradually with increasing addition of BA. In addition, the water/cement ratio (W/C) had a significant influence on the compressive strength: the larger the W/C, the smaller the compressive strength. The concrete samples with the addition of 10%, 15%, and 20% BA at a W/C of 0.35 met the strength requirement of C30, at 37 MPa, 34 MPa, and 32 MPa, respectively. When the W/C was 0.40, only the concrete samples with the addition of 10% and 15% BA met the strength requirement, at 35 MPa and 31 MPa, respectively. Unfortunately, the compressive strengths of all concrete specimens at a W/C of 0.35 were less than 30 MPa. This shows that the addition of BA could not improve the strength performance of the concrete due to its relatively low hydration activity.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**Flexural Strength (MPa)**} & \multicolumn{2}{c}{**Compressive Strength (MPa)**} & \multirow{2}{*}{**Activity Index (\%)**} \\ \cline{2-2} \cline{4-5}  & \multirow{2}{*}{**Average Value**} & \multicolumn{1}{c}{**Standard**} & & \multirow{2}{*}{**Average Value**} & \multicolumn{1}{c}{**Standard**} \\  & & & & & **Deviation** \\ \hline Without BA & 9.0 & 0.8 & 57.1 & 2.9 & — \\ C90 BA10 & 5.2 & 0.3 & 34.6 & 2.6 & 61 \\ C85 BA15 & 5.0 & 0.2 & 32.0 & 2.7 & 56 \\ C80 BA20 & 5.0 & 0.5 & 27.6 & 2.2 & 48 \\ C75 BA25 & 4.8 & 0.4 & 26.8 & 2.5 & 47 \\ C70 BA30 & 4.5 & 0.3 & 24.4 & 2.1 & 43 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Flexural strength and compressive strength of mortar samples with various contents of BA.