# Binary Mixes of Self-Compacting Concrete with Municipal Solid Waste Incinerator Bottom Ash
## 3 Results and Discussion
### Mechanical Performance  
#### 3.4.1 Compressive Strength  
Table 5 shows the compressive strength development of cubic (f\({}_{\text{cm,cube}}\)) and cylindrical (f\({}_{\text{cm}}\)) specimens. As expected, strength increased over time. However, it decreased with increasing replacement levels of cement with MIBA, as shown in Figure 3. There are several combining factors that led to this decline. Since cement and FA are being directly replaced with a mineral addition with little to no binding ability upon hydration, a strong strength reduction was expected with increasing MIBA contents [10; 40]. Moreover, in spite of the pozzolanicity observed in MIBA, it was clearly inferior to that of FA and showed higher content of crystalline phases in the XRD, thereby resulting in slower strength development over time. As mentioned, the water content of MIBA-containing mixes was slightly higher than that of the RC in order to achieve comparable self-compacting behaviour. Naturally, the increase in the water to binder ratio led to a greater porosity of the cementitious microstructure, thus leading to lower strength.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mixes**} & \multicolumn{2}{c}{**Slump-Flow**} & \multicolumn{2}{c}{**V-Funnel**} & \multicolumn{1}{c}{**L Box**} \\ \cline{2-5}  & **t\({}_{\text{500}}\) (s)** & **SF (mm)** & **Tv (s)** & **PL (H\({}_{2}\)/H\({}_{1}\))** \\ \hline RC & 2.7 & 680 & 5.6 & 0.9 \\ C20 BA & 2.7 & 600 & 3.6 & 0.8 \\ C30 BA & 2.9 & 645 & 2.7 & 0.9 \\ C40 BA & 3.4 & 560 & 2.5 & 1.0 \\ C50 BA & 3.7 & 550 & 2.2 & 0.8 \\ \hline \hline \end{tabular} Note: t\({}_{\text{500}}\) = flow time; SF = slump flow diameter; Tv = time that the sample takes to fully flow through the V-funnel; PL = passing ability index in the L-box.  
\end{table}
Table 4: Fresh concrete properties, slump-flow, V funnel and L box.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sive Size (mm)**} & \multicolumn{4}{c}{**Cumulative Pass Amount (\%)**} \\ \cline{2-5}  & \multicolumn{2}{c}{**Fine Aggregate**} & \multicolumn{2}{c}{**Coarse Aggregate**} \\ \cline{2-5}  & **Sand 0/2** & **Sand 0/4** & **Gravel 1** & **Gravel 2** \\ \hline
22.4 & 100.0 & 100.0 & 100.0 & 100.0 \\
16 & 100.0 & 100.0 & 100.0 & 83.4 \\
11.2 & 100.0 & 100.0 & 80.1 & 30.0 \\
8 & 100.0 & 100.0 & 39.2 & 4.4 \\
5.6 & 100.0 & 99.5 & 5.7 & 2.6 \\
4 & 100.0 & 98.2 & 1.2 & 1.6 \\
2 & 99.9 & 89.1 & 0.7 & 1.3 \\
1 & 99.4 & 59.4 & 0.5 & 0.9 \\
0.5 & 80.7 & 21.5 & 0.5 & 0.5 \\
0.25 & 16.1 & 3.5 & 0.4 & 0.3 \\
0.125 & 0.3 & 0.3 & 0.2 & 0.1 \\
0.063 & 0.0 & 0.0 & 0.0 & 0.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Particle size distribution of natural aggregates.  
Another factor that contributed negatively to the specimens' strength development is the presence of metallic aluminium (Al), the reaction of which with OH\({}^{-}\) ions in the mixes' solution generated hydrogen (H\({}_{2}\)) in the fresh state, resulting in entrapped gas pockets. This led to an increase in porosity of the MIBA-containing SCC in the hardened state, being one of the main factors of strength decline [41; 42; 43]. This inverse proportionality, between MIBA content and compressive strength, had been reported in the study of [PERSON] et al. [14], wherein ternary mixes of SCC were produced with MIBA and FA. The main results showed that the highest percentage of MIBA caused an average compressive strength loss of 74% when compared to the RC. A factor that contributed considerably to this loss was the expansion of specimens from the oxidization of Al particles. [PERSON] et al. [44], when studying the behaviour of concrete with MIBA, also found that the presence of Al led to reduced compressive strength.  
Figure 3 also presents the relationship between the average compressive strength in cylindrical and cubic specimens. Little dispersion is observed, and the linear regression presents high correlation coefficients (R\({}^{2}\) = 0.97 after 28 days and R\({}^{2}\) = 0.98 after 91 days).