## 3 Results and Discussion
### Effect of Different Factors on Properties of Fired Bricks
#### 3.3.1 Glass Powder Content  
The addition of glass powder can promote vitrification and, thus, improve the performance of sintered bricks [25]. Figure 6a indicates the influence of glass powder content between 0 wt% and 10 wt% on the compressive strength and water absorption of sintered brick. The compressive strength of sintered brick increases with the increasing addition of glass powder. When the content of glass powder is increased from 0 wt% to 10 wt%, the compressive strength is increased by 56.8%. An increase in compressive strength was due to the addition of glass powder, which promotes the vitrification of the sintered brick. At the same time, the glass phase can fill the internal pores of the sintered brick, enhance the connectivity between particles and improve the compressive strength of the sintered brick [24]. The water absorption of sintered bricks decreases with the increasing glass powder content. When the content of glass powder was 10 wt%, the water absorption decreased to 23.64%. At this stage, the water absorption is higher than 19% of the Chinese standard. This is because the water absorption of the sintered brick is related to the size of the pores. The sintered brick combined with glass powder will form more glass phase to seal the internal pores at high temperatures, reduce the porosity and lead to the decrease of the water absorption [32].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline CaO/\% & 0 & 0.5 & 1 & 1.5 & 2 & 2.5 & 3 \\ \hline pH & 3.3 & 3.7 & 3.9 & 4.1 & 4.6 & 6.6 & 7.8 \\ \hline \end{tabular}
\end{table}
Table 4: Effect of quicklime addition on pH value of polyaluminum chloride slag.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Element**} & \multirow{2}{*}{**pH**} & **Cl** & **Pb** & **Cr** & **Cd** & **Cu** & **Mn** & **Zn** \\  & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** & **(mg/L)** \\ \hline polyaluminum chloride slag & 3.5 & 1671.3 & \textless{}0.02 & 0.05 & \textless{}0.02 & 0.96 & 2.7 & 0.61 \\ GB8978-1996 & 6–9 & - & 1.0 & 1.5 & 0.1 & 1.0 & 5.0 & 5.0 \\ \hline \end{tabular}
\end{table}
Table 3: pH and mass concentration of pollutants in leaching solution of polyaluminum chloride residue.  
The linear shrinkage is related to the properties of the materials. Excessive linear shrinkage will lead to the deformation of the sintered bricks, so the linear shrinkage should be controlled. In general, the linear shrinkage of fired bricks is lower than 8% [33]. The influence of glass powder on the linear shrinkage and bulk density of sintered bricks is shown in Figure 6b. The linear shrinkage of sintered brick decreases with the increasing glass powder content. When the content of glass powder reaches 10 wt%, the linear shrinkage decreased to 4.44%. This is as a result of the fact that the shrinkage is related to the loss of water between the particles. The more the content of glass powder, the less the mass of the polyaluminum chloride slag will be. The less the loss of water during sintering, the lower the shrinkage. The bulk density of sintered bricks is in direct proportion to the content of glass powder. As the addition of glass powder increases from 0 wt% to 10 wt%, the bulk density of the fired brick increases from 1.56 g/cm\({}^{3}\) to 1.61 g/cm\({}^{3}\).  
Considering that the water absorption of sintered brick does not meet the Chinese standard, the glass powder content is determined as 10 wt% for further research.