# Study on Compressive Strength and Water Permeability of Steel Slag-Fly Ash Mixed Permeable Brick
## 3 Results and Discussion  
### Effects of Different Variables on Compressive Strength  
The compressive strength tests were carried out for different variables. The test results are summarized in Table 4.  
Figure 1: The water permeability coefficient test device (**1**—water supply system, **2**—overflow port, **3**—overflow water tank (the water tank with the drain port and maintaining a certain water level), **4**—bracket, **5**—permeable brick sample, **6**—measuring cylinder, **7**—water level difference, **8**—water cylinder (a cylinder with an overflow and maintaining a certain water level)). As shown in Figure 2, when the water-cement ratio is less than 20%, with the increase of water-cement ratio, the compressive strength of the permeable bricks increases, but the range is considerably small. When the water-cement ratio is about 28%, the compressive strength after seven days (28 days of curing) reaches the maximum, which is 27.54 MPa (27.23 MPa). When the water-cement ratio is larger than 28%, the compressive strength decreases significantly with the increase of the water-cement ratio. This is because when the water-cement ratio is less than 20%, the slurry prepared for water-permeable brick is insufficiently hydrated, its fluidity is weak, the workability of concrete is poor, and concrete vibration and compaction are very difficult. The coating layer and bonding layer on the surface of aggregate particles are not evenly distributed. Therefore, the mechanical performance is relatively weak. When the water-cement ratio is about 28%, the slurry is fully hydrated, the structural layer is evenly distributed, the overall structural stability is reliable, and the effective void ratio of the formed brick is less, so the compressive strength exhibits a maximum value. When the water-cement ratio continues to increase, the cementitious material located at the upper part flows to the lower part of the structural layer under the action of gravity, resulting in blockage of the concrete bottom and the insufficient thickness of the upper surface slurry, which reduces the compressive strength of the permeable brick [4]. As seen from Table 5 and Figure 3, when the proportion of steel slag is less than 6%, the compressive strength of permeable brick will increase with the increase of steel slag proportion. When the content of steel slag continues to increase, the compressive strength increases within a small range. This is because part of the steel slag can fill the voids during the preparation of the brick, increasing the contact point between the slurry particles, thereby causing the compressive strength increase within a certain range. When the internal voids of the slurry are fully filled, the compressive strength reaches the maximum value and it no longer increases [12]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Water-Cement** & **Porosity** & **Compressive Strength** & **Compressive Strength under** \\  & **Ratio (\%)** & **(\%)** & **under 7 Days Curing (MPa)** & **28 Days Curing (MPa)** \\ \hline
1 & 15 & 22.3 & 17.69 & 21.06 \\
2 & 20 & 22.1 & 18.68 & 21.35 \\
3 & 22 & 22.0 & 23.57 & 23.68 \\
4 & 25 & 21.8 & 22.86 & 24.63 \\
5 & 28 & 21.6 & 27.54 & 27.23 \\
6 & 32 & 20.8 & 25.63 & 26.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The compressive strength of permeable bricks at different water cement ratios. Figure 2: Compressive strength under different water-cement ratio conditions. In order to make the permeable brick having a certain compressive strength, in the subsequent tests, the proportion of the steel slag was adjusted to 6.5%. It can be seen from Table 6 and Figure 4 that the fly ash content has a great influence on the compressive strength of the permeable brick. With the increase of the fly ash content, the compressive strength of the permeable brick shows a downward trend. When the fly ash content is 30%, the compressive strength of the 28 days curing permeable brick reaches a maximum value of 26 MPa. This is because of the poor plasticity of fly ash.