# Study on Compressive Strength and Water Permeability of Steel Slag-Fly Ash Mixed Permeable Brick
## 3 Results and Discussion
### The Influence of Different Variables on Permeability  
In the analysis of the water permeability of the molded permeable brick, only three variables (steel slag content, fly ash content, and porosity) were selected. A suitable mix design scheme is obtained by variable analysis. The test results are organized as shown below. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Target Porosity (\%)** & **10** & **12** & **14** & **16** & **18** & **20** & **22** & **25** \\ \hline
**Observed Porosity (\%)** & 11.4 & 12.8 & 14.2 & 16.3 & 17 & 19.4 & 21.3 & 23.2 \\
**Compressive Strength (MPa)** & 28.6 & 27.5 & 26.4 & 25.8 & 23.5 & 20.3 & 18.5 & 17.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The 28 days curing compressive strength at different permeable brick porosity. Figure 4: The compressive strength at different fly ash contents. Figure 5: The 28 days curing compressive strength of different observed porosity. As seen from Figure 6, the permeability coefficient of permeable brick fluctuates between 1.08 cm/s and 1.16 cm/s when the proportion of steel slag content changes, and there is no obvious alteration. This is because the steel slag has a small particle size and is wrapped by fly ash. At the end of the molding, a reasonable space is still left so that the water can still be infiltrated through the pores [5]. As seen from Figure 7, when the fly ash content rises from 30% to 55%, the permeability coefficient of the permeable brick increases gradually from 1.12 cm/s to 1.21 cm/s. This is because the fly ash particles are mixed with other materials during the slurry forming process to form colloidal particles which have a smaller particle size. No further polymerization occurs between such colloids. When the content of fly ash increases, the number of colloidal particles grows and the space between the particles expands, so the water permeability coefficient also increases [3]. The purpose of this study was to use fly ash and steel slag to make permeable bricks. The key objective was to improve the proportion of fly ash and steel slag in permeable brick as much as possible. When the content of fly ash is 30%, the compressive strength of permeable brick can meet the requirement to use this material. As can be seen from Table 8 and Figure 8, when the porosity is constant, the permeability coefficient increases with the rise of water level difference. When the water level difference is constant,  
Figure 6: Permeability coefficient corresponding to different steel slag content. Figure 7: Permeability coefficient corresponding to different fly ash content. the permeability coefficient increases with the higher porosity. When porosity is less than 16%, the difference of permeability coefficient under different water level difference is very small, because when porosity is low, there are fewer effective pore channels in the material, which cannot reflect the actual effect of water head difference on permeability coefficient [9]. When the porosity is more than 16%, the influence of water level difference on the permeability coefficient becomes obvious, indicating that the permeable brick has good permeability. When the porosity is more than 25%, the permeability coefficient changes little with the increase of porosity, indicating that the internal pores of the permeable brick are fully utilized, and the permeable brick has the best permeability. Therefore, porosity had the greatest influence on the permeability coefficient of permeable bricks. When porosity increased from 12% to 25%, the permeability coefficient increased from 0.4 cm/s to 1.2 cm/s. In the test, the data when the water level difference of 6 cm was selected for polynomial fitting. The fitting results are as follows:  
\[P=0.039+0.011x+0.001x^{2} \tag{6}\]  
where \(P\) denotes the permeability coefficient, \(x\) denotes the porosity. The coefficient of determination R\({}^{2}\) = 0.98. The formed permeable bricks prepared by the test basically meet the requirements of GB/T 25993-2010 and the ASTM C902-15.