# Characteristics of Previous Concrete with Activated-Sludge Biological Fermentation
## 3 Results and Discussion
### BET Surface Area  
The efficacy of pervious concrete in treating pollutants is heavily influenced by its specific surface area. Our observations from the specific surface area test of the samples (see Table 7) indicate that the sample with 8% AS added had a specific surface area of 9.14 m\({}^{2}\)/g at 30 \({}^{\circ}\)C, which is 98.7% higher than that of the sample without AS added. This study examined the pore size distribution of the micropores (Figure 7) and found that the AS-added samples had a higher pore size distribution curve compared to the unadded samples overall. There was a notable increase in the number of samples with added sludge in the micropore size range below 10 nm, while the increase was less pronounced in the micropore size range above 30 nm. The results indicate that the addition of sludge to permeable concrete can result in an increase in the number of micropores, with the greatest increase observed in micropores below 10 nm. In contrast, the number of micropores above 30 nm exhibited a relatively low increase. This increase in the number of micropores led to an increase in the specific surface area of the concrete, which effectively enhanced its adsorption capacity. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample Number**} & \multicolumn{5}{c}{_w/\%_} & \multirow{2}{*}{\(R_{\text{WP}}\)**\%**} \\ \cline{2-2} \cline{5-8}  & **C-S-H** & & & & & & & \\ \hline SPC-0\# & 16.95\% & 36.63\% & 2.87\% & 2.10\% & 5.28\% & 26.17\% & 5.2782 & 2.3653 & 2.09 \\ SPC-1\# & 21.57\% & 32.53\% & 4.15\% & 3.21\% & 4.89\% & 23.65\% & 4.2632 & 2.6215 & 2.31 \\ SPC-2\# & 31.25\% & 27.60\% & 3.24\% & 3.16\% & 5.14\% & 19.62\% & 5.3698 & 3.7276 & 1.28 \\ SPC-3\# & 20.90\% & 35.26\% & 3.32\% & 2.20\% & 4.36\% & 23.94\% & 5.1875 & 2.9185 & 1.53 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Quantitative results of each physical phase obtained by Rietveld method. Figure 7: Pore size distribution curves of SPC at different curing temperatures. Table 7 illustrates that the addition of a similar proportion of activated sludge results in the sample cured at 30 \({}^{\circ}\)C exhibiting the largest specific surface area (9.14 m\({}^{2}\)/g). Notably, the specific surface area of the sample cured at 20 \({}^{\circ}\)C is not significantly different, while the sample cured at 40 \({}^{\circ}\)C exhibits the smallest specific surface area (7.54 m\({}^{2}\)/g). Figure 7 illustrates that the number of micropores smaller than 10 nm in the SPC-3# sample is significantly less than in SPC-1# and SPC-2#, while the number of micropores in the 30-100 nm range is slightly higher than in the latter two. The SPC-2# sample exhibits a greater number of micropores in the 3-7 nm range relative to SPC-1#, while exhibiting a smaller number of micropores in the 7-30 nm range. The variations in other micropore sizes are approximately equivalent. These observations indicate that the samples cured at 20 \({}^{\circ}\)C and 30 \({}^{\circ}\)C have a higher quantity of small-diameter pores, resulting in a larger specific surface area. In addition, the 40 \({}^{\circ}\)C sample has fewer small-diameter pores and relatively more large-diameter pores, leading to a lower specific surface area. This finding also corroborates the loose structure observed in the electron microscope images in Section 3.1, which explains the limited improvement in compressive strength over time for samples cured at 40 \({}^{\circ}\)C. As the strength of pervious concrete cured at 20 \({}^{\circ}\)C is low and does not meet construction standards, it is recommended that 30 \({}^{\circ}\)C be considered the optimal curing temperature for achieving the maximum specific surface area. Adding 8% activated sludge and curing at 30 \({}^{\circ}\)C significantly increased the specific surface area of permeable concrete, especially in the micropore range below 10 nm, and enhanced its adsorption capacity.