# Characteristics of Previous Concrete with Activated-Sludge Biological Fermentation
## 3 Results and Discussion  
### Compressive Strength  
In general, the strength of pervious concrete is low due to the lack of fine aggregates. This study was no exception. The compressive strengths of SPC samples at different curing temperatures are shown in Figure 3. Surprisingly, the compressive strength of concrete without AS was 13 MPa after 28 days of curing, while the strength of concrete with AS was 20.8 MPa under the same conditions. The use of 8% AS as cement at a holding temperatureof 30 \({}^{\circ}\)C increased the compressive strength to some extent, presumably due to the volcanic ash reaction of the activated SiO\({}_{2}\) and alumina in the AS with the calcium oxide in the aggregate [24] (see Table 1). The internal view of the sample was generated using SEM (Figure 4). Figure 4a-c indicate that the inclusion of AS leads to an increase in the amount of fluffy hydration product C-S-H gel and a decrease in the lamellar Ca(OH)\({}_{2}\), as compared to the sample without AS. Because the addition of AS increased the rate of volcanic ash reaction and the rate of Ca(OH)\({}_{2}\) consumption [25], it generated hydration products and reduced calcium hydroxide, ultimately increasing the strength of the samples. It was observed that the curing temperature had an impact on the rate of hydration reaction of concrete. Upon analyzing Figure 3, the sample strength reached a maximum of 22.3 MPa at 40 \({}^{\circ}\)C and a minimum of 15.2 MPa at 20 \({}^{\circ}\)C. It is important to note that the strength increased by 36.8% when the curing temperature was raised from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C. However, the growth rate was only 7.2% when the temperature was increased from 30 \({}^{\circ}\)C to 40 \({}^{\circ}\)C. Similar results were observed by other researchers [26]. Additionally, the growth rates of the samples cured at different temperatures were 167%, 151%, and 41% for curing times ranging from 7 to 28 days. These results suggest that once the curing temperature reaches 30 \({}^{\circ}\)C, further increases in temperature have no significant effect on the strength of the concrete. To investigate the underlying reasons, we conducted a further analysis of the micromorphology diagrams of the samples with varying conditioning temperatures, as shown in Figure 4. Figure 4b shows needle-like AFt and Ca(OH)\({}_{2}\) in a lamellar structure, with a relatively low distribution of fluffy C-S-H gel in the structure. In Figure 4c, the proportion of C-S-H gel has significantly increased, indicating sufficient hydration reaction. In Figure 4d, the needle-bundle-like AFt is crisscrossed with lamellar Ca(OH)\({}_{2}\), and the uneven distribution of gel results in a loosely structured sample. The results indicate that a curing temperature of 30 \({}^{\circ}\)C is optimal for achieving a better degree of hydration reaction and increased strength. However, at 40 \({}^{\circ}\)C, the sample experiences a rapid early hydration reaction rate, resulting in high early compressive strength. However, a brief period for generating the distribution of hydration products results in non-uniformity, leading to a porous structure that hinders the subsequent development of compressive strength [27]. Considering the cost, it is recommended to cure at a temperature of 30 \({}^{\circ}\)C.  
The interfacial transition zone (ITZ) between the aggregate and cement is one of the most important components of concrete and affects the mechanical properties of concrete. ITZ often has the characteristics of large CH crystal growth and development. Compared with cement paste matrix, ITZ has high porosity, large pore size, good connectivity, and low mechanical strength. Therefore, cracks are more likely to occur and expand in the ITZ, ultimately affecting the strength, elastic modulus, crack resistance, shrinkage performance,  
Figure 3: 7 d and 28 d compressive strength and splitting tensile strength of SPC at different curing temperatures. transmission performance, and other engineering properties of concrete.