## 3 Results and Discussion
### Main Performance Characteristics of Ceramsite  
The main performance indexes of ceramicsite before and after calcining are shown in Table 3.  
It can be seen from Table 3 that the overall porosity of the ceramicsite is small, which is related to the determination method. The vacuum water absorption meter is easy to measure the macropore, while it is not easy to obtain the micropore and mesopore. With the increase of calcining temperature, the porosity of the ceramicsite is increasing, the reason is that in the calcining process, the water and volatile substances in the billet gradually volatilize, which has a certain pore-forming effect. At 950 \({}^{\circ}\)C, the volatiles in the sludge will be decomposed, and the billet will show partial melting and shrinkage. Moreover, CaO and MgO produced by calcination will be dissolved in water resulting in a certain amount of pores. It can be seen from the BET data that the specific surface area of the ceramicsite increases at first and then decreases with the increase of calcining temperature. High temperature can greatly increase the specific surface area. [PERSON] et al. [22] roasted the waterworks sludge to make ceramicsite, which was used to purify sewage. Although the specific surface area of the ceramicsite is only 6.07 m\({}^{2}\)/g, the roasting make the specific surface area increase by 34.27%, and had a good removal rate on COD. The specific surface area increases with the increase of temperature for the same reason as the increase of porosity, which is mainly due to the pore formation of water and volatile components. While at 950 \({}^{\circ}\)C, the components in the ceramicsite show partial melting, although macropores are generated, but the fluid will also make the micropores and mesopores disappear. The unfired sludge has fine particles, complex composition, good cohesiveness, and high compressive strength. There are large amounts of organic matters in the unfired sludge, which can serve as the organic binder. So when mixed with the fine particles, it will \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Elements** & **Content** & **Elements** & **Content** \\ \hline Fe & 1612.8 \(\pm\) 8.7 & Ni & 79.3 \(\pm\) 1.7 \\ Al & 3143.5 \(\pm\) 10.2 & Cr & 187.6 \(\pm\) 0.9 \\ Zn & 937.7 \(\pm\) 5.8 & Cd & 21.7 \(\pm\) 0.5 \\ Cu & 134.3 \(\pm\) 2.3 & Pb & 52.2 \(\pm\) 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Contents of main elements in surplus sludge mg/kg dried basis. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Firing** & **Porosity (\%)** & **BET (m\({}^{2}\)/g)** & **Compressive** & **Point Zero** \\
**Temperature** (\({}^{\circ}\)C) & & **Strength (MPa)** & **Charge (pH\({}_{\text{pzc}}\)) * \\ \hline
105 & - & 201.1729 \(\pm\) 10.3 & 11.33 \(\pm\) 1.14 & 6.77 \(\pm\) 0.11 \\
350 & 6.3 \(\pm\) 0.2 & 186.2960 \(\pm\) 8.7 & 6.77 \(\pm\) 1.10 & 6.21 \(\pm\) 0.05 \\
650 & 7.9 \(\pm\) 0.2 & 401.7924 & 8.25 \(\pm\) 0.83 & 5.34 \(\pm\) 0.03 \\
950 & 12.7 \(\pm\) 0.3 & 292.6853 & 10.77 \(\pm\) 0.55 & 8.12 \(\pm\) 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The main performance characteristics of ceramicsite. become a dense agglomeration, similar to the cementation. At 350 \({}^{\circ}\)C, water volatilization, organic components start to volatilize, and porosity begins to increase. At this temperature, the organic components either evaporate or carbonize, and the clay components (kaolin, Ili, etc.) serve as inorganic binder that has not been crystal transformation through the removing of the structural water. Thus, this is a critical point; at this time, the ceramics is at the worst stage of compressive degree. When the temperature rises to 650 \({}^{\circ}\)C, the inorganic components begin to porcelain, and the compressive strength increases with the cohesiveness increasing. When the temperature rises to 950 \({}^{\circ}\)C, some components of the sludge begin to become liquid phase and play the role of bonding particles, and the mechanical strength is greatly increased.