## 2 Materials and Methods  
### Modification of Industrial Solid Wastes  
The red mud used for our experiment consisted of red mud waste from aluminum oxide production by Bayer process at Nanshan Aluminum General Plant of Nanshan Aluminum in Longkou, Shandong province. The slag and fly ash consisted of bottom slag from the main MSWI boilers and fly ash collected by bag filters at Hengyuan Thermal Power Waste Incineration Power Plant in Qingdao, Shandong province. The sludge consisted of sludge precipitated from acrylic production sewage treatment at the acrylic plant of PetroChina Daqing Petrochemical Co., Ltd in Daqing, Hei Longjiang province. For four kinds of solid wastes, XRF analysis was carried out using a Brucker AXS X-ray fluorescence analyzer (XRF, SSTIGER4 kW), as shown in Table 1.  
As indicated, all four solid wastes contain high levels of metal oxides and SiO\({}_{2}\). Hence, it is necessary to acidify-alkalize them to generate metal oxides useful for suppressing explosion. Slag and fly ash contain a level of element Cl since both contain some dioxin, a carcinogenic toxic substance; hence, they have to be detoxified [18]. The material analysis of sludge shows that the content of organic matter in sludge is 50.2%, PH value is 5.56 (acidic), total nitrogen is 12.4, total phosphorus is 0.152 and total potassium is 0.236. The sludge contains 50.2% of organic matter such as additives in the production process of acrylic fiber; hence, it should be treated with decombustion. According to the related literature [19; 20; 21], the dioxin in slag and fly ash can be fully decomposed by high-temperature melting, and high-temperature combustion is also needed to remove the burning materials from sludge. Hence, a Muffle roaster was used to combust the slag, fly ash and sludge under high temperature before a planetary ball mill was used to crush them to 75 um to form the preliminary modified products.  
Next, the red mud and the preliminarily modified slag, fly ash and sludge were acidified-alkalized in the following procedure (in the case of red mud as an example):  
1. [label=(0)]
2. Into 100 mL deionized water, 30 g CN was added. To this, 6 mol/L HCl was added dropwise, and the metal salt solution was obtained by stirring at 90 \({}^{\circ}\)C for 1.5 h with a magnetic stirrer.
3. Ammonia water was added to the metal salt solution to the pH value of 7.8; then, 150 mL of Absolute ethanol (C\({}_{2}\)H\({}_{6}\)O) was added, and the mixture was stirred at 60 \({}^{\circ}\)C for 1 h.
4. After the substance in the solution was precipitated, it was washed and filtered with non-ionic water using a vacuum pump. The filtered mud was dried in vacuum drying oven for 24 h, and the modified red mud (MRM) with particle size less than 75 um was obtained by grinding with ball mill.  
In the same way, modified slag (MSA), modified fly ash (MFA) and modified sludge (MSU) were obtained. Figure 1 compares the SEM results of the microstructure morphologies of the four industrial solid wastes before and after modification.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Substance** & **Fe\({}_{2}\)O\({}_{3}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **CaO** & **TiO\({}_{2}\)** & **Na\({}_{2}\)O** & **MgO** & **Cl** \\ \hline Red mud & 33.3 & 18.2 & 15.3 & 16.3 & 6.73 & 8.19 & 0.30 & 0 \\ \hline Slag & 12.7 & 26.3 & 41.2 & 7.17 & 6.73 & 1.20 & 2.35 & 3.22 \\ \hline Fly ash & 16.923 & 13.834 & 2.69 & 11.7 & 1.22 & 5.6 & 1.88 & 22.6 \\ \hline Sludge & 36.8 & 12.4 & 12.4 & 2.89 & 0.68 & 5.72 & 1.01 & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: XRF analysis of solid wastes.  
Figure 1: SEM analysis of four kinds of industrial solid wastes before and after modification.  
As illustrated, after modification, MRM, MSA, MFA and MSU have become porous materials with a much higher porosity, which greatly enhanced the loading capacity of the materials.