## Materials and Methods  
### Material Characterization  
The raw materials used in this study were total tailings, and the moisture content was less than 10% through drying in the open air and a drying oven. Particle size distribution (PSD) of the tailings sample was analyzed using a LS-POP (VI) powerful laser diffraction particle size (Figure 1) under dry conditions appropriate for the ASTM D421 standard procedure [49]. The LS-POP (VI) (OMEC, Zhuhai, China) is a centrifugal particle size analyzer which combines particle sedimentation with photometric detection. Particle sizes can be measured over a very wide range from 2 nm to 500 \(\upmu\)m, depending on the particle density, dispersant density, and viscosity. By using a single light source (power 2 mW) and eliminating the requisite for laser switching, one-second measuring intervals can be achieved, allowing for real-time monitoring of the particle size. Sample concentration in the dispersant is lower than 0.01 wt% (differs with sample). Sample particles are settled in any of four modes: The gravimetric sedimentation mode, the centrifugal sedimentation mode, the multi mode (combining gravitational sedimentation and centrifugal sedimentation), and the centrifugal lift mode. The main reasons behind why the centrifugal sedimentation mode is used for PSD analyses are that (i) it increases the sedimentation speed of fine particles by several orders of magnitude, and (ii) it greatly moderates the effect of Brownian motion. Moreover, the particle size distribution and fractal fitting curves of the studied tailings sample are shown in Figure 2. Combined with Figure 2a and the interpolation calculation of the grading curve, the amount of fine mud less than 0.1 mm reached 21%, the weighted average particle size was 0.361 mm, and the unevenness coefficient was 53.4, which far exceeded the optimum grading range  
Figure 1: Photo of the LS-POP (VI) laser particle sizer. of 4 to 5 in the Thabo equation. As shown in Figure 2b, the fractal dimension of the studied tailings sample was 2.527, and the fitting correlation coefficient reached 96.35%, which could replicate the real PSD of the whole tailings. In general, a greater fractal dimension led to the fine tailings particles used in experiments. In conclusion, the particle size distribution of the whole tailings sample was not uniform, the content of fine mud was high, and the filling material of the tailings demonstrated seriously poor grading. Desulfurized ash (DA) is grayish with a fine grain size, and the average particle size is 17.5 \(\upmu\)m. Desulfurization through a gas-solid reaction at a certain temperature has been proven advisable as it yields effective results and oaths that the ash content does not increase. The most direct and effective means of desulfurization is calcination at a temperature between 850 \({}^{\circ}\)C and 900 \({}^{\circ}\)C, which proposes a good desulfurization rate and a relatively low cost [50]. Note that a high temperature leads to a high desulfurization rate. However, the relatively high-temperature calcination results in a low yield and diseconomy. After the calcination process, the mineral components were effectively improved, while the hydration process was facilitated. X-ray diffraction (XRD) of the powder samples was evaluated using a MXP21 VAHF diffractometer (MAC Science, Tokyo, Japan) over the range of 10-80 degrees 2\(\theta\) at a scanning speed of 0.5 deg/min. The tube anode was copper, and the K\(\alpha\) radiation (0.15 nm) was monochromatized with a graphite crystal. The pattern was collected using a tube voltage of 21 kV and a 500 mA tube current. The pattern was scanned by using a step scan mode (step size 0.02 degree, counting time 1 s per step). XRD is a rapid analytical system principally used for the phase identification of a crystalline material. Both XRD and semi-quantitative analyses showed that the sulfur-containing phase in DA was dominated by CaSO\({}_{3}\), and its mass fraction reached 10-50%. By contrast, the calcium sulfate content was relatively very small.