## 3 Sulfur Cake
### Processing and Modification Technologies  
The processing and modification technologies of sulfur cake are pivotal for its effective utilization in the construction and chemical industries. The purification and activation of sulfur cake includes removal of the impurities like unreacted calcium compounds, carbonaceous residues, and water. Sulfur cake modification has three major categories: thermal treatment, chemical treatment, and blending with additives. Thermal treatment involves the withdrawal of moisture and volatile impurities through heating; chemical pretreatment is based on the ability of acids or reagents to react with to rid a particular contaminant; and functional additives introduce into a blend to provide mechanical, chemical, or thermal performance enhancement in a specific application. The critical comparison of these treatment methods, as well as their mechanisms, advantages, and limitations, is provided in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline
**Properties** & **Elemental Sulfur** & **Sulfur Cake** & **Remarks** & **Ref.** \\ \hline Sulfur content (wt\%) & \textgreater{}98 & 40–65 & \begin{tabular}{c} Sulfur cake contains lower purity sulfur \\ due to industrial by–product origin \\ \end{tabular} & [20; 23] \\ \hline CaSO4 content (wt\%) & 0 & 20–30 & \begin{tabular}{c} Gypsum phase contributes to structural \\ and chemical performance in concrete \\ \end{tabular} & [20; 27] \\ \hline Ca(OH)2 content (wt\%) & 0 & 5–10 & \begin{tabular}{c} Provides alkalinity and reactive \\ filler behavior \\ \end{tabular} & [20] \\ \hline Other impurities (SiO2, metals, organics) & \textless{}1 & 5–10 & \begin{tabular}{c} Impurities vary by origin and may have \\ pozzolanic or inert behavior \\ \end{tabular} & [20; 26] \\ \hline Density (g/cm3) & 2.07 & 1.6–2.1 & \
\begin{tabular}{c} Lower bulk density in sulfur cake \\ contributes to better workability in mixes \\ \end{tabular} & [20] \\ \hline Compressive strength of concrete (MPa) & 25–35 & 35–45 &
\begin{tabular}{c} Sulfur cake improves strength due to \\ synergistic phase interactions \\ \end{tabular} & [25] \\ \hline \end{tabular}
\end{table} 
Table 2: Comparative properties of elemental sulfur and sulfur cake in terms of chemical composition, physical characteristics, and performance. The thermal treatments are normally performed at temperatures exceeding 120 \({}^{\circ}\)C, where the moisture is volatilized and some impurities are partially decomposed resulting in high purity of sulfur and high reactivity [30]. Experimental measurements have also shown that under thermal conditions, moisture can be removed from above 5 wt% to below 0.5 wt%, which is important to optimize downstream processing operational efficiency and the stability of the material itself [31]. The major techniques for sulfur cake modification are presented in Figure 2, including thermal, chemical, and additive-based treatments, supporting the role of each route in impurity removal and performance enhancement. The chemical treatment procedures augment the thermal procedures by tackling particular impurities. An example of acid washing is the use of dilute HCl, which washes out calcium hydroxide and calcium sulfate [32], which can be used to enhance the sulfur level by as much as 80%. Moreover, the adoption of selection agents, like cyclo-dextrin polymers, has been shown to effectively remove tricyclic organic sulfur pollutants, with a purification efficiency of over 90% [33].