## 3 Sulfur Cake  
### Composition and Properties  
Sulfur cake is a solid byproduct commonly produced in sulfuric acid manufacture and purification processes and there has been an emerging interest in the composition and properties of the product as it has the potential to be used as an infrastructure additive, such as in concrete or bitumen system applications. Sulfur cake usually contains a heterogenous mixture of calcium sulfate (CaSO4), calcium hydroxide (Ca(OH)2), and elemental sulfur, together with other inorganic and organic impurities whose make-up varies depending on the industrial process in which it was produced [20]. Sulfur cake mainly contains an amount of elemental sulfur comprising 40-65% of its weight. Calcium sulfate is found predominantly in the form of gypsum (CaSO4\(\cdot\)2H2O), comprising 20-30% and calcium hydroxide can be found in weights of 5-10%. What is left behind are the minor components like silica, unreacted lime, heavy metals, and organic residues. CaSO4 and Ca(OH)2 are formed during the neutralization of flue gas (sulfur dioxide (SO2) or hydrogen sulfide (H2S) gases via lime or limestone in flue gas desulfurization or an analogous treatment system [21; 22]. The exact balance of these constituents not only influences the physicochemical behavior of the cake but also its feasibility of use in material science. In addition to sulfur cake, there exist several categories of sulfur-containing wastes, including recovered sulfur in wastewater, sulfur in coke and pyrolysis residue waste, and hydrothermally treated sulfur sludges. These differ significantly in terms of sulfur content, impurities, and their reusability. Table 1 shows a comparative overview of these types in terms of their composition, sources, volume of production, and their applicability in environmental and constructive applications. \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Types of** **Sulfur Waste** & **Typical Components** & **Production Sources** & **Comparison Notes** & **Annual Production (Mt/Year)** & **Ref. ** \\ \hline \multirow{4}{*}{Sulfur cake (industrial by-product)} & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & Sulfuric acid plants (e.g., filtration, SO2, scrubbing) & Contains Ca-based binders that may enhance strength; suitable for concrete/bitumen after drying and partial purification. Minor metal content should be controlled to ensure long-term stability. & \multirow{4}{*}{12} & \multirow{4}{*}{[20; 23]} \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 (5–10\%), SiO2 (5–10\%), SiO2, metals & & & & \\ \cline{1-1}  & S (40–65\%), CaSO4 (20–30\%), Ca(OH)2 In terms of physico-chemical properties, sulfur cake exhibits a porous, friable structure with a specific surface area typically ranging from 10 to 25 m\({}^{2}\)/g, depending on the cooling and precipitation conditions. The material is typically yellow-gray and hydrophobic due to its sulfur content, though calcium-based components impart partial hydrophilicity. Its bulk density varies from 1.6 to 2.1 g/cm\({}^{3}\), lower than that of pure gypsum but higher than that of purified elemental sulfur [23]. More importantly, the sulfur phase in sulfur cake exists primarily in orthorhombic crystalline form (\(\alpha\)-S\({}_{8}\)), which exhibits good thermal stability and chemical resistance but poor compatibility with polar materials.