## 3 Sulfur Cake
### Composition and Properties
However, in sulfur cake, the presence of calcium compounds significantly modifies interfacial properties, improving adhesion and composite performance in mixtures with concrete or asphalt binders. The phase behavior and thermal stability of sulfur cake components are complex due to the interaction between sulfur and the mineral matrix. Elemental sulfur has a well-known allotropic transformation at \(\sim\)95 \({}^{\circ}\)C, transitioning from orthorhombic to monoclinic form, and eventually melts at 115.2 \({}^{\circ}\)C. Upon heating, sulfur cake exhibits a multiphase transformation profile. Differential scanning calorimetry (DSC) studies have shown distinct thermal events at 100-120 \({}^{\circ}\)C and 600-700 \({}^{\circ}\)C, corresponding to sulfur melting and CaSO\({}_{4}\) decomposition, respectively [27]. Moreover, the hydroxide component decomposes above 400 \({}^{\circ}\)C, releasing water vapor and forming reactive CaO. These transformations offer unique opportunities for tailoring sulfur cake for thermally processed composites, such as sulfur concrete and bitumen modifiers. Compared to elemental sulfur, sulfur cake has several distinct advantages, particularly in terms of cost, availability, and environmental benefits. Elemental sulfur, though relatively abundant as a petroleum refining by-product, often requires costly purification to remove organic and metal impurities [28; 29]. In contrast, sulfur cake, being a waste material from sulfuric acid or metal smelting plants, can be directly utilized with minimal processing, thus reducing raw material costs and addressing waste management issues simultaneously [26]. Moreover, studies indicate that sulfur cake retains many of the desirable characteristics of elemental sulfur--such as impermeability to water, chemical resistance, and self-healing thermal behavior--while its calcium content enhances chemical bonding in cementitious systems and acts as a latent hydraulic binder [20]. In the context of infrastructure applications, sulfur cake has demonstrated strong potential for use as a modifier in both concrete and bitumen formulations. In concrete, its combination of sulfur- and Ca-based compounds provides a dual function: elemental sulfur contributes to corrosion resistance and water impermeability, while CaSO\({}_{4}\) and Ca(OH)\({}_{2}\) participate in pozzolanic or latent hydraulic reactions, improving mechanical strength. A detailed comparison of the key properties of elemental sulfur and sulfur cake is presented in Table 2, highlighting their compositional differences and performance. Sulfur-based concrete made using sulfur cake can reach compressive strengths exceeding 35 MPa after 28 days, with enhanced durability under acidic or saline conditions compared to ordinary \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Types of Sulfur Waste** & **Typical Components** & **Production Sources** & **Comparison Notes** & **Annual Production (Mt/Year)** & **Ref.** \\ \hline Hydrothe-rmaally treated sulfur residues & S (\(\sim\)70–85\%), hydrated organics, trace metals, oxidized sulfur species & Hydrothermal treatment of sulfur-rich industrial waste & Improved purity compared to raw residues; suitable for moderate-performance applications, though residual organics may reduce mechanical strength. & 1.2 & [26] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Sulfur cake is a cheap and environmentally friendly extender or partial substitute of binders in asphalt systems. The sulfur component helps in minimizing the viscosity of bitumen and enhancing rutting resistance, whereas the calcium sulfate enhances the interaction between the filler matrix and provides a load-carrying presence. Field tests have demonstrated that modifications of sulfur cake-modified asphalt may increase Marshall stability (up to 20%) and reduce temperature sensitivity, thereby lengthening pavement life in hot climates [24].