## 4 Technological Developments of Sulfur in Construction
### Modified Sulfur Cake Bitumen Technologies
However, in Figure 6d, the DSC curve of modified sulfur shows broader thermal transitions, extending up to 277 \({}^{\circ}\)C, with a reduced peak at 113.9 \({}^{\circ}\)C, and the emergence of a new thermal event at 250-277 \({}^{\circ}\)C. This indicates the formation of polymeric sulfur or sulfur-hydrocarbon copolymers with higher thermal stability and delayed crystallization, resulting from the prolonged reaction and chemical integration of hydrocarbon chains. Such thermal behavior is advantageous for infrastructure applications, as it ensures greater stability during mixing and laying operations in road construction and reduces the risk of thermal cracking [103]. The morphological and thermal enhancements observed validate the modified sulfur's potential as a sustainable, high-performance alternative in sulfur concrete and sulfur-modified asphalt. The smoother matrix and improved thermal resistance lead to stronger adhesion, better compaction, and superior resistance to environmental degradation [104,105]. This modified binder aligns with current innovations in green infrastructure materials, offering a viable route for valorizing sulfur waste while enhancing mechanical performance and service life in demanding construction environments. The primary role of modified sulfur in asphalt binders lies in its ability to act as a crosslinking agent, establishing molecular bridges between bitumen components. This crosslinking enhances the elastic recovery and stiffness of the asphalt, contributing to improved high-temperature performance and rutting resistance [106]. The Density Functional Theory (DFT) and macroscopic experimental investigations confirmed that sulfur atoms form stable S-S and C-S bonds within the binder matrix, strengthening its network and reducing phase separation [107]. The blending behavior and compatibility of sulfur cake with conventional bitumen are critical to achieving desired binder performance. Studies have shown that sulfur can be successfully blended at temperatures between 120 \({}^{\circ}\)C and 150 \({}^{\circ}\)C without significant segregation or exudation issues when properly formulated [108]. In hot mix asphalt production, sulfur content ranging from 10% to 40% by weight of the binder has been reported to be compatible, with 20% being the optimum for balancing workability and performance [109]. The presence of calcium hydroxide and calcium sulfate in the sulfur cake enhances the dispersion of sulfur within the bitumen matrix, acting as stabilizing agents and improving interfacial bonding [110]. The softening point of bitumen increases significantly with the inclusion of sulfur. For example, a base bitumen with a softening point of 48 \({}^{\circ}\)C can reach up to 61 \({}^{\circ}\)C after modification with 20% sulfur cake, indicating enhanced thermal resistance [111]. Similarly, penetration values decline, showing increased hardness, critical for high-temperature stability. Modified sulfur also increases binder viscosity, particularly at elevated temperatures, helping resist flow and deformation under load. At 135 \({}^{\circ}\)C, the viscosity of sulfur-modified bitumen rose from 0.35 Pa-s (heat bitumen) to 0.49 Pa-s with 20% sulfur addition [108]. Aging resistance is another critical factor improved by sulfur modification. Sulfur's crosslinking reduces the volatilization of lighter components during thermal aging, leading to a lower aging index. The aging index based on dynamic shear rheometer (DSR) data dropped from 2.1 in unmodified bitumen to 1.5 in sulfur-modified samples after RTFO aging, indicating better durability [107]. Furthermore, the modified binder exhibits enhanced stiffness modulus and maintains viscoelastic properties under prolonged oxidative conditions. To more accurately quantify the high-temperature performance and viscoelastic behavior of sulfur-modified asphalt binders, dynamic shear rheometer (DSR) testing provides essential parameters. The complex shear modulus (G) and phase angle (\(\delta\)) are key indicators in this context.