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Materials 2020, 13, 4712 8 of 23 Table 2. Melting point of various allotropes of sulfur. Allotrope of Sulfur α-sulfur β-sulfur γ-sulfur δ-sulfur ω-sulfur Fibrous Hexasulfur Heptasulfur Cyclo-S12 Cyclo-S18 Cyclo-S20 Melting Point (◦C) Refs. 110.06 [36] 115.1 [13] 112.8 114.6 [69] 119.6 [71] 120.4 [13] 133 [69] 106.8 [72] 108 [73] 108.6 [13] 160 [13] 77 90 [73] 160 104 [69] 75 [73] 104 [72] 50 [74] 39 [75] 148 [74] 128 124 [76] [36,69] 4.2. Viscosity The viscosity of sulfur depends to a large extent on the temperature. For instance, at 160 ◦C the viscosity of sulfur decreases by up to 7–8 centipoise, after which viscosity of sulfur increases significantly (approximately 930 poise) at 190 ◦C and then plummets again. The increase/decrease in viscosity also depends on the concentration/intensity and total length of sulfur chains in the liquid. In view of this, decrement in viscosity (at 160 ◦C) can be attributed to increase in concentration/intensity and total length of sulfur chains while the decrement in viscosity (after 190 ◦C) can be justified by the decrease in total length of sulfur chains. 4.3. Density Like viscosity, density of sulfur also depends on the temperature. The density of sulfur increases with decrease in temperature as shown in Figure 6. It is reported that as the temperature increases, the polymerization form will be changed from 8 membered rings of sulfur atoms to a long chain with around 106 million atoms which this new polymerization shape reduces the density of sulfur [77]. However, there is a constant temperature at which the polymerization of sulfur changes its several properties (like viscosity and density). This temperature is known as Lambda Temperature [69]. Sulfur is known as an element which has the largest number of solid allotropes and most of them have cyclic molecules with ring size range between 6 and 10 [78]. The density of various allotropes of sulfur in Lambda Temperature is given in the Figure 7.PDF Image | Critical Review on the Properties and Applications of Sulfur-Based Concrete
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