EVALUATION OF PHYSICOCHEMICAL PROPERTIES OF SULFUR-MODIFIED BND 60/90 BITUMEN

ABSTRACT

This study investigates the modification of BND 60/90 road bitumen using varying proportions (2%, 4%, 6%, and 8%) of elemental sulfur to enhance its physicochemical properties. The modified samples were subjected to standard tests, including penetration, softening point, ductility, brittleness temperature, and flash point, to evaluate performance improvements. The results showed a consistent increase in penetration and ductility values with higher sulfur content, indicating enhanced flexibility. The softening point rose to 45 °C, reflecting improved heat resistance, while the brittleness temperature decreased to –24 °C, demonstrating better low-temperature performance. Additionally, the flash point significantly increased, confirming improved thermal stability. The findings suggest that sulfur is an effective and sustainable bitumen modifier, with 6–8% content offering optimal balance in mechanical and thermal properties for road applications.

АННОТАЦИЯ

В этом исследовании изучается модификация дорожного битума BND 60/90 с использованием различных пропорций (2%, 4%, 6% и 8%) элементарной серы для улучшения его физико-химических свойств. Модифицированные образцы подвергались стандартным испытаниям, включая пенетрацию, температуру размягчения, пластичность, температуру хрупкости и температуру вспышки, для оценки улучшений характеристик. Результаты показали последовательное увеличение значений пенетрации и пластичности при более высоком содержании серы, что указывает на повышенную гибкость. Температура размягчения повысилась до 45°C, что отражает улучшенную термостойкость, в то время как температура хрупкости снизилась до –24°C, что демонстрирует лучшие низкотемпературные характеристики. Кроме того, температура вспышки значительно повысилась, что подтверждает улучшенную термическую стабильность. Результаты показывают, что сера является эффективным и устойчивым модификатором битума, при содержании 6–8%, обеспечивающим оптимальный баланс механических и термических свойств для дорожного применения.

Keywords: sulfur-modified bitumen, penetration, softening point, ductility, brittleness temperature, flash point.

Ключевые слова:  модифицированный серой битум, пенетрация, температура размягчения, пластичность, температура хрупкости, температура вспышки.

Introduction

Bitumen is one of the most widely used binding materials in road construction; however, its physical and mechanical properties tend to vary under climatic influences and traffic loads [1]. Traditional bitumen often softens at high temperatures and becomes brittle at low temperatures, which limits its performance and durability. As a result, modifying bitumen to enhance its thermal stability and mechanical strength has become a pressing necessity in modern road engineering [2].

Various materials such as polymers, rubber, industrial waste, and inorganic compounds have been explored for bitumen modification. Among these, sulfur has garnered particular attention as an effective and economical modifier [3]. Sulfur is not only inexpensive and environmentally friendly but also contributes to structural enhancement of bitumen when incorporated into its matrix. Specifically, the presence of sulfur facilitates cross-linking reactions within the bitumen, resulting in improved thermal resistance and increased mechanical strength [4].

Recent studies have shown that sulfur-modified bitumen demonstrates enhanced properties such as more stable penetration values [5], increased softening point, higher viscosity at elevated temperatures, and reduced brittleness at low temperatures. Furthermore, utilizing sulfur, especially as a recycled byproduct from industrial processes, contributes to sustainable practices and reduces environmental impact [6-7].

This study focuses on the preparation of sulfur-modified bitumen samples and investigates their key physicochemical properties, including penetration, softening point, ductility, brittleness temperature, and flash point [8-9]. The objective is to assess how varying sulfur content affects the quality indicators of bitumen and to identify the optimal modification parameters for enhanced road performance [10].

Materials and methods

In this study, road bitumen of grade BND 60/90 was used as the base binder for modification. Elemental sulfur (S), with a purity of at least 99.5%, was employed as the modifying agent. To investigate the effect of sulfur content on the performance of bitumen, four modified samples were prepared by adding 2%, 4%, 6%, and 8% sulfur by weight of the bitumen. A control sample containing no sulfur was also included for comparison.

The modification process was carried out by heating the bitumen to 160–180 °C in a laboratory-scale mixing vessel equipped with a mechanical stirrer. Sulfur was gradually added to the hot bitumen under continuous stirring to ensure homogeneous dispersion. The mixing continued for 30 minutes after the complete addition of sulfur to facilitate proper reaction and integration.

Once prepared, the modified bitumen samples were subjected to a series of standardized tests to evaluate their physicochemical properties. The following parameters were analyzed:

• Penetration at 25 °C (GOST 11501): to assess the consistency and hardness of the bitumen.
• Softening Point (Ring and Ball method) (GOST 11506): to determine the temperature at which the bitumen begins to soften.
• Ductility at 25 °C (GOST 11505): to evaluate the bitumen's ability to stretch without breaking.
• Brittleness Temperature (GOST 11507): to assess the temperature at which the bitumen becomes brittle and cracks.
• Flash Point (GOST 4333): to determine the temperature at which the bitumen emits flammable vapors.

All tests were conducted in accordance with relevant GOST standards under controlled laboratory conditions. The results of these tests provided comparative insights into how increasing sulfur content affects the thermal and mechanical performance of road bitumen.

Results and discussion

The impact of sulfur content on the physical properties of road bitumen was assessed by testing the modified samples prepared with varying sulfur concentrations (2%, 4%, 6%, and 8%). The results of these evaluations are presented and discussed in this section to determine how sulfur influences the performance of the binder.

Figure 1 presents the penetration values at 25 °C for the sulfur-modified bitumen samples. The data clearly indicate that sulfur content has a measurable effect on the hardness and plasticity of bitumen, reflecting changes in its consistency as the modifier concentration increases.

Figure 1. Penetration values of bitumen modified with different sulfur contents at 25 °C

As illustrated in Figure 1, the penetration values of the modified bitumen samples increase steadily with rising sulfur content. At 2% sulfur, the penetration is 85 (0.1 mm), increasing to 95, 105, and reaching 115 at 8%. This trend indicates that sulfur reduces the stiffness of bitumen, enhancing its plasticity and flexibility. Improved penetration suggests better low-temperature performance, as the binder becomes less prone to cracking. However, excessive softness could potentially compromise rutting resistance under heavy traffic, emphasizing the need for balanced formulation.

Figure 2. Softening point values of bitumen modified with varying sulfur content

As shown in Figure 2, the softening point of the bitumen increases with sulfur content up to 6%, reaching a maximum of 45 °C. The initial value at 2% sulfur is 35 °C, which rises to 40 °C at 4% and stabilizes at 45 °C from 6% to 8%. This indicates that sulfur significantly enhances the thermal resistance of bitumen, making it less susceptible to deformation at elevated temperatures. The plateau beyond 6% suggests a saturation point, where additional sulfur has minimal further effect on softening temperature.

Figure 3. Ductility values of sulfur-modified bitumen at 0 °C

Figure 3 shows that the ductility of bitumen improves consistently with increasing sulfur content. Starting from approximately 2.1 cm at 2% sulfur, the ductility rises to 3.0 cm at 4%, 4.2 cm at 6%, and reaches 4.8 cm at 8%. This positive trend indicates enhanced elasticity and resistance to cracking under stress, which is beneficial for road performance, especially at lower temperatures. The sharp increase up to 6% suggests that sulfur effectively improves the bitumen’s stretchability, while further gains beyond this point are moderate.

Figure 4. Effect of sulfur content on the brittleness temperature of modified bitumen

As illustrated in Figure 4, the brittleness temperature of bitumen decreases with increasing sulfur content. At 2% sulfur, the temperature is around –12 °C, while it drops significantly to –22 °C at 6%, and further to –24 °C at 8%. This trend indicates improved low-temperature flexibility and reduced tendency to crack under cold conditions. The sharp decrease up to 6% sulfur highlights its effectiveness in enhancing cold resistance, making the binder more suitable for use in colder climates.

Figure 5. Flash point values of sulfur-modified bitumen samples

Figure 5 demonstrates that the flash point of the bitumen remains constant at 210 °C for 2% and 4% sulfur content. However, a sharp increase is observed at higher sulfur levels — reaching 233 °C at 6% and 245 °C at 8%. This rise indicates improved thermal stability and safety during high-temperature handling and processing. The significant jump after 4% sulfur suggests that higher sulfur content reduces the volatility of the bitumen matrix, making it more resistant to ignition.

Conclusion

The conducted study has demonstrated that sulfur is an effective modifier for enhancing the physicochemical properties of BND 60/90 road bitumen. By incorporating 2% to 8% elemental sulfur, significant improvements were observed in multiple performance indicators relevant to road applications.

Penetration values increased from 85 to 115 (0.1 mm), indicating enhanced flexibility and reduced stiffness of the binder. The softening point also showed a notable rise, reaching 45 °C at 6% sulfur and maintaining that level at 8%, suggesting improved resistance to thermal deformation. Ductility results revealed a steady increase from 2.1 cm to 4.8 cm, reflecting better elasticity and resistance to cracking under mechanical stress.

Brittleness temperature decreased from –12 °C to –24 °C with increasing sulfur content, indicating superior performance at low temperatures and enhanced durability in cold climates. Additionally, the flash point rose sharply from 210 °C to 245 °C, confirming greater thermal stability and safety during handling and application.

Among the tested formulations, sulfur content between 6% and 8% yielded the most balanced and optimal improvements across all key parameters. These findings support the potential use of sulfur-modified bitumen in road construction, particularly in regions experiencing temperature extremes. Moreover, utilizing sulfur, which is often an industrial byproduct, contributes to both economic efficiency and environmental sustainability in pavement technologies.

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