Study of the possibility of using sulfur asphalt and sulfur concrete in road construction

Ferrous cakes are a finely dispersed, almost inert material, similar in chemical composition to the compacting additive ferrous sulfate (FS). Iron cakes can be used as a sealing additive in concrete and as the main raw material for the production of many building materials and products. Ferrous cakes are iron hydroxosulfate Fe(OH)n(SO4)m, a powdered material with a specific surface area of 7400 cm²/g.

The problem of using ferruginous cakes is connected with the need for a complete study: determination of the chemical, mineralogical composition; assessment of the ratio of sanitary-toxicological and explosion-fire requirements; testing for chemical and destructive resistance and interaction with other raw materials in building materials. For successful planning of the research course, it was first necessary to determine what type of waste ferruginous cakes belong to.

An analysis of literary sources has shown that at present, in both our country and abroad, there is no unified universal classification of waste, but the following are put forward as the main classification features: physical and chemical composition, method of processing and disposal, type of formation and their volume, degree of sanitary, and toxicological hazard. Man-made products of the metallurgical complex are divided into non-ferrous and ferrous metallurgical waste, as well as hydrometallurgic sludge. The latter includes ferruginous cake, a waste of hydrometallurgical production and obtained from the purification of nickel electrolyte (iron-cobalt pulp) from iron hydroxide [5].

Sulfuric acid is used to precipitate iron from iron-cobalt pulp and soda is used as a neutralizing agent. The process can be represented by the following chemical reactions: 1 2Fe(OH)3+3H2SO4⇄Fe2(SO4)3+6H2OFe2(SO4)3+4H2O⇄Fe(OH)SO4+2H2SO4Fe2(SO4)3+H2O⇄2Fe(OH)SO4+H2SO4H2SO4+Na2CO3⇄Na2SO4+H2O+CO2 \[\begin{array}{*{35}{l}} 2\text{Fe}{{(\text{OH})}_{3}}+3{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}} & \rightleftarrows & \text{F}{{\text{e}}_{2}}{{(\text{S}{{\text{O}}_{4}})}_{3}}+6{{\text{H}}_{2}}\text{O}\\ \text{F}{{\text{e}}_{2}}{{(\text{S}{{\text{O}}_{4}})}_{3}}+4{{\text{H}}_{2}}\text{O} & \rightleftarrows & \text{Fe}(\text{OH})\text{S}{{\text{O}}_{4}}+2{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}\\ \text{F}{{\text{e}}_{2}}{{(\text{S}{{\text{O}}_{4}})}_{3}}+{{\text{H}}_{2}}\text{O} & \rightleftarrows & 2\text{Fe}(\text{OH})\text{S}{{\text{O}}_{4}}+{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}\\ {{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}+\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}} & \rightleftarrows & \text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}+{{\text{H}}_{2}}\text{O}+\text{C}{{\text{O}}_{2}}\\ \end{array}\]

From the chemical reactions describing the technological process, it follows that according to the chemical classification of by-product raw materials, involving the separation of raw waste according to the content of the main part, the iron cake belongs to sulfate products.

Iron cake, like all slimes, has a high degree of dispersion. Chemical composition, method and formation volume, and a high degree of dispersion are the main factors that determine the direction of their disposal. Therefore, ferruginous cakes can be used as an additive in concretes and mortars as a filler.

To confirm this premise, a study of a complex of ferruginous cakes as raw material for obtaining the above materials was carried out. For the research of raw materials, complex methods were used to determine the raw materials suitability. Sampling for the study of dump ferruginous cakes was carried out in accordance with the current regulatory documents. The preparation of ferruginous cakes samples for testing consisted in drying to an air-dry state in an oven.

The macrostructure was determined by visual inspection of the raw materials by the nature of the structure.

The granulometric composition of the ferruginous cake was determined by sedimentation analysis, based on the fact that the rate of grains fall in a liquid medium varies depending on their size. Based on the data of the grain analysis of the material by the sedimentation method, the specific surface area was calculated with the following formula: 2 Sy=(6/100d)∑ (mn/Dcp) \[{{S}_{y}}=\left( {6}/{100d}\; \right)\sum{\left( {{{m}_{n}}}/{{{D}_{cp}}}\; \right)}\] where S_y – specific surface of the material, cm²/g; d – specific gravity of the material; m_n – mass content of this fraction; D_cp – average grain diameter of a given fraction, cm.

The mineralogical composition of raw materials was determined by the petrographic method with the introduction of data from thermogravimetric, x-ray diffraction, and electron diffraction analyses. Petrographic analysis was carried out using an optical microscope.

A sample of a certain volume was heated at a rate of 20°C/min, and the weight loss was recorded as a function of temperature. Three temperature intervals at which significant weight losses occur were identified: T1 = 234°C (7%); T2 = 406°C (10.9%); T3 = 692°C (20.6%). It should be noted that when heated, the color of the starting material changed from olive green to red-black. The temperature intervals are not specified precisely enough, but they are within the limits: up to 200°C -almost no color change is observed; 200°C–300°C - the sample becomes light brown; 300°C–420°C - the sample becomes red-brown; 420°C–600°C - the sample becomes red-black.

A sample of a certain volume in closed aluminum containers was heated at a constant rate and the heat release in the sample was recorded. The first tests were carried out at temperatures from −70°C to 600°C. Since no temperature transitions were found in this temperature range, the samples were further studied from 0°C. Different heating rates were used: 10°C/min and 20°C/min.

The greatest overlap on the temperature scale is due to kinetic reasons (different heating rates) [9, 16–18]. Subsequently, a standardized heating rate of 20°C/min was used.

In conventional asphalt mixes consisting of crushed stone, sand, mineral powder, and bitumen, the framework base is made up of crushed stone and sand grains, and the gaps between them are filled with smaller particles, usually mineral powder. The denser the mineral structure, the greater an asphalt concrete’s performance. At the same time, the ratio of bitumen content to mineral powder has great influence on the properties of asphalt concrete. Therefore, under production conditions, a high dosage accuracy of the amount of these two components is required, which, unfortunately, is not always done. Deviations in the dose of at least one of the components can lead to a sharp deterioration in the quality and durability of asphalt concrete [19].

In asphalt containing sulfur, a peculiar concrete structure is formed, where sulfur plays the role of filling fine particles and at the same time binds mineral particles and large grains. Bitumen, introduced into the mixture together with sulfur, envelops mineral particles and large grains and plays the role of a viscoelastic binder, which gives asphalt concrete strength and flexibility and reduces fatigue strength. Visual studies show that in the structure of asphalt concrete, large and small grains of sand have a bitumen shell, and when the mixture is cooled, sulfur is in the voids between the mineral particles covered with bitumen. Sulfur penetrates the voids and crystallizes there in accordance with their configuration. Thus, sulfur plays the role of filling the cavities and replaces the amount of free bitumen, usually found in the mineral cavities.

The addition of molten sulfur to the mineral part of the asphalt mixture or bitumen increases the fluidity of this mixture so an asphalt mixture can be formed (cast precast elements in molds). In this case, almost no compaction of the mixture is required. The hot mix containing sulfur in the structure differs in high convenience. From the point of view of convenient processing, the amount of bitumen in the asphalt mix should be more optimal, which is determined from the conditions of strength and stability of asphalt concrete. But, as is clear, the introduction of an overestimated amount of bitumen worsens the asphalt concrete parameters, primarily increasing the deformability of the road surface, which, as is clear, is not acceptable from the point of view of saving bituminous binder. The addition of sulfur contributes to the satisfaction of these requirements. Firstly, sulfur increases the fluidity of the mixture, which reduces the need for energy during compaction, and secondly, after curing in the pores, sulfur gives asphalt concrete high deformation stability. Thus, ease of workability can be used as a criterion for selecting the composition of the asphalt mix, and the requirements for its density can be more easily connected with the requirements of paving.

Compositions consisting of sand, bitumen and sulfur are homogeneous, since small particles are evenly distributed throughout the mass. These mixtures are easily transported and there is no need to take any special measures when concluding them. By varying the percentage of sulfur content, cast mixtures can be obtained that do not require compaction with rollers or leveling. Strength and deformation properties were studied by standard methods, including frost resistance.

Based on the spectroscopy data of the original sample (the presence of absorption bands for S-O and OH groups), it can be assumed that the sample contains the main iron salt, Fe(OH)SO₄. Then it is possible to imagine the processes that occur when the sample is heated through the following reactions (only the most significant ones are indicated): 3 3Fe(OH)3SO4=Fe2(SO4)3+Fe(OH)32Fe(OH)3=Fe2O3+3H2O2Fe(OH)SO4=Fe2O3+H2O+SO3Fe(SO4)3=Fe2O3+3SO3 \[\begin{array}{*{35}{l}} 3\text{Fe}{{(\text{OH})}_{3}}\text{S}{{\text{O}}_{4}} & =\text{F}{{\text{e}}_{2}}{{(\text{S}{{\text{O}}_{4}})}_{3}}+\text{Fe}{{(\text{OH})}_{3}}\\ 2\text{Fe}{{(\text{OH})}_{3}} & =\text{F}{{\text{e}}_{2}}{{\text{O}}_{3}}+3{{\text{H}}_{2}}\text{O}\\ 2\text{Fe}(\text{OH})\text{S}{{\text{O}}_{4}} & =\text{F}{{\text{e}}_{2}}{{\text{O}}_{3}}+{{\text{H}}_{2}}\text{O}+\text{S}{{\text{O}}_{3}}\\ \text{Fe}{{(\text{S}{{\text{O}}_{4}})}_{3}} & =\text{F}{{\text{e}}_{2}}{{\text{O}}_{3}}+3\text{S}{{\text{O}}_{3}}\\ \end{array}\]

The calculated weight loss for reactions 1-2 is 5%, which is an acceptable value.

Table 6 presents the values of temperature transitions and their heat.

The largest scatter in the values of the heat of transitions and temperatures is observed. The observed temperature transitions are nonequilibrium. Upon cooling from 600°C, only one exothermic peak was recorded at 517°C. A repeated heating cycle leads to the disappearance of temperature peaks, a single peak is observed in the region of 150°C with a slight thermal effect. Note that the color of the sample remains unchanged (dark red). The absence of recurrence indicates that the observed thermal effects should be attributed to chemical reactions that occur in the sample upon heating. Endothermic transition in the temperature range of 60°C–100°C should be attributed to the release of water due to crystallization of the sample. The decomposition of the salt by the reaction probably occurs >400°C.

The main requirement for fillers is their dispersion and chemical resistance. Therefore, special attention was paid to these properties in the study of ferruginous cakes. The action of acids HCl, HNO3, and H2SO4 was evaluated since the production environment of metallurgical shops contains vapors of these acids. Laboratory samples were prepared as follows: sulfur was melted at a temperature of 140°C–150°C, DCPD was added and stirred for 8– 10 minutes, then dicyclopentadiene was added, and the sample was mixed again. Then, crushed stone and sand heated to 120°C were added. The compositions were selected, taking into account that the mixture was cast in order to simplify the technology of product preparation and exclude compaction by vibration. The mobility of the sulfur concrete mixture was determined with a standard cone, the draft of which was >15 cm. The hot sulfur concrete mixture was poured into heated molds 7x7x7 cm and tested for strength within a day. The selection of compositions and test results are shown in Table 7.

The following data were taken as the optimal compositions of sulfur concrete (Table 8). In the compositions table, the optimal ratio of sulfur, filler in aggregates, in which is confirmed by the structure of these samples.

One of the main indicators of sulfur concrete is its strength, which for the most part determines its reliable operation in structures.

The structure of sulfur bacon is heterogeneous in its properties. Large filler, the soluble part, and the mastic part differ from each other in their ability to take load. This difference is especially evident when using high-strength fillers. Therefore, structural concrete should be produced only with a modified sulfur binder.

The use of sulfur binder has a significant impact on the strength of sulfur concrete. We selected the optimal composition of the sulfuric binder and the quantity needed in sulfuric concrete (29.6%) to provide sulfuric concrete with high strength (up to 70 MPa) and sufficient mobility (cone draft more than 15 cm). It does not need to be compacted by vibration, which simplifies the manufacturing technology of products, since the sulfur concrete mixture is cast. The homogeneity of the samples obtained without vibration is confirmed by electron microscopic analysis (Figure 2), and the compressive strength of samples of the same composition with and without subsequent vibration of the melt was 70 MPa and 67.1 MPa, respectively. Studies of the features of the samples were carried out by scanning electron microscopy (SEM) on a JSM-6390LV scanning electron microscope (JEOL, Japan). The studies were carried out in the secondary electron mode at an accelerated voltage of 15 kV and low beam currents. This observation mode was chosen to minimize the effects of recharging.

Fig. 2.

Visual examination of the structure of sulfur concretes using microphotography showed that the effect of compaction (vibration) does not significantly change the structure or, accordingly, the characteristics.

The effect of filler dispersion on the strength of sulfur concrete in this case is not significant, since the specific surface area of cakes and tailings is slightly different (7400 cm/g and 6200 cm/g, respectively).

The effect of the modifier on strength is more significant. On the iodine modifier, the strength is much less than on diclopentadine: Rct = 42.5 MPa and 67.1 MPa, respectively. The homogeneity of the compositions turned out to be almost the same (Figure 3), which is confirmed by the strength characteristics of sulfur concrete samples on cakes and tails (42.5 and 42.3 MPa, respectively) during the chemical modification of the sulfur melt with iodine.

Fig. 3.

It has been established that modified concrete has much better performance than unmodified. The flexural strength of concrete increased as the amount of dicyclopentadiene increased and at 5% reached 20% of the compressive strength (around 13 MPa). The optimal modification time of the sulfur binder dicyclopentadiene is 8–10 min.

One of the most important advantages of sulfur concrete compared to cement concrete is the rapid increase in strength. With an increase in temperature from 20°C to 77°C, the strength of sulfur concrete increases abnormally, and then, reaching the initial strength at 90°C, it begins to decline sharply. This leads to the temperature limit of operation of such concretes, which is 80°C. The anomalous change in the strength of concrete in the temperature range of 20°C–80°C can be explained by the processes of sulfur recrystallization. The decrease in strength at 77°C occurs due to a sharp increase in the volumetric expansion of sulfur and is explained by first-order melting.

The test results showed that when the temperature rises from +20°C to +74°C, the modulus of elasticity, as well as the compressive strength, increases from 4.5–104 to 5.1–104 MPa and only at temperatures above +75°C, a decrease in the modulus of elasticity is observed.

The study of sulfur concrete temperature deformations showed that the CLTE (coefficient of linear thermal expansion) value decreases when aggregates are introduced into the sulfur binder and reaches values comparable to cement concretes [16, 20, 21] (10-6 deg-1 for similar cement concrete), and also approaches the CLTE of steel (10 * 10-6 deg-1), which makes it possible to reinforce sulfur concrete with steel and also use it to protect cement concretes from corrosion (CLTE of about (10 * 10-6 deg-1) in multilayer structures. Swelling deformations of sulfur concretes in the process of water absorption are inversely proportional to the sulfur content in the compositions. That is, with an increase in the sulfur content, the swelling deformations decrease. This is explained by a decrease in the porosity of the compositions due to an increase in the amount of sulfur in samples [22].

The type of filler also affects the water absorption of sulfur concrete. The hemicall composition of cakes and “tailing” implies their hydrophobicity, and high dispersion provides an additional clogging effect. As a result, the porosity of sulfur concrete and water absorption are minimal, which implies a high coefficient of water (Table 9) and high frost resistance.

Tests of samples in the water-saturated state after 30 days of storage in water showed that their strength decreases slowly. The coefficient of water resistance for compositions from 13% sulfur is 0.80, from 15% sulfur – 0.90, and from 19% ~ 0.95 (Table 9). However, the decrease in the coefficient of water resistance of samples with 13% sulfur once again confirms the sulfur concrete optimal composition selection (19 wt.% sulfur).

Studies to determine frost resistance and resistance to low temperatures were carried out on samples of 7x7x7 cm, made from sulfur concrete of the optimal composition on fillers cakes and tailings. Frost resistance was determined according to GOST 10060.4-95.

Determination of resistance to alternating influences of freezing-thawing was carried out according to the standard method when cooled to −15°C and according to the accelerated method when cooled to −50°C. Samples of sulfur concrete (composition A with a filler – cakes) and concrete (composition B with fillers “tailing” of metallurgical production) on a modified PPR sulfur binder, as well as composition C on an unmodified sulfur binder with a filler under conditions of alternate freezing at a temperature of −50°C and thawing one part of the samples in water, the other part in air. The stability of the samples was assessed according to the results of mechanical tests for compression according to the standard method.

The stability coefficient of samples after 50, 100, 200, and 300 freeze-thaw cycles is presented in Table 10.

Sulfur concrete compositions A and B, when thawed in air after 300 cycles, had a resistance coefficient Ka equal to 0.87 and 0.96, respectively. When thawing in water, samples of composition A and B after 300 cycles also had a slight decrease in strength. Composition B on an unmodified sulfur binder had a lower stability coefficient.

Table 11 shows the changes in the strength of the samples according to the standard method.

After 50 cycles, no external changes were observed on the samples when tested for frost resistance. The edges and corners remained sharp, peeling was not observed, and samples were not destroyed during ordinary tests. After 100 test cycles, the samples darkened, the corners remained sharp, no peeling was observed; the same was observed after 300 cycles. After 500 cycles on the samples, the corners crumbled, which was accompanied by a further decrease in the concrete strength.

Corrosion resistance tests were carried out according to GOST 25881-83 on samples 4x4x16 cm in two directions: a) on an unmodified binder; b) using dicyclopentadiene as a modifier. Corrosion resistance was determined by changing the mass, appearance and strength characteristics of prototypes when exposed to liquid aggressive media in accordance with the test procedure for polymer concrete. The coefficient of chemical resistance of CCR was determined by the change in compressive strength after exposure to saline, acidic and alkaline media. The following media were chosen: water, 10% solutions of sulfuric, hydrochloric, and nitric acids, 10% solutions of NaCl, Na₂SO₄, NaF salts, and 10% NaOH alkali solution. Samples of concrete on a sulfur binder without a modifier, as well as compositions with the addition of DCPI, were studied. Selection of mix compositions of road concrete based on modified sulfur-bitumen binder with applied technological and physicalmechanical properties.

Ordinary asphalt concrete according to the requirements of DSTU B V.2.3-119, compacted in a hot state at a temperature of 140°C–160°C, must have the following composition (Table 12) and properties:

K_st at 0°C not less than 9 Mpa;

Based on the data of local raw materials and industrial waste, various composite mixtures were selected for the manufacture of laboratory samples in order to obtain high-quality road sulfur concrete with applied properties on a modified sulfurbitumen binder. Iodine was used as a sulfur modifier [23].

The mixture included the following components (industrial waste):

technical sulfur as a binder;

The main task in the selection of compositions, we determined the maximum savings of bitumen without weakening such properties of road concrete as density, frost resistance, and durability. To do this, we first studied the effect of the addition of bitumen on the strength characteristics of concrete with a modified sulfur-bitumen binder.

As a criterion for evaluating the optimal composite compounds, sulfur concrete mixture quality indicators (obtaining the specified physical and mechanical properties of sulfur asphalt concrete) as well as workability were taken. The mixture should be cast to maximize the simplification of its laying technology. The main physical and mechanical characteristics of the compositions are given in Table 13.

According to modern concepts, the most effective way to increase the durability of concrete is to obtain a dense, impervious to water, aggressive liquids and gases structure, which is achieved in two main ways: introduction into concrete during the manufacture of various additives and clogging of the capillary-porous space with various substances and compositions, followed by curing.

The sulfur melt combines high binding properties with the ability to penetrate into the cellular structure of concrete, has high adhesion and inertness to the components of concrete and reinforcement, good wettability of the concrete matrix, low melting point, and high strength and hardness upon transition from liquid form. Sulfur melt can be successfully used as a thermoplastic binder and impregnation material [15, 24, 25].

The prerequisites for using sulfur as a binder are equally important: the ability of the melt to quickly solidify upon cooling and reliably bind aggregates; high chemical resistance of sulfur to a number of aggressive environments, especially high resistance to acid aggression; low energy costs for the production of sulfur concrete products when compared to cement-concrete products. Sulfur binder can be used instead of cement to obtain cementless compositions [26, 27].

The solidified melt of pure sulfur does not have sufficient strength. Therefore, it is modified with mineral powder and, in some cases, chemical additives, to obtain a sulfur binder. Modifying additives introduced into the sulfur melt are divided into plasticizers, stabilizers, flame retardants, antiseptics, and complex additives [28, 29]. These additives change both the physical properties and chemical properties of sulfur in the right direction, some in the direction of strengthening, others in the direction of reducing chemical activity.

Structural building materials are subject to increased requirements for strength, frost resistance, deformability, and corrosion resistance. To improve these characteristics, structural sulfur concretes are produced only on a modified sulfur binder. Considering the properties of purchased concretes, hardened sulfur concrete reaches its design strength in just a few hours, and the humidity and ambient temperature do not affect the hardening process [30].

The effect of negative temperatures on the strength of sulfur concrete was studied. The strength characteristics of sulfur concrete were determined at low temperatures from 0°C to −60°C with an interval of 20°C on prism samples 7x7x7 cm, three samples for each point. Cooling was carried out in a climatic chamber. The time required to equalize the set temperature over the entire cross section of the sample was determined using chromel - a drop thermocouple placed in the center of the control sample, which was placed in the chamber along with the samples to be tested. Upon reaching the predetermined temperature, the samples were tested for compression in a hydraulic press from the entire section. The test results showed that with decreasing temperature, the strength characteristics of sulfur concrete in the temperature range 0 to −60 ° C increased from 67 MPa to 72 MPa (7%). This is due to the fact that sulfur concretes behave like thermoplastic polymers, the strength of which increases in direct proportion to a decrease in temperature.

The deformative properties of sulfur concrete, which manifest themselves during operation (shrinkage, creep) must be taken into account when determining the crack resistance and structural rigidity [31]. The evaluation of these sulfur concretes parameters on the basis of the features of their production technology makes it possible at the first stage to give comparative data with respect to similar properties of traditional known materials (such as cement concrete, polymer concrete, etc.) [32]. Hardened sulfur concrete is practically not subject to shrinkage phenomena. As tests have shown, the dimensions of the samples stored indoors under normal temperature and humidity conditions practically did not change. Thus, according to the test results, it was found that a slight saturation of samples of the optimal composition with water leads to a small loss of strength, and its value is inversely proportional to the amount of sulfur in the composition [33].

It should be noted the low water absorption of sulfur concrete (0.1%–0.3%) versus cement concrete (2%–4%). To reduce the water absorption of cement concretes, special additives are added or concretes are impregnated with them, which complicates the technology of their manufacture and increases the cost. In this sense, sulfur concretes differ favorably.

Crystalline sulfur has a high CTE, which causes internal stresses as a result of temperature changes. Studies have shown that high frost resistance of sulfur concrete can be obtained by introducing plasticizers and stabilizers in the polymer state into the sulfur melt.

During cyclic freezing and thawing, the thermal compatibility of sulfur and filler is also of paramount importance. Modified sulfur binder with properly selected inert fillers provides concrete with high density, hydrophobicity, closed porosity, and increased frost resistance. When using the same fillers as for cement concrete, concretes with improved properties can be obtained. The compositions of sulfur concrete on a modified sulfur binder were the most resistant to frost. Keeping samples of sulfur concrete indoors for a year showed that the strength of sulfur concrete does not change.

When sulfur is mechanically modified and its melt is filled with finely dispersed acid-alkali-resistant fillers, the latter are a structure-forming component. Being centers of crystallization, they contribute to obtaining a homogeneous, dense, fine-grained structure. However, crystalline sulfur, being chemically resistant to acid solutions, was “washed out” with alkali. A study of the chemical composition of efflorescences formed in the upper third of the samples in a sodium sulfate medium showed that they represent the original Na₂SO₄ salt. Efflorescence on sulfur concrete in the NaCl medium in the form of a yellowish coating is also the starting material: sodium chloride NaCl.

Thus, in solutions of salts there is no appearance of neoplasms, but the salt accumulation process, which is characteristic of capillary-porous bodies, proceeds. It should be noted that the crystallization process takes place mainly in the surface layer, and there is no accumulation of salts in the bulk of the sample. This suggests that if the sulfur binder forms pores, they are closed and do not communicate with each other. Since the process of salt accumulation occurs only on the concrete surface, the strength of the samples stabilizes by 90 days and does not change further during the entire test period (Kxc = 0.7–0.75).

In engine oil, sulfur concretes showed very high resistance. Throughout the tests, the stability coefficient remained at the level of 0.9–0.94. In conventional asphalt mixes consisting of crushed stone, sand, mineral powder and bitumen, the framework base is made up of crushed stone and sand grains, and the gaps between them are filled with smaller particles, particularly mineral powder. The denser the mineral part structure, the higher asphalt concrete performance. At the same time, the ratio of the bitumen and mineral powder content has a great influence on the properties of asphalt concrete. Therefore, under production conditions, a high dosage accuracy of the amount of these two components is required, which, unfortunately, is not always done. Deviations in the dose of at least one of the components lead to a sharp deterioration in the quality and durability of asphalt concrete.

In asphalt containing sulfur, a peculiar concrete structure is formed, where sulfur plays the role of filling fine particles and at the same time binds mineral particles and large grains. Bitumen, introduced into the mixture together with sulfur, envelops mineral particles and large grains and plays the role of a viscoelastic binder, which gives asphalt concrete strength, flexibility and reduces fatigue strength. Visual studies show that in the structure of asphalt concrete, large and small grains of sand have a bitumen shell, and when the mixture is cooled, sulfur is in the voids between mineral particles covered with bitumen. Sulfur penetrates the voids and crystallizes there in accordance with their configuration. Thus, sulfur plays the role of filling the cavities and replaces the amount of free bitumen, usually found in the cavities of the mineral part.

The addition of molten sulfur to the mineral part of the asphalt mixture or bitumen increases the fluidity of this mixture, so such an asphalt mixture can be formed (cast precast elements in molds). In this case, almost no compaction of the mixture is required. The hot mix containing sulfur in the structure differs in high convenience.

From the point of view of convenient processing, the amount of bitumen in the asphalt mix should be more optimal, which is determined from the conditions of strength and stability of asphalt concrete. But, as is clear, the introduction of an overestimated amount of bitumen worsens a number of asphalt concrete parameters, primarily increases the deformability of the road surface, which, as is clear, is not acceptable from the point of view of saving bituminous binder. The addition of sulfur contributes to the satisfaction of these requirements. Firstly, sulfur increases the fluidity of the mixture, which reduces the need for energy during compaction, and secondly, after curing in the pores, sulfur gives asphalt concrete high deformation stability. Thus, ease of workability can be used as a criterion for selecting the composition of the asphalt mix, and the requirements for its density can be more easily connected with the requirements of paving.

Compositions consisting of sand, bitumen and sulfur are homogeneous, since small particles are evenly distributed throughout the mass. These mixtures are easily transportable and no special measures need to be taken during their confinement. By varying the percentage of sulfur content, cast mixtures can be obtained that do not require compaction with rollers or leveling.