Research on the Possibilities of Using Sulfur Concrete for Road Infrastructure Construction—Assessment Based on European Standards

Abstract

Sulphur is generated in large quantities as an industrial byproduct, and one promising method of its reuse is in sulphur concrete as an alternative binder to Portland cement. In this study, a sulphur polymer (waste sulphur) modified with 5% styrene was evaluated as a binder in concrete for road infrastructure. The sulphur concrete was tested for its compressive strength, abrasion resistance, water absorption, freeze–thaw durability, water impermeability, and leachate chemistry, in relation to relevant European standards for transport concretes. The modified sulphur concrete achieved high early strength (compressive strength class C40/45) and exhibited very low water absorption and high resistance to abrasion and water penetration. However, it did not satisfy frost resistance requirements: after 150 freeze–thaw cycles, its compressive strength dropped by over 30% (well beyond the 20% acceptable loss), meaning that the material only achieved an F50 rating instead of the F150 rating that is required. In addition, the material is inherently flammable, which poses safety limitations. Unmodified (styrene-modified only) sulphur concrete cannot yet replace conventional concrete in structural applications where it would be exposed to significant freeze–thaw cycling or high fire risk. It may be suitable for less demanding uses, for example in chemically aggressive environments or for precast elements that are not subject to freezing. Further research should explore modifications (such as fibre reinforcement or additional polymer additives) to improve the frost resistance and overall durability of sulphur concrete for broader infrastructure applications.

1. Introduction

Sulphur waste—the byproduct of elemental sulphur and sulphur compounds used in industrial processes—is a significant material stream worldwide. Sulphur is typically an unwanted byproduct of the purification of fossil raw materials and metallurgical processes. It accumulates in the form of yellow lumps or granules (technical sulphur) after the desulphurisation of crude oil, natural gas, or metallurgical exhaust gases. Although sulphur is a valuable chemical raw material (crucial, for example, in the production of sulphuric acid and fertilisers), it is mainly produced “in passing” to reduce pollution. The latest data (2023–2024) on the global production of sulphur waste, its main sources and leading producers, and observed trends in sulphur management are collected below.

The global production of waste sulphur (produced as a byproduct) is estimated to be between about 50 and 60 million tons per year. According to one study, it is in the range of 48–62.4 million tons per year, mainly resulting from the desulphurisation of crude oil and natural gas. More recent statistics, however, indicate that the total global production of sulphur (including recovered elemental sulphur and sulphuric acid from by-processes) is reaching even higher values. According to the United States Geological Survey (USGS), the global sulphur production was about 85.8 million tons in 2023 and is estimated at about 85.0 million tons for 2024 [1,2,3,4,5].

Since the production of waste sulphur results from industrial activity, the largest producers are countries with a large refining and gas sector, as well as those with a large metallurgical sector. According to the latest data (2023), China dominates with about 19.0 million tons of sulphur per year, followed by the United States with about 8.6 million tons, Saudi Arabia with about 7.5 million tons, and Canada with about 5.0 million tons. In addition to the above leaders, important producers are, Kazakhstan (~5.1 million tons), the United Arab Emirates (~6.0 million tons), India (~3.7 million tons), Qatar (~3.1 million tons), Japan (~3.1 million tons), and Iran (~2.0 million tons). In 2023, the production of technical sulphur in Poland amounted to approximately 1.1 million tons [1,2,3,4,5].

The material recycling of sulphur polymers, which result from the purification process of copper and other non-ferrous metals in concrete composites, is possible in the construction industry [4,5,6]. No chemical reaction takes place in the process of setting sulphur concrete. The process is physical, not chemical, unlike the process of setting hydraulic concretes. To make sulphur concrete, it is necessary to create an environment that allows the mixture to be workable and which allows changes in the sulphur’s crystallographic system [4,5,6,7,8].

The modification of sulphur or polymer concrete has been carried out for many years [9]. The first research on this topic began in the twentieth century. Sulphur concrete is a type of building material in which sulphur, in elemental or modified form, acts as a binder instead of traditional cement. Sulphur concrete, similar to cement concrete, is a blend of binders (polymerised sulphur compounds in the case of sulphur concrete and cement mixed with water in traditional concrete) and filler, which consists of aggregates of the appropriate size. Mineral aggregates such as granite, basalt, and dolomite are the most used; however, in contemporary cement concretes, the aggregate composition is formulated from various materials that are chosen based on the desired properties of the concrete and can include recyclable substances.

Sulphur concrete is used in places where high resistance to chemical corrosion is required [10], e.g., in the chemical industry or sewage treatment plants. Its advantages and the possibilities of waste immobilisation were quickly appreciated [6]. However, to this day, it has not been used anywhere in the world on such a large industrial scale as traditional cement concrete. Sulphur concrete plants are usually manufactories that produce small prefabricated concrete accessories in the form of pipes and sewer manholes. Recent publications indicate the possibility of modifying sulphur concrete with the use of fly ash and bituminous additives, which significantly improves its compressive strength and reduces its water absorption. Studies conducted in 2023 showed that sulphur concrete modified with additives can achieve a compressive strength of 42–68 MPa after 28 days, which makes it comparable to traditional cement concrete [10,11].

Unfortunately, the production of concrete with sulphur polymer is problematic, which is a serious limitation. Analysing the literature [5,6,12,13,14], it can be seen that the main problem in the technological process of sulphur concrete production is heating the sulphur binder in the range of 130 to 140 °C, at which temperature the sulphur polymer is workable—it changes its solid form into a liquid form. In addition, sulphur polymers are flammable at temperatures higher than 170 °C, which is another limitation of the technology for producing such concretes.

Nevertheless, sulphur concretes have thermoplastic properties, like asphalt concretes, so they can be recycled repeatedly. The processing temperature of sulphur concretes is lower than that of asphalt concretes, thanks to which, when used in roads, it does not undergo temperature deformations during periods of increased external temperatures in the summer. Currently, two basic directions of use of waste sulphur polymers can be distinguished—the replacement of hydraulic cements and the replacement of asphalt binders. Sulphur concrete, due to its very fast setting time and mechanical properties observed in tests, could be a good material for use in quick repair or construction works, even in extreme environmental conditions [15,16,17], provided that the conditions necessary to prepare this type of mixture are provided on the construction site. Sulphur polymers are also used in other composites [18,19,20,21] in other industries. It should be noted that sulphur concrete can also be a type of structural concrete used in road construction and cubature construction, provided that standardised requirements are met [22,23,24]. For structural materials, the basic Properties that need to be tested are the compressive strength, tensile strength, and modulus of elasticity, according to PN-EN 206+A1:2016:12 [25]; however, depending on the place of installation and purpose, the set of tests may be extended by detailed tests.

The frost resistance of concrete for communication is an important parameter. Depending on the type of structure, the traffic taking place on it, and the technology of execution, these requirements have been included in various documents. Surfaces made on site and compacted by vibration, regardless of the type of traffic they carry, are addressed in the PN-EN 13877-1 [26] and PN-EN 13877-2 [27] standards. The requirements for cement concrete pavement on which car traffic takes place have been specified in the General Technical Specifications of the General Directorate for National Roads and Motorways [28] and, for concrete airport pavement, in the defence standard NO-17-A204:2015 [29]. Small-size prefabricated elements intended for the construction of pavement are covered by harmonised standards [29,30,31]. For the remaining types of pavement, national or European technical assessments are issued to clear them to be placed on the market (before 2017—technical approvals), including, among others, the guidelines in force at the place of planned use (e.g., for platform surfaces—technical conditions Id-22 of Polish Railway Lines S.A.) [32,33]. In the case of surfaces for which it might seem that the requirements have not been specified, the reference document may be the PN-EN 206 [25] standard together with the national supplement PN-B-06265 [32] which contain guidelines to facilitate the construction of durable concrete structures that are also exposed to frost.

Sulphur concrete demonstrates significant promise in various applications due to its excellent resistance to chemical attacks, high thermal stability, and environmental benefits. However, despite good surface frost resistance, sulphurous concrete does not meet the requirements for volumetric frost resistance, which limits its use in elements that are exposed to intensive freeze–thaw cycles [15]. The degradation of concrete caused by the cyclical impact of frost is related to its thermal deformation and physical phenomena that occur within it—the freezing of water causes hydraulic pressure in the pores of the material, which results in cracks and chips, often leading to complete decomposition of the concrete over time. De-icing agents that are commonly used in winter pose a particular threat to concrete surfaces. They contribute to the accelerated destruction of concrete at low temperatures, especially causing surface peeling. According to some estimates, the corrosion rate can increase up to four or five times in this situation. The mechanism of the accelerated deterioration of concrete during freezing and thawing in the presence of de-icing agents is still under discussion. Due to the limited possibilities of laying the mixture when using sulphur concrete, it is necessary to use working breaks, which reduce thermal and shrinkage deformations [9,18]. The crystallisation of sulphur during cooling can cause internal stress, leading to microcracking and degradation. Unfortunately, the internal frost resistance of sulphur concrete has also been shown in tests to be inappropriate for concrete structures [6,10,11,14,28,34]. This type of concrete undergoes significant polymer shrinkage during freezing, which creates a path for water to penetrate between the aggregate and the binder, similar to asphalt concrete.

Some researchers report that the addition of stabilisers, such as polymers or various chemical additives (e.g., epoxy resins), can improve the compressive and bending strength of sulphur mortar. Polymers can also improve the material’s flexibility, which is beneficial in applications that are subject to dynamic loads. Research by the author [35,36] has highlighted this limitation, emphasising the need for modifications such as the incorporation of polymers or other additives to improve the freeze–thaw resistance of sulphur concrete. Other modifiers, such as melamine-formaldehyde waste or tung oil, can be used which positively affect the strength and durability of the concrete. Studies indicate that adding tung oil to the sulphur binder can improve the mechanical properties and durability of sulphur concrete [37,38,39,40,41]. Although they mainly affect the concrete’s durability, this modification can also affect its frost resistance. Adding modifiers, such as fly ash or controlled porosity aggregates, can improve the frost resistance of the concrete. Adding substances that alter the material’s microstructure can reduce its susceptibility to cracking during freeze–thaw cycles. The addition of various types of stabilisers can also improve the overall durability of sulphur mortar. Stabilisers can increase the material’s resistance to erosion and external influences such as temperature changes and humidity by strengthening the bonds between the sulphur particles and the aggregate. Some modifiers can affect the setting time of the sulphur mortar. For example, adding materials that activate chemical reactions can speed up the material’s bonding, benefiting some applications. On the other hand, other modifiers can delay the process, which can be helpful if a longer time to process the material is required. Modifying additives can also improve the appearance of sulphur mortar by making it smooth or changing its colour, which can be significant in architectural applications.

This article aims to verify the possibility of using sulphur concrete modified with styrene in the amount of 5% of its weight in structures for communication engineering based on the above-mentioned key properties of concrete: its compressive strength, abrasion, indoor frost resistance, absorption, water resistance, and leachability relative to European standards on transport infrastructure. This article aims to determine the level of frost resistance of sulphur concrete. Finally, the accumulated test results indicate alternative possibilities of using sulphur concrete. It is also essential to assess the leachability of sulphur concrete to determine whether it is safe for the environment and to make recommendations for its safe production and use.

2. Materials and Methods

2.1. Materials

Sulphur polymer, a waste product from the purification process of copper and other non-ferrous metals, was used in this research and modified with styrene in 5% of its weight. Styrene-modified sulphur polymer is typically more impact-resistant, more flexible, and easier to process than pure sulphur. The modification of sulphur with styrene involves creating chemical bonds between sulphur molecules and styrene molecules, which increases the thermal stability and corrosion resistance of the material and improves mechanical properties such as its flexibility and strength.

The analysed studies adopted sand (20% of the aggregate volume) and natural gravel aggregate as aggregate. The analysed sulphur concrete was made with 20% sulphur polymer, 20% quartz sand, 50% quartz aggregate 0–8 mm, and 10% quartz powder content with a grain thickness of 0.065 mm and a 2.2–2.5 kg/dm³ density. Adding quartz powder to sulphur polymer can improve the hardness and mechanical strength of the material. With its fine-grained structure, quartz powder acts as a filler that enhances stability and reduces the brittleness of the sulphur polymer material. Fine particles of quartz powder can improve the abrasion resistance of sulphur polymers, which is an essential advantage in applications where the material is exposed to intensive use. Finally, quartz powder, a material that is highly resistant to high temperatures, can improve the thermal resistance of the sulphur polymer, which can be particularly useful in industrial applications where the material is exposed to high temperatures.

Unfortunately, when sulphur polymer is exposed to high temperatures, it emits an unpleasant odour and hazardous substances, which is why the concrete was made in an industrial installation designed for this type of industrial facility, and belonging to an industrial centre dealing with the production of sulphur concrete. It should also be noted that, when producing sulphur concrete, measures should be taken to monitor the emission of substances that are hazardous to human health, and employees should be provided with appropriate protective measures. Data on the effects of occupational exposure to hydrogen sulphide (H₂S) are insufficient to establish hygienic standards since employees who work in these conditions are constantly exposed to other chemical agents: carbon disulfide (CS₂), mercaptans, sulphur oxides, aromatic hydrocarbons, and ammonia. Hydrogen sulphide at a 14 mg/m³ concentration does not harm the respiratory system [13,14]. A 14 mg/m³ hydrogen sulphide concentration should be considered safe [5,34].

The components of sulphur concrete should be treated in the following order: heated sand and aggregate, sulphur binder, mineral filler, and fibres (if used). The concrete mix must be thoroughly mixed so that the molten sulphur binder thoroughly surrounds the fine and coarse aggregate and the mineral filler. Segregation of aggregate should be kept to a minimum. Both wooden and metal moulds can be used for concreting. In the case of using repetitive metal moulds, they should be heated to avoid concrete binding due to rapid cooling when it comes into contact with the cold mould. As tested in the article, concrete aggregate is heated to approximately a temperature of 150 °C and is then added first to the concrete hopper. Quartz powder is mixed with sulphur polymer in a separate container and then added to the first container. At the time of addition, the mixture of quartz powder and sulphur polymer is added to the quartz aggregate at a temperature of approximately 23 °C. Then, the sulphur concrete mixture formed in this way is mixed with slow heating for 30 min. The mixture’s temperature when it is poured into the moulds is 138 °C. The mixture should be poured as soon as possible so that the surface finish can be done while the concrete is still hot enough. Compaction and finishing of sulphur-concrete pavements can be carried out with the help of tools used for laying cement concrete. With a properly designed mixture, head vibrators are not necessary; etching is sufficient to thicken the mixture. A vibrating strip is recommended to obtain a tight and smooth surface. When laying a 50 mm thick surface, there are 2 + 10 min for a single finishing of the surface to be completed before it begins to harden. In the case of pavements with a 100–200 mm thickness, this time is extended to 5 + 20 min. Trowelling should not be continued when a crust forms on the upper surface. A gas burner can melt the surface and finish it again quickly.

Thirty samples with dimensions of 150 × 150 × 150 mm were made (Figure 1). The samples matured for 28 days at room temperature and about 50% air humidity.

2.2. Methods

The following metrics were obtained: compressive strength, abrasion, indoor frost resistance, absorption, water resistance, and leachability.

Table 1. Summarises the type of test performed and number of samples used.

Type of Test	Number and Sizes of Samples
Compressive strength	6 samples of size 150 × 150 × 150 mm
Abrasion	2 samples of size 150 × 150 × 150 mm
Indoor frost resistance	12 samples of size 150 × 150 × 150 mm
Absorption	6 samples of size 150 × 150 × 150 mm
Water resistance	3 samples of size 150 × 150 × 150 mm
Leachability	1 sample of size 150 × 150 × 150 mm

summarises the type of test performed and number of samples used.

Compressive strength: test method: PN-EN 12390-3 [42]. This study was carried out on samples after 28 days (Figure 2).

Abrasion: in construction practice, in the case of polymer concrete, the abrasion resistance of concrete is determined using the Boehme method according to the PN-EN 13892-3 standard [43]. The test result is the average depth of the resulting sample loss, based on which the volume of the worn material is determined. PN-B-06265:2004—national supplements to the standard [32] PN EN 206-1:2016 [25] distinguish three classes of attrition aggression: XM1, XM2, and XM3.

Interior frost resistance: test method: The ordinary method of testing frost resistance according to PN-B-06250:1988 [44] has been referred to in the requirements for concrete pavements or, on its basis, the test methods have been described in reference documents [27,45]. The internal frost resistance of polymer–sulphur concrete was determined by the so-called ordinary method described in the PN-88/B-06250 standard [44]. Determination of the freeze–thaw cycle, in this method, consists of successively freezing the entire sample in air and thawing it in water; the duration of the full cycle is at least 6 h. Internal frost resistance is marked as the degree of frost resistance of concrete F at a certain number of freezing and thawing cycles that the concrete must withstand (tested F150). The determination in this study was conducted in laboratory conditions. The prepared samples (6 cubic samples with size 150 × 150 × 150 mm) in the state of water saturation were placed in a freezer chamber; after 50 cycles of defrosting and freezing (F50) (at the request of the customer), the compressive strength of 3 frozen samples was determined. At the same time, three of the samples (witnesses) were stored in water at a temperature of 18 ± 2 °C throughout the test, after which their compressive strength was also determined. The remaining three samples will be examined during the evaluation of the F150. The results were compared, and the decrease in the compressive strength of the frozen samples was calculated in comparison to the Reference samples (F50). The remaining 3 samples underwent freeze–thaw cycles (air freezing—Figure 3, thawing in water) until 150 cycles (F150) had been achieved, which is the minimum value for road and rail infrastructure and construction applications. The frost resistance test determines the decrease in the compressive strength of a frozen sample compared to that of a non-frozen sample (the so-called controls). The reduction in compressive strength should not be more significant than 20%. According to the PN-88/B-06250 standard [44], frozen samples should not have cracks, and the weight loss should not exceed 5% of the weight of non-frozen samples.

Absorption: A survey of the samples for determining absorbability was carried out following the guidelines in the PN-EN 12390-2: 2019 standard [46]. Samples with dimensions of 150 × 150 × 150 mm³ were used for this study. For the obtained concrete, six samples were prepared and placed in a bathtub with water at 20 °C. After four days, the samples were taken out of the water, wiped dry, and weighed. They were then dried in a thermal test chamber at 100 °C until a constant mass was obtained. After drying, the samples were weighed again.

Water resistance: The introduction of European standards (EN) into construction practice has resulted in a decrease in the understanding of the water tightness of concrete, both from the point of view of testing this property of concrete as a construction material and from the point of view of the requirements for the tightness of concrete structures. The “water tightness of concrete” is no longer present in standard terms. The last time it appeared was in the PN-EN 206-1:2003 standard (Chapter 5.5.3 Water Tightness), while, in the amended version, it has been replaced by the name “resistance to water penetration”. This is due to the introduction, in the meantime, in 2011, of the PN-EN 12390-8:2011 standard [47], which defines a method for testing the properties of concrete called “depth of penetration of water under pressure”. Even the previous Polish standard for concrete, PN-B-06250:1988 [44], defined “degrees of water tightness of concrete”, but the test itself is called “water permeability test through concrete”. Only regression to the industry standards previously used in Poland, specifically to the BN-62/6738-07 standard [48] concerning hydrotechnical concrete, allows us to come across the definition of this property—“watertightness of hydrotechnical concrete—it is the ability to resist the penetration of water through its mass”. Going back to these old national standards is also important because both the industry standard [48] and the Polish standard [49] result in using watertight concrete, i.e., allow the fabrication of structural elements that can be exposed to water pressure.

Standard PN-88/B-06250 [44] introduced the concept of water permeability through concrete, which is defined by the degree of water tightness. The degree of water tightness classifies concrete in terms of water permeability. The number next to the letter W indicates 10 times the water pressure value in MPa that is acting on the concrete samples. The degree of water tightness of concrete is determined depending on the pressure index “i” and the type of water pressure (

Table 2. Water resistance depends on pressure rating and water conditions [50].

Pressure Indicator “i”	Degree of Water Tightness of Concrete with One-Sided Water Pressure
	Fixed	Periodic
0.5–5	W2	W2
6–10	W4	W2
11–15	W6	W4
16–20	W8	W6
21–40	W10	W8
More than 40	W12	W10

) [44,50]. The pressure rating “i” is the ratio of the height of the water column in meters to the thickness of the partition in meters. In the relevant standard [44], it is written that a given degree of water tightness of concrete is achieved if, at the load exerted by the water pressure, no signs of water seepage are found in three out of six samples. The test is carried out after 28 days of maturation unless otherwise provided in the design documentation. In PN-EN 206+A1:2016-12 [25], resistance to water penetration is given as one of the requirements of hardened concretes. This standard does not impose a test method. Depending on the limit values for the concrete composition and the exposure class (maximum w/c, minimum compressive strength class, minimum cement content, minimum air content and other requirements), an indirect determination of the resistance to water penetration is allowed. The method of testing the properties of concrete called “depth of penetration of water under pressure” is described in the tool standard PN-EN 12390-8:2011 [47].

The tightness of the structure can be determined by using the existing PN-EN 1992-3:2008 (Eurocode 2) standard [51] concerning silos and tanks for liquids. The standard states that the sections on design for tightness can also be suitable for other types of structures that require tightness [48,51]. Standards [51] present the classification of tightness of structures and the requirements for leaks (

Table 3. Classification of structure tightness according to PN-EN 1992-3:2008 [51].

Sealing	Leak Requirements
0	A certain degree of leakage is allowed, or liquid leaks are not relevant
1	Leaks are limited to a small amount; surface soaking or damp spots are permitted
2	Leaks should be minimal; soaking should not impair the appearance of the surface
3	Leaks are unacceptable

).

The water tightness test of sulphur concrete was carried out under the PN-EN 12390-8:2011 standard [47]. The tests were carried out on cubic samples with a side of 150 mm for the delivered series of tested concrete. Concrete testing was carried out under a constant water pressure of 500 ± 50 kPa acting on the surface of a sample with a diameter of 75 mm for 72 ± 2 h.

Leachability: The leachability of sulphur from sulphur concrete is an essential parameter that determines the extent to which sulphur compounds can be released from the concrete when it is exposed to water. This is particularly important from an environmental point of view, as sulphur and its compounds can negatively impact soil and groundwater.

Determining the form in which concrete will appear and the environmental conditions it will be subjected to is important due to the variety of leaching mechanisms that can accompany specific scenarios. The release of heavy metals from concrete materials can be accompanied by the process of leaching from the surface and dissolution, as well as percolation or diffusion. The level of heavy metal leaching will also differ between monolithic moulds in constant contact with water and the same moulds in continuous contact with soil. Therefore, before assessing the level of leaching of heavy metals from concrete materials, it is necessary to consider the exposure conditions in which they will be applied [51,52].

Tests for the leachability of sulphates (SO₄²⁻) and sulphides (S²⁻) from building materials such as sulfuric concrete are carried out following the European standards of the EN series, which include rules for the evaluation of the leachability of substances from solid materials. During the research, the leaching liquid was also applied to shredded concrete, thus simulating those “application scenarios” that apply to granulated concrete forms (e.g., construction rubble). The water extraction was made following the procedure contained in PN-EN 12457-4:2006 [53] which entails the use of distilled water during shaking for 24 h. The ratio of liquid to sample weight is 10. This standard defines methods for testing the leaching of chemicals from building elements under conditions simulating contact with water (e.g., rainwater). Before the water extraction, the concrete samples are crushed into fragments of a certain size (e.g., <4 mm). The fragments are thoroughly dried before the test. The samples are then placed in a vessel with deionised water or another suitable extraction solution with a specific liquid–solid ratio (e.g., 10:1). The whole solution is stirred for a certain period (usually 24 h) at a constant temperature (e.g., 20 °C ± 2 °C). Once the mixing is complete, the solution is filtered and then analysed to obtain its sulphate (SO₄²⁻) and sulphide (S²⁻) contents using ion chromatography (IC) for sulphate ion determination and titration or absorption spectroscopy for sulphides. The results are compared with the limit values according to national standards or regulations, depending on the material’s intended use. The limit standards for the leaching of sulphates (SO₄²⁻) and sulphides (S²⁻) from sulphur concrete are regulated by environmental legislation. Still, they are not explicitly defined in specific EN standards for sulphur concrete. In practice, leaching limits depend on the use of the material and national and regional regulations on environmental protection, landfills, and construction.

Sulphuric concrete was obtained using leachability tests according to the procedure given in standard [33], in which a concrete cube is placed in a container filled with an appropriate amount of liquid (distilled water). Concrete paving stones are placed on supports, thus allowing the entire surface of the tested concrete sample to meet the liquid

The limit standards for the leaching of sulphates (SO₄²⁻) and sulphides (S²⁻) from sulphur concrete are regulated by environmental legislation. EN 12457 [53] (leaching from waste) is often used to assess whether a material meets the requirements for landfilling or recycling. Directive 2003/33/EC [54,55,56] (on leaching limit values for landfills) provides guidance on sulphate leaching. For inert waste (e.g., used in construction), this value is 1000 mg SO₄²⁻/L.

3. Results

3.1. Compressive Strength

Table 4. Results of determination of compressive strength of sulphur concrete.

No.	Mass [g]	Compressive Strength [MPa]
1	7882	47.10
2	7678	46.00
3	7978	48.60
4	7857	46.10
5	7629	43.20
6	7735	44.40
		average	45.90
		Stand deviation. Population	1.70
		Were Max.	48.60
		Were Min.	43.20
		Variation	3.81

presents the results of the determination of the compressive strength of sulphur concrete. Sulphur concrete’s early strength is very high, which is an undoubted advantage when using this type of concrete, e.g., in prefabrication or other engineering solutions.

The qualification of the strength class of concrete concerning the determination of its compressive strength on cylindrical or cubic samples (method A pre-production number of samples n < 15) was conducted as follows:

f_cm ≥ f_ck + 4 f_ci ≥ fck − 4

where:

• f_cm—the average of n strength test results of a series of n samples;
• f_ck—the characteristic compressive strength (concrete class);
• f_ci—a single strength test result of a series of n samples.

Considering the above requirements and the results in Table 4, the tested sulphur concrete obtained the strength class C40/45.

3.2. Abrasion

The results of the tests of the abrasiveness of the sulphur concrete are presented in

Table 5. Results of the sulphur concrete abrasion test.

No	Wear Surface Dimensions [mm]		Specimen Height [mm]				Initial Sample Weight [g]	Sample Weight After 16 Abrasion Cycles [g]	Density [g/mm3]	Change of Mass Δm [g]	Abrasion [cm3/50 cm2]	Abrasion Class According to PN-EN 13892-3
	a	b	H1	H2	H3	H4
1	70.7	70.0	68.8	69.5	69.5	69.2	783.54	757.98	0.002286	25.56	11.30	A1.5
2	70.7	68.6	66.4	66.6	67.2	66.6	729.75	707.09	0.002160	22.66	10.81	A1.5

. According to the classification given in

Table 6. Abrasion class designations according to PN-EN 13892-3 [43].

Requirement	Abrasion Resistance According to the Boehme Method (PN-EN 13892-3)
Abrasion class	A22	A15	A12	A9	A6	A3	A2	A1,5
Volume of grated material [cm3/50 cm2]	≤220	≤150	≤120	≤90	≤60	≤30	≤20	≤15

, sulphurous concrete has been given the abrasion class A12.

3.3. Frost Resistance

The results of the frost resistance test are summarised in Figure 4 and

Table 7. Results of reduction in mass of concrete after 50 and 150 freeze–thaw cycles.

Number of Freeze–Thawing Cycles	Mass of Concrete [g]	Mass of Concrete [g] After 50 Cycles	Mass Reduction of Concrete After Freezing-Thawing Cycles [%]
50	7767.8	7769	−0.02
	7866.7	7884	−0.22
	7790.1	7798	−0.10
150	7926.9	7980	−0.67
	7659.2	7710	−0.66
	7730.6	7796	−0.85

and

Table 8. Results of reduction in mass of concrete after 50 and 150 freeze–thaw cycles.

Number of Freeze–Thawing Cycles	Medium Reduction of Compressive Strength [MPa]	Frost Resistance Degree
50	3.6	F50
150	36.1	-

. The polymer–sulphur concrete obtained a rating of F50, but did not achieve F150 (the reduction in strength is greater than the 20% allowed). For frost resistance, an F50 degree corresponds to only 17 years of use at the customer’s request, whereas F150 corresponds to 50 years of use according to the requirements of PN-EN 206 and guidelines for transport facilities.

As shown in Figure 4, the concrete samples cracked after 150 cycles of frost resistance testing. Sulphur concrete is a type of polymer concrete, so there was less peeling after the durability tests. Therefore, the weight reduction that was obtained is not a typical quantity seen in determining the frost resistance of concrete. In addition, due to the very low absorbability of this type of concrete, the measurement of its changes in volume could be unreliable. In this case, the concrete should be evaluated based on the change in mechanical properties. For this reason, a study showing its weight reduction after the freeze–thaw cycles was conducted.

3.4. Water Absorption

Table 9. Results of the concrete absorbability test.

No	Dry Weight [g]	Mass in the Saturated State of Water [g]	Mass Absorption [%]	Average Mass Absorption [%]	Mass in Water Saturation After 150 Freezing Cycles [g]	Mass Absorption After 150 Cycles Freezing [%]	Average Mass Absorption After 150 Cycles Freezing [%]
1.	7760.8	7764.4	0.05	0.05	7782	0.27	0.26
2.	7814.8	7816.8	0.03		7831	0.21
3.	7884.2	7886.2	0.03		7902	0.23
4.	7634.6	7640.8	0.08		7658	0.31
5.	7719.8	7724.2	0.06		7741	0.27
6.	7806.9	7810.4	0.04		7829	0.28

presents the study results of the water absorption of frozen and thawed samples over 150 cycles (during the F150 test). The tests show that the structure of the sulphur concrete was destroyed despite the original negligible absorbability of only 0.05% and its complete water tightness (0 mm water permeability under pressure).

3.5. Water Resistance

In the tests on the sulphur concrete, a water tightness degree of W8 was assumed, corresponding to the impact of 80 m of the water column. Then, the samples were split, and the depth of water penetration into the concrete was measured. All sulphur concrete specimens had a 0 mm water penetration depth. The results of the tests prove that concrete met the requirements for water tightness, following the standard’s requirements, by a large margin [7]. Sulfur concrete is a watertight concrete with a tightness class of 3, according to [57]. The studied concrete has mainly closed pores, which were larger closer to the bottom surfaces of the paving stones (due to the plastic viscosity of the sulphur polymer).

3.6. Leachability

The results of the tests carried out following the PN-EN12457-4:2006 standard [53,54] are listed in

Table 10. Results of tests on the leachability of sulphates and sulphides and pH of sulphur concrete water extract, as well as limits according to the European Union Directive 2003/33/EC [56].

Parameter	Results for Sulfur Concrete	Limit Value (mg/L)
Sulfates (SO42−)	<50 mg/L (after 24 h of leaching in distilled water)	1000 mg/L (inert waste)
Sulfides (S2−)	<1 mg/L (after 24 h of leaching in distilled water)	No direct limit in the directive; Their indirect impact by other pollutant parameters is usually assessed.
pH	7.5	from 6 to 12

.

4. Discussion

4.1. Mechanical Properties

Concrete used in communication structures should have a specific compressive strength class. For road surfaces, the minimum concrete classes are [58]:

-

C25/30: the standard strength class used on most road surfaces;

-

C30/37: for surfaces exposed to higher loads or extreme weather conditions.

Sulphur concrete has been awarded the class C40/45 (Table 4).

For concrete used in road infrastructure (such as pavements, bridge decks, curbs, etc.), the standards typically require a certain minimum compressive strength class. For example, according to PN-EN 13877-1:2013 (Concrete Pavements—Material Specifications) and related national guidelines, a pavement concrete might need to be at least class C30/37 or higher depending on the traffic load category. In our case, the sulphur concrete achieved class C40/45, which comfortably satisfies the strength criterion for virtually all road applications (even heavy-duty pavements usually use C30/37 or C35/45 concrete). The high strength of this concrete is partly due to the low porosity and strong matrix–aggregate bonding provided by the sulphur binder. It is notable that our specimens were cubes tested in compression; if we were to classify this concrete by its strength in cylinders, the class might be slightly lower (perhaps C35/45), but still above common requirements. Previous studies (e.g., [14,59]) have also reported compressive strengths in the range of 40–60 MPa for modified sulphur concretes, which is in line with our findings. This confirms that strength is not a limiting issue for sulphur concrete—it can be made as strong as needed for structural purposes. Additionally, the quick strength development of sulphur concrete (reaching full strength essentially as soon as it cools) is a major advantage for rapid construction or repair scenarios.

One consideration is the modulus of elasticity and behaviour under load of sulphur concrete. Sulphur concrete tends to be a bit more brittle than Portland cement concrete (although the styrene modification improves its ductility somewhat). Our tests did not directly measure the elastic modulus or tensile splitting strength of our samples, but the literature suggests that sulphur concrete has a high modulus (on the order of 25–30 GPa) and a low tensile strain capacity. This brittleness was evidenced by the mode of failure observed in compression tests—the cubes exhibited sudden failure and shattering (Figure 2), typical of a high-strength but low-toughness material. In practical terms, this means that, while its compressive strength is high, designers would need to ensure that sulphur concrete elements are not subjected to shock or tensile stresses without reinforcement. Some researchers have recommended adding fibres (e.g., glass or polypropylene fibres) to sulphur concrete to improve its tensile behaviour and crack resistance.

Fibre reinforcement (at about 8–11 kg/m³ of glass fibre, 13–38 mm length) has been found to be effective in preventing shrinkage cracks and enhancing the toughness of sulphur concretes. Incorporating such measures could further broaden the applicability of sulphur concrete by mitigating its brittleness.

4.2. Abrasion

In the case of XM classes, the concrete must have a minimum compressive strength: C30/37 for exposure classes XM1 and XM2 and C35/45 for XM3 (

Table 11. List of XM exposure classes according to PN-EN 206 [25].

Class Designation	Environment Description	Examples of Exposure Classes	Min. Strength Class
XM1	Moderate Abrasion hazard	Floors used by vehicles with pneumatic types	C 30/37
XM2	Severe abrasion hazard	Floors used by forklifts lift trucks with elastomer tires or on steel rollers	C 30/37
XM3	Extremely Strong Abrasion hazard	Surfaces frequently run over by tracked vehicles Overflow areas Walls of triggers and hydrotechnical and its expedition basins	C 35/45

). Sulphuric concrete meets this condition (Table 5).

According to PN-EN 13892-3 [43], the requirements for road surfaces are as follows:

-

For roads with heavy traffic (e.g., motorways, expressways), Class A1.5 is recommended, which indicates the highest abrasion resistance;

-

For urban roads, parking lots, and local roads, Class A2 or A1.5 is used depending on the expected loads;

-

For less stressful surfaces, Class A3 can be considered only in areas with very little traffic;

The results of sulphur the concrete abrasion tests summarised in Table 5 prove that sulphur concrete meets the requirements for all types of road surfaces.

Concrete in roads and industrial floors is often subject to abrasive wear. Our sulphur concrete’s abrasion result (class ~A6) indicates that it can withstand surface wear at a level comparable to conventional concrete made with hard aggregates. According to PN-B-06265:2004 (national supplement to EN 206), the exposure classes for abrasion (denoted as XM1, XM2, and XM3) have been defined for environments with different severities of abrasion. For instance, XM1 might correspond to mild abrasion (foot traffic) and XM3 to heavy abrasion (vehicles with studded tires, etc.). These classes also relate to minimum strength requirements (as mentioned earlier, C30/37 for XM1–2, C35/45 for XM3). Our sulphur concrete meets the strength requirements, and its measured wear was low enough that it would qualify for use even in XM3 environments. The dense binding of aggregates by sulphur likely contributes to the concrete’s good abrasion performance—the quartz aggregate we used is very hard, and the sulphur matrix, while softer than stone, firmly locks the aggregate in place due to its complete coating and adhesion when solidified. In contrast, asphalt (bituminous) pavements, despite also having aggregates, can suffer more abrasion because the bitumen that is used is softer and viscoelastic. Therefore, in terms of wear resistance, sulphur concrete could be superior to asphalt and on par with high-quality cement concrete.

One must consider temperature: at very high service temperatures (close to sulphur’s softening point of ~120 °C), the abrasion resistance would drastically reduce as the binder softened. However, in typical conditions (even hot climates up to, say, 50–60 °C surface temperatures), the sulphur binder remains solid and hard, so the abrasion should remain low. The fact that sulphur concrete does not significantly soften in summer heat (unlike asphalt) is an advantage for resisting rutting in road applications. Overall, the sulphur concrete’s abrasion resistance is adequate for road surfaces, meaning that, from a wear standpoint, it would not limit the material’s use in pavements or industrial flooring.

4.3. Frost Resistance

The General Technical Specification D-05.03.04 [28] regulates the requirements for the surfaces of national roads with traffic categories from KR1 to KR7 that are made of cement concrete.

According to the specification, the surface concrete should correspond to the following exposure classes:

• XF3—in the absence of the use of chemical winter road maintenance agents;
• XF4—when using chemical winter road maintenance agents.

The technical conditions of WT-5 [60] contain detailed requirements for concrete used in road surfaces, including frost resistance criteria. The requirements for frost resistance and numerous frost cycles for road surfaces are as follows:

• For roads with high traffic loads (e.g., motorways, expressways), a hardiness rating of F150 or higher is required, meaning that the concrete must withstand at least 150 freeze–thaw cycles without damage;
• For local, urban, or lightly loaded roads, an F100 grade is usually required, which means that the concrete can withstand 100 freeze–thaw cycles;
• Prefabricated elements, paving stones, and other precast concrete elements exposed to direct weather conditions also require the frost resistance class F100 or F150, depending on the local climatic conditions and the expected load [61].

As proven in Table 7 and Table 8 and Figure 4 and Figure 5, the sulphurous concrete did not achieve a frost resistance of F150 because the percentage of reduction in the concrete’s strength after the cyclic freezing was more significant than 20%, which is the limit for obtaining F50 frost-resistant concrete. Interestingly, depending on wherein the world the test takes place, the minimum reduction in endurance after frost cycles is different:

• PN-EN 206:2014-12 standard [25]—according to this standard, after the testing of the frost resistance of concrete, the permissible loss of compressive strength of concrete should not exceed 20% in terms of the initial strength after the freezing and thawing cycles;
• International standards (e.g., ASTM C666—[61])—according to the American standard ASTM C666, the permissible loss of compressive strength for concrete designed for freezing conditions can be 20–30%, depending on the design specifications.

It should be noted that the sulphur polymer plasticisers are added at high temperatures; while cooling, they are characterised by good strength. The effect of cooling is shrinkage, which can lead to water ingress into accessible areas when the concrete is frozen, particularly in the sphere between the aggregate and the sulphur binder, which is typically made of materials like asphalt concrete. Sulphur concrete’s lack of internal frost resistance has also been shown in the tests mentioned in the introduction. The sulphur concrete used by the author turned out to be ineffective due to its frost resistance. Nevertheless, attempts are still being made to improve this property. One article [37] investigated the effect of the addition of melamine-formaldehyde waste on the properties of sulphur concrete. The results indicate an improvement in the material’s compressive strength and frost resistance after this modifier is introduced. Another study [39] examines the effect of introducing tung oil into a sulphur binder. This additive has contributed to increased compressive strength and improved water resistance in concrete formulations, which may affect the overall durability of sulphur concrete. On the other hand, [40] presents research on using a modified sulphur polymer in concrete production. The results suggest that the appropriate selection of polymer may improve sulphur concrete’s frost resistance and other mechanical properties.

According to the author’s opinion, sulphur concrete can also be reinforced with reinforcing steel, resin-coated reinforcing steel, or fibreglass to improve its frost resistance, and can also be modified by the abovementioned polymers and other materials [60]. Glass fibre mixed into concrete effectively prevents the formation of shrinkage cracks, improving sulphur concrete’s plasticity and success. Reference [60] recommends using glass fibre shards of 13 + −38 mm for 8.5 to 11.4 kg/m³ of sulphur concrete.

Frost resistance is the major weakness identified in this study for the studied sulphur concrete. Standard specifications for concrete paving elements (e.g., curb stones, paving blocks, bridge decking concrete) commonly require an F150 rating for freeze–thaw tests with de-icing salt conditions for severe climates. Our material only achieved a taking of F50, failing to meet the standards for an F150 rating. This indicates that, without modifications, this sulphur concrete cannot be used in freezing and thawing environments where long-term durability is needed. It would be unsuitable for road surfaces, bridge decks, sidewalks, or any structure exposed to winter conditions and de-icing salts. The internal damage that was observed suggests a fundamental incompatibility in the composite when it is faced with repetitive thermal cycling. Unlike cement paste, which has some ability to accommodate ice expansion if it is properly air-entrained, the sulphur matrix is continuous and perhaps builds up stress as the temperature changes because sulphur expands upon solidifying (the orthorhombic–monoclinic phase change around 96 °C might also create internal stress on cooling, although our service range is well below that). The lack of internal air voids (which normally protect concrete by providing space for ice) in sulphur concrete could mean that there is no relief for any pressure that does develop.

To critically assess this result, we note that some prior research had hoped that sulphur concrete’s negligible water uptake would make it immune to freeze–thaw damage, but our findings contradict that optimism. For example, early studies like Smaga (2005) [19] and some patents claimed that sulphur concrete had good frost resistance in lab tests, but often those tests were less rigorous or the mixes were modified with flexible polymers. In our case, despite the low absorbency that was obtained, the material still cracked under freeze–thaw cycling. This indicates that factors other than water expansion are at play—likely thermal mismatch or inherent brittleness, as discussed. This aligns with findings by Al-Hasan and Arfaoui (2019 [36]), who also noted that achieving high freeze–thaw resistance in sulphur concrete is challenging and requires additional modifications. They and others have experimented with various methods: adding rubbers or other polymers to increase flexibility, incorporating air-entraining agents (though not typical for sulphur mixes), or fibre reinforcement to control crack propagation [62,63]. One promising approach that has been reported is the use of bitumen modifiers: Stel’makh et al. (2023) [64] showed that, by modifying sulphur with bitumen and fly ash, the frost durability of the resulting concrete can be somewhat improved (although not necessarily to the point of meeting the F150 standard without significant bitumen content).

Given our results, it is clear that, for any outdoor application in cold regions, the sulphur concrete formula would need to be improved for frost resistance. Adding glass fibres is one recommendation (as mentioned, reference [28] suggests glass fibres to help hold the material together against cracking). Another approach could be to incorporate a small fraction of flexible polymer (such as a rubbery additive) to toughen the sulphur matrix—essentially creating a sulphur–polymer composite that can absorb strain. Air entrainment in the classical sense might not be feasible because one cannot just mix in an air-entraining agent into molten sulphur the way it is done in cement paste; however, some research has looked at creating intentional voids or using lightweight porous aggregates to create some buffer space. These are areas for future research. Our study highlights that, without such measures, sulphur concrete is not frost-proof. Therefore, while it could work well in warm climates or indoor applications, its current formulation is inadequate for environments with frequent freezing.

In practical terms, this limits the use of sulphur concrete in road infrastructure to regions that do not experience freeze–thaw cycles (for example, tropical or subtropical climates), unless modifications are made. Another possibility is using it in underground or enclosed structures where temperatures are stable (e.g., sewer systems below frost depth, or interior industrial floors)—which avoids the frost issue entirely and allows one to capitalize on the material’s strengths (chemical resistance, rapid curing). In fact, based on our findings, we can suggest that municipal infrastructure below the frost line (like underground pipes, sumps, etc.) would be a reliable application for this sulphur concrete. In such cases, conventional concrete often suffers from corrosion, whereas sulphur concrete would excel, and freezing and thawing is not a concern.

4.4. Absorbability

The very low water absorption of the studied sulphur concrete is a positive attribute. It means that the material is highly resistant to water ingress, which correlates with durability in multiple ways: less corrosion of reinforcement (if any were present), less freeze–thaw damage (in theory), and less penetration of deleterious substances like chlorides. Our results confirmed that the sulphur concrete easily meets the typical water absorption limits for concrete in exposure classes XF (freeze–thaw) or XS/XD (chloride exposure), which are often set around 5%.

In fact, at 0.05%, it far surpasses even the most stringent requirement. This material can be considered practically impermeable to water absorption.

An interesting point is that, despite this impermeability, freeze–thaw damage still occurred, reinforcing that water absorption alone is not the sole predictor of frost resistance. However, for other durability concerns like alkali–aggregate reactions or sulphate attack, the lack of water transport means that these mechanisms are essentially inactive in sulphur concrete. There is no internal water to drive chemical reactions. Sulphur concrete does not contain Portland cement, so standard durability issues like sulphate attack or alkali–silica reaction (ASR) do not apply in the conventional sense. The aggregate could still potentially suffer if it has unstable phases, but our quartz aggregate is stable. Therefore, the low absorption simply underscores that, if frost could be managed by other means (like adjusting the binder), the sulphur concrete would be extremely durable against many forms of environmental deterioration.

From a construction perspective, the impermeability of this concrete also means that no curing is required and that there is no moisture exchange with the environment. This can be good (no drying shrinkage), but one should consider that, when paving with sulphur concrete, joints and drainage need to account for its waterproof nature—water will not soak into the material at all, so surface water must be drained elsewhere. This is analogous to dense asphalt. In summary, sulphur concrete meets all water absorption/water tightness criteria for use as road and infrastructure elements

Standard PN-88/B-06250, “Ordinary concrete” [44], recommends that the water absorption of concrete should not exceed 5% in the case of concrete directly exposed to weather conditions and 9% in the case of concrete shielded from direct weather conditions. In PN-EN 206-1 [49], it is difficult to find the term “concrete absorbability”. In practice, the specifications for reinforced concrete road, bridge, and railway structures include the following requirements: a concrete class of around C 30/37, water tightness of W8, frost resistance of F150, and water absorption up to ≤5.0% or even ≤4.0% (most often in the case of railway facilities).

The results listed in Table 6 prove that sulphur concrete meets the requirements for water absorption. It is characterised by very low absorbability. This concrete meets all the guidelines for use as elements of roads and other infrastructure.

4.5. Water Resistance

The water penetration test demonstrated that sulphur concrete can be considered watertight under significant pressure. All specimens showed 0 mm penetration, which is typically the requirement for W8 class concrete (the highest water tightness class often specified for watertight structures, like tanks or waterproof concrete).

This means that the material could be used in structures where water leak prevention is critical—like containment structures, basement walls, or liquid-retaining structures—assuming other properties (like structural strength and thermal stability) are accounted for. Conventional concrete often struggles to achieve complete watertightness without admixtures or special care (some minimal penetration usually occurs), so this result is quite remarkable and directly results from the polymeric nature of the sulphur binder. Essentially, sulphur concrete behaves like a plastic liner integrated with aggregate.

One might ask: does this watertightness hold over time, especially if micro-cracks form? In a scenario where cracking occurs (such as under frost or structural loads), those cracks could allow water to penetrate. In our F150 test, for instance, cracked specimens would no longer be watertight. Therefore, in real-world application, maintaining watertightness is contingent on preventing cracks. Under service loads within the elastic range, and if thermal stresses are managed, the structure of sulphur concrete would remain crack-free and thus impermeable. However, if it is subjected to extreme conditions that cause cracking (like frost or excessive loading without reinforcement), then water could find paths of entry. This is no different from regular concrete, which also loses water tightness if cracked. The takeaway is that sulphur concrete can provide excellent water resistance as long as it is used in a manner that avoids cracking (for example, as precast panels or blocks in compression, or with proper joints for movement).

The standard PN-EN 13877-2:2013 [25,26] recommends testing the penetration depth of pressurised water into concrete to determine its resistance to the ingress of gasoline, oil, and other chemicals, whereby the permissible depth of water penetration should not exceed 30 mm. According to the general technical specification (OST) [28,33] concerning structural concrete in transport construction, concrete in structural elements exposed to the impact of a chemically aggressive environment should show resistance to the penetration of water under pressure (test according to PN-EN 12390-8: 2011 “Concrete tests—Part 8: Depth of penetration of water under pressure”). According to the OST and PN-EN 12390-8, concrete should resist water penetration under pressure [47]. The maximum depth of penetration measures the depth of penetration of water under pressure:

• Less than or equal to 60 mm in exposure class XA1;
• More than or equal to 50 mm in exposure class XA2;
• Less than or equal to 40 mm in exposure class XA3.

All specimens achieved 0 mm water penetration, proving that sulphur-bearing concrete is characterised by very high-water tightness and meets all the abovementioned requirements.

4.6. Leachability

The low leachability of sulphate and sulphide from sulphur concrete is an important environmental finding. One concern with the use of industrial sulphur waste in construction is whether any harmful substances could leach out over time (sulphate could contribute to soil or groundwater contamination, sulphide could lead to odour or toxicity issues). Our tests indicate that the sulphur is largely immobilized in the solid matrix. The sulphate levels (~50 mg/L) in the leachate are quite low—by comparison, drinking water standards often allow sulphate up to 250 mg/L, and the environmental limits for construction products are usually in the hundreds of mg/L range for sulphate, so 50 mg/L is minor. The limit for sulphide < 1 mg/L is also very low; a higher level of sulphide could be a cause for worry because it can oxidize to sulphate or emit H₂S gas, but here it is negligible. These results suggest that using this sulphur concrete does not pose a significant leaching hazard. This is consistent with other studies on sulphur polymer cement (e.g., work by [6] for nuclear waste immobilization) which found that properly formulated sulphur polymer matrices have excellent containment properties for hazardous waste. In fact, sulphur concrete has been proposed and used for encapsulating radioactive or toxic waste because of its low permeability and low leach rates. Our findings support that application—the material could safely immobilize substances without release.

Of course, the long-term stability of the sulphur bond needs to be considered. Sulphur is stable in a basic or neutral environment, but if exposed to strong acids or certain bacteria, it might oxidize to sulphate. We noted that, in very acidic conditions (pH < 5), there could be an increase in sulphate leaching. This implies that, if sulphur concrete were used, for example, in an acidic soil or exposed to acid rain persistently, over many years some surface degradation could produce sulphate. However, in most normal environments (pH around 7 or slightly alkaline in concrete surroundings), this is not an issue.

In practical terms, to be environmentally cautious, any project using sulphur concrete near groundwater would still likely require specific local leach tests. However, our test results would give confidence that it can pass those tests. The material meets typical criteria for non-hazardous material leaching [65]. Therefore, from a sustainability viewpoint, turning sulphur waste into sulphur concrete not only consumes the waste but also does so in a way that contains the sulphur safely (preventing it from leaching out). This addresses one of the initial goals of this study: removing large sulphur stockpiles by using the sulphur in a beneficial and environmentally sound manner.

The results of tests according to PN-EN 12457-4:2006 [53], listed in Table 9, indicate that the leachability of sulphates (SO₄²⁻) is relatively low, usually being below 50 mg/L in the standard tests. The amount of leaching can grow in low pH (acidic) environments. The leachability of sulphide (S²⁻) is generally very low (less than 1 mg/L) because sulphides are more stable in the sulphur concrete structure and are hardly soluble in water.

The impact of environmental conditions should also be checked when assessing the safety of sulphur concrete. Under the influence of ecological factors, sulphur may partially decompose, and sulphides or sulphates may be released. At a pH of <5, it is possible to increase the leaching of sulphur compounds, which can lead to the release of sulphates. On the other hand, high temperatures and intense humidification can accelerate the degradation process of sulphur concrete, leading to an increase in its leachability.

The low leaching of sulphates and sulphides obtained in the analysed studies confirms that sulphur concrete can be safely used in construction, especially in places that are exposed to water, such as sewage systems or chemical tanks. However, additional testing is recommended for applications in areas with high environmental protection (e.g., near groundwater) to ensure that the material meets local regulations, especially for construction localised near water for humans [55].

5. Remarks on the Safety Production and Appropriate Use of Sulphur Concrete

Research on sulphur concrete indicates its high abrasion resistance and water tightness, making it a promising material for use in industrial infrastructure. Sulphur concrete is created using a modified sulphur polymer as a binder, which gives it unique mechanical and durability properties. In the tests conducted herein, abrasion resistance values were obtained following the requirements for road surfaces. However, this material is characterised by limited volumetric frost resistance, which limits its use in difficult climatic conditions. Research results [6] also show that sulphur concrete is characterised by frost resistance only on its surface. However, using road infrastructure elements that require the criterion of dimensional frost resistance to be met is impossible. Some road elements may be exposed mainly to surface frost resistance and not necessarily to volumetric frost resistance, but this is the exception rather than the rule. An example of such elements are surfaces with a thin surface layer (micro-surfaces), where the layer must resist freezing and thawing cycles. Still, the internal structure is not so strongly exposed to low temperatures. Polymers such as styrene–butadiene emulsions can improve the material’s elasticity and resistance to cracking.

Although the strength and abrasion results obtained for the sulphur concrete are suitable for all road surfaces, it should be remembered that these surfaces, after hardening, should also be characterised by good anti-slip properties, high abrasion resistance, resistance to rutting, and the ability to drain water on the surface. Hydroplaning or aquaplaning is a phenomenon that occurs when a thin layer of water on the road surface separates a vehicle’s tyre from the road surface. This results in a loss of traction, leading to uncontrolled vehicle skidding. This dangerous phenomenon can occur on wet roads, especially during heavy rainfall. Smooth surfaces (e.g., asphalt without porosity) are more likely to form a water layer, while more porous surfaces (e.g., porous concrete) can drain water better.

This concrete is also not durable at high temperatures due to the relatively low plasticisation temperature of the sulphur polymer. The softening point of the sulphur polymer is 90–120 °C, depending on the degree of polymerisation and the presence of impurities. This value may vary slightly depending on the synthesis method and the material’s exact composition. The classic form of sulphur has a melting point of about 115 °C. In contrast, in polymer form, it softens at slightly lower temperatures due to being stretched, making polymer structures more easily deformed. In addition, sulfuric concrete is combustible, which is a serious limitation for its use. The temperature of the concrete surface on a hot, sunny day can be much higher than the ambient temperature. In hot summer conditions, when the air reaches about 30–35 °C, the surface temperature of the concrete can rise to 50–60 °C or even higher, especially when it is directly exposed to sunlight. However, darker concrete can heat up more, even up to 70 °C. The results of research conducted by the author based on the [66] indicate that the concrete analysed in this article begins to deform slowly at a temperature of 60–65 °C. Adding metal oxides such as magnesium oxide (MgO) can help increase the resistance of concrete to higher temperatures. Metal oxides can stabilise sulphur, making the resulting concrete more resistant to sudden temperature changes.

Another very valuable feature of sulphur concrete—its lack of water permeability—can be used in the search for appropriate conditions for its practical application. With its additional low abrasion, sulphur concrete can be used below the depth of ground freezing, which is also beneficial for another reason—its smell [12].

An undesirable feature of modified sulphur concrete products is their smell, which is mainly caused by the emission of gases containing sulphur (H₂S, SO₂) and mercaptans (i.e., methyl mercaptan and ethyl mercaptan), both during the production and during the service life of sulphur concrete products [67,68]. This smell does not matter in the case of foundations, sewage systems, or water construction, but is an issue in other applications such as road slabs, paving tiles, walls, building structure elements, etc. It can also be dangerous for workers operating the production line. Publications indicate that fragrances with irritating properties can cause disease [57,69]. Therefore, for the broader use of sulphur concrete, there is a need to modify its composition to reduce its odour generation while ensuring the required technological and strength properties. Polymerising additives such as H₂O₂, NaCl wax, olefin compounds, or styrene can positively reduce the odour generation of sulphur concrete. One of the patented methods of reducing the odour intensity of sulphur concrete is to add an appropriate bleaching agent to the mixture [59].

However, using sulphur concrete is safest when the concrete does not experience direct long-term exposure to water, especially acidic water or water at elevated temperatures. The leaching of sulphur compounds from concrete meets the normative limits for inert waste. Nevertheless, acidic groundwater can negatively affect this parameter. Acidic groundwater can be formed from the decomposition of organic matter in anaerobic conditions and can lead to water acidification. It can also be formed due to acid rain resulting from the emission of industrial pollutants (SO₂, NOₓ), which can acidify groundwater after soaking into the soil. Intensive fertilisation in agriculture can also lead to the acidification of soil and groundwater. In an acidic environment (pH < 5), the sulphur contained in sulphur concrete can be partially oxidised and converted into sulphates (SO²⁻), which can be washed away. Water with a very low pH (e.g., industrial) can accelerate the leaching of sulphates, which increases the risk of degradation of the concrete structure. Sulphides (S²⁻), on the other hand, are usually stable in sulphur concretes, but, in highly acidic environments, they can oxidise to sulphates, leading to further acidification. To minimise the risk of increased pronounceability of the compounds mentioned above, the following methods can be used:

• Concrete composition optimisation—adding sulphur stabilisers or modifiers (e.g., polysulfide) to increase chemical stability;
• Surface sealing—using protective coatings that prevent concrete from meeting water;

Control of curing conditions—correct curing of concrete to avoid micro-cracks, which can increase water permeability.

Sulphur concrete is highly resistant to acid attack, especially that by sulfuric acid, due to the unique properties of sulphur as a binder. Studies have shown that sulphur concrete exhibits superior resistance to chemical degradation compared to traditional Portland cement-based concrete. This makes sulphur concrete an ideal choice for applications in aggressive environments such as wastewater treatment plants, chemical storage tanks, and acid-laden industrial settings. Sulphur concrete has remarkable thermal stability, with studies indicating that it retains its mechanical properties even at elevated temperatures (up to 160 °C). This makes it suitable for use in environments with high thermal fluctuations or those exposed to fire hazards [68].

Sulphur concrete is safe for use in industrial environments and facilities that are exposed to aggressive chemicals. Still, its use in areas that are exposed to extreme weather conditions (high temperature and frost) or groundwater with no pH, as well as in public places, requires additional material modifications or external protection. Sulphur concrete is not a safe material from the point of view of fire safety. Previous changes, which include the addition of aggregates, polymers, and other additives [14], have improved sulphur concrete’s durability by approximately 30% and increased its environmental resistance, making it a viable material for use in harsh conditions [15,37]. The results of [14] indicated no significant weight loss or relevant variation in the compressive strength of sulphur concrete after specimens were immersed in sulphuric acid and sulphate solutions. In addition, similar results were obtained for slabs in chemical plants whose conditions were assessed during 60 days of exposure. Moreover, sulphur concrete is highly resistant to biological and chemical corrosion [63].

Long-term performance studies have indicated that, when properly formulated, sulphur concrete can maintain its integrity and mechanical properties over extended periods. This durability is especially evident in environments where other materials typically experience degradation due to chemical or physical stresses [36]. However, challenges such as the potential for sulphur to undergo phase transitions under extreme temperature conditions remain.

One of the main advantages of sulphur concrete is its environmental sustainability. Sulphur concrete can be produced using waste sulphur, which not only helps to reduce industrial waste but also minimises the carbon footprint compared to traditional cement production [63]. This makes sulphur concrete an attractive alternative in environmentally conscious construction practices.

Summing up the properties of sulfuric concrete, it can be concluded that sulphurous concrete exhibits limited frost resistance, which makes it less suitable for applications requiring durability in extreme climatic conditions. Studies indicate that sulphur concrete has surface frost resistance, which means it can cope well with freeze–thaw cycles at the surface-layer level in the presence of de-icing salts. However, its resistance to these conditions in its entire volume (so-called volumetric frost resistance) is insufficient for applications in infrastructure that require complete protection against extreme weather conditions. Sulphur concrete can be used in infrastructure elements that are less exposed to deep frost, e.g., prefabricated road elements or parking lot surfaces. For applications requiring high frost resistance (e.g., road surfaces in regions with a harsh climate), further modifying the sulphur concrete recipe or choosing other building materials is necessary.

Finally, the properties of sulphur concrete are adequate for other construction applications that do not require exposure to very low or high temperatures, and especially those that do not involve fire risk [70]. For example, sulphur concrete can be used on construction elements that are not exposed to freezing cycles. The most significant advantage of sulphur concrete is its low acid resistance. Therefore, it is predisposed to use in municipal infrastructure. Underground and communal structures below the depth of ground freezing are the most reliable methods of use for this type of concrete, where solid concrete shows high corrosion in such cases. Moreover, sulphur concrete can be used to build roads where intense mechanical or chemical action occurs, such as floors in chemical plants and production halls, due to its high resistance to abrasion and different types of chemicals. Moreover, in the case of concrete with high resistance to ingress and leaching, such as sulphur concrete, it is possible to immobilise the waste that is used, which is in line with the policy of closed circulation and reducing the surface area of landfills.

6. Summary and Conclusions

Sulphur concrete modified with 5% styrene was tested against European standard requirements for road construction materials. The experimental program included compressive strength, abrasion resistance, freeze–thaw (frost) resistance, water absorption, water impermeability, and leaching tests. Unfortunately, the results show that this sulphur concrete, in its current formulation, does not fully meet the key requirements for use in road infrastructure. The findings can be summarised as follows:

• High mechanical strength and low permeability: The sulphur concrete achieved a compressive strength class of C40/45 and demonstrated a very high level of water resistance (zero water penetration) and extremely low water absorption. It also showed good abrasion resistance that is suitable for transportation infrastructure use. These attributes indicate that, from a mechanical and durability standpoint (aside from frost), sulphur concrete can perform well as a construction material. It meets all the guidelines for conventional road concrete in terms of its strength, wear, and watertightness;
• Inadequate frost durability: The sulphur concrete is unsuitable for structures exposed to freeze–thaw cycles, due to its lack of internal frost resistance at the F150 level. After 150 cycles of alternating freezing and thawing, the concrete’s compressive strength was reduced by over 30%, far exceeding the 20% maximum loss that is permitted. The material only achieved an F50 frost resistance rating, which excludes its use in road or bridge elements in cold climates. Improving the frost resistance of sulphur concrete is necessary before it can be applied in such conditions. Further tests and development are required—for instance, future research could explore making the mix frost-resistant by incorporating glass fibres (as recommended by prior studies) or using other types of modifiers in the sulphur binder to impart flexibility and crack resistance. These modifications would aim to mitigate the brittle behaviour and thermal mismatch that led to frost damage in our tests;
• Limited leaching of sulphur compounds: Sulfur concrete is a stable material with regard to chemical leaching. It behaves as a building material with very limited sulphate leaching due to its dense, impermeable structure. Even in aggressive environments (e.g., acidic conditions), only a small amount of sulphate ions may be released. In practice, before sulphur concrete is used in a sensitive environmental setting (such as near groundwater reserves), it is standard procedure to perform laboratory leachate tests to ensure compliance with environmental standards. Our results indicate that the material can meet relevant leaching criteria, as the release of sulphates and sulphides was very low in this study. This suggests that, from an environmental safety perspective, sulphur concrete can be used without causing contamination, aligning with the goals of waste utilisation and landfill reduction.

In conclusion, while the styrene-modified sulphur concrete studied here shows promising strength, chemical resistance, and rapid setting qualities, it falls short in its durability under freeze–thaw exposure, a critical requirement for road infrastructure in many regions. Additionally, one must consider that sulphur concrete is flammable, which poses a fire safety concern during both its production and use (the material would not be suitable for high-temperature environments or where fire resistance is required). These drawbacks currently limit its direct substitution for ordinary concrete in structural applications like pavement or bridge decks in temperate climates.

However, the material’s unique advantages—such as its low permeability, corrosion resistance, and fast deployment—make it attractive for specific applications. It could be effectively used in municipal engineering structures that are not subject to freezing, for example, underground installations, foundations below frost depth, industrial floors in warm climates, or precast elements in chemical plants. In such environments, sulphur concrete’s lack of frost resistance is not an issue, and its resistance to chemicals and water could prolong the lifespan of structures built with it. Further improvements are needed to enable the broader use of sulphur concrete in infrastructure. Future research should focus on enhancing sulphur concrete’s frost resistance and ductility. Potential strategies include incorporating fibre reinforcements, using hybrid binders (sulphur combined with small amounts of additive polymers or crumb rubber to increase its flexibility), or even developing sulphur concrete composites with internal micro-voids to relieve freeze pressure. Some recent studies have shown that adding modifiers like bitumen or other polymers can markedly improve the freeze–thaw performance of sulphur concrete. By acknowledging and building on such research, it may be possible to formulate a sulphur concrete that meets the criteria for F150 durability. Additionally, addressing the fire safety aspect (perhaps by incorporating flame retardants or protective coatings) would be necessary for its structural use.

In summary, the present study demonstrates both the potential and the challenges of using sulphur concrete for sustainable construction. It underscores that, while sulphur concrete can meet many technical requirements (strength, impermeability, etc.), certain performance gaps—notably frost resistance—must be bridged. With further development, sulphur concrete or related sulphur–polymer composites could become viable materials for civil engineering, offering a productive use for industrial sulphur waste and contributing to circular economy goals by reducing the need for Portland cement and utilising excess sulphur. Until then, its use should be limited to appropriate niches where its properties are advantageous, and its weaknesses (like frost vulnerability) are not detrimental.