Investigating applications of mineral bitumen sulfur in asphalt reinforcement and its production process
The effects of mineral bitumen and petroleum bitumen sulfur, engineering properties of bitumen with sulfur (SEA) such as shear strength, modulus
hardness, change in formability, viscosity, as well as different methods of producing sulfur asphalt and safety points in preparation
It has been investigated. Replacing mineral bitumen containing sulfur or mixing 30 to 50% of consumed bitumen with sulfur causes the asphalt resistance to increase significantly.
About 92% of the sulfur produced in the world is used to produce sulfuric acid, and only 8% of sulfur is directly used in various industries. Among the applications of sulfur, we can mention such things as production of sulfuric acid, paper industry, sulfur dioxide, vulcanization of tires, agriculture, concrete pouring and construction coating.
Another use of sulfur and bituminous sulfur is its use in the construction of asphalt pavements. The reason for this is the increase in the price of petroleum products, the gradual reduction of hydrocarbon resources and especially excess sulfur. Replacing mineral bitumen with sulfur (natural bitumen) or bitumen with sulfur causes the resistance of asphalt to increase significantly. In the construction of road pavements, the quality of stone materials can be improved by using sulfur. Therefore, we will first discuss the engineering and technical properties of bitumen and sulfur mixture or SEA, and then we will examine its production process.
Combined effect of sulfur with bitumen and mineral bitumen with sulfur
At ambient temperature, sulfur consists of a compound ring of 8 atoms arranged in an octagonal shape, and for this reason, it has a high chemical resistance. When its temperature exceeds 118 degrees Celsius, it melts quickly and its chemical structure is broken, and its ability to combine, especially with unsaturated hydrocarbons of bitumen, increases greatly. The retardation of sulfur decreases up to a temperature of 157 degrees Celsius, so adding molten sulfur to pure bitumen at a temperature between 118 and 157 degrees Celsius reduces the retardation of bitumen and it also combines with bitumen. As a result, it will be possible to combine and mix sand and bitumen materials at a lower temperature and it will be easier to work.
Sulfur is polymerized at temperatures higher than the melting point and becomes a chain of two radicals.
Radicals may act on bitumen in two ways:
- a) They form sulfur carbon bonds.
- b) They absorb hydrogen as a result of dehydrogenation.
- Performing these reactions depends on the temperature. At a temperature of 5 ± 140 sulfur atoms enter the structure
Molecular chemistry combines with the naphthenic group of aromatics in bitumen and causes the formation of polysulfides. In terms of polarity, polysulfides are in the group of polar aromatics, as a result, bitumen will become softer and have more elasticity. Therefore, it can be said that the reaction between sulfur and aromatic naphthenic group leads to the formation of polar aromatic compounds with a high percentage of sulfur.
At a temperature of more than 157 degrees Celsius, due to the high hardness of sulfur, sulfur no longer plays the role of bitumen lubricant, and on the contrary, it hardens the mixture, and at the same time, hydrogen sulfide is released during the hydrogenation process. The rheological changes of bitumen at this temperature are due to the increase in the ratio of asphaltene to resin.
The action of sulfur in pure bitumen is as follows:
Between zero and 4 percent by weight of sulfur is combined with bitumen. Between 4 and 25 percent of sulfur is dissolved in bitumen, but no other chemical composition is formed. Between 25 and 50 percent of sulfur can be dispersed in bitumen as fine particles. For grades of more than 50% sulfur, even if it is spread in bitumen, it tends to settle.
The experiment
NAT is used by Shell company to measure the resistance for stability against deformation. In this test, Marshall samples with a diameter of 102 mm are subjected to axial load at a temperature of C.
are placed The number of loads is up to 3600, and the pressure of each pulse is 100kpa, that is, one minute under pressure and one minute of rest, and this period continues.
None of the laboratory samples made of sulfur-free bitumen can fully withstand loading up to 3600 pulses. But SEAM mixtures (with the same type of bitumen as sulfur-free mixtures) can not only withstand loading repetitions up to 3600 pulses, but will have a relatively small and stable shape change compared to PMB mixtures.
-Relative shape change of bitumen with penetration degree of 400/300 in sulfur-free mixture and SEAM mixture
Coefficient of elastic bounce The coefficient of elastic bounce is obtained from the following equation. This coefficient is related to the change of the return shape Er = (P/Hr h)(v +0.27). P is the amount of load applied to the sample, Hr is the amount of reversible horizontal shape change, h is the height of the sample and v is Poisson's ratio.
The NAT test by Shell is used to measure the rebound coefficient (MR). In this test, Marshall samples with a diameter of 102mm are subjected to repeated indirect tensile loading with a sinusoidal frequency of 5HZ.2.
- The results of the test for the comparison of the coefficient of growth in SEAM mixture and traditional mixture at 20 C temperature
The results of these experiments show that the replacement of a part of bitumen with sulfur causes the coefficient of elastic bounce to increase, especially for softer bitumens. The amount of this increase is relatively small for 20% sulfur, but for 50% sulfur. It is worth considering. Therefore, the ability to spread load in SEAM paving is significantly higher than traditional mixtures.
Resistance to fatigue
The samples were placed under a constant pressure at a temperature of 1 NAT test by Shell, fatigue life 20 C are measured.
- Fatigue test results on the samples
According to the conducted tests, we conclude that sulfur-containing asphalt pavements resist fatigue caused by loads relatively more than sulfur-free asphalt pavements.
Investigating the change in formability of SEA mixture
In an experiment using two parallel plates 30mm long and 20mm wide with a distance of 6mm called rheometer, 6mm thick samples of different mixtures of bitumen and sulfur were subjected to shear stress in such a way that one of the two plates in place It was fixed and the other plane was subjected to shear force in the cutting direction.
- The ratio of shape change to loading time at a constant temperature of 15 degrees Celsius under the effect of loads of 0.4 grams per square millimeter - 0.21 grams per square millimeter - 0.12 grams per square millimeter loading times until breaking tensile stress
The texture of sulfur bitumen
Sulfur is added to the asphalt mixture in two ways, and we will examine these two cases:
1-Adding sulfur to hot materials dipped in bitumen:
In this method, stone materials are mixed together at a temperature of 149 degrees and bitumen at a temperature of 140 degrees Celsius. First, the bitumen surrounds the aggregates and creates a coating around the material and fills the corresponding voids. When the molten sulfur is added to this hot material, it covers the bituminous material.
A part of the bitumen layer is filled with sulfur. Since the combined layer of bitumen and sulfur in this case is very thin and small, in addition, this layer cannot show much effect in sticking and putting stone materials together, but the sulfur present in the space between the materials is only in the form of Sulfur crystals will be present, in fact they will act like fine aggregates. In order to increase the effect of the thin layer of bitumen and sulfur and to reduce the empty space between the stone materials, it is necessary to increase the amount of sulfur consumed, which is not economically viable, and we do not expect anything from the added sulfur other than the role of fine-grained materials.
2- Mixing bitumen with sulfur before adding it to hot materials:
In this case, the sulfur element in the mixture will exist in three different states:
- A) A small amount of it enters the chemical composition with bitumen.
- b) Another part is dissolved in bitumen.
- c) The residue appears as sulfur crystals, which are generally in the form of small asphalt particles.
The amount of sulfur dissolved in bitumen depends on the type of bitumen. For example, a type of bitumen at a temperature of 150
degrees Celsius dissolves up to 20% of sulfur.
Design of sulfur asphalt pavements
For the preparation of sulfur asphalt or mineral bitumen sulfur, it is better to mix bitumen and sulfur or mineral bitumen together and then add it to the ingredients.
In this part, we will examine the preparation of several different designs of sulfur asphalt, which are:
1) Sulfur asphalt prepared with molten sulfur
2) Sulfur asphalt prepared with solid sulfur
3) Sulfur asphalt emulsion
Appropriate sulfur percentage
The replacement of bitumen by sulfur is based on volume. If the amount of sulfur is less than 25 percent by weight, the desired texture properties are inappropriate because the sulfur particles in the mixture will not be enough, and if the amount is above 60 percent by weight, the additional amount of sulfur is added to the grain. The dispersed particles stick and cause the mixture to become unfavorable, in addition, the dispersed phase of sulfur in bitumen turns into bitumen-in-sulfur. Therefore, the appropriate ratio to reach the desired texture includes 35 to 50 parts of sulfur in 50 to 65 parts of bitumen.
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