Gilsonite mixtures and binders can be improved by modifying them with various additives. It is a natural bituminous hydrocarbon used as an additive to hot mix asphalt. It is used as an asphalt binder modifier and asphalt mix modifier to improve mix performance. Offers options to improve rheological properties, economic stability, strength rutting resistance and moisture sensitivity. Since gilsonite is recognized as an additive material that contributes to the modification of bitumen binders and thus improves the performance of asphalt mixtures, several research projects have conducted further research to develop bitumen binders by combining other additives with gilsonite. 
Most of these studies decided to choose SBS as an additional additive for diaspore, because SBS improves the performance of HMA, and also because SBS is an expensive additive and uses diaspore instead of SBS. Based on rheological evaluation of the modified binder, the use of diaspore and SBS in the same mix reduces the viscosity of the binder, which helps to improve the workability of the asphalt mix by lowering the mixing and compaction temperature. According to the research reviewed, modification of asphalt binders with hard asphalt and SBS improved stability, ITS, moisture sensitivity and fatigue life. It also improves the storage stability of bitumen-modified binders, thereby extending the life of the HMA. The durability and moisture resistance of asphalt concrete significantly increase the service life of the pavement. When using silicate aggregates, there is a clear need to prevent moisture damage and debonding. Generally, peeling can be prevented by using anti-peeling agents or by modifying the adhesive. Anti-stripping agents achieve their purpose by altering the zeta potential to alter the affinity of the aggregates for water. On the other hand, binder modification promotes the bonding between aggregate particles and asphalt. However, the process of applying anti-stripping agents or traction enhancers is often expensive and, in some cases, adversely affects other pavement performance characteristics. Therefore, this paper explores the value of using natural bitumen instead of commonly used anti-stripping chemicals to improve bitumen resistance to moisture damage and reduce bitumen construction costs. In addition to being considerably less expensive, the chemical composition of hard bitumen is almost similar to petroleum-based bitumen, which makes it very compatible with bituminous binders. 
Gilsonite modified bitumen
Various gilsonites have been added to bituminous binders and modified bituminous mixtures by wet and dry processes, respectively. Different processes and a range of sources can result in different properties of modified asphalt mixtures. The modified asphalt mixture in the wet process provided better performance compared to the control asphalt mixture. However, the dry process is cost-effective and easier for manufacturers to produce asphalt mixtures because it does not require mixing tanks. Most of the studies used a modified wet process, and the studies reviewed showed that hard asphalt pavements using the wet process performed well. These methods have been developed and have undergone extensive research. Several tests were carried out to evaluate the performance of the resulting product according to different criteria. Physical property tests such as penetration, softening point (SP), ductility, and viscosity are common tests considered in most review studies to evaluate the rheological properties of modified adhesives. According to the reviewed studies, the addition of diaspore decreased the penetration value and increased the softening point, ductility and viscosity values. Based on these test results, modifying the asphalt binder with hard bitumen can improve the quality of the asphalt mixture. However, an increase in the viscosity of the modified adhesive leads to an increase in mixing and compaction temperature. Therefore, diaspore modification can lead to binder oxidation and reduced design life due to reduced ductility and fatigue cracking. therefore, To reduce viscosity, another additive, such as a wax, must be added to the stearite-modified binder. 
The Marshall test is considered the basic unit and generally accepted measurement method for determining the quality of asphalt mixtures, as shown in the Common Standard. Regarding the Marshall test, the use of hard bitumen in an asphalt mixture can improve the Marshall performance of the mixture by increasing Marshall stability. In addition, when the content of hard asphalt increased to 10%, the stability value continued to increase, which enhanced the deformation resistance of the asphalt mixture. Other tests are also used to evaluate the performance of asphalt mixtures. For example, Indirect Tensile Strength (ITS) testing was performed on most of the resulting asphalt mixtures, as this test demonstrates the mechanical properties and moisture sensitivity of asphalt mixtures. According to the research reviewed, mixtures containing diaspore have higher ITS values compared to mixtures without diaspore. This is due to the better adhesion of the hard bitumen modified bitumen binder to the aggregate resulting in improved adhesion of the aggregate to the binder. In addition, the addition of hard bitumen to the asphalt binder increases the moisture sensitivity of the asphalt mixture based on the TSR index. However, the content of hard bitumen above 5% in the asphalt binder has no significant effect on the TSR index. Therefore, there is no effect on humidity sensitivity. At the same time, the addition of more than 5% hard bitumen decreased the TSR index, which negatively affected moisture damage due to the increased brittleness of the mixture. In general, additions of up to 5% diaspore can improve moisture damage resistance. Therefore, it is recommended to use hard bitumen modified bitumen binders in wet areas. The main important result obtained from the creep test is to show that the permanent deformation depends to some extent on the rutting resistance of the asphalt mixture.
The results of DCT, MSCR and WTT tests show that asphalt mixtures prepared from hard bitumen-modified binders have improved rutting resistance. This improvement is believed to be due to the high content of asphaltenes in hard asphalt, which increases the rutting resistance of the asphalt-modified mixture. Likewise, the addition of hard bitumen increases the stiffness of the bitumen-modified mixture at low temperatures and reduces elasticity, thereby improving rutting resistance. Interestingly, based on the literature reviewed, it should be noted that the results of dynamic testing of bituminous mixtures and rheological testing of bituminous binders are consistent and confirm that with increasing hard bitumen content, there is an increase in rutting resistance. similar trends observed. Based on the reviewed studies, many researchers have considered the use of elastic modulus testing to evaluate elastic deformation of asphalt-modified mixtures. The results show that the elastic modulus parameter increases with the increase of the hard asphalt content in the asphalt mixture. This reflects the positive effect of hard bitumen on bitumen binders. However, the elastic modulus test results showed that the flexural properties of the gilsonite-modified binder bitumen mixture decreased significantly. It is worth noting that an excess of hard bitumen in the bituminous binder can lead to too high modulus values, resulting in brittle bitumen mixtures. 
Gilsonite economic and environmental assessment
The criteria for evaluating economic and environmental aspects in this study are the use of gilsonite in the asphalt industry alone and in combination with other additives. From an economic point of view, better pavement quality and longer service life meet safety and economic requirements. The use of hard asphalt increases the tensile strength of the asphalt mixture, which increases pavement life and reduces repair and maintenance costs. Also, diaspore is a suitable and economical alternative to improve adhesive performance, for example, diaspore is less expensive than other modifiers such as EVA (ethylene vinyl acetate). According to the elastic modulus results, the hard bitumen exhibited higher performance compared to the bituminous mixture made from pure bituminous binder. Since asphalt pavements are required to have a modulus of elasticity, this result can have a significant impact on the economic design of roads. From an environmental perspective, the processes of the bitumen industry play an important role in contributing to the causes associated with global greenhouse gas emissions. Therefore, the asphalt industry is actively seeking alternative materials and technologies for sustainable development. Considering the amount of recycled material contained in the asphalt mix, hard asphalt improves the performance of porous HMA when 100% recycled concrete is used as aggregate. Therefore, it is a potential material for sustainable pavement systems. However, adding hard bitumen to bituminous binders reduces penetration values and increases SP and viscosity. This results in increased mixing and compression temperatures, which in turn lead to increased energy consumption and associated increased CO2 emissions. This can be avoided by using hard bitumen and Sasobit modified bitumen binders. In addition, DSR tests show that the viscosity of using gilsonite is lower than that of SBS, so gilsonite has an energy consumption advantage. Finally, an evaluation of the environmental impact and economic viability of bituminous mixtures containing hard bitumen shows that the combination of hard bitumen with other optional additives offers additional economic and environmental advantages. However, more research is needed to fully examine the economic and environmental perspectives. The cost-effectiveness and carbon footprint of using hard bitumen as an asphalt modifier requires further research as only a few publications address this aspect. 
Gilsonite in tropical countries
In tropical countries, roads constructed with layers of asphalt should be constructed using an asphalt mixture containing gilsonite, which is hard enough to increase resistance to permanent deformation. When the available bitumen is not hard enough, another method is to modify it with hard bitumen. This paper presents the results of laboratory tests performed on samples of hard bitumen modified hot mix asphalt modified by wet and dry methods. Gilsonite increases stiffness and improves the performance of the original adhesive at high operating temperatures. In addition, Gilsonite modified hot mix asphalt has higher strength and stiffness under monotonic and dynamic loading. Tropical countries, such as South America, are characterized by high temperature climates. In these countries, roads built with asphalt layers must be constructed using an asphalt mixture containing asphalt hard enough to increase resistance to permanent deformation (i.e., rutting). Despite this requirement, in countries such as Colombia, the hardest asphalt cement (AC) produced is AC 60-70 [Performance Grade (PG) 58-18]. This asphalt is not hard enough to resist rutting in hot areas. Another way to harden bitumen is to use hard bitumen (G) modified bitumen. In Colombia, there are many natural deposits of hard bitumen type bitumen. The deposits are mainly distributed in the departments of Boyaca, Caquetá, Caldas, Cundinamarca, Tolima, Santander and Cesar. Hard bitumen is one of the purest natural bitumens with low fixed carbon and low sulfur content. Rond´on and Reyes 2012 use this material to produce asphalt concrete mixtures and evaluate their performance under monotonic and dynamic loads. The results obtained from their study showed that the use of hard bitumen to modify asphalt mixtures using a wet process, i.e. adding hard bitumen to asphalt cement, resulted in a harder bituminous concrete mixture, leading to the conclusion that these mixtures were Performs well in climate. 
The Marshall stability and stiffness values were greater for any percentage of asphalt and hard rubber-modified mixtures compared to the reference mixture (i.e., no hard asphalt). The elastic modulus of the modified mixture was better than that of the reference mixture, and a greater increase was observed when the test temperature was increased. These results demonstrate that hard bitumen can be used as an asphalt modifier to improve the stiffness properties and resistance to permanent deformation of mixtures used in hot climates. Similar conclusions were reported by Liu and Li (2008) who used wet-modified Alaska asphalt cement to diaspore ratios ranging from 3% to 12% relative to the total weight certainly. Esfee et al. (2011) reported that adding hard bitumen to bitumen helps to increase viscosity, reduce penetration and harden the binder. Ameri et al. (2011a), based on rheological tests, reported that two asphalts (PG 58-22 and PG 64-22) modified with 4%, 8% and 12% percentages of hard asphalt relative to asphalt mass improved performance and the performance of both asphalts at high operating temperatures.