Highlights

2.1. Modified and fluxed bitumen

Bitumen modification, with the addition of one or more constituents, is a standard practice to change and improve the rheological properties of the base material. To achieve high road pavements durability under severe service conditions, such as temperatures and vehicle loads, bituminous binders should have a wide linear visco-elasticity range (Karlsson and Isacsson, 2006). For example, the addition of polymers either virgin or recycled, such as elastomers and plastomers, allows the improvement of the material response at high temperatures, increasing its elasticity, and also at low temperatures, reducing the resulting stiffness of the binder (Rossi et al., 2015). The polymeric material added to the bitumen absorbs the oil phase of the binder in the preparation process (temperature range 160 °C to 200 °C), forming a gel-like (cross-linked) structure. This cross-linking results in a smaller intraparticle distance and an increase in the viscosity of the binder (Lo Presti, 2013). The styrene-butadiene-styrene polymer (SBS) is commonly used as a modified agent. This is due to the excellent mechanical properties such as high fatigue resistance, elastic recovery and high resistance at extreme temperatures. It is possible to classify a large amount of various wasted elastomeric products that could be and were employed as constituent material for bitumen modification (Airey, 2004). For example, the rubber from end-of-life tyres (ELTs) is a high valued product that can be further exploited after the tyres’ service life. Furthermore, the use of these waste materials entails a double benefit, the improvement of the performance of the bitumen and the possibility of recycling materials. Therefore, modification of bitumen using crumb rubber presents itself as a sustainable approach (Ibrahim et al., 2013).

Several studies also reported the use of bio-polymers for the modification of bitumen, including poly (lactic) acid (PLA). PLA is a bio-based green thermoplastic polyester obtained from sugar cane or corn-starch; these elements can be easily found in plant biomasses (Auras et al., 2004; Rasal et al., 2010). The presence of carboxyl and hydroxyl groups within its structure allows the development of chemical reactions with the bitumen, defining its compatibility. Moreover, this bio-polymer is an interesting candidate for replacing traditional polymers, due to its high tensile strength and high modulus. Indeed, the mechanical tests conducted on bitumen modified with PLA (<10 wt%) showed good results in terms of hardness, stiffness and ductility (Jailani et al., 2021).

The bitumen modification can be also carried out with bio-chars aiming to improve the binder properties. Bio-char is another product of thermochemical conversion processes. It comes in the solid form and it is rich in carbon, but it also contains nitrogen, hydrogen and traces of heavy metals (Rondón-Quintana et al., 2022). Before being added to the binder mixture, it is reduced to particles of the order of μm (75–150 μm) and subsequently added to the bitumen. The bio-char content in bitumen varies in a range between 2 wt% and 20 wt%, nevertheless studies have shown that 10% by weight may represent the optimal content (Walters et al., 2015; Zhao et al., 2014a). Finally, a bitumen modified with bio-char has been proven to provide a higher resistance to permanent deformation (rutting), a better resistance to aging and also higher viscosity (Zhang et al., 2022a).

In order to reduce the stiffness of the final bituminous binders specific chemical additives could be also incorporated in the bitumen as fluxing or rejuvenating agents. Nowadays, due to the increased approach to the recyclability of RAP, several chemical additives with rejuvenating properties are employed as a partial substitution for neat bitumen. Different types of products either bio-based or fossil-based can partially, or almost fully restore the properties of the bitumen, such as oleic acids, or linoleic acids (Cao et al., 2019). The use of biological materials is growing rapidly because, if treated with thermochemical conversion processes, they can positively influence the properties of the bitumen. Following this approach, it was determined that 2.5% of rejuvenator by weight, in the RAP, gives a mixture development with properties comparable to those of a mixture with fresh bitumen (Barzegari and Solaimanian, 2020). In particular, these properties are attributed to the presence of amine and amide groups within the composition of bio-oils. Formulations containing bio-oils with these groups can be characterized as self-rejuvenating bio-binders, as they exhibit greater durability and resilience to aging, extending the life of various infrastructure applications such as road pavements (Kazemi et al., 2022, 2021).

The natural compounds derived from biomass that are incorporated in bitumen need to be previously converted into oils (bio-oils). Among the materials widely used, of natural origin, as bitumen modifiers are WCO and swine manure. Studies on bio-oils obtained from swine manure, if used to replace bitumen, with percentages of approximately 5% by weight of the total blend, allow an improvement in properties. This percentage can improve the low-temperature cracking properties and confers a decrease in the viscosity of the bitumen (Fini et al., 2012). In addition to the performance advantages of the mixtures, life cycle assessment (LCA) studies demonstrate that substituting fossil components with bio-oils has the potential to significantly decrease both energy consumption and global warming impact. This reduction is attributed to the diminished release of pollutants associated with bio-oil utilization, highlighting its environmental and sustainable benefits (Kazemi et al., 2022; Samieadel et al., 2018).

2.2. Extended bitumen and alternative binders

Increasing the percentage of bitumen substitution above 25 wt% and up to 75 wt% the resulting bituminous binders are commonly known as extended bitumen. The extenders should therefore replace a significant portion of the traditional bituminous binders. Thus, if modifiers would enhance the final performance of a base binder, the extenders should at least replicate the physio-mechanical response of traditional paving binders (Dhasmana et al., 2023; Yang et al., 2015). This substitution usually contributes to the sustainability of the final material. In order to limit the consumption of fossil sources, and the related issues, a possible material proposed for the extension of bitumen is lignin. Several studies were conducted on lignin by the federal highway administration (FHWA) demonstrating the feasible use of the material as a replacement of bitumen fraction, but also as an additive for the improvement of the binder’s properties. Lignin not only acts on the physio-mechanical response of the extended bitumens, it also has an antioxidant effect, therefore it counteracts the aging phenomenon (Pérez et al., 2019; Sun et al., 2017). However, the use of lignin alone does not allow the development of full alternative binders. In fact, during the preparation of the extended bitumen, high percentages of lignin may lead to the formation of foam and crystallization of the material, which limit the maximum percentage of the substitution (Xu et al., 2017).

Striving to the green transition of the paving sector, the studies on more sustainable bituminous binders have been intensified in recent years to make the fraction of standard bitumen within the binders even smaller. Researchers are committed to developing binders, where the fossil contribution is lower than 25 wt% aiming to zero. The materials that contain a very small amount or no quantity of bitumen are called alternative binders. Usually, the replacement takes place with waste materials, which can be of natural origin, both vegetal and animal, but also municipal wastes, to give them a second life and minimize their disposal in landfills (Aziz et al., 2016).

Today, in the pavement industry sector, the term bio-binders has reached a great level of attention. Indeed, it is meant as a binding material that is formulated in the presence of material from biological sources, in particular biomass. Therefore, all the classes of binders described above, being formulated with materials of biological origin, fall into the group of bio-binders with no recognized threshold value.

4.1. Pyrolysis

Pyrolysis is one of the most promising TCC processes and plays an important role in biomass conversion. However, the pyrolysis process is very complex because a different path is followed based on the type of component being processed (Yang et al., 2007). The pyrolysis products mainly depend on the temperature and heating rate. They occur, in different percentages by weight, in all three states of aggregation: non-condensable gases (e.g., CO, CO₂), a liquid obtained from the condensation of gases (hydrocarbons and water) and finally the solid fraction, given by char (coal and ashes). Depending on the desired product, it is possible to carry out different types of pyrolysis. The goal is to obtain material from biological sources that can be used in the production of paving compounds. Indeed, depending on the type of desired product, it is possible to enrich the different fractions by modifying the process conditions. For example, to maximize the amount of char, hence of the solid fraction, the heat is transferred to the sample more slowly. In this case, the so called slow pyrolysis is adopted (Hernandez-Mena et al., 2014). If instead, the desired product is the fraction of condensable gases (bio-oils) as in the case of the paving industry, the heating rate is very high, and the fast pyrolysis is applied (Graham et al., 1984).

It is possible to divide the pyrolysis process into several phases, which are related to the different temperatures encountered during the process. The initial phase of the process is defined as drying (≃100 °C) that is a preliminary phase. The temperatures are lower and in this first part, the humidity and any present water are removed. After drying, another phase starts between 100 °C and 300 °C, where the dehydration of the biomass takes place allowing the removal of further low molecular weight molecules such as CO₂ and CO. This is followed by the intermediate phase where temperatures range from 200 °C to 600 °C. Condensable vapours are generated, from which bio-oils and incondensable gases are obtained and the biomass begins to transform into a primary char. Eventually, the final phase (≃300 °C-900 °C). If the vapours that develop reside for a sufficiently long time inside the biomass, the formation of secondary char and further incondensable gases is generated. The formation of these is given by the cracking of the species that have vaporized (Basu, 2018; Boateng, 2020; Diebold and Bridgwater, 1997).

The most relevant pyrolysis process for paving applications is the FasP. During the FasP the heating time is much shorter than the reaction time, indeed, the generated vapours remain inside the reaction system for a few seconds (Graham et al., 1984). These conditions allow for the maximization of the production of condensable gases by avoiding secondary reactions such as the cracking of larger molecules into non-condensable gases. With this type of process, the percentage of bio-oil obtained varies in a range between 60%-75% by weight. The other products obtained are incondensable gases and solids, char, in which the percentage ranges of these by-products are respectively 10%–20% by weight (gases) and 15%–25% by weight (solids) (Barik, 2019; Jiang et al., 2016). The biomass heating rates are very high, approximately equal to 200 °C/s, with maximum temperatures ranging from 450 °C to 750 °C (Safdari et al., 2019).

The fast pyrolysis process for the production of bio-oils was carried out using different types of technologies. Peralta et al. (2012b) have developed a pilot plant (at the Iowa State University) consisting of a fluidized bed reactor (Fig. 3(a) and (b)). The use of this type of plant is able to process about 10 kg/h of biomass converting it up to 70 wt% of the resulting bio-oil with an operating temperature of 450 °C. The study was conducted by processing organic biomasses, mainly of plant nature, such as red oak wood, switchgrass and corn stover. These are lignocellulosic biomasses, made up of bio-polymers such as cellulose, hemicellulose and lignin (Greenhalf et al., 2013). The same system configuration was also adopted using wood previously treated in the laboratory with preservatives. These compounds would have preventive action against the attack, for example, of fungi. These preservatives are inorganic complexes of metals such as copper, chromium and boron. Downstream of the fast pyrolysis process, the metals content is very low in the oily fraction, while it is high in the solid one (bio-char). Thanks to this analysis, the effectively of the fast pyrolysis to treat contaminated lignocellulosic biomasses has been understood. Hence, a wide variability of vegetable biomasses can be used in this process. The obtained bio-oils, purified from traces of heavy metals, can still be used for the development of further products. Previous studies also focused on empty fruit bunches (EFB). The reactor configuration adopted is that of a fluidized bed reactor. Studies on the bio-oils produced by EFB fast pyrolysis was carried out at different temperatures (400 °C, 500 °C, 600 °C). EFBs have been shown to be well-pyrolyzed. Furthermore, maximum yields of bio-oils were obtained at temperatures close to 450 °C (Ciuta et al., 2014; Sembiring et al., 2015). The fast pyrolysis can be carried out in production plants that exploit fluidized bed reactor, which is the most used application, and the ablative one. In ablative plants, the heat is transferred by conduction to the biomasses, which are subjected to a rotating movement and high pressures (about 50 bar) (Peacocke, 1996). The high operating conditions allow to prevent the possible condensation of the produced high-boiling products. The bio-char obtained is removed by gravity; while the developed gases are condensed to obtain bio-oils. Auersvald et al. (2020) tested ablative pyrolysis with different types of lignocellulosic biomass (beech wood, poplar wood, straw and miscanthus). The obtained results from fast ablative pyrolysis were found to be comparable to those of a fluidized bed reactor. Indeed, the obtained liquid phase yields are very close between the two configurations, around 60 wt%. Among the quantity of obtained liquid phase, a fraction consists of water, which is in greater quantity in the case of straw and miscanthus. The limited use of this type of technique is given to the high operating and production costs. However, there are other limitations related to the fast pyrolysis process in general. The composition of the bio-oil has a high content of oxygenated compounds and water, that turn into a highly viscous, corrosive mixture capable of damaging the equipment in which it is contained (Auersvald et al., 2020; Bridgwater et al., 1999; Greenhalf et al., 2013; Mohan et al., 2006; Peacocke, 1996; Peralta et al., 2012a, b; Sembiring et al., 2015).

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Figure 3

4.2. Hydrothermal liquefaction

Another TCC process widely studied by researchers for the production of bio-oils and their subsequent applications is the so-called HTL or hydrated pyrolysis. The HTL process is a thermochemical depolymerization process where the reactor converts the wet biomasses under more moderate conditions than the fast pyrolysis; in fact, in the HTL temperatures vary between 200 °C and 400 °C and pressures are variable in the 10-25 MPa range (Zhang and Chen, 2018). The process tries to simulate natural degradation that converts dead plants and living matter buried underground into possible fuels. Furthermore, given the quantity of water present in the biomasses, the HTL acts as a reaction medium for the development of the products (Dhasmana et al., 2015). Fini et al. (2011) applied the HTL process to pig manure. The starting material is loaded into the mixed reactor (continuous stirred tank reactor CSTR) at 300 °C. The whole system is kept under pressure thanks to a current of nitrogen at 10.3 MPa for removing the residual air. Upon completion of the reaction, the system is rapidly cooled. The process produces gases that are condensed into bio-oils and a viscous residue. Differently than fast pyrolysis process, where the bio-oils are used for the development of the bio-binders, in the application of HTL process on swine manure, the residue is used for their development. In general, the bio-binders that has been developed using this bio-source fall into the class of modified bitumen. Hence, it was used in a limited quantity with the aim of improving the rheological properties of the starting binder (Bridgwater et al., 1999; Fini et al., 2011; Oldham et al., 2015). Other HTL processed biomass products that exhibited good properties for the development of bio-binders are obtained from microalgae. Indeed, it has been determined that it is possible to obtain similar properties to those of bitumen from the residues of processed microalgae. The application of the process on microalgae, in particular Scenedesmus species, has been extensively studied by catalysing the HTL process. Among these (Geantet et al., 2022), compared the performance of the products obtained from the HTL processes with and without a catalyst. The use of catalysts allows the modification of the viscosity and rheology of the final product, since it uses cerium nitrate hydrate, Ce(NO₃)₂∙6H₂O, which leads to a growth of the molecules such as longer-chains hydrocarbons. The product obtained from the catalytic process showed good performance. Indeed, the properties of the final material exhibited similar stiffening and rheological properties to those of bitumen, which were better replicated in respect to the products of non-catalytic process. Thus microalgae can be used to develop alternative binders in which the bitumen content is zero (Elliott et al., 2015; Geantet et al., 2022; Maag et al., 2018).

6.1. Rheological properties of bio-binders with low amount of bio-materials

The development of bio-based modified binders, with low percentages of replacement (about 10 wt%) of the bituminous component with biological materials, can be considered as the basis for more marked fossil replacements. In the case of bio-chars, their use in the development of bio-binders is mainly for bio-based modified ones. In literature, some studies report formulations developed with bio-char having a percentage of biological components ranging from 5 wt%-20 wt%. From the analyses conducted through the characterization of the binder mixtures, it was observed an overall improvement of binder properties thanks to the presence of the biological source. The final bio-binders with bio-char were found more viscous than the traditional bitumen. In addition to this, studies have reported an increase in stiffness, resistance to deformation, and wear, and in some cases having an antioxidant property. However, the bio-char was also reported to have a negative effect on the performance at low temperatures (Dong et al., 2020; Zhao et al., 2014b; Zhou et al., 2020).

In case the biological component is a viscous liquid, it is essential to understand the physical properties that should be met by binders to be used in the paving industry. Bio-oils are chemically complex, and it is challenging to perform chemical analysis or determine their performance. Density and viscosity are the most widely considered physical properties. The density is higher than the raw material from which they are obtained; instead, the viscosity is a factor that depends on the water content (14 wt%-33 wt%) and the presence of light compounds (Raouf et al., 2010).

Unlike bio-char, the addition of bio-oil showed a negative action on the final mixture (Fig. 4) due to its softening effect. In fact, studies report that substitutions of the fossil component with bio-oils with percentages ranging from 2 wt% to 15 wt% influence viscosity. The action of the bio-oils within the binder mixture leads to a change in the performance grade (PG) index. There is a reduction in performance at high temperatures, so the bio-binders are more prone to rutting. Conversely, the lower PG value decreases so the bio-binders appear to have greater resistance to cracking. Technically, there is a reduction in the softening point (°C) and an increase in the penetration value (dmm) (Table 3) (Fini et al., 2012; Gondim et al., 2016; Kowalski et al., 2017; Singh-Ackbarali et al., 2017; Sun et al., 2016).

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Figure 4

Table 3

Bio-oil (wt%)	High-temperature property (°C)	Low-temperature property (°C)
0	61.7	-29.9
5	56.3	-31.3
8	51.8	-33.3
10	48.9	-34.2
15	44.7	-35.8

Furthermore, these characteristics can also be observed through the construction of master curves, if the bio-binders show simple thermo-rheological behaviour. In fact, a reduction in the complex modulus G* is observed as the biological component increases compared to bitumen (Gong et al., 2016; Lei et al., 2017). The reduction of the complex modulus can be counteracted through the addition of traditional polymeric materials (i.e., SBS). Other than these, polymers of natural origin such as PLA and lignin also find their use for the above purpose (Gong et al., 2016; Ingrassia et al., 2019; Kowalski et al., 2016; Raouf and Williams, 2010a; Zhang et al., 2017).

6.2. Rheological properties of binders with high amounts of bio-materials

The use of plant-based biomasses to replace fossil constituents is commonly adopted by researchers developing bio-binders with a significant quantity of bitumen replacement. Given the chemical compatibility of the bio-oils and the standard paving bitumen, in terms of composition, the bitumen replacement may exceed the 25 wt%.

The addition of bio-oils from vegetal biomasses usually varies the complex shear modulus, G*. In fact, in the case of bio-binders, the value of G* at high frequencies and/or low temperatures is greater than the targeted value of standard bitumen; vice versa, at low frequencies and/or high temperatures the value of G* is lower than that of bitumen (Barzegari and Solaimanian, 2020; Gaudenzi et al., 2022; Yang et al., 2014). Nevertheless, the observed trends of different bio-binders from rheological viewpoint are not always the same. Rheological studies showed that the addition of bio-oils to bitumen improved the properties of the final binder. In particular, bio-oils obtained from vegetal biomasses (e.g., waste wood) increased high-temperature performance and viscoelastic properties (Raouf and Williams, 2010a; Yang and Suciptan, 2016). However, at low temperatures, these properties are not affected by any benefit using these types of bio-oils. Other rheological studies on bio-binders developed with swine manure and waste cooking oil do not improve the performance of the bio-binder at high temperatures, but the rheological performances benefit at low temperatures (Fini et al., 2011; Onochie et al., 2013; Sun et al., 2016). These inconsistencies are given by the different biomasses that have been used, and hence, their chemical composition. On this basis, the bio-binders made from raw materials such as switchgrass, pinewood and oakwood have differing values in terms of PG. The PG values of the bio-binder obtained with switchgrass bio-oils are lower than the bio-binders obtained with pinewood and oakwood bio-oils. The reason of the higher PG value for the other samples can be ascribed to the more solid chemical structure of pinewood and oakwood and a higher lignin content (Raouf and Williams, 2010b; Zhang et al., 2022b; Zhao et al., 2014b). For this reason, many researches are being developed in search of an evaluation system that allows to characterization of bio-binders of different nature (Zhang et al., 2022a).

Other materials of biological origin that show appreciable performance when converted to develop binders are algae. In fact, their master curves, constructed through rheological tests, appreciably describe those of pure bitumen at high frequencies (and/or at low temperatures) (Dhasmana et al., 2015; Geantet et al., 2022).

When it comes to the rheological properties of bonding materials, including bio-binders, it is important to make some clarifications. For example, when the replacement of fossil constituents is small, in some countries such as the United States, strategic highway research program (SHRP) regulations continue to be followed (Kennedy et al., 1994). If the degree of substitution within the binder material is higher, the principle of time temperature superposition (TTSP) is not longer valid and the frequency sweep tests for the construction of master curves cannot be applied as for the neat bitumen. Indeed, rheological tests conducted on binders with high percentages of the biological fraction show non-compliant results. Therefore, on this basis, for bio-binders, it is not possible to define a simple thermo-rheological behavior, so the TTSP is not valid (Airey et al., 2016; Yang and You, 2015).

6.3. Aging tendency of bio-binders

Another fundamental aspect that must be considered is the long-term performances of bio-binders is their tendency to age. In fact, due to their nature, bio-materials are very susceptible to aging phenomenon. The elementary analysis highlights the chemical structure of these bio-products showing their greater amount of heteroatoms with oxygen than that available in neat bitumen. In fact, in these conditions the light fractions will be displaced and the oxygen content of the bio-binder is greater in respect to standard bitumen. The aging process and therefore the further removal of the light fractions, leads to an increase in the stiffness of the material. This behavior is more evident when the biological component of the bio-binder formulation comes from plant biomass, such as wood waste. On the contrary, studies conducted on bio-binders developed with bio-oils derived from animal waste show a reduced susceptibility to aging (Fini et al., 2015). These changes are detected through comparison with bitumen. In fact, both at low frequencies (and/or high temperatures) and at high frequencies (and/or low temperatures), the behavior closely resembles that of bitumen, despite the reduction in fossil fraction (Barzegari and Solaimanian, 2020; Dhasmana et al., 2015; Ingrassia et al., 2019). In the literature, there are relatively few studies on the aging tendency of bio-binders, indicating a need for further investigation

6.4. Mechanical properties of paving materials with bio-binders

The mechanical behaviour of the paving bio-concrete should be fully characterized in order to assess the feasibility of using the innovative bio-based binders. However, the mix design, production and characterization of the bituminous mixtures made with bio-binders in respect to traditional ones may represent some uncertainties since there are no widely-known protocols/guidelines and the existing experiences are limited. Depending on the type of adopted bio-binder (i.e., the nature of the starting biomass and the production process), the behaviour of bio-concrete may be unchanged or improved if compared to the traditional ones. Macro properties such as workability and compaction, showed improvements due to the presence of the bio-material. This is followed by a reduction in the laying temperature of the mixture of about 20 °C-30 °C (Ingrassia et al., 2019; Kowalski et al., 2017; Su et al., 2018; Zhang et al., 2022b). Several studies have shown that the presence of biological material affects the properties of the mixture. Fatigue improvements have been observed, but also others advantages related to cracking at low temperatures; conversely, a deterioration related to rutting resistance was observed (Cooper et al., 2013; Wen et al., 2013).

The assessment of bituminous concrete performances was studied in different conditions. The response at high-temperature was mainly given by dynamic stability tests and resistance tests against rutting. Promising performance grades of asphalt mixes with bio-binders have been demonstrated through dynamic stability. The low softening point value guarantees the best performance of the bituminous mixture at high temperatures. In addition, the high viscosity of bio-oils gives a good adhesion between the binder and aggregates. The phenomenon is also supported by the amount of functional groups containing oxygen that allow making the binder more polar (Ingrassia et al., 2019; Mogawer et al., 2016; Su et al., 2018; Wen et al., 2013; Zhang et al., 2022b).

Regarding the low-temperature behaviour of concrete produced with bio-based binders, the resistance against thermal cracking was assessed by determining the critical cracking temperature. From comparisons with traditional bituminous mixtures, the material of biological origin within the mix increases the critical cracking temperature value of approximately 30%, although this is not always the case. What has been seen from the rheological analyses in terms of performance at high and low temperatures and the dependence on the type of biomass, was also reflected in the mixtures (Su et al., 2018; Zhang et al., 2022a).

Another critical aspect of paving materials is water damage. As for their performance at high and low temperatures, mechanical tests are also carried out for this aspect. The tests often include Marshall and/or wet indirect tensile strength ones. The scientific literature on these types of tests on bituminous mixtures with bio-binders is not wide. Studies report that samples made with a binder containing 50% by weight of bio-extender (obtained from Oakwood and Pinewood) meet the requirements of the Marshall test specifications. Similarly, indirect tensile tests also show higher results than conventional tests (Mogawer et al., 2016; Su et al., 2018; Zhang et al., 2022b).