C N Chailleux

C Uguna

G Snape

Airey

A Weir

Jiménez Del

Barco Carrión

C Queffélec

E Chailleux

C Snape

G Airey

Renewable binders from waste biomass for road construction: a review on thermochemical conversion technologies and current developments

Keywords: biomass, bio-oil, thermochemical conversion, biobinder, sustainability, recycling Carbon dioxide LCA: Life Cycle Assessment Rotating Thin Film Oven Test Pressure Ageing Vessel RA: Recycled Asphalt PEA: Polyethyl acrylate PMA: Polymethyl acrylate PBA: Polybutyl acrylate DBP: Dibutyl phthalate EMS: Epoxidized Soybean Soyate

HAL is

Introduction

Most bituminous binders used for pavement materials are derived from fossil fuels [START_REF] Peralta | Development of a rubber-modified fractionated bio-oil for use as noncrude petroleum binder in flexible pavements[END_REF].

Petroleum bitumens and modified bitumens (bitumens designed to change the performance of straight run bitumen altered by one or more chemical agents such as polymers, waxes and crumb rubber) [START_REF] Self | Introduction[END_REF], are the most common materials used as binders in asphalt mixtures. In Europe, the overall consumption of bitumen remains high and relatively stable, varying from 12.89 million tonnes in 2016 to 10.74 million tonnes in 2019 [START_REF]Asphalt in Figures[END_REF]. In addition, bitumen and asphalt mixture demand is predicted to further increase after years of low spending and will be driven by increased expenditure on road maintenance to accommodate for repair backlogs [START_REF] Group | World Asphalt (Bitumen)[END_REF].

In the US, the production of hot and warm mix asphalt mixtures has been on the rise since 2016 due to increased construction activity, highlighting their high demand for bitumen and bituminous mixtures (Figure 1) [START_REF]Asphalt in Figures[END_REF]. Fortunately, efforts to use more sustainable materials in all areas of construction have greatly increased [START_REF] Olaoye | A method for establishing a hygrothermally controlled test room for measuring the water vapor resistivity characteristics of construction materials[END_REF]. This rise in production coupled with the urge to minimise the usage of fossil fuels has developed a drive to produce binders from alternative sources globally, particularly from bio-renewable materials or biobinders.

Figure 1: Total production of hot and warm mix asphalt (in million tonnes

) between 2016 and 2019 [START_REF]Asphalt in Figures[END_REF].

Biobinders are defined as asphalt binder alternatives made from non-petroleum-based renewable sources, which should not impact on food production, and have environmental and economic benefits [START_REF] Peralta | Development of a rubber-modified fractionated bio-oil for use as noncrude petroleum binder in flexible pavements[END_REF]. They can be produced from a range of sources including vegetable oils, algae and swine manure [START_REF] Ingrassia | Renewable materials in bituminous binders and mixtures: Speculative pretext or reliable opportunity?[END_REF]. Not only do they have the potential to reduce petroleum bitumen demand, biobinders have also exhibited good performance in terms of resisting the common distresses affecting pavements depending on their composition [START_REF] Ingrassia | Renewable materials in bituminous binders and mixtures: Speculative pretext or reliable opportunity?[END_REF]. Therefore they are receiving increasing attention in pavement engineering as effective alternatives to petroleumbased binders. Manufacturing biobinders from biomass has gathered interest in recent years in light of environmental concerns with conventional bituminous binders. According to the US National Academy of Sciences, approximately 550 million dry tons per year of cellulosic biomass can be produced by 2020 without any major impact on food production or the environment [START_REF] Council | Liquid transportation fuels from coal and biomass: technological status, costs, and environmental impacts[END_REF][START_REF] Perlack | US billion-ton update: biomass supply for a bioenergy and bioproducts industry[END_REF].

Fossilised biomass, like bitumen, has been heavily exploited for decades as coal and oil. The burning of fossil fuels uses 'old' biomass and converts it to 'new' CO2, contributing to increased greenhouse gas emissions. On the other hand, burning 'new' biomass contributes no new CO2 to the atmosphere as the released CO2 is absorbed and recycled back into replanted biomass through photosynthesis in a cyclical process; this balance makes 'new' biomass carbon neutral [START_REF] Mckendry | Energy production from biomass (part 1): overview of biomass[END_REF][START_REF] Zhang | Review of biomass pyrolysis oil properties and upgrading research[END_REF]. Unlike fossil fuels, biomass is abundantly available around the world on a renewable basis, either through natural processes or as a by-product of human activities i.e.

organic wastes [START_REF] Mckendry | Energy production from biomass (part 1): overview of biomass[END_REF][START_REF] Isa | High conversions of biomass and pyrolysis oil using sub-and supercritical water above 400[END_REF].

Biomass sources

Sources of biomass can be categorised into four main groups: woody plants and agricultural products, herbaceous plants/grasses (all perennial crops), aquatic plants and manures (waste biomass) [START_REF] Mckendry | Energy production from biomass (part 1): overview of biomass[END_REF]. Within this grouping, biomass sources can be further subdivided into those with high-and low-moisture contents. 'Dry' biomass such as wood chips and sawdust are naturally more suited for gasification, pyrolysis or combustion, whereas aquatic plants and manures are inherently high moisture materials and therefore more suited to 'wet' processing techniques [START_REF] Mckendry | Energy production from biomass (part 1): overview of biomass[END_REF]. The herbaceous plant sugarcane has a high-moisture content and so would be suitable for a 'wet' conversion process. On the other hand, switchgrass, another herbaceous plant, has a much lower moisture content. Apart from moisture content, other factors should be considered when selecting an appropriate conversion method, especially in relation to those sources of biomass which lie between 'wet' and 'dry'. These include:

1. Moisture content (intrinsic and extrinsic) 2. Calorific value 3. Proportions of fixed carbon and volatile components 4. Ash/residue content 5. Alkali metal content 6. Cellulose/lignin ratio For 'dry' biomass conversion processes such as pyrolysis, the first five properties are of interest while for 'wet' processes like hydrothermal liquefaction (HTL) the moisture content and the cellulose/lignin ratio are the most important [START_REF] Mckendry | Energy production from biomass (part 1): overview of biomass[END_REF].

Over the last decade, there has been an increase in biobinder development technologies from biomass that are applicable to road construction. These technologies have evolved taking into account the available biomass and physico-chemical treatment needed to be applied to reach an optimum, consistent state. Biomass used as a source of road biobinders can be divided in two categories: liquid hydrophobic (mainly lipidic) biomass and solid biomass that needs to be converted (i.e. pyrolysed or liquefied). Liquid biomass includes vegetable oils and wood byproducts such as pine rosin and pitch. Today, major full-scale developments have been made

with this type of biomass where the processes are mainly physical blending and well-known chemical modifications such as transesterification of vegetable oil and polymerisation of rosin [START_REF] Colas | Obtention de liants routiers a`base de bitume et d'une nouvelle gamme de fluxantsd'origine naturelle fonctionnalises[END_REF][START_REF] Pasquier | Liant bitumineux, composition et utilisation[END_REF][START_REF] Pinomaa | Dyeable pavement material[END_REF]. Considering the global consumption of bitumen, liquefied biomass cannot cover the demand alone as there are many other competing uses including the increasing demand to produce biofuels and chemicals such as biodiesel and bioethanol [START_REF] Demirbaş | Biomass resource facilities and biomass conversion processing for fuels and chemicals[END_REF]. This is why more studies have shifted the focus to the second category (solid biomass). Conventional agricultural and wood by-products can provide large quantities of biomass so long as they are not used for the food industry. Depending on the moisture content of biomass, two main types of thermochemical processes can be used: solvent liquefaction and pyrolysis. Using biogenic sources for biobinder production can help reduce emissions and diverts the biomass away from combustion and into sustainable development and circular economy. In terms of producing asphalts with novel biomaterials, waste biomass products are particularly interesting.

Recycling biomass waste products can minimise waste destined for landfill and reduce greenhouse gas emissions by substituting the biowaste into conventional asphalt binders [START_REF] Azahar | The potential of waste cooking oil as bio-asphalt for alternative binder-an overview[END_REF].

Life cycle assessment (LCA) in the road industry is an important tool used to measure and compare the key life-time environmental impacts of asphalt products and laying processes [START_REF] Bird | Life cycle and sustainability indices for road paving materials[END_REF].

This includes energy consumption and greenhouse gas emissions. The production of 1 tonne of bitumen (including crude oil production, transport, refinery and storage) amounts to a total of approximately 226 kg of CO2 emissions to the atmosphere [START_REF] Association | Life Cycle Inventory: Bitumen[END_REF]. As more novel materials and laying techniques emerge, LCA provides a framework for assessing the environmental benefits of biobinders and so researchers and LCA practitioners should focus on expanding the current databases to accommodate these novel materials [START_REF] Huang | Development of a life cycle assessment tool for construction and maintenance of asphalt pavements[END_REF]. Albeit novel, biobinders have proven to be beneficial in reducing emissions. One study which investigated the production of a biomodified binder via HTL from swine manure observed a significant reduction in greenhouse gases with an 81% drop in CO2 emissions, whilst also reducing the emission factors of the conventional binder in the mixture (Figure 2) [START_REF] Samieadel | Comparative life cycle assessment (LCA) of biomodified binder and conventional asphalt binder[END_REF]. Similarly, the index for the Environmental Impact of Biomass Pre-processing (EIBP) is a comparable method to LCA that includes carbon footprint reduction and pollutant impacts from by-products and residuals [START_REF] Tao | Biomass combustion: Environmental impact of various precombustion processes[END_REF].

It is considered to be a more feasible method to make a more generalised comparison between various pre-combustion processes, including HTL, pyrolysis, gasification and anaerobic digestion. A lower EIBP value suggests a better environmental impact. In this study, anaerobic digestion process has the lowest EIBP for most feedstocks studied, whereas HTL had the highest values. Despite this, HTL had an "extremely high environmental impact improvement potential" [START_REF] Tao | Biomass combustion: Environmental impact of various precombustion processes[END_REF]. The authors found no strong correlations between energy conversion efficiency and EIBP and concluded that environmental impact should be considered separately when optimising biomass pre-combustion processes.

This review attempts to provide a link between biomass thermochemical conversion technologies and their respective products that may be used as biobinders in pavement engineering. For this purpose, firstly, the main biomass thermochemical conversion technologies are compared, and a review of the parameters influencing liquefaction products is provided. Next, it presents an overview of the recent biobinder studies reported in literature with an emphasis on the feedstock used and their performance-related properties. The studies are classified depending on the technology used to obtain the biobinder, including a specific section dedicated to the use of biobinders with recycled asphalt. The limitations to why biobinders have not been produced at industrial scale are also presented. Based on this, summary tables of the currently known properties of different biobinders have been compiled from the literature. The tables aim at providing a quick but insightful way of identifying potential biobinder feedstocks with certain desirable properties and help researchers identify gaps and potential research opportunities in the field.

Comparison of main biomass thermochemical conversion technologies

The main biomass thermochemical conversion technologies for biofuels and chemicals are gasification, pyrolysis and solvent liquefaction. The conversion of carbon-containing solids at high temperatures (700-1000 °C) and under oxygen-starved conditions is referred to as thermal gasification [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF]. Biomass gasification for the purpose of producing biobinders for road construction has not been studied and is therefore not relevant in this case. As a result, the focus in this section is on pyrolysis and solvent liquefaction as these are the techniques primarily used to produce biobinders.

Pyrolysis and liquefaction are two direct methods considered to be both time saving and relatively simple, and so they are used extensively [START_REF] Bridgwater | Review of fast pyrolysis of biomass and product upgrading[END_REF]. They are comparable technologies as they both extract bio-based intermediate products, referred as a bio-oil or biocrude product respectively. There are complex reaction pathways associated with these technologies and currently many research groups are focusing on understanding them [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF].

Although similar, there are considerable differences between these two conversion technologies. During pyrolysis, biomass is decomposed in the absence of oxygen within temperature and heating rates ranging from 300-900 °C [START_REF] Guo | Bioenergy and biofuels: History, status, and perspective[END_REF] and less than 0.005 °C/s to more than 10,000 °C/s respectively [START_REF] Collard | A review on pyrolysis of biomass constituents: Mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin[END_REF]. Pyrolysis can be classified as slow, intermediate, fast and flash pyrolysis depending on the operating conditions, with fast and slow pyrolysis being the most commonly used processes [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF]. Slow pyrolysis operates at relatively low heating rates and temperatures with long residence time. The main target product is often solid char i.e.

traditional charcoal making process [START_REF] Al Chami | Evaluation of flash and slow pyrolysis applied on heavy metal contaminated Sorghum bicolor shoots resulting from phytoremediation[END_REF]. Intermediate and fast pyrolysis use moderate to high heating rates and temperatures, with fast pyrolysis characterised by shorter residence time.

Flash pyrolysis uses the highest heating rates and shortest residence time, with a reaction time only lasting several seconds or less. Flash pyrolysis requires the use of special reactors and a sample particle size of approximately 105-250 μm [START_REF] Goyal | Bio-fuels from thermochemical conversion of renewable resources: a review[END_REF][START_REF] Zhang | Essential scientific mapping of the value chain of thermochemically converted second-generation bio-fuels[END_REF].

In pyrolysis, low temperatures and long residence times favour the production of char, whereas high temperatures and long residence times favour the production of the gaseous products. If the purpose is to maximise the yield of the liquid product (bio-oil), moderate temperatures, high heating rates and short residence times are required [START_REF] Al Chami | Evaluation of flash and slow pyrolysis applied on heavy metal contaminated Sorghum bicolor shoots resulting from phytoremediation[END_REF][START_REF] Demirbas | Pyrolysis of ground beech wood in irregular heating rate conditions[END_REF]. Pyrolysis oil or bio-oil, recovered from the condensable vapours and aerosols produced during the process, is composed of a complex mixture of oxygenated organic compounds, including alcohols, aldehydes, esters, saccharides, phenolic compounds, carboxylic acids and lignin oligomers [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF]. Bio-oil usually has a high yield of 70-80 wt. % [START_REF] Zhang | Essential scientific mapping of the value chain of thermochemically converted second-generation bio-fuels[END_REF].

Liquefaction, also referred to as solvent liquefaction, converts biomass into liquid fuels by processing in a hot, pressurised liquid environment for a period of time, in order to break down the solid biopolymeric structure to mainly liquid components [START_REF] Elliott | Hydrothermal liquefaction of biomass: developments from batch to continuous process[END_REF]. Various solvents can be utilised such as water and methanol, non-polar solvents like toluene and tetrahydronaphtalene and ionic liquids like 1-ethyl-3-methylimidazolium chloride [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF]. Solvent liquefaction undertaken in water is typically referred to as hydrothermal processing, hydrothermal liquefaction or hydrous pyrolysis. It is particularly attractive for wet feedstocks that are handled as slurries. Unlike pyrolysis, HTL does not required feedstock drying, therefore saving on high dewatering costs. This in turn increases the economic return of fuel production largely due to the wet nature of biomass feedstocks [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF].

Solvent liquefaction can produce fractionated and hydrolysed plant polymers [START_REF] Allen | Fractionation of sugar cane with hot, compressed, liquid water[END_REF], partially deoxygenated liquid product (biocrude) [START_REF] Elliott | Developments in direct thermochemical liquefaction of biomass: 1983-1990[END_REF] or syngas (gasification product) [START_REF] Elliott | Chemical processing in high-pressure aqueous environments. 7. Process development for catalytic gasification of wet biomass feedstocks[END_REF]. As reaction temperature increases, pressure must be increased in order to avoid the boiling of the water in the biomass (Figure 3). Extraction of high-value plant chemicals including resins, phenolics, phytosterols and fats occur at around 100 °C. At around 200 °C and 20 atm, fractionation of biomass takes place to yield cellulose, lignin and hemicellulose degradation products like furfural. According to Elliott et al. [START_REF] Elliott | Hydrothermal liquefaction of biomass: developments from batch to continuous process[END_REF], a further increase in temperatures and pressures (300-350 °C, 120-180 atm) can hydrolyse the cellulose to glucose and more extensive chemical reactions take place, yielding a hydrocarbon-rich liquid product known as biocrude. For the purpose of biobinders, the biocrude liquid is the desired product. It is important to note that although visually resembling bio-oil, biocrude has a lower oxygen content and is less miscible in water, making it more amenable to hydrotreating [START_REF] Elliott | Hydrothermal liquefaction of biomass: developments from batch to continuous process[END_REF]. Gas products with a significant fraction of methane are primarily achieved at around 600-650 °C and 300 atm [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF].

Figure 3: Temperature/pressure regimes for hydrothermal processing [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF].

The use of catalysts is not common in pyrolysis, whilst the solvents used during liquefaction can act as catalysts and deliver premium products in comparison to those acquired through pyrolysis [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF]. Additionally, the biocrude oil produced from liquefaction is far more stable and less corrosive than the one obtained from fast pyrolysis. This is due to a higher heating value and lower oxygen and moisture content. Equipment handling and storage costs are therefore reduced [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF][START_REF] Bensaid | Direct liquefaction of ligno-cellulosic residues for liquid fuel production[END_REF][START_REF] Haarlemmer | Analysis and comparison of bio-oils obtained by hydrothermal liquefaction and fast pyrolysis of beech wood[END_REF][START_REF] Lédé | Properties of bio-oils produced by biomass fast pyrolysis in a cyclone reactor[END_REF]. The less oxygenated and more stable liquefaction product could therefore be more easily stored, transported and upgraded [START_REF] Boateng | Fast pyrolysis of biomass thermally pretreated by torrefaction[END_REF]. Studies report that pyrolysis oils can contain between 35-50% oxygen on a water free basis [START_REF] Zhang | Review of biomass pyrolysis oil properties and upgrading research[END_REF][START_REF] Haarlemmer | Analysis and comparison of bio-oils obtained by hydrothermal liquefaction and fast pyrolysis of beech wood[END_REF][START_REF] Doassans-Carrère | Comparative study of biomass fast pyrolysis and direct liquefaction for bio-oils production: products yield and characterizations[END_REF]. In comparison, HTL biocrude produced from a continuous-flow reactor contains between 5 and 18% oxygen for lignocellulosic, macro and microalgal biomass feedstocks [START_REF] Elliott | Hydrothermal liquefaction of biomass: developments from batch to continuous process[END_REF][START_REF] Elliott | Hydrothermal processing of macroalgal feedstocks in continuous-flow reactors[END_REF][START_REF] Elliott | Process development for hydrothermal liquefaction of algae feedstocks in a continuous-flow reactor[END_REF][START_REF] Jazrawi | Pilot plant testing of continuous hydrothermal liquefaction of microalgae[END_REF][START_REF] Tews | Biomass Direct Liquefaction Options. TechnoEconomic and Life Cycle Assessment[END_REF]. The high-water content found in the fast pyrolysis product is also a major concern as it would be immiscible with hydrocarbon products including bitumen. As a result, a lower moisture content in the product yield is desirable, such as those achieved via HTL. Overall, liquefaction can be considered a more competitive technology for biomass conversion to bio-bitumen-like products than pyrolysis, as indicated by their typical parameters in Table 1 proposed by Dimitriadis and

Bezergianni [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF].

However, the high-pressure conditions during liquefaction raise investment costs of the equipment units. Solvent liquefaction has some key engineering challenges that must be overcome in order to make this technology commercially viable. These include the continuous feeding of biomass slurries into high-pressured reactors, efficient energy integration and product separation from solvent [START_REF] Brown | Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals and Power[END_REF].

Table 1: Typical hydrothermal liquefaction and pyrolysis parameters for biomass conversion [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF].

Parameter Pyrolysis Liquefaction

Drying Necessary Unnecessary

Pressure (MPa) 0.1 -0.5 5 -20 Both pyrolysis and liquefaction product yield and quality are governed by a range of factors, including biomass feedstock, temperature, pressure, catalyst and residence time. These parameters are highly dependent on the chemical characteristics of the feedstock such as the ratio of protein, lipid and carbohydrate fractions present in the biomass [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF]. As liquefaction can provide a more stable product in high yields with relatively low oxygen contents than pyrolysis, it therefore seems to be more suited for the production of biobinders and so it is important to examine it in more detail in order to optimise the technology. As a result, the following section focuses on the effects of the operating parameters that come into play in solvent liquefaction.

Liquefaction Operating Parameters

Biocrude product yields vary considerably according due to the nature of the biomass feedstock and the operating parameters during liquefaction, such as temperature and pressure. Each parameter is inter-connected with one another, but their effects are ultimately influenced by the biomass feedstock. Each biocrude product must therefore be individually studied, even if the starting material is the same. The key operating parameters that affect liquefaction products are summarised in Table 2. It is important to note that these studies were not conducted for the sole application of road biobinders. However, it is useful to understand the influence of HTL parameters on product yield, and it is safe to assume they will affect biobinder production in a similar way.

Liquefaction parameter

Main factors affecting biocrude yield

Temperature

Can have a substantial effect on both product yield and properties of liquefaction biocrude

No ideal set temperature for a potential biobinder as product depends heavily on biomass feedstock and all operating parameters involved General consensus that increasing reaction temperature increases yield up to a certain point, where the biocrude yield then begins to level off or decrease [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF], with some attributing a decrease in yield with temperatures over 300 °C to a competition between hydrolysis and repolymerisation reactions involved in the liquefaction process [START_REF] Zhong | A comparative experimental study on the liquefaction of wood[END_REF][START_REF] Sun | Direct liquefaction of paulownia in hot compressed water: Influence of catalysts[END_REF]

Pressure

High pressure increases solvent density, with the resulting high-density medium penetrating efficiently into molecules of biomass components, resulting in enhanced decomposition and extraction The rate of biomass dissolution can be controlled by maintaining pressure above the critical point, helping to boost the favourable reaction pathways to increase oil or gas yields [START_REF] Akhtar | A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass[END_REF] The effect of pressure on product yield becomes negligible once supercritical conditions for liquefaction are achieved [START_REF] Akhtar | A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass[END_REF] Catalyst Mostly used to reduce char formation and boost biocrude yield by reducing condensation and/or repolymerisation reactions of intermediate products [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF] Catalyst selection dependent on feedstock used. Selection can have either a positive or negative effect on the desired chemical reactions [START_REF] Xu | Hydro-liquefaction of woody biomass in sub-and super-critical ethanol with iron-based catalysts[END_REF] Different types of catalysts can influence yield even if the same feedstocks are used [START_REF] Xu | Hydro-liquefaction of woody biomass in sub-and super-critical ethanol with iron-based catalysts[END_REF][START_REF] Wang | Effects of solvents and catalysts in liquefaction of pinewood sawdust for the production of bio-oils[END_REF], highlighting the importance of studying each liquefaction product (and potential biobinder) individually

Residence time Similarly dependent on feedstock used and other parameters involved Short residence times (a few minutes rather than tens of minutes) have been reported to decompose biomass effectively and produce high yields for algal and evergreen-type feedstocks [START_REF] Akhtar | A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass[END_REF][START_REF] Qu | Experimental study on the direct liquefaction of Cunninghamia lanceolata in water[END_REF][START_REF] Valdez | Hydrothermal liquefaction of Nannochloropsis sp.: Systematic study of process variables and analysis of the product fractions[END_REF] Like temperature, increasing residence time can increase biocrude yields until a certain threshold [START_REF] Dimitriadis | Hydrothermal liquefaction of various biomass and waste feedstocks for biocrude production: a state of the art review[END_REF][START_REF] Xu | Hydro-liquefaction of woody biomass in sub-and super-critical ethanol with iron-based catalysts[END_REF][START_REF] Xu | Conversion of secondary pulp/paper sludge powder to liquid oil products for energy recovery by direct liquefaction in hot-compressed water[END_REF] This levelling-off possible accounted to cracking of the liquid products to gases and the formation of char [START_REF] Xu | Hydro-liquefaction of woody biomass in sub-and super-critical ethanol with iron-based catalysts[END_REF] Solvent Water is the most popular solvent used in HTL due to being readily available, environmentally benign and inexpensive. Water acts as a solvent, a reactant and a catalyst during HTL, making the process significantly different from pyrolysis [START_REF] Toor | Hydrothermal liquefaction of biomass: a review of subcritical water technologies[END_REF] Organic solvents such as methanol and ethanol have lower boiling and critical points than those of water and so milder reaction conditions can be used [START_REF] Singh | Effect of solvent on the hydrothermal liquefaction of macro algae Ulva fasciata[END_REF] High molecular weight products have been obtained from organic solvents in comparison to water for the liquefation of pinewood and algae [START_REF] Singh | Effect of solvent on the hydrothermal liquefaction of macro algae Ulva fasciata[END_REF][START_REF] Liu | Effects of various solvents on the liquefaction of biomass to produce fuels and chemical feedstocks[END_REF], while others have reported lower yield biocrude products with organic solvents, such as Cheng et al. [START_REF] Cheng | Highly efficient liquefaction of woody biomass in hot-compressed alcohol-water co-solvents[END_REF], who studied the liquefaction of white pine sawdust, reinforcing the idea of treating each feedstock individually, even if it comes from the same type of biomass Combining more than one solvent has also been explored, and Cheng at al. [START_REF] Cheng | Highly efficient liquefaction of woody biomass in hot-compressed alcohol-water co-solvents[END_REF] was able to show the advantage of using co-solvent systems over single solvents to produce a greater product yield and biomass conversion Hydrogen-donor solvents like tetralin can also enhance the yield of liquid products [START_REF] Akhtar | A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass[END_REF][START_REF] Li | Investigation of solvent effect on the hydroliquefaction of sawdust: an innovative reference approach using tetralin as chemical probe[END_REF] Biomass-tosolvent ratio Generally, the optimum biomass-to-solvent ratio varies accordingly to the feedstock used and the operating parameters [START_REF] Vassilev | Advantages and disadvantages of composition and properties of biomass in comparison with coal: An overview[END_REF].

Most types of biomass contain water and due to its threefold functionality, many researchers have evaluated the overall effect of biomass-to-water content Similar to temperature and residence time, biocrude yield increases with increasing biomass/water ratio until a certain point, with a too-high water content ratio being undesirable [START_REF] Jin | Co-liquefaction of micro-and macroalgae in subcritical water[END_REF]. Smaller ratios can sometimes lead to higher heavy oil yields [START_REF] Qu | Experimental study on the direct liquefaction of Cunninghamia lanceolata in water[END_REF].

Summary and findings

Liquefaction biocrude products are overall found to be both more deoxygenated and hydrophobic and contain less water than pyrolysis oils. They are physically less dense but in fact more viscous than the latter [START_REF] Elliott | Hydrothermal liquefaction of biomass: developments from batch to continuous process[END_REF]. As a result, they are more suited for the production of biobinders. Operating parameters during liquefaction are all closely interrelated, and each play a key part in determining yields. For instance, temperature, residence time and biomass-tosolvent ratio generally increase yield until a certain threshold. The liquefaction process mechanisms and interactions between the parameters have not been clarified much in the literature, although three major steps take place: depolymerisation followed by decomposition and recombination [START_REF] Toor | Hydrothermal liquefaction of biomass: a review of subcritical water technologies[END_REF]. These processes along with the critical parameters explained above vary extensively according to the feedstock used. As biomass is a complex mixture of carbohydrates, lignin, proteins and lipids, the reaction chemistry and mechanisms of biomass liquefaction are consequently also complex [START_REF] Mckendry | Energy production from biomass (part 1): overview of biomass[END_REF][START_REF] Gollakota | A review on hydrothermal liquefaction of biomass[END_REF][START_REF] Mckendry | Energy production from biomass (part 2): conversion technologies[END_REF]. This coupled with the variability of feedstocks makes it difficult to predict what happens during and after liquefaction. This is a challenge that exists in this field and the complexities of the critical parameters should be studied individually for each feedstock [START_REF] Gollakota | A review on hydrothermal liquefaction of biomass[END_REF].

Biobinders

There are three ways in which biobinders can be utilised to decrease the demand of petroleumbased bitumen: (1) as bitumen modifiers (<10% bitumen replacement), (2) as bitumen extenders (20-75% bitumen replacement), and (3) as a direct alternative binder (100% replacement) [START_REF] Airey | Rheological characteristics of synthetic road binders[END_REF][START_REF] Aziz | An overview on alternative binders for flexible pavement[END_REF][START_REF] Metwally | Development of non-petroleum binders derived from fast pyrolysis bio-oils for use in flexible pavement[END_REF][START_REF] Sun | Evaluation of optimized bio-asphalt containing high content waste cooking oil residues[END_REF]. According to the literature, most current studies focus on using biobinders as modifiers, and adding <10% to the mixture [START_REF] Mamat | A review of performance asphalt mixtures using bio-binder as alternative binder[END_REF]. This is perhaps because of the increase in uncertainty in rheological properties when higher amounts of biobinder are used.

Research is still vital to use biobinders as bitumen extenders or direct replacements [START_REF] Aziz | An overview on alternative binders for flexible pavement[END_REF], and therefore there is a need to study further these materials in order to maximise their potential.

Biobinders can be produced from a range of natural resources including agricultural crops, municipal wastes, forestry by-products, sugar, molasses and rice, natural tree and gum resins, natural latex rubber and vegetable oils, amongst many others [START_REF] Peralta | Development of a rubber-modified fractionated bio-oil for use as noncrude petroleum binder in flexible pavements[END_REF]. However, Kluttz [START_REF] Kluttz | Considerations for Use of Alternative Binders in Asphalt Pavements[END_REF] highlighted some issues around substituting other materials for some or all of the bitumen in conventional mixtures. He pointed out that the alternative binder should have predictable properties regarding rheology, adhesion to aggregates, coating behaviour in a mix plant and flow characteristics during construction. Kluttz [START_REF] Kluttz | Considerations for Use of Alternative Binders in Asphalt Pavements[END_REF] further added other less obvious assumptions to take into consideration such as predictable leaching characteristics, water solubility, interactions with fuels or oils, environmental issues, odour, mixing with virgin binders and interaction with contiguous mixes. All of these points should serve as starting points when evaluating a new material for a pavement binder. It is important to note that bituminous binders and biobinders greatly differ in terms of their chemical compositions and properties. Bitumen is mostly composed of hydrocarbon molecules with some heterocyclic species and functional groups containing sulphur, nitrogen and oxygen atoms [START_REF] Romberg | Some Chemical Aspects of the Components of Asphalt[END_REF][START_REF] Traxler | The Physical Chemistry of Asphaltic Bitumen[END_REF]. On the other hand, biobinders are generally composed of a mixture of fatty acid derivatives with an array of compounds, including aromatic and nitrogenous compounds, esters, aldehydes and ketones [START_REF] Zhang | High temperature performance of SBS modified bio-asphalt[END_REF]. This difference in chemical composition presents an added challenge with the development of biobinders, and stresses the importance of characterizing the chemical properties of the biobinder prior to blending with bitumens.

When dealing with the blend of bitumen and biobinders, mixing speed and temperature are important parameters to consider in order to obtain a homogeneous, consistent material that could mimic the rheological properties of petroleum bitumen. Table 3 presents the mixing conditions used by different authors when working on the partial replacement of petroleumbased binders. The following section presents current biobinder studies organised according to the thermochemical technique employed to make the biobinder (where applicable and mentioned).

A summary has been compiled from the literature displaying the currently known properties of various biobinders (Appendix A). It aims at providing a quick but insightful way of identifying potential biobinder feedstocks with certain desirable properties and help researchers identify gaps and potential research opportunities in the field. In addition, Table 4 to Table 8 present a short summary of Appendix A, with key improvement and drawback properties of each biobinder.

Pyrolysis-based biobinder studies

Wood-derived pyrolysis oils have shown to enhance high temperature stability and elasticity, fatigue and rutting resistance [START_REF] Yang | Mechanical performance of asphalt mixtures modified by bio-oils derived from waste wood resources[END_REF][START_REF] Yang | High temperature performance evaluation of bio-oil modified asphalt binders using the DSR and MSCR tests[END_REF] and have been recommended as good bitumen modifiers or extenders. Bio-oils can be successfully combined with other waste materials such as crumb rubber from used tires at lower temperatures (around 125 °C ) than those used for blending traditional bitumens, with the developed bio-bitumens performing as well or better than conventional asphalt mixtures with ground tire rubber, in terms of rutting, fatigue, moisture sensitivity and low temperature cracking [START_REF] Peralta | Development of a rubber-modified fractionated bio-oil for use as noncrude petroleum binder in flexible pavements[END_REF][START_REF] Williams | Development of non-petroleumbased binders for use in flexible pavements-Phase II[END_REF].

Similarly, commercially made biobinders such as Eco-Biopave™, made from a mixture of rosin oil, pyrolysed waste materials and natural rubber (Figure 4) have also shown better behaviour at high temperatures with limited emissions of volatile organic compounds (VOCs), as well as good properties against fatigue and cracking after short-term ageing [START_REF] Owerhall | Bio Bitumen Binder Composition[END_REF]. However, it is agreed that sufficient commercial production of fractionated bio-oil and bio-oil pavement trials are necessary to further understand the ageing mechanisms of these new materials for the technology to be freely applicable. Biobinders developed from switchgrass bio-oil, oak wood and corn stover blended at under 10% with bitumen can also result in improvement of the rutting parameter (|G*|/sinδ) of the base bitumen [START_REF] Williams | Utilization of fractionated bio oil in asphalt[END_REF]. In particular, it has been noted that the rheology of switch grass bio-oils is similar and comparable to that of bituminous binders and can therefore be considered a feasible alternative solution [START_REF] Yang | High temperature performance evaluation of bio-oil modified asphalt binders using the DSR and MSCR tests[END_REF][START_REF] Raouf | General rheological properties of fractionated switchgrass bio-oil as a pavement material[END_REF][START_REF] Williams | Bio-oil formulation as an asphalt substitute[END_REF] . However, such bio-oils cannot be used as direct replacements on their own due to their low viscosity and high-water content, and therefore require upgrading.

Low fractions of biobinders (~10% or less) tend to show the most promising results in terms of improving binder performance without lowering viscosity too much, ensuring a stable performance. Yang et al. [START_REF] Yang | Chemical characterization and oxidative aging of bioasphalt and its compatibility with petroleum asphalt[END_REF] suggest that increasing the bio-oil fraction decreases the compatibility with petroleum bitumen, as adding higher dosages increases conglomeration of the asphaltenes, leading to possible stiffening effects or loss of elastic behaviour.

There are studies that have also found promising results with higher blending proportions of biobinders (up to 50% replacement), making the case for treating each biobinder individually and trialling different proportions. Both Mohammad et al. [START_REF] Mohammad | Laboratory evaluation of asphalt mixtures that contain biobinder technologies[END_REF] and Yang and Suciptan [START_REF] Yang | Rheological behavior of Japanese cedar-based biobinder as partial replacement for bituminous binder[END_REF] manufactured biobinders using up to 50% fast pyrolysis bio-oil obtained from pine wood chips.

The biomodified mixtures had rutting performance that was similar or better than that of the base binder. In the case of Mohammad et al. [START_REF] Mohammad | Laboratory evaluation of asphalt mixtures that contain biobinder technologies[END_REF], almost all the mixtures had adequate moisture susceptibility and showed improved low-temperature performance. However, the mixtures with biobinders revealed less fracture resistance at intermediate temperatures.

Similarly, Zhang et al. [START_REF] Zhang | High temperature performance of SBS modified bio-asphalt[END_REF] studied how a styrene-butadiene-styrene (SBS) modified binder with pyrolysis-derived bio-oil added up to 20% can improve high temperature performance.

After RTFOT ageing, temperature sensitivity was lower than that of the base binder but increased with increasing bio-oil content. On the other hand, rutting resistance increased in comparison to the base binder and improved with increasing bio-oil content. Overall, the high temperature performance of SBS-bio-modified binders is promising in this study, but further work is needed to improve its performance and investigate its modification mechanisms as well as storage stability. This emphasises the need for characterising binders before and after ageing, as the biggest challenge with biobinders lies with their ageing susceptibility. Table 4 presents a summary of the pyrolysis-derived biobinder studies with the primary property improvements and drawbacks.

Liquefaction-based biobinder studies

HTL-derived biocrude from swine manure has shown to enhance low temperature performance, decrease the rate of ageing, and allow for reduced mixing and compaction temperatures due to the decrease in viscosity, when used at <10% by weight of the base binder [START_REF] Mills-Beale | Aging influence on rheology properties of petroleum-based asphalt modified with biobinder[END_REF][START_REF] Fini | Chemical characterization of biobinder from swine manure: Sustainable modifier for asphalt binder[END_REF][START_REF] Fini | Partial replacement of asphalt binder with bio-binder: characterisation and modification[END_REF][START_REF] You | Evaluation of low-temperature binder properties of warm-mix asphalt, extracted and recovered RAP and RAS, and bioasphalt[END_REF][START_REF] Fini | Reducing asphalt's low temperature cracking by disturbing its crystallization[END_REF]. The addition of the HTL biocrude from swine manure can yield a more robust binder against oxidation compared to bio-oil from pyrolysis of corn stover or miscanthus [START_REF] Fini | Source dependency of rheological and surface characteristics of bio-modified asphalts[END_REF].

A life cycle analysis was carried out to determine the environmental impact [START_REF] Samieadel | Comparative life cycle assessment (LCA) of biomodified binder and conventional asphalt binder[END_REF]. Even though only 10% of swine manure biocrude was added to the bituminous binder, it was proven to reduce by half the energy consumption and improve the global warming potential index by 7.8%. In other words, this process reduces the emission of carbon dioxide by 5 times compared to the production of bitumen from petroleum. Life cycle studies such as this one present an area of opportunity for biobinders in order to quantitatively show the benefits of their implementation in asphalt binders, and discovering their future potential.

Similarly to pyrolysis oils, HTL biocrudes combined with other modifiers have exhibited good performance. Swine manure biocrude modified with crumb rubber, Gilsonite, SBS and polyphosphoric acid at low proportions (<10%) show improved low temperature performance and enhanced temperature sensitivity, although results become less obvious at higher temperatures [START_REF] Fini | Synthesis and characterization of biomodified rubber asphalt: Sustainable waste management solution for scrap tire and swine manure[END_REF][START_REF] Aflaki | Comparing effects of biobinder with other asphalt modifiers on low-temperature characteristics of asphalt[END_REF].

Algae-derived biocrudes are also very popular biobinders. Studies that display the potential of microalgae and HTL to produce binders for pavements, show how the rheological behaviour of the developed biocrude is heavily affected by the operating conditions used.

Dhasmana et al. [START_REF] Dhasmana | Rheological and chemical characterization of biobinders from different biomass resources[END_REF] produced bio-crude from HTL at 300 °C of different algae feedstocks including spirulina, a nanoalgae strain and swine manure. Although all the biobinder blends exhibited similar viscoelastic properties before and after ageing, the algae-derived biobinders were stiffer than those obtained from swine manure. Other algae biobinder studies have claimed a rheological simple material with similar viscoelastic properties to bitumen, if lower temperature ranges (240-260 °C) are used [START_REF] Borghol | Biosourced analogs of elastomer-containing bitumen through hydrothermal liquefaction of Spirulina sp. microalgae residues[END_REF][START_REF] Audo | Subcritical hydrothermal liquefaction of microalgae residues as a green route to alternative road binders[END_REF]. High molecular weight species that fragment at the higher temperature result in less viscous material that is no longer thermorheologically simple. It is crucial to preserve these heavy species which appear to function similarly to asphaltenes and resins in petroleum derived binders. Future work involves studying how the properties of the hydrophobic fraction vary with microalgae strain. This would helpful in understanding how the main elements and their molecular weight distributions impact the final biobinder viscosity profile. Once the most suitable microalgae residues are chosen, an assessment of their economic viability for road pavement compared to the petroleum distillation process would be beneficial. Table 5 presents a summary of the liquefaction-derived biobinder studies with key property improvements and drawbacks.

Biobinder and recycled asphalt studies

The use of bio-oils and biocrude as rejuvenators for recycled asphalt mixtures have also been a focus in recent years. Increasing the recycled asphalt (RA) content is environmentally valuable since it reduces long distance aggregate transport and amount of new bitumen needed in asphalt mixtures [START_REF] Blanc | Bio materials with reclaimed asphalt: from lab mixes properties to non-damaged full scale monitoring and mechanical simulation[END_REF][START_REF] Santos | Life cycle assessment of low temperature asphalt mixtures for road pavement surfaces: A comparative analysis[END_REF][START_REF] Santos | Environmental and economic assessment of pavement construction and management practices for enhancing pavement sustainability[END_REF]. In order to make the most of the RA, researchers use new binders and/or additives that re-activate the aged binder by increasing its viscous fraction [START_REF] Blanc | Bio materials with reclaimed asphalt: from lab mixes properties to non-damaged full scale monitoring and mechanical simulation[END_REF][START_REF] Del Barco Carrión | Binder design of high RAP content hot and warm asphalt mixture wearing courses[END_REF][START_REF] Presti | Towards 100% recycling of reclaimed asphalt in road surface courses: binder design methodology and case studies[END_REF]. In this case, bio and conventional rejuvenators typically improve cracking performance (fatigue and low temperature behaviour) but can be detrimental to rutting performance. Table 6 presents a summary of the biobinder studies with recycled asphalt with the main property enhancements and drawbacks.

Biobinders from swine manure and pongamia oil have been considered suitable rejuvenators with RA, with improved rutting and fatigue resistance and adequate thermal stability [START_REF] Mogawer | Performance characteristics of high reclaimed asphalt pavement containing bio-modifier[END_REF][START_REF] Nayak | A rheological study on aged binder rejuvenated with Pongamia oil and Composite castor oil[END_REF]. Apart from swine manure, other studies have examined thermochemically treated wood, miscanthus and corn stover with RA, with the objective of understanding how feasible these mixtures could be in cold regions prone to thermal cracking. Results showed that the presence of biobinders improved fracture resistance, and overall it was observed that the biomodified RA mixtures exhibited better low-temperature cracking behaviour to that of conventional hotmix asphalt [START_REF] Hill | Low-temperature performance characterization of biomodified asphalt mixtures that contain reclaimed asphalt pavement[END_REF][START_REF] Hill | Evaluation of low temperature viscoelastic properties and fracture behavior of bio-asphalt mixtures[END_REF].

Nevertheless, limited data on accelerated pavement testing is currently available as most studies are restricted to laboratory evaluations [START_REF] Blanc | Full-scale validation of bio-recycled asphalt mixtures for road pavements[END_REF][START_REF] Ding | Laboratory investigation of the recycled asphalt concrete with stable crumb rubber asphalt binder[END_REF][START_REF] Mogawer | Evaluating the effect of rejuvenators on the degree of blending and performance of high RAP, RAS, and RAP/RAS mixtures[END_REF][START_REF] Tran | Effect of rejuvenator on performance characteristics of high RAP mixture[END_REF][START_REF] Zaumanis | Influence of six rejuvenators on the performance properties of Reclaimed Asphalt Pavement (RAP) binder and 100% recycled asphalt mixtures[END_REF]. One study that has undertaken a full scale accelerated pavement test was Blanc et al. [START_REF] Blanc | Full-scale validation of bio-recycled asphalt mixtures for road pavements[END_REF], where three mixtures were designed incorporating 50% RA content with three innovative bio-materials and compared with a reference high modulus asphalt mixture. They were tested for one year in order to speed up rutting and fatigue cracking. The authors concluded that all three bio-mixtures present similarly or better performance than the control and confirmed that they can be effectively used in road construction, but further investigation should focus on long-term monitoring to evaluate ageing performance.

Fatty acids from vegetabe oils like soybean and sunflower oils have also been shown to soften aged bitumen [START_REF] Seidel | Soy fatty acids as sustainable modifier for asphalt binders, Alternative Binders for Sustainable Asphalt Pavements[END_REF][START_REF] Somé | Effect of vegetable oil additives on binder and mix properties: laboratory and field investigation[END_REF]. Field investigations have found the biomodified mixtures to have adequate performance after 5 years of service, despite the biomodified binders being more sensitive to ageing than traditional binders. Nevertheless, the authors agree that five years is not long enough to draw definitive conclusions.

The use of biobinders to enhance the properties of petroleum bitumen has been a topic of discussion over several years, however there is limited knowledge on the adhesion properties of bitumens blended with biobinders. Gong et al. [START_REF] Gong | Evaluation of bio-binder modified asphalt's adhesion behavior using sessile drop device and atomic force microscopy[END_REF] and Jiménez del Barco Carrión et al.

[116] looked at characterizing the adhesion behaviour of bio-modified bitumen using contact angle measurement and surface free energy. Gong et al. [START_REF] Gong | Evaluation of bio-binder modified asphalt's adhesion behavior using sessile drop device and atomic force microscopy[END_REF] used dosages of 1%, 2%, and 3% of bio-modifier (produced from natural bean oil) with two base binders. They found that the adhesion properties depended on the compatibility between the biobinder and base bitumen used. Jiménez del Barco Carrión et al. [START_REF] Del Barco Carrión | Intrinsic adhesive and cohesive assessment of the moisture sensitivity of bio-rejuvenated recycled asphalt binders[END_REF] combined biobinders with RA binders and concluded that biobinders had great potential to maintain moisture damage resistance of such type of asphalt mixtures. As a result, it is critically important to characterize these properties for innovative materials used.

A main conclusion drawn from RA studies is that high amounts (i.e. 50% RA) could be incorporated into asphalt mixtures with suitable biobinders, restoring their rheological properties and enhancing the performance of the mixture [START_REF] Del Barco Carrión | Intrinsic adhesive and cohesive assessment of the moisture sensitivity of bio-rejuvenated recycled asphalt binders[END_REF][START_REF] Del Barco Carrión | Evaluation of bio-materials' rejuvenating effect on binders for high-reclaimed asphalt content mixtures[END_REF][START_REF] Del Barco Carrión | Linear viscoelastic properties of high reclaimed asphalt content mixes with biobinders[END_REF][START_REF] Chailleux | Biorepavation: Innovation in bio-recycling of old asphalt pavements, comparison between EU and US mix design specification systems[END_REF][START_REF] Sotoodeh-Nia | Effect of two novel bio-based rejuvenators on the performance of 50% RAP mixes-a statistical study on the complex modulus of asphalt binders and asphalt mixtures[END_REF]. Such bio-derived mixtures with RA can sufficiently pass the design requirements for pavements and perform well at low, intermediate and high temperatures without the need of neat bitumen [START_REF] Manke | Performance of a sustainable asphalt mix incorporating high RAP content and novel bio-derived binder[END_REF]. Table 6 summarises the results of the recycled asphalt and biobinder studies found.

Miscellaneous biobinder studies

Some studies are considered miscellaneous in the sense that they do not adhere or refer to one specific thermochemical technology. Materials such as algae [START_REF] Chailleux | Alternative binder from microalgae: Algoroute project[END_REF], residues from the forestry industry, waste cooking oils, some agricultural wastes and natural fibres such as sugarcane, rice husk and hemp as well as synthetic binders have been used as bitumen modifiers at different percentages [START_REF] Ingrassia | Renewable materials in bituminous binders and mixtures: Speculative pretext or reliable opportunity?[END_REF][START_REF] Caro | Studying the impact of biomodifiers produced from agroindustrial wastes on asphalt binders[END_REF][START_REF] Martínez-Echevarría-Romero | Valorization of vinasse as binder modifier in asphalt mixtures[END_REF].

Overall, the materials have shown adequate rheological performance and improve some mechanical properties compared to bitumen (see Annex A for details). They seem to perform better at lower temperatures than conventional bitumens, due to their increased workability, making them more suited to be used in colder climates or as fluxing agents for stiff binders such as those found in recycled asphalt [START_REF] De Toulouse | Coating material for road construction[END_REF][START_REF] Avantium | Asphalt composition comprising humins obtained from dehydration of carbohydrates[END_REF]. This renders the study of high-temperature performance more critical with biobinders [START_REF] Wen | Laboratory evaluation of waste cooking oil-based bioasphalt as an alternative binder for hot mix asphalt[END_REF][START_REF] Sun | Investigation of the potential application of biodiesel by-product as asphalt modifier[END_REF][START_REF] Gong | Physical-chemical properties of aged asphalt rejuvenated by bio-oil derived from biodiesel residue[END_REF]. When ageing was considered, most of them revealed faster ageing rates [START_REF] Pouget | Thermo-mechanical behaviour of mixtures containing bio-binders[END_REF][START_REF] Xue | Effects of two biomass ashes on asphalt binder: Dynamic shear rheological characteristic analysis[END_REF]. Ageing of biomodified binders and mixtures is therefore an unavoidable issue which needs to be carefully studied. A summary of the miscellaneous biobinder studies' key properties is shown in Table 7.

Synthetic binder studies

Synthetic binders have also been studied as alternative binders. Although these are not derived from natural sources and have not been studied extensively, they can still affect the rheological properties of asphalt binders. In particular, Airey and Mohammed [START_REF] Airey | Rheological properties of polyacrylates used as synthetic road binders[END_REF] investigated the rheological properties of polyacrylates binders, which consisted of polyethyl acrylate (PEA), polymethyl acrylate (PMA) and polybutyl acrylate (PBA). Results indicated that PEA could simulate a 'soft' 100/150 penetration grade bitumen, while PMA showed stiff 10/20 penetration grade bitumen characteristics. Airey et al. [START_REF] Airey | Rheological characteristics of synthetic road binders[END_REF] further studied these binders and blended the polyacrylates with conventional bitumens, which were found to be rheologically similar to SBS polymer modified bitumens.

Bio-rejuvenators manufactured from cotton oil and the plasticizer dibutyl phthalate (DBP) have also been used to restore the properties of conventional and SBS-modified binders [START_REF] Gong | Physical-chemical properties of aged asphalt rejuvenated by bio-oil derived from biodiesel residue[END_REF][START_REF] Zhu | Recycling long-term-aged asphalts using biobinder/plasticizer-based rejuvenator[END_REF].

Results showed that the 10% dosage of bio-rejuvenator helps to restore workability and rutting resistance of the long-term aged bitumen to original levels. Low-temperature cracking and fatigue resistance also improved but not to the level of the virgin conventional and SBSmodified bitumens. A summary of these studies is presented in Table 8.

Current limitations of biobinders

Although the field of biobinders has drastically taken off in the last ten years, the use of bioderived additives to enhance petroleum bitumen has been around for much longer. The first patent dealing with the production of an alternative binder based on compounds partially derived from biomass was published in 1991 in the US [START_REF] Pinomaa | Dyeable pavement material[END_REF]. This binder was obtained from a mixture of natural or modified vegetable resins (tall oil, wood, or turpentine) and was considered an added-value product with high abrasion resistance. Since then, various companies have developed such biobinders based on oils, resins and polymers. The key benefit is reduced asphalt manufacturing process temperatures compared to petroleum binders, corresponding to significant energy reductions [START_REF] Colas | Obtention de liants routiers a`base de bitume et d'une nouvelle gamme de fluxantsd'origine naturelle fonctionnalises[END_REF][START_REF] Colas | Liant de nature végétale pour la réalisation de matériaux pour le bâtiment et/ou les travaux publics[END_REF][START_REF] Eiffage | Composition comprenant une fraction organique pour la réalisation d'une couche et/ou d'un revêtement de voie ou de bâtiment[END_REF][START_REF] Eurovia | Liant synthétique essentiellement à base de matières issues de ressources renouvelables, en particulier d'origine végétale, et ses applications en technique routière[END_REF][START_REF] Shell | Binder composition and asphalt mixture[END_REF][START_REF] Valagro | Procédé de préparation d'une composition tensioactive à base de biotensioactifs non-ioniques d'origine naturelle[END_REF].

At present, essentially all studies (academic or commercial) that use biobinders in road construction are limited to small-batch production, and so bio-bitumen, whether blended with bituminous binders or not, cannot compete with traditional bitumen. One of the key obstacles to the deployment of biobinders compared to conventional binders is their price. Indeed, the price of vegetable binders is equivalent to that of synthetic binders and is 3 to 10 times higher than that of petroleum bitumen [START_REF] Yvelines | Catalogue des revêtements adaptés aux véloroutes, voies vertes, pistes cyclables et bandes cyclables[END_REF]. This results in an asphalt mixture approximately four times more expensive than the traditional product. Unlike bitumen, it can be safe to assume that the price of biobinders will not be affected by the volatile price of crude oil. A high oil price coupled with further development of renewable technologies will result in a favourable environment for developing biobinders at a much larger scale. Due to the fact that the biobinder technologies are at an early stage, its price is unlikely to fall until a commercial market has been developed and economies of scales take off.

In addition, the process of manufacturing biobinders and then combining them with bitumen tends to be time-consuming and stability of the biobinders is a major issue. Each biobinder has to be carefully studied in detail to understand its chemical and rheological properties, as well as their ageing mechanisms. This building of knowledge is what will help make biobinders more acceptable for widespread use in the future.

Carrying out more studies that look at the sustainability impact and overall carbon footprint of biobinders in comparison with bitumen, such as LCA, will also help build the case for biobinders. In order to help LCA to be useful as a decision-making tool for practitioners and road administrations, cradle-to-grave analysis should be performed, and therefore, more data about the durability of asphalt mixtures containing biobinders should be produced, including full-scale trials and trial sections.

Conclusions and final remarks

This review aimed, for the first time, to bring together studies across different disciplines and investigate their effects on biobinders for road construction.

The utilisation of biomass is beneficial to the environment and society as a sustainable form of energy. A vast range of biomass feedstocks can be thermochemically treated and the derived products subsequently used for various applications, including biobinders for road construction. Its availability worldwide makes it an attractive option for researchers looking to find more environmentally-friendly alternatives to bituminous binders.

Liquefaction and pyrolysis are two effective biomass thermochemical conversion technologies that can be used to produce biobinders. Unlike pyrolysis, liquefaction does not require feedstock drying, saving on high drying costs. The recovered products seem to be more stable and less corrosive than the ones obtained from pyrolysis, due to lower oxygen and moisture contents and higher heating values, and are therefore more suitable as biobinders for asphalt mixtures.

However, liquefaction is considered a less developed technology than pyrolysis and so understanding the effects of the operating parameters is essential to optimise biocrude yields and ultimately advance the process. Product yield and quality are impacted by a range of factors, including temperature, pressure, residence time and type of biomass source. These parameters are highly reliant on the nature of the feedstock as well as each other and can vary extensively. Numerous complex reactions take place during the conversion of biomass into biocrude products. The wide variation in different feedstock types and reaction conditions generates a broad but fragmented spectrum of knowledge and makes it essential to individually study each material to produce biobinders.

The concept of biobinders has definitely gained momentum in pavement engineering and a range of studies have been carried out evaluating their influence on asphalt mixtures performance. Biobinders have shown promising performance as bitumen modifiers and extenders. They can enhance the chemical and mechanical properties of conventional asphalt binders. Extensive research has been found on these categories, but less confident results are found in terms of total replacement of conventional bitumen.

The review carried out reveals that biobinder behaviour is dependent on biomass composition.

While some biobinders improved rutting resistance, others improved fatigue cracking.

However, most biobinders seem to enhance low temperature performance and lower the viscosity of the bitumen. There are no reports of a single biobinder improving all of the desired performance parameters. Therefore, before these materials can be further implemented, their performance in asphalt mixtures needs to be always fully characterised in terms of their chemical, rheological, mechanical and ageing properties. These need to be tested over a whole range of service temperatures depending on the behaviour and nature of the biobinder. Due to their recent use, most of the studies found are focused on laboratory properties. In order to provide confidence to the use of these materials, more data about long-term performance and durability are required; and attention should be paid to future issues such as their recyclability.

Biobinders that are available in large quantities such as through industrial processes, i.e. waste feedstocks, are more attractive in terms of commercial viability than those derived from more involved processes. Due to their availability, these are more likely to become a practical reality in the near future. Comprehensive environmental assessment tools like LCA and EIBP should be incorporated when studying thermochemical processes and biobinders as they can highlight potential research opportunities and help provide a clearer picture of where the technology is heading in the long term. Environmental impact studies should consider the whole life cycle of asphalt mixtures containing biobinders, accounting for their impact during the whole service life of the pavement. For this purpose, future research should focus on their long-term performance and recyclability.

Finally, the price and length of time required to make biobinders are major barriers limiting their widespread use. Current biobinder materials are mostly used for research purposes either at lab or pilot-scale, and so are not presently found at industrial scale. For these reasons, successfully produced biobinders almost exclusively apply to small cycle and pedestrian paths, sidewalks and car parks. Nevertheless, there is a great opportunity for biobinder production costs to decrease as the technology becomes mainstream, their combination with reclaimed asphalt to reduce the final price of the mixture and also for the implementation of a more biobased and circular economy in the future, where suitably efficient bio-products can then compete and capture markets dominated by petroleum bitumen.

[

Figure 2 :

 2 Figure 2: Greenhouse gas emissions from an HTL-derived swine manure biobinder compared to a conventional asphalt binder [20].

Figure 4 :

 4 Figure4: On the left, biobinder test road section near Adelaide, Australia. On the right, the Ecopave™ biobinder cross-section specimen[START_REF] Ecobiopave™ | GEO320 MRH Research Line Marking field testing[END_REF][START_REF] Ecobiopave™ | Acknowledgement of organizations involved in the evaluation of GEO320™ and MRH™ over the years[END_REF].

Table 2 :

 2 Summary of liquefaction parameters and their effects on product yield.

Table 3 :

 3 Main mixing conditions used in biobinder studies.

	Mixer type	Temperature	Mixer	Time	Type of biomass Reference
		(°C)	speed	(min)
			(rpm)
	Shear mixer	120	5000	20	Pyrolysed oil	[70]
	Shear mixer	180	4000	45	Used oil	[71]
	Ribbon mixer	125	3000	30	Tall oil	[72]
	Mechanical mixing	125	3000	30	HTL oil from manure	[73]
	High speed shear mill	145	3000	5	HTL oil from manure	[73]
	stirrer Mechanical	150	2000	minute each 0.1 ml	ethylene glycol dissolved in Agricultural waste	[74]
	Low shear mixing reactor	160	1500	60	Dehydrated sap	[75]

Table 4 :

 4 Summary of pyrolysis-derived biobinder studies.

	Biobinder	Replacement	Key property	Key property	References
			improvement	drawback/reduction
	Oakwood and crumb	100% binder	None	None	[1]
	rubber	replacement
	Oakwood and crumb		Fatigue & moisture
	rubber	20% binder extender	damage	None	[78]
	Switchgrass oil		None
		100% binder		Temperature	[87]
		replacement		susceptibility

Table 5 :

 5 Summary of liquefaction-derived biobinder studies.

	Biobinder	Replacement	Key property	Key property	References
			improvement	drawback/reduction

Table 6 :

 6 Summary of recycled asphalt and biobinder studies.

	Biobinder	Replacement	Key property	Key property	References
			improvement	drawback/reduction
	Bio-rejuvenator	Up to ~5% added to	Rheological properties	None	[99, 108]
	SYLVAROAD™	mixtures with 50%	including complex
	Biobinder	RA	modulus and phase
	Biophalt®		angle, fatigue, rutting,
	Bio-additive		thermal cracking
	Epoxidized Soybean		resistance & durability in
	Soyate (EMS)		field
	Rejuvenator A	6,12 and 18%	Thermal cracking,	None	[102]
	(regenerated oil and	Rejuvenator A and	rutting & fatigue
	a Fischer-Tropsch	9, 18 and 27%	resistance
	wax) and	Rejuvenator B added
	Rejuvenator B	to RA mixture
	(highly viscous
	material free of
	polycyclic aromatic
	hydrocarbons)
	Waste vegetable oil,	12% added to RA	Rutting, fatigue &	Viscosity, moisture	[112]
	waste vegetable	mixture	thermal cracking	damage & ageing
	grease, organic oil,		resistance	resistance
	distilled tall oil,
	aromatic extract and
	waste engine oil

Table 7 :

 7 Summary of miscellaneous biobinder studies.

	Biobinder	Replacement	Key property	Key property	References
			improvement	drawback/reduction
	Microalgae	100% binder	None	None	[123]
		replacement
	Forestry industry	5.7%	None	Ageing resistance	[130]
	by-products
	Waste cooking oil	10,30 and 60%	Rheological properties &	Fatigue & rutting	[127]
		binder modifier and	thermal cracking	resistance
		extender	resistance
	Biodiesel by-product	2-8% binder	Fatigue & thermal	Rutting & moisture	[128]
	from waste cooking	modifier	cracking resistance	damage
	oil (as aged binder
	rejuvenator)
	Biodiesel by-product	1.5,1.75 and 2%	Fatigue & thermal	Rutting & moisture	[129]
	from waste cooking	binder modifier	cracking resistance	damage
	oil (as aged binder
	rejuvenator)

Table 8 :

 8 Summary of synthetic binder studies.

	Biobinder	Replacement	Key property	Key property	References
			improvement	drawback/reduction
	Polyethyl acrylate	100% binder	Rheological properties	None	[133]
	(PEA), polymethyl	replacement	including complex
	acrylate (PMA) and		modulus and phase angle
	polybutyl acrylate
	(PBA)
	Polyethyl acrylate	25-75% binder	Rheological properties	None	[61]
	(PEA), polymethyl	extender	including complex
	acrylate (PMA) and		modulus and phase angle
	polybutyl acrylate
	(PBA)
	Cotton oil by-	5 and 10% binder	Viscosity, thermal	Rutting resistance	[134]
	product and	modifier	cracking & fatigue
	dibutylphthalate		resistance
	(DBP) (as aged
	binder rejuvenator)

Acknowledgements .................................................................................................................. 36 References ................................................................................................................................ 37 Acknowledgements This research is part of the Carbon Capture and Storage and Cleaner Fossil Energy Centre for Doctoral Training and the Nottingham Transportation Engineering Centre research group at the University of Nottingham. The Authors would like to acknowledge the Engineering and Physical Sciences Research Council (EPSRC) and Tarmac, a CRH company, for providing financial support for this project. This project has also received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 754446 and UGR Research and Knowledge Transfer Found -Athenea3i.

Appendix A. Summary of the influence of biobinder composition on bituminous binders' properties and performance