5 Must-Have Features in a China Iron-carbon bed filler

- Liquid locks and vacuum barriers

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Basic liquid lock

This is probably the most basic liquid lock. I used naphtha and petroleum to build this liquid lock, but you can use a wide variety of liquids. Keep in mind that certain liquids are more susceptible to temperature changes and may go through state changes if something very hot or cold is carried through the liquid lock. If that happens, the liquid lock collapses instantly. A liquid lock consisting of petroleum and naphtha can handle temperatures between -50°C and 538°C.

Naphtha is arguably the best liquid for liquid locks, especially if you place it at the bottom of the liquid lock. It handles extreme temperatures very well, and its high viscosity means that it won't start to spread to nearby tiles until it exceeds 40 kilograms per tile. In comparison, crude oil and water spreads to adjacent tiles when they exceed 400 and 40 grams respectively. That makes naphtha very resistant to vaporization due to its high mass, and it also prevents debris and bottled polluted water from offgassing. The only real drawback to naphtha is that it can be displaced if you have a crude oil spillage nearby, since it has lower density than crude oil.



Many liquid locks are temporary constructions used to build vacuum rooms, but some liquid locks are meant to stay forever. In that case, it's a good idea to build a double liquid lock and use atmo sensors to ensure that the locks are intact. If either atmo sensor detects gas pressure, the notifier activates, which causes the game to pause and zooms in on the faulty liquid lock. The signal switch can be used to close the door manually until the liquid lock has been fixed if necessary.

Important: The space between the two liquid locks becomes a vacuum when the door opens. Since heat can't transfer through vacuum, this design prevents the heat from the steam room to transfer to the room on the left.


A screenshot of a construction site where the liquid locks haven't been removed yet. The liquid locks show where the dupes would enter the buildings during the construction phase.

Dry liquid lock

It looks pretty wet for a dry liquid lock, but surprisingly, the dupes don't get the Sopping wet debuff when they walk through the liquid lock. Apparently, dupes "teleport" when they jump over an empty tile. This can obviously be exploited but it can also cause problems. For instance, if you have a 1-tile wide nature reserve then it's entirely possible for the dupes to jump through it without getting the +6 morale bonus.

You can build this type of liquid lock using two or three different liquids, but the former requires vastly more liquid as you need to completely fill the bottom tile. It is highly recommended to have at least 2 kilograms of liquid in the bottom tile, as that will prevent polluted dirt, oxylite, bleach stone etc. from offgassing inside the liquid lock. I used water and brine to build the liquid lock in the picture.

P-trap with vacuum barrier

Probably the least "exploity" of the liquid locks shown so far, but there's a twist. When you deconstruct the floor tile in the middle, the naphtha drops down and leaves a vacuum behind. Due to the way gases and heat moves, ie. horizontally and vertically, but not diagonally, this liquid lock is also a vacuum barrier. Some might consider that exploity.

Naphtha is usually the superior liquid when building P-traps. The high mass per tile (up to, but not including, 40 kilograms of naphtha) makes it highly resistant to extreme temperatures and offgassing. It's not a good choice for places with lots of crude oil lying around (oil wells come to mind), since crude oil has higher density and is able to displace naphtha.



As previously mentioned, liquid locks are susceptible to vaporization due to their relatively low mass. In addition to using liquids with high viscosity (did I mention naphtha?), it's recommended to use bridges as heatsinks.

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are outstanding for this purpose, but conductive wire bridges will do just fine in most cases. Bridges reliably spread the thermal energy across the entire length of the bridge, conducting heat into the surrounding structures. It's basically a lightning rod for liquid locks, except it protects against flash heating, not lightning strikes.

Tempshift plates are also great for protecting liquid locks, but can be a little troublesome when used in conjunction with P-traps.

Mechanical vacuum barrier

If you only care about heat transfer, and don't mind that gases are getting mixed, then maybe all you need is a mechanical vacuum barrier. A couple of weight plates connected directly to an AND gate controls the mechanized airlock in the middle. When the mechanized airlock closes it destroys the gas occupying the same tiles, and turns the room into a vacuum.

Cold vacuum barrier

The small room is at a temperature where it flash freezes gases and turns the room into a vacuum, and since the room is a vacuum, no heat can be transfered in either direction. The cold vacuum barrier is extremely situational, since it pretty much requires that you have a reservoir with super coolant nearby.

It may not be the most convenient build, but I find it oddly satisfying using cold vacuum barriers in my late game colonies. Probably for the very reason that it isn't convenient.

Waterfall liquid lock

Two waterfalls protect the greenhouse from the vacuum of space. Again, this is a very situational type of liquid lock. I included it mostly because the guide is called Compendium of Amazing Designs, and 25-tile high liquid locks in space fits that description.

One of the most useful tools in the toolbox must be the liquid lock. They come in different shapes and sizes (and colours), but they all serve the same purpose: To confine the movement of gases and occasionally also to prevent heat to transfer from one area to another.This is probably the most basic liquid lock. I used naphtha and petroleum to build this liquid lock, but you can use a wide variety of liquids. Keep in mind that certain liquids are more susceptible to temperature changes and may go through state changes if something very hot or cold is carried through the liquid lock. If that happens, the liquid lock collapses instantly. A liquid lock consisting of petroleum and naphtha can handle temperatures between -50°C and 538°C.Naphtha is arguably the best liquid for liquid locks, especially if you place it at the bottom of the liquid lock. It handles extreme temperatures very well, and its high viscosity means that it won't start to spread to nearby tiles until it exceeds 40 kilograms per tile. In comparison, crude oil and water spreads to adjacent tiles when they exceed 400 and 40 grams respectively. That makes naphtha very resistant to vaporization due to its high mass, and it also prevents debris and bottled polluted water from offgassing. The only real drawback to naphtha is that it can be displaced if you have a crude oil spillage nearby, since it has lower density than crude oil.Many liquid locks are temporary constructions used to build vacuum rooms, but some liquid locks are meant to stay forever. In that case, it's a good idea to build a double liquid lock and use atmo sensors to ensure that the locks are intact. If either atmo sensor detects gas pressure, the notifier activates, which causes the game to pause and zooms in on the faulty liquid lock. The signal switch can be used to close the door manually until the liquid lock has been fixed if necessary.Important: The space between the two liquid locks becomes a vacuum when the door opens. Since heat can't transfer through vacuum, this design prevents the heat from the steam room to transfer to the room on the left.It looks pretty wet for a dry liquid lock, but surprisingly, the dupes don't get thedebuff when they walk through the liquid lock. Apparently, dupes "teleport" when they jump over an empty tile. This can obviously be exploited but it can also cause problems. For instance, if you have a 1-tile wide nature reserve then it's entirely possible for the dupes to jump through it without getting the +6 morale bonus.You can build this type of liquid lock using two or three different liquids, but the former requires vastly more liquid as you need to completely fill the bottom tile. It is highly recommended to have at least 2 kilograms of liquid in the bottom tile, as that will prevent polluted dirt, oxylite, bleach stone etc. from offgassing inside the liquid lock. I used water and brine to build the liquid lock in the picture.Probably the least "exploity" of the liquid locks shown so far, but there's a twist. When you deconstruct the floor tile in the middle, the naphtha drops down and leaves a vacuum behind. Due to the way gases and heat moves, ie. horizontally and vertically, but not diagonally, this liquid lock is also a vacuum barrier. Some might consider that exploity.Naphtha is usually the superior liquid when building P-traps. The high mass per tile (up to, but not including, 40 kilograms of naphtha) makes it highly resistant to extreme temperatures and offgassing. It's not a good choice for places with lots of crude oil lying around (oil wells come to mind), since crude oil has higher density and is able to displace naphtha.As previously mentioned, liquid locks are susceptible to vaporization due to their relatively low mass. In addition to using liquids with high viscosity (did I mention naphtha?), it's recommended to use bridges as heatsinks. Conveyor bridges are outstanding for this purpose, but conductive wire bridges will do just fine in most cases. Bridges reliably spread the thermal energy across the entire length of the bridge, conducting heat into the surrounding structures. It's basically a lightning rod for liquid locks, except it protects against flash heating, not lightning strikes.Tempshift plates are also great for protecting liquid locks, but can be a little troublesome when used in conjunction with P-traps.If you only care about heat transfer, and don't mind that gases are getting mixed, then maybe all you need is a mechanical vacuum barrier. A couple of weight plates connected directly to an AND gate controls the mechanized airlock in the middle. When the mechanized airlock closes it destroys the gas occupying the same tiles, and turns the room into a vacuum.The small room is at a temperature where it flash freezes gases and turns the room into a vacuum, and since the room is a vacuum, no heat can be transfered in either direction. The cold vacuum barrier is extremely situational, since it pretty much requires that you have a reservoir with super coolant nearby.It may not be the most convenient build, but I find it oddly satisfying using cold vacuum barriers in my late game colonies. Probably for the very reason that it isn't convenient.Two waterfalls protect the greenhouse from the vacuum of space. Again, this is a very situational type of liquid lock. I included it mostly because the guide is called, and 25-tile high liquid locks in space fits that description.

Due to population growth and infrastructure development, increasing demand for construction materials is expected to cause substantial consumption of natural resources and greenhouse gas emissions (China Association of Building Energy Efficiency ). It is crucial to save the limited natural resources from manufacturing construction materials and reduce carbon emissions throughout the entire life cycle. Developing new and sustainable construction materials with low or even negative carbon footprint is an effective way to achieve carbon neutrality, which has increased interest in scientific and industrial communities (Churkina et al. ). To alleviate the issue of substantial CO2 emissions associated with cement and concrete, several research schemes such as the development of alkali-activated materials, limestone calcined clay cement (LC3) (Scrivener et al. ), waste utilisation for cement replacement (Yin et al. ; Maljaee et al. ), have been intensively investigated and practically applied in recent years. In the latest development, biochar as a carbon-negative material has been considered a promising candidate for cement and aggregate substitution in construction materials (Akinyemi and Adesina ). Figure 3 shows the conversion of waste biomass into biochar and the proposed biochar construction materials as a carbon sink. Fine-grained biochar can act as supplementary cementitious material, whilst coarse-grained biochar can partially substitute aggregate in concrete.

Fig. 3

Upcycling of waste biomass into carbon-negative biochar construction materials

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4.1

Biochar-cement composites

4.1.1

Biochar as a filler in cement composites

Biochar can be a promising filler in cement-based composites. The roles of biochar in cement composites have been investigated regarding rheology, cement hydration, and mechanical properties. For example, Gupta and Kua () investigated the yield stress and plastic viscosity of biochar-cement composites by comparing coarse (2&#;100 μm) and fine (0.10&#;2 μm) biochar as a filler, where macroporous coarse biochar influenced the flowability and viscosity of cement paste to a greater extent than fine biochar. It was also found that fine biochar particles exhibited higher early strength (i.e., 1-day and 7-day) and better water tightness than macroporous biochar (Gupta et al. a). Fine biochar would fill in pores and voids between solid particles in the cement system, enhancing the compactness and early strength of the biochar-cement composites. Compared with conventional fillers, biochar has a large pore volume that can retain the surrounding water. The retained water would slowly be released and contribute to internal curing, facilitating the hydration of biochar-cement composites (Wang et al. b). Furthermore, the biochar exhibited a more pronounced enhancement in the long-term strength development via dry curing compared to water curing (Sirico et al. ). Nonetheless, excessive introduction of biochar would increase the overall porosity and compromise the mechanical strength and workability of biochar-cement composites. The effect of different curing schemes on the long-term mechanical performance of biochar-cement composites deserves further investigation.

Biochar can facilitate CO2 diffusion and regulate moisture content in the biochar-cement composites during accelerated carbonation (Praneeth et al. ; Wang et al. a). Wang et al. (a, b) suggested that the combination of biochar and CO2 curing can enhance the properties of biochar-cement composites, which is especially effective for Mg-based cement. This is because Mg-based cement would expand after hydration and further fill in the pores of biochar, thus counteracting the adverse effects of large pores. This novel integration of biochar with CO2 curing can serve as a promising technique for producing sustainable construction materials. However, CO2 pre-saturated biochar displayed a detrimental effect on the development of compressive strength due to weak bonding between cement and CO2 pre-saturated biochar (Gupta et al. a, b; Kua et al. ). Thus, the CO2 curing and pre-saturation schemes should be carefully designed to achieve the required mechanical performance and carbon sequestration capacity. The ability of biochar for internal curing and maintaining high relative humidity can also mitigate autogenous shrinkage and dry shrinkage, thus improving the durability of biochar-cement composites. By combining biochar with MgO expansive additive, Mo et al. () solved the autogenous shrinkage of cement and enhanced the compressive strength. The reduction of autogenous shrinkage could reach 16.3% at the age of 180 h with an addition of 2 wt% biochar into the cement. Similarly, a 6-week observation in another study proved that the autogenous shrinkage was eliminated using a combination of rice husk biochar and rice husk ash (Muthukrishnan et al. ). It should be noted that rice husk biochar is rich in active silica, which may facilitate pozzolanic reactions and further relieve the autogenous shrinkage.

Benefiting from the abundant micropores/mesopores and high specific surface area of biochar, the water adsorption/retention capacity, thermal insulation, and temperature regulation ability of biochar-cement composites can be further enhanced. For instance, Gupta and Kua introduced 40 wt% rice husk biochar as a porous micro-filler in cenosphere-containing (10&#;40 wt%) lightweight cement mortar, demonstrating 15&#;20% higher strength retention and 9&#;25% lower permeability, giving evidence to the significant enhancement of thermal stability of biochar-cement composites at 450 °C (Gupta and Kua ). Meanwhile, the biochar-cement composites with 10&#;30 wt% silica fume exhibited a 28-day compressive strength of up to 66 MPa with a density of less than  kg m&#;3. Nevertheless, the relatively high price of cenosphere and silica fume would unavoidably increase the cost of manufacturing lightweight biochar-cement composites, and the density for lightweight concrete should be less than  kg m&#;3 (ACI 213 ; ACI 213R-03 ). Therefore, it is necessary to reduce the consumption of cenosphere and silica fume in future studies, increase the biochar dosage, and improve the properties of engineered biochar for better composite performance. The biochar-cement composites were also tested at 550 °C, showing that the stability was still maintained due to the function of biochar in reducing capillary porosity and redistribution of water (Gupta and Kua ). Furthermore, biochar as a hygroscopic filler has been applied in pervious concrete to regulate the temperature and purify contaminated water, thus contributing to developing sponge cities. By incorporating 5% biochar, the total water adsorption of pervious concrete reached 117 kg m&#;3, and the enhancement of water adsorption, in turn, decreased the surface temperature of pervious concrete by 6 °C (Tan et al. ).

Fine biochar (<&#;125 μm) as an alternative for cement was applied for manufacturing ultra-high performance concrete (UHPC) with lightweight and high strength (Dixit et al. ). It was found that biochar with internal curing and nucleation sites can improve the hydration and alleviate the brittle nature of biochar, making it feasible for manufacturing UHPC. Dixit et al. also found that the addition of 2 wt% and 5 wt% biochar reduced the autogenous shrinkage of calcined marine clay-based UHPC by 21% and 32%, respectively (Dixit et al. ). Using 5 wt% biochar in UHPC also contributed to carbon sequestration of approximately 115 kg CO2 per m3 of UHPC (Dixit et al. ).

Replacing cement partially with biochar is a win&#;win strategy in respect of sustainable waste management and carbon sequestration. Optimising the biochar dosage in the admixture would benefit the enhancement of compressive strength, flexural strength, toughness, ductility, and durability of biochar-cement composites. The optimal dosage of biochar recommended as a filler in the biochar-cement composites was 0.5&#;2 wt% in consideration of the improvement in mechanical performance (Maljaee et al. ). The incorporated dosage of biochar could be further increased to reduce the CO2 emissions associated with construction and buildings, even though this would lead to an inevitable strength loss (within an acceptable range). Biochar with a relatively large particle size was not recommended as a filler because it could not efficiently fill the pores, leading to low strength and high capillary pores (Akhtar and Sarmah ). Meanwhile, the O/C atomic ratio in biochar was strongly associated with its hydrophilicity, such that high ratios may ensure good water retention capacity and facilitate internal curing (Karnati et al. ).

Few studies investigated the customisation of engineered biochar for improving the performance of biochar-cement composites. Engineered biochar can be tuned to possess hydrophilic functional groups, which may be compatible with cement and promote hydration reaction. The mineral-rich engineered biochar may be able to facilitate the pozzolanic reactions further. For example, the Si released from Si-enriched biochar could form additional C-S-H with Al and Ca in the cement system, densifying the structure and enhancing the mechanical performance (Wang et al. b, d; Chen et al. a). The composites of Mg/Al layered double hydroxides (Mg/Al-LDHs) impregnated biochar feature smaller crystallite sizes, larger interlayer spacing, higher surface area, and more exposed active sites (Peng et al. ), which could provide additional nucleation sites and promote the hydration rate. Meanwhile, the Mg/Al-LDHs impregnated biochar has the potential to be used as a corrosion control additive in concrete because free Cl can be captured in the interlayer of LDHs (Cao et al. ; Ye ). The engineered biochar can also be combined with Al-rich minerals (e.g., kaolinite) via cation bridging, ligand exchange, and Van der Waals attraction (Yang et al. ), enhancing the carbon stability of biochar in the composites. A modification of the electronegativity of biochar surface may also regulate the cement hydration process. Therefore, incorporating engineered biochar into cement composites exhibits a good research potential and warrants further investigations.

4.1.2

Biochar as an aggregate in concrete

Biochar can be used to substitute for aggregate, especially in lightweight concrete. Previous research has investigated the application of hollow cenospheres, wood, and fibres (e.g., reed fibres and milled fibres) as low-density aggregate in construction sectors for achieving lightweight and high performance (Wang et al. a; Shon et al. ; Chen et al. a; Lu et al. ). Biochar can also be incorporated into the concrete as a porous and lightweight fine aggregate. It was found that replacing sand with 20% biochar with an average particle size of 26 μm could enhance the flexural strength by 26% while reducing the bulk density by 10% (Praneeth et al. ). Restuccia et al. adopted biochar derived from hazelnut shells and coffee powder as nano-aggregates (10&#;15 μm), where the rupture modulus and fracture energy of samples increased by 22% and 61%, respectively (Restuccia and Ferro ). These results indicated that biochar could provide a ductile behaviour and strengthen the interfacial transition zones, thus improving the bending strength and fracture energy. Carbon-negative concrete can also be developed by incorporating 30 wt% biochar as aggregates, providing both environmental benefits and economic profits, and revolutionising the development of the concrete industry (Chen et al. b). Therefore, the application of biochar as an alternative aggregate and the associated environmental benefits (e.g., net-zero CO2 emissions, moisture regulation) are worth further substantiation.

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In the future, large-scale development of biochar-cement composites should be achieved through advancing our scientific understanding of the interfacial reactions and further optimising the pore structure and physicochemical properties of engineered biochar. It is suggested that engineered biochar should be selected from an appropriate feedstock and can be reinforced with chemical additives or physical approaches to maximise the value-added performance of biochar-cement composites.

4.2

Biochar-polymer composites

The adoption of carbon-based fillers (nanotubes, graphene and its derivatives, graphite, activated carbon, and biochar) in polymer composites has gained great interest due to good interfacial bonding with polymer, availability in different forms, etc. Table 1 compares the performance and characteristics of biochar-polymer composites investigated in the latest studies. Incorporating biochar enhanced the mechanical performance (e.g., tensile strength, flexural strength, and elongation) and functional performance (e.g., electrical conductivity and rolling resistance) of biochar-polymer composites.

Table 1 Performance and characteristics of biochar and properties of biochar-polymer composites

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4.2.1

Biochar-resin composites

Biochar can be incorporated into epoxy resin to enhance its mechanical and electrical properties, extending resin composites&#; applications in structural applications, surface coating, and laminating electronic circuit boards in industries such as automobiles and aerospace. Giorcelli et al. found that a low biochar dosage had no significant effect on the mechanical properties of the biochar-epoxy resin composites, whilst a high biochar dosage significantly enhanced the mechanical properties in terms of toughness and resilience 105. The electrical performance of the composites was also investigated, in which the degree of conductivity enhancement was associated with the carbon content of biochar (Giorcelli et al. a, b).

The shape (i.e., spherical and cylindrical) of biochar affects the mechanical properties of epoxy resin. The biochar-epoxy resin composites exhibited remarkable elongation properties (up to 8.2%) and low friction coefficients (reaching 0.37) when incorporating 2 wt% of spherical biochar, while the two parameters reached 4.0% and 0.22, respectively, when adopting 10 wt% of cylindrical biochar (Bartoli et al. ). Therefore, the pyrolysis procedure, types of feedstock, and characteristics of the resultant biochar primarily affected the performance of biochar-epoxy resin composites, especially for electrical properties. A thoughtful selection of suitable biochar is a prerequisite for effective incorporation.

4.2.2

Biochar-rubber composites

Carbon black is a conventional filler used in rubber composites representing approximately 90% of the rubber filler market (Fan et al. ). However, carbon black is a fossil fuel-derived product with considerable carbon emissions. Some researchers have replaced carbon black with renewable bioresources (e.g., starch, lignin, soy protein, and eggshell) in the rubber composites (Jong ; Barrera and Cornish ; Cao et al. ; Du et al. ), yet they exhibited low reinforcement efficacy on rubber owing to the brittle nature and strong hydrophilicity (Jiang et al. ).

Biochar with similar properties to carbon black has been applied to rubber composites. Jong et al. () found that coconut shell biochar exhibited a fivefold increase in tensile modulus of the rubber composites compared to natural rubber. The importance of filler size and filler surface properties was emphasised for the strength enhancement of rubber (Jong et al. ). Biochar larger than 10 µm in diameter would introduce localised stress in the rubber composites, adversely influencing reinforcement properties. To address this issue, the reinforcement efficacy of particle size on the biochar-rubber composites could be adjusted by using nano-silica as the co-milling material and controlling ball milling time (Xue et al. ; Peterson and Kim ). Both studies confirmed that smaller particle sizes (<&#;1 μm) improved the elongation and toughness properties of biochar-rubber composites.

Generally, biochar as a filler incorporated into polymer could improve the mechanical, thermal, and electrical properties and concurrently reduce the production cost. However, some intrinsic drawbacks of biochar (e.g., relatively large particle size, low surface activity, variable components and properties) would require adequate and scientific designs before industrial applications. Therefore, the properties of polymer-composites can be further improved by tailor-making the biochar properties such as porous structure, surface functionalisation, chemical composition, mineral speciation/crystallinity, carbon structure/reactivity, etc. Such enhancement could strengthen the biochar-polymer interactions and impart superior properties for the composites. Recently, molecular dynamics simulations have been applied for evaluating the mechanical performance of carbon materials-polymer interfaces under different conditions (Zhou et al. ; Tam et al. ), which are also recommended for improving our scientific designs of the biochar-polymer composites. Combining the experimental analysis at the macro scale and computational simulations at both micro and nanoscale, a comprehensive understanding of the biochar-polymer composites can be achieved. In the future, the multi-scale investigation will be an essential study for next-generation biochar-polymer composites.

4.2.3

Biochar-plastic composites

Biochar was employed as a filler in wood-plastic composites (WPC) (Das et al. a). By increasing the biochar content to 24 wt%, the tensile and flexural strength of biochar-modified WPC improved comparing with conventional WPC (Das et al. b). This was attributed to the porous biochar, which allowed molten polypropylene to fill in and created a mechanical/physical interlocking (Das et al. a).

Destructions due to building fire highlight the importance of flame-retardant polymer composites. Biochar with a stable porous honeycomb structure and no flammable volatiles is qualified with considerable thermal resistance and can be used as excellent fire-resistance materials (Babu et al. ). The highest thermal stability was observed in the case of WPC with 18 wt% biochar incorporation (Zhang et al. a). The biochar application into WPC would synergistically preserve mechanical properties and reduce flammability. Poultry litter biochar was found to impart the optimal tensile and flexural properties of composites due to the Ca-rich ash in poultry litter biochar (Das et al. a). Besides, biochar addition could save production cost by approximately 18% as the dosage of coupling agent (i.e., maleic anhydride grafted polypropylene) could be reduced from 3 to 1 wt% without significant deterioration in mechanical performance (Das et al. b). Conventional flame retardants (i.e., ammonium polyphosphate and magnesium hydroxide) were introduced into the biochar-modified WPC to further impede its flammability. Considering both enhancements of resistance to radiative heat and economic benefits, the loading amount of magnesium hydroxide was suggested to be 20 wt% (Das et al. a, b). A higher dosage of flame retardants (e.g., magnesium hydroxide at a high loading rate of 50 wt%) may further strengthen the thermal stability of biochar-modified WPC; however, the excess flame retardants would be trapped in the biochar pores and obstruct the infiltration of polypropylene, which consequently reduced the mechanical bonding/interlocking between biochar and polypropylene (Das et al. a). The employment of biochar for enhancing the flame resistance of WPC is a promising approach concerning both environmental sustainability and economics. The effect of biochar on the WPC manufacturing process (e.g., extrudability) requires further investigation before industrial applications.

The electrical conductivity of biochar-plastic composites is also attracting extensive attention regarding their various applications, such as electrostatic dissipation materials, electromagnetic interference shielding materials, and semiconducting layers to prevent electrical discharge. Poulose et al. () applied biochar to manufacture biochar polypropylene composites to enhance the electrical properties and tensile modulus, but the agglomeration and the high ash content of biochar would hamper the conductivity enhancement. In general, the integrated properties of the biochar-plastic composites are associated with the dispersion of biochar and the network formation in the polymer matrix (Khushnood et al. ).

Other parameters (e.g., characteristics of biochar, polymer viscosity, and types of coupling agents) would affect the integrated properties of the biochar-plastic composites. The addition of coupling agents, wood and biochar was crucial for the tensile and flexural strength of composites but had little effect on the flammability (Ikram et al. ). Polar wood biochar exhibited no effect on the melting temperature of high-density polyethene, but it promoted the early crystallisation of biochar-plastic composites (Zhang et al. b). Dynamic mechanical analysis revealed that biochar incorporation enhanced the stiffness, elasticity, creep resistance, and stress relaxation of the biochar-plastic composites (Zhang et al. a). However, other properties of biochar, such as ash content, specific surface area, surface functional groups, etc., are not clearly stated regarding the performance of biochar-plastic composites and require further investigations. In future research, it is also necessary to identify the optimum levels of various factors to achieve the desirable properties of biochar-plastic composites for either mechanical performance or flammability.

4.3

Biochar-asphalt composites

Asphalt mixtures, consisting of asphalt binder and aggregates, are principal construction materials employed in the highway and pavements. Some carbon-based materials (e.g., carbon black, carbon fibre, and carbon nano-tubes) have been introduced to improve the properties (e.g., rutting resistance, stripping resistance, and durability) of asphalt binders in previous studies (Cong et al. ; Wang et al. b; Ziari et al. ). However, the high cost and limited enhancement of the aforementioned carbon-based materials inhibited their large-scale applications. A few researchers have adopted biochar as an economic modifier to enhance the properties of asphalt binders and mixtures, such as durability, temperature sensitivity, and fatigue performance (Fig. 4). Biochar was more effective in strengthening temperature susceptibility and rutting resistance of asphalt binders than carbon black or carbon fibre (Zhao et al. a, b). Biochar with a particle size less than 75 μm could be a favoured asphalt binder modifier for achieving satisfactory rotational viscosity and low-temperature crack resistance (Zhang et al. ).

Fig. 4

Biochar as a modifier for asphalt manufacture

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The ageing of the asphalt binder would lead to cracking, fatigue, and raving (Nazari et al. ). Ageing also contributes to an increase in viscosity, affecting the stiffness of the asphalt binders and mixtures (Pasandín et al. ). Therefore, more interest has been gained in enhancing the ageing resistance and susceptibility of asphalt (Cong et al. ; Kumar et al. ; Dong et al. ). Pyrolysis biochar could primarily improve the ageing resistance of asphalt binders by mitigating the oxidative ageing of asphalt binder components rather than reducing the volatilisation of lightweight components (Dong et al. ). Furthermore, pyrolysis biochar, having carbon as the primary composition, can shield the surface of asphalt from ultraviolet light, prevent photo-oxidative ageing and improve the high-temperature stability of asphalt (Zhou and Adhikari ). Bio-oil, one of the by-products generated during pyrolysis of biochar, can also be applied as a rejuvenator for aged asphalt (Zhang et al. b), enabling a combination usage of biochar and bio-oil for manufacturing sustainable asphalt.

Hydrochar also exhibits good compatibility with asphalt owing to the micron-sized pits, voids and abundant functional groups on the surface. The high-temperature performance of asphalt was significantly improved by incorporating hydrochar. The optimum dosage of hydrochar was 6 wt% with rutting index reaching 76 °C and penetration and softening point reaching 31.7 (0.1 mm) and 54.7 °C, respectively (Hu et al. ). However, incorporating hydrochar hindered the workability of asphalt under low temperatures, which requires further investigation and improvement by adopting tailored hydrochar.

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