Biochar as construction materials for achieving carbon neutrality

Abstract

Biochar is a waste-derived material that can sequester carbon at a large scale. The development of low-carbon and sustainable biochar-enhanced construction materials has attracted extensive interest. Biochar, having a porous nature and highly functionalised surface, can provide nucleation sites for chemical reactions and exhibit compatibility with cement, asphalt, and polymer materials. This study critically reviewed the state-of-the-art biochar-enhanced construction materials, including biochar-cement composites, biochar-asphalt composites, biochar-plastic composites, etc. The efficacies and mechanisms of biochar as construction materials were articulated to improve their functional properties. This critical review highlighted the roles of biochar in cement hydration, surface functional groups of engineered biochar for promoting chemical reactions, and value-added merits of biochar-enhanced construction materials (such as humidity regulation, thermal insulation, noise reduction, air/water purification, electromagnetic shielding, and self-sensing). The major properties of biochar are correlated to the features and functionalities of biochar-enhanced construction materials. Further advances in our understanding of biochar’s roles in various composites can foster the next-generation design of carbon–neutral construction materials.

Graphical Abstract

Highlights

• Carbon-negative construction materials can be realised by emerging biochar applications.

• Incorporation of biochar can mitigate CO₂ emissions and natural resource depletion.

• Biochar as construction materials foster the attainment of Sustainable Development Goals.

1 Introduction

Global CO₂ emissions increased by 53% from 1990 to 2020 and reached nearly 31.5 giga-tonnes (International Energy Agency 2021), triggering climate change and adversely impacting the ecosystems. According to Global Alliance for Buildings and Construction, approximately 40% of energy-related CO₂ emissions result from construction and building activities (Global Alliance for Buildings and Construction 2017). The manufacture of cement (i.e., the most consumed construction material worldwide) accounted for ~ 36% of CO₂ emissions in construction activities and ~ 8% of total anthropogenic CO₂ emissions (Habert et al. 2020). To achieve ambitious goals of carbon neutrality on a global scale, it is imperative to take unprecedented actions for innovating the design and promoting the use of carbon-negative construction materials.

Biochar, as a negative emission technology (NET) highlighted by the Intergovernmental Panel on Climate Change (IPCC), is a carbon-enriched material produced from the thermochemical conversion of various biomass waste in an oxygen-limited environment (Intergovernmental Panel on Climate Change 2018). The carbon present in biomass can be transferred into a more stable form in biochar, which persists one to two orders of magnitude longer than biomass precursor, resulting in long-term carbon sequestration (Fawzy et al. 2021; Lehmann and Stephen 2015). Recent literature indicated that the average CO₂ footprint of biochar ranged from − 2.0 kg to − 3.3 kg of CO₂-eq per kg of biochar, relying on the locations and properties of feedstocks and the target applications (Azzi et al. 2019). Globally, biochar technology can reduce greenhouse gas emissions by 3.4–6.4 PgCO₂-eq, of which 1.7–3.7 PgCO₂-eq (49–59%) is attributed to the CO₂ withdrawal from the atmosphere (Lehmann et al. 2021). Although there is variation in the total CO₂ reduction depending on different circumstances, extensive use of biochar is widely considered efficient for mitigating climate change and achieving net-zero emissions.

Biochar can be converted from various waste biomass, such as wood waste, agro-waste, food waste, manure waste, and municipal/industrial sludge. The global generation of biomass waste (derived from agricultural and forestry practices) is estimated to be 140 giga-tonnes yr⁻¹, causing severe greenhouse gas emissions and environmental pollution issues (Tripathi et al. 2019). It was estimated that producing 373 million tonnes yr⁻¹ of biochar derived from agro-waste could sequester approximately 500 million tonnes of CO₂ yr⁻¹, equivalent to 1.5% of total annual global CO₂ emissions (Windeatt et al. 2014; Yang et al. 2019). Carbon emissions would be further reduced when other types of biomass waste are transformed into biochar for carbon sequestration. In this context, converting various biomass wastes into biochar and expanding new applications of biochar could provide a win–win strategy for realising long-term decarbonisation and fostering a circular economy.

Integrating biochar into innovative biochar construction materials could facilitate waste valorisation and reduce CO₂ emissions of construction materials. This strategy could also bring additional economic benefits through carbon trade, incentivising the construction and building industry to curtail overall CO₂ emissions (Wang et al. 2021a, b). Recent studies have reviewed the influences of incorporating biochar in concrete composites regarding workability, hydration, and mechanical properties (Akinyemi and Adesina 2020; Maljaee et al. 2021). Although it has often been overlooked in the construction-related literature, the feedstock of biomass and manufacturing technologies of biochar would significantly affect the properties of biochar construction materials, which should be critically investigated. To the best of our knowledge, no literature review has articulated the state-of-the-art knowledge about designing different types of engineered biochar as construction materials, especially for maximising their technical benefits, value-added functionality, and decarbonisation capacities. Hence, in this tutorial review, we firstly introduced different processes for biochar production and emphasised the efficacy of biochar construction materials in achieving carbon neutrality. We comprehensively evaluated the environmental impacts of biochar production and customisation as well as the decarbonisation capacity of biochar as filler and aggregate in comparison to conventional materials. Afterwards, we critically reviewed the manufacturing technologies of biochar construction materials, including biochar-cement composites, biochar-asphalt composites, biochar-plastic composites, etc. Lastly, we identified the grand challenges in employing biochar as supplementary cementitious materials, fine/coarse aggregate, and functional additives in the construction materials, and suggested the prospects for future research directions in this review. As illustrated in Fig. 1, we highlighted sustainable waste management towards a circular economy and proposed a novel strategy for achieving carbon neutrality by adopting biochar construction materials.

Fig. 1

Sustainable waste management towards circular economy and carbon neutrality by adopting biochar construction materials

2 Biochar production

Biochar production processes can be divided into three categories: pyrolysis, gasification, and hydrothermal carbonisation (HTC). Figure 2 is a schematic diagram of the production processes for different types of biochar, with their yields following the order of HTC > pyrolysis > gasification. These biochars differ widely in chemical and physical properties due to their production conditions and feedstock selection. Factors such as pyrolysis temperature, heating rate, residence time, pyrolytic atmosphere, gas pressure, and types of biomass waste play critical roles in the yield and physicochemical characteristics of biochar for promoting the performance of biochar construction materials (including pore size distribution, specific surface area, cation exchange capacity, water retention capacity, etc.) (He et al. 2022; Leng and Huang 2018; Maljaee et al. 2021).

Fig. 2

Pyrolysis and hydrothermal carbonisation of biomass for biochar production

Biochar is generally a product from partially carbonised biomass to highly refractory carbon with distinctive heterogeneity in physicochemical properties, primarily dependent on the feedstocks, production conditions, and various modification methods (Lehmann and Stephen 2015; Wang et al. 2020a, b). As an easily tuneable carbon-based material, biochar can be applied for a wide range of emerging applications, including additives and raw materials in construction materials, by engineering its porous structure, surface functionality, and aromatic/graphitic carbon structure with fit-for-purpose designs (Chen et al. 2022b; Maljaee et al. 2021; Sajjadi et al. 2019). Different thermochemical treatments, including conventional pyrolysis (300–800 °C), gasification (> 700 °C with a fast-heating rate of tens of °C min⁻¹), and HTC (180–250 °C, 2–10 MPa), have been applied to produce biochar (or hydrochar from HTC) with the yields following the order of HTC > pyrolysis > gasification (Fig. 2) (Cao et al. 2021; Wang et al. 2020a, b; You et al. 2018). During biochar production, bio-oil and pyrolytic gas are the co-products that can be applied for bioenergy applications or chemical upgrading (Tomczyk et al. 2020; Wang and Wang 2019). Overall, thermochemical technology should be customised depending on the nature of biomass waste (e.g., moisture content and carbon/mineral compositions) such that we can achieve the maximum reduction of CO₂ emissions and upgrade the technical performance of biochar construction materials.

2.1 Pyrolysis biochar

Conventional pyrolysis with a slow heating rate (5–10 °C min⁻¹) in an oxygen-limited environment is the most widely adopted thermochemical technology to produce biochar owing to its technical simplicity of operation and economic feasibility for upscaling (Tripathi et al. 2016; Yang et al. 2021b). The physicochemical properties of pristine biochar are typically regulated by the compositions of feedstocks and the pyrolysis conditions, including temperature, duration, activation, and modification methods (Cha et al. 2016; Wang and Wang 2019). Pyrolysis temperature plays a critical role in the carbonisation process, which affects the energy value, yield, carbon stability, porous structure, and the pH value of biochar (He et al. 2021b). Increasing pyrolysis temperature could remarkably increase the pore volume and surface area of biochar due to the carbon phase change from amorphous to graphitic form and the driving off of pore-blocking substances (Lian and Xing 2017). For instance, by increasing the pyrolysis temperature, the specific surface area increased by 3.9-fold for sawdust-derived biochar (400–700 °C; 147.4–572.6 m² g⁻¹) (Zhu et al. 2019), 5.2-fold for rice straw-derived biochar (500–700 °C; 22.4–115.5 m² g⁻¹) (Shen et al. 2019), and 29.5-fold for wood waste-derived biochar (650–950 °C; 10.5–309.2 m² g⁻¹) (He et al. 2021b). Longer residence time promotes the repolymerisation process and the development of the porous structure of biochar; slow pyrolysis with residence time longer than 1 h has been regarded as a dominant technology to produce biochar due to the higher economic feasibility and technological maturity (Chen et al. 2019c). Various activation or modification methods, including physical activation (CO₂ or steam), chemical activation by acid, alkaline, and oxidising/reducing agents, have been conducted to augment the surface functionality of engineered biochar (Sajjadi et al. 2019; Xu et al. 2021; Wan et al. 2021). Appropriate specific surface area, pore structures, and surface functionalities of biochar can enhance the water holding and CO₂ storage capacity that would further promote the performance of biochar construction materials via internal curing and accelerated carbonation (Chen et al. 2020a, b; Praneeth et al. 2020).

2.2 Gasification biochar

Gasification with a high reaction temperature (> 700 °C) in the presence of gasifying agents produces pyrolytic gas as the primary products (CO, H₂, CO₂, and CH₄) and biochar with high aromaticity and porosity by four reaction stages, including drying (100–200 °C), pyrolysis (200–700 °C for fixed bed, 700–910 °C fluidised bed, and > 1400 °C for entrained flow), combustion (700–1500 °C), and reduction (800–1000 °C) (You et al. 2018). The production of pyrolytic gas and biochar is typically controlled by the gasification conditions such as temperature, properties of feedstocks, gasifying agents (air, O₂-enriched air, O₂, CO₂, and steam), and the gas pressure (Shayan et al. 2018). High temperature promotes the release of volatiles with a higher yield of pyrolytic gas at the expense of a lower biochar yield, facilitating the formation of micropores/mesopores with a higher specific surface area of biochar (Qi et al. 2021). Compared to the pyrolytic biochar, the gasification biochar with abundant pore structures might be a better choice as additives in construction materials for the sake of internal curing and accelerated carbonation. For the feedstocks with high ash content and alkali and alkaline earth metals (AAEMs) such as sewage sludge and food waste digestate, the solid–solid interaction between carbon and AAEMs (especially for ion-exchangeable Na⁺, Mg²⁺, and Ca²⁺) could weaken the C–C bond and improve the reactivity of gasification, hence enhancing the functionalisation of biochar (Mafu et al. 2018).

2.3 Hydrothermal carbonisation biochar (hydrochar)

HTC exhibits advantages in dealing with wet and bulky biomass such as wet yard waste, food waste, and wastewater sludge, producing carbonised solids (hydrochar) without an energy-intensive drying pretreatment (Cao et al. 2021; Chi et al. 2021). The HTC is carried out in a subcritical water system; ~ 1000-fold of ionic products (K_w = [H⁺][OH⁻]) can be dissociated from water by increasing temperature from 25 °C to 250 °C, which makes water a suitable medium for base- or acid-catalysed reactions (Yang et al. 2021b). In the HTC process, the carbohydrates, protein, lipid, lignin, and humic substances from the feedstocks would be depolymerised to the intermediates by hydrolysis, dehydration, decarboxylation, deamination, and decomposition processes; then, the intermediates could form hydrochar by aromatisation, condensation, and polymerisation (Liu et al. 2021). A higher heating rate was also found to facilitate feedstock decomposition while suppressing the formation of hydrochar; for instance, Wang et al.(2019a) found a decrease in hydrochar yield from 10.3% to 5.0% by increasing the heating rate from 8 °C min^-1 to 50 °C min⁻¹. It is noteworthy that higher levels of nutrients, AAEMs, transition metals, and polycyclic aromatic hydrocarbons (PAHs) may accumulate in the hydrochar, which should be carefully investigated and properly addressed before potential use in construction materials.

3 Carbon neutrality and biochar-enhanced construction materials

Carbon neutrality entails efficiently reducing greenhouse gas emissions and sequestering/capturing CO₂ from the atmosphere (Wang et al. 2021a). Biochar as a soil amendment to curtail CO₂ emissions and accomplish carbon sequestration has been intensively investigated since its first proposal nearly 15 years ago (He et al. 2022; Lehmann et al. 2021). More recently, biochar as a carbon-negative material has been employed in construction materials to enable buildings to become a carbon sink and facilitate the attainment of carbon neutrality targets (Danish et al. 2021; Maljaee et al. 2021).

3.1 Decarbonisation by biochar production

3.1.1 Decarbonisation by converting biomass waste into biochar

The extent of carbon reduction by biochar is determined by its production conditions and application environment that would affect the amount of CO₂ emissions during the entire life cycle (Lehmann et al. 2021; Puettmann et al. 2020; Yang et al. 2021a, b). Meanwhile, the yield, carbon content, and stability of biochar, as well as the energy conversion efficiency of pyrolytic gas and bio-oil as renewable energy, are pivotal factors in determining the CO₂ sequestration potential of thermochemical systems converting biomass into biochar (Yang et al. 2021a, b). The life cycle impacts of biochar systems also encompass CO₂ emissions associated with the transport and storage of biochar, which could be kept minimum as a marginal contribution to the overall emissions (Matuštík et al. 2020). Notably, the large-scale deployment of converting waste biomass to biochar has been considered a ready-to-implement NET for achieving carbon neutrality targets (Azzi et al. 2019; Yang et al. 2021a, b). The pyrolytic gas and bio-oil generated by pyrolysis/gasification/HTC of biomass waste can be used for electricity, heat generation, and alternative fuels. More importantly, biochar/hydrochar can be used in the agriculture and construction industry to enhance their environmental performance and sequester carbon in the natural or built environment. The decarbonisation intensity (including fossil fuel offset) of pyrolysis systems varies from 36.6 CO₂-eq to 136.5 g CO₂-eq MJ⁻¹ in accordance with different production processes and biochar applications (Roberts et al. 2010; Peters et al. 2015). Among them, the biomass intermediate pyrolysis poly-generation (BIPP) system, referring to multiple outputs of an integrated technology system, features the highest CO₂ reduction capacity (Yang et al. 2021a, b). It is believed to be profitable without subsidies in China, reducing 61% carbon emissions per unit of gross domestic product and alleviating air pollution. Overall, converting biomass waste into biochar can foster resource circulation and the attainment of the United Nations’ Sustainable Development Goals (SDGs) and global carbon neutrality.

3.1.2 Decarbonisation of biochar customisation

The engineered biochar exhibits an auspicious future due to its superior adsorptive, catalytic, and electrochemical characteristics after proper customisation (Klüpfel et al. 2014; Qiu et al. 2020; Kumar et al. 2020a; Xu et al. 2022). For instance, ball milling is a practical industry process which can now be assisted by automated technology minimising manual labour. As an emerging mechanochemical activation technology, ball milling has been widely employed in sustainable chemistry and, more recently, in customising biochar to increase its fineness, reactivity, and specific surface area (Kumar et al. 2020b). The ball-milled biochar with customised size, porosity, and surface functional groups can be adopted as fillers in cement composites and carbon-based modifiers in phase change materials (PCMs), which could facilitate their physicochemical performance based on the recent findings (Lyu et al. 2017). Featuring environmentally benign (e.g., ambient temperature, solvent-free), short duration and high reaction selectivity, the mechanochemical methods for customising biochar improve overall sustainability criteria (Lyu et al. 2017; Howard et al. 2018). For example, the electricity consumption associated with chopping and grinding of feedstocks ranged from 5 kWh t^-1 to 200 kWh t⁻¹ depending on their initial and targeted particle size, while the energy consumption related to biochar grinding should be minimal due to its brittle nature (Onarheim et al. 2015; Cheng et al. 2020). The CO₂ emissions from the biochar grinding process for manufacturing biochar-cement composites are negligible compared to the CO₂ emissions from cement clinker production (Reis et al. 2020). Meanwhile, the CO₂ emissions of biochar customisation can be compensated by the ability of biochar to enhance the CO₂ uptake capacity of cementitious materials. In recent studies, for example, incorporating 4 wt% tailored biochar in cement could store an additional 0.12 kg of CO₂ and incorporating 8 wt% ball-milled biochar in cement could reduce CO₂ emissions by 15% compared to plain cement (Praneeth et al. 2021; Tan et al. 2022). However, the commercial-scale application of ball-milled biochar is still in its infancy and requires extensive studies to validate the cost-effectiveness and environmental impacts.

3.2 Decarbonisation of biochar-enhanced construction materials

Cement and concrete are critical components widely applied in the construction industry, which inevitably present tremendous environmental impacts worldwide. In addition to particulate matter and potentially toxic elements that result in human health impacts, the most intractable environmental issue is the high level of CO₂ emissions associated with cement and concrete production, accounting for ~ 8% of the total anthropogenic CO₂ emissions (Schneider 2015; Miller et al. 2018; Zou et al. 2018). In a newly constructed building, approximately 40% of CO₂ emissions come from concrete, and at least 70% of CO₂ emissions of concrete come from cement, followed by the transport of raw materials and aggregate production (Habert et al. 2020).

3.2.1 Decarbonisation of biochar as a filler in cement systems

The adoption of fillers as a partial clinker substitution is a promising method to reduce CO₂ emissions and energy consumption (John et al. 2018). Grinding is crucial for ensuring the high mechanical performance of fillers with high fineness, which together with raw material collection and transportation, contributes to CO₂ emissions in filler production (Batuecas et al. 2021; Ige et al. 2021). The calcination of fillers (e.g., limestone) requires lower temperature and less energy, thereby less CO₂ emissions (John et al. 2018). The cement system incorporating 10% limestone and 5% silica fume as fillers can reduce CO₂ emissions by more than 13% while having comparable mechanical strength with plain cement (Li et al. 2019). In recent years, biochar has been adopted as a filler in cement systems, demonstrating good performance owing to its internal curing effect and provision of nucleation sites. The lightweight and brittle nature of biochar can alleviate the burden on transportation and energy consumption compared to conventional fillers. Meanwhile, a high dosage of fillers (up to 60%) can make carbon-negative concrete feasible (Habert et al. 2020). Overall, biochar as a carbon-negative filler can revolutionise the construction industry by improving the properties of cement-based composites and environmental sustainability.

3.2.2 Decarbonisation of biochar as aggregates in concrete systems

It is estimated that approximately 17.5 Gt of aggregates are utilised to manufacture concrete each year, whose crushing and transportation induce considerable CO₂ emissions (Miller et al. 2018). The over-extraction of natural aggregates (e.g., sand and gravel) has already caused massive environmental damage with cascading effects that affect human well-being (Ioannidou et al. 2017; Aurora et al. 2017). Crushing the demolition waste into smaller particles for reuse as recycled aggregates is a sustainable approach to reducing the CO₂ emissions associated with natural aggregates (Zhan et al. 2018, 2020). The recycled aggregates with porous nature and increased surface area are suitable for CO₂ curing that can densify the matrix, enhance mechanical performance, and increase the lifetime CO₂ uptake (Zhan et al. 2019; Habert et al. 2020). Accelerated carbonation is a promising approach for CO₂ mineralisation and performance enhancement of cementitious materials (Chen and Gao 2019a, 2020; Pan et al. 2020). The CO₂ utilisation potential of cementitious construction materials is expected to remove 100–1400 tonnes CO₂ by 2050 while generating the highest breakeven cost of US$70 per ton of CO₂ utilised (Hepburn et al. 2019). However, the carbonised areas are concentrated on the surface of cementitious materials due to the dense structure and closed pores, which under-utilises the decarbonisation potentials.

When serving as aggregates, biochar with a hierarchical porous structure can improve the pore size distribution of the matrix and promote the deep carbonation of cementitious materials. The biochar pores can interlink with those in the cement systems, facilitating the diffusion and dissolution of CO₂ and enhancing carbonation progress. By incorporating biochar as aggregates, the biochar-enhanced concrete can sequester 59–65 kg CO₂ ton⁻¹ whilst delivering a maximum benefit of 41.1 USD m⁻³ (Chen et al. 2022b). The life cycle analysis proved that the biochar incorporation significantly reduces the CO₂ emissions, whereas biochar-enhanced concrete with 30 wt% biochar can be effectively converted into carbon-negative products (Chen et al. 2022b). However, the kinetics of CO₂ adsorption and water release of biochar in cementitious materials under CO₂ curing lack investigations and sufficient understanding (Chen et al. 2019b; Chen and Gao 2019b). Future research should intend to tailor the adsorption and desorption kinetics of different engineered biochar to control the CO₂ diffusion and regulate the crystalline forms of CaCO₃ (i.e., calcite, aragonite, and vaterite) for desirable interfacial chemistry and optimal performance.

4 Biochar as construction materials

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 2020). 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. 2020). 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. 2018), waste utilisation for cement replacement (Yin et al. 2018; Maljaee et al. 2021), 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 2020). 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

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 (2019) 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. 2018a). 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. 2021b). Furthermore, the biochar exhibited a more pronounced enhancement in the long-term strength development via dry curing compared to water curing (Sirico et al. 2021). 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. 2020; Wang et al. 2020a). Wang et al. (2020a, 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. 2018a, b; Kua et al. 2020). 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. (2019) 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. 2019). 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 2020). 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 2102 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 1920 kg m⁻³ (ACI 213 2003; ACI 213R-03 2003). 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 2020). 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. 2021).

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. 2019). 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. 2021). Using 5 wt% biochar in UHPC also contributed to carbon sequestration of approximately 115 kg CO₂ per m³ of UHPC (Dixit et al. 2021).

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. 2021). 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 2018). 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. 2020).

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. 2019b, d; Chen et al. 2022a). 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. 2021), 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. 2017; Ye 2021). 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. 2018), 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. 2016a; Shon et al. 2019; Chen et al. 2020a; Lu et al. 2021). 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. 2021). 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 2016). 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. 2022b). 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.

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

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. 2019a, 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. 2020). 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. 2020). 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 2015; Barrera and Cornish 2016; Cao et al. 2018; Du et al. 2019), yet they exhibited low reinforcement efficacy on rubber owing to the brittle nature and strong hydrophilicity (Jiang et al. 2020).

Biochar with similar properties to carbon black has been applied to rubber composites. Jong et al. (2014) 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. 2014). 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. 2019; Peterson and Kim 2020). 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. 2015; Tam et al. 2019), 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. 2015a). 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. 2015b). This was attributed to the porous biochar, which allowed molten polypropylene to fill in and created a mechanical/physical interlocking (Das et al. 2016a).

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. 2020). The highest thermal stability was observed in the case of WPC with 18 wt% biochar incorporation (Zhang et al. 2020a). 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. 2016a). 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. 2016b). 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. 2017a, 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. 2017a). 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. (2018) 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. 2015).

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. 2016). 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. 2019b). Dynamic mechanical analysis revealed that biochar incorporation enhanced the stiffness, elasticity, creep resistance, and stress relaxation of the biochar-plastic composites (Zhang et al. 2020a). 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. 2014; Wang et al. 2016b; Ziari et al. 2018). 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. 2014a, 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. 2018).

Fig. 4

Biochar as a modifier for asphalt manufacture

The ageing of the asphalt binder would lead to cracking, fatigue, and raving (Nazari et al. 2018). Ageing also contributes to an increase in viscosity, affecting the stiffness of the asphalt binders and mixtures (Pasandín et al. 2015). Therefore, more interest has been gained in enhancing the ageing resistance and susceptibility of asphalt (Cong et al. 2014; Kumar et al. 2018; Dong et al. 2020). 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. 2020). 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 2019). 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. 2020b), 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. 2021). However, incorporating hydrochar hindered the workability of asphalt under low temperatures, which requires further investigation and improvement by adopting tailored hydrochar.

5 Environmental and technical advantages of biochar-enhanced construction materials

5.1 Humidity regulation and cooling effect

Excessive artificialisation of the ground (e.g., the extensive application of pavement) caused by the rapid urban expansion has disrupted the balance of moisture and heat transition between the ground and the atmosphere, leading to a series of thermal environment issues in urban (He et al. 2021a). For example, pavement materials involving concrete and asphalt mixtures would adsorb and store solar energy and then release the stored energy into the city as sensible heat, contributing to the urban heat island effect (Qin et al. 2018). Pervious concrete features a porous structure that can provide channels for heat and moisture transfer in road pavement, enabling evaporative cooling and alleviating the urban heat island effect in hot seasons (Park et al. 2021a). Incorporating biochar into the pervious concrete could reduce albedo and increase water adsorption capacity, mitigating urban heat island issues (Park et al. 2021b). The biochar-pervious concrete could absorb more solar radiation than plain pervious concrete, keeping cool through water evaporation and creating an urban microclimate. These findings were validated by Tan et al. (2021) by adopting a low-speed straight flow climatic wind tunnel to investigate the temperature regulation capacity of biochar-pervious concrete. Their results demonstrated that the maximum temperature decreased by 6 °C for 18 h (Tan et al. 2021). Therefore, biochar is considered a promising hygroscopic filler for manufacturing humidity- and temperature- regulating construction materials, enhancing hygrothermal performance and alleviating the urban heat island issues (Park et al. 2021b). Figure 5a illustrates the mechanisms of humidity regulation and urban microclimate of biochar pervious concrete. The hygrothermal properties of biochar-pervious concrete should be optimised to enhance the capabilities for regulating humidity and temperature.

Fig. 5

Environmental and technical advantages of biochar composites: a humidity regulation and urban microclimate; b thermal insulation and noise reduction; c contaminant immobilisation and indoor air quality improvement; d electromagnetic shielding; e biochar-added 3D printable concrete; f biochar-enhanced phase change materials; g self-sensing cement composites; h bacteria cargo for self-healing cement composites

5.2 Thermal insulation and noise reduction

Customised biochar possesses 3D porous and 2D flake-like structures, which contribute to the formation of additional pathways for heat transfer (Xiong et al. 2022). When porous biochar is uniformly distributed throughout the construction materials, it can induce scattered heat propagation, render the heat propagation routes multi-directional, and hinder the effect of unidirectional heat propagation (Jiang et al. 2022; Wu et al. 2022; Xiong et al. 2022). This phenomenon effectively slows the expected propagation of heat flow and inhibits the heat transfer through solids, enhancing the thermal insulation potential of biochar-cement composites (Fig. 5b). It was found that the addition of biochar decreased the thermal conductivity of biochar-cement composites by 25% (Rodier et al. 2019). Similar results were also obtained in biochar-clay composites, where the maximum thermal conductivity decreased by 67% (Lee et al. 2019). The addition of 2 wt% biochar resulted in a low thermal conductivity [0.192 W (m·K)⁻¹], and its incorporation also improved the acoustic performance of biochar-cement composites across the frequency range of 200–2000 Hz (Cuthbertson et al. 2019). The introduction of biochar can increase the porosity of cement-biochar composites. The pores in biochar would break the thermal bridging within the biochar-cement composites, which is responsible for the low thermal conductivity and thermal insulation improvement. Therefore, biochar with high porosity and 3D pore structure is more favoured for enhancing the thermal insulation and noise reduction of construction materials.

5.3 Contaminant immobilisation and indoor air quality improvement

Biochar has been widely applied for water purification and soil remediation due to its peculiar and tuneable properties, such as high porosity, good stability, and high cation exchange capacity. Many studies have focused on incorporating biochar into construction materials to impart the functional properties (e.g., contaminant immobilisation and indoor air quality improvement) to the composites (Fig. 5c). For instance, the water purification process of biochar-pervious concrete was mainly controlled by adsorption ability and microbial degradation of biochar. Wang et al. also applied the biochar-modified binder for the contaminated sediment immobilisation, suggesting that biochar enhanced the immobilisation efficacy of potentially toxic elements (PTEs) and other organic contaminants in the sediment (Wang et al. 2019c). The environmental merits of biochar make the sediment products environmentally acceptable as construction materials, such as fill materials and paving blocks. Biochar cement is a climate-positive and robust binder for immobilising municipal solid waste incineration fly ash, where biochar promotes the hydration of cement, resulting in a denser matrix for encapsulation of PTEs (Chen et al. 2019a, 2022a). These studies provided new insights into adopting climate-positive binders to treat hazardous waste. As an economical and highly efficient adsorbent for volatile organic compounds (VOCs), biochar also has the potential to be applied in biochar particleboard for adsorbing VOCs from interior finishing, contributing to indoor air quality improvement (Zou et al. 2019; Xiang et al. 2020). However, very few studies concerning incorporating biochar into particleboard to eliminate VOCs were performed, requiring further relevant investigations.

Asphalt pavement production would inevitably generate VOCs, posing health risks to construction labours (Cui et al. 2020). To solve this problem, some researchers introduced biochar for removing VOCs in asphalt. Biochar could reduce the VOCs emissions by half, and the adsorption mechanisms depend on the types of biochar (Zhou et al. 2020). For example, chemical adsorptions occurred between straw/wood-derived biochar and VOCs, while physical adsorptions typically occurred with pig manure-derived biochar (Zhang et al. 2020b; Zhou et al. 2020). The adsorption capacity of biochar and biochar asphalt towards VOCs can be improved by increasing specific surface area, pore volume, and the amount of surface chemical functional groups but decreasing pore size (Li et al. 2020). Although biochar exhibits a promising perspective as a VOCs scavenger, few studies have considered the synergy of both VOCs removal efficiency and asphalt performance improvement by adopting biochar.

5.4 Electromagnetic shielding

Electromagnetic radiation caused by wireless and communication devices is an increasing public concern. The ability to attenuate or hinder electromagnetic interferences is defined as electromagnetic shielding. The shielding performance of cement composites is expected to be increased by incorporating carbon-based materials, such as graphene and carbon nanotubes (Chen et al. 2015; Nam et al. 2018). The graphene and carbon nanotubes qualified with high specific surface area, low density, and high electrical conductivity could enhance the electromagnetic shielding efficacy (Zhou et al. 2018). However, the high cost and agglomeration of graphene and carbon nanotubes limit their large-scale applications in cement composites. Biochar as a cost-effective material was proposed for improving the electromagnetic interference shielding efficacy of cement-based composites (Fig. 5d). A maximum increase of shielding effectiveness was up to 353% at 1.56 GHz frequency by incorporating 0.5 wt% biochar compared to plain cement (Khushnood et al. 2015). The electromagnetic shielding performance of a sustainable lightweight biochar-cement-gypsum composite was facilitated by increasing biochar content, which became more pronounced at frequencies above 4 GHz (Natalio et al. 2020). The biochar composite exhibited high shielding efficacy in the microwave range; the mechanisms behind it were not interpreted clearly but attributed to some conventional reasons, such as natural alignment of carbon ultrastructure (e.g., lignin), dissipation of surface currents and polarisation in the electric field.

5.5 Biochar-enhanced 3D concrete printing

The three-dimensional (3D) concrete printing involving layer-by-layer concrete deposition by a 3D printer without framework support or vibration processes has gained tremendous interest in recent years. Construction materials plus 3D printing could reduce construction waste by 30–60%, labour expense by 50–80%, and production time by 50–70% (Zhang et al. 2019a). However, the high-efficiency 3D concrete printing requires excellent pumpability, extrudability, and buildability, making conventional cement-based materials difficult to satisfy the requirements (De Schutter et al. 2018). The appropriate dosages of polymeric fibres and nano-sized additives (e.g., nano-silica, graphene-based materials, and nano-clay) could improve the thixotropy and buildability of 3D concrete printing (Sikora et al. 2022). For instance, the nano graphene-based materials at the dosage of 1 wt% enhanced the flexural strength of composites by 89% and compressive strength by 28% whilst demonstrating excellent shape retention and buildability (Chougan et al. 2020).

Lightweight concrete with a porous structure is a versatile material applicable for specific structural and insulating purposes. Recently, 3D printing of lightweight foam concrete with a density ranging from 800 to 1200 kg m⁻³ was manufactured for exterior wall elements without an extra thermal insulation layer (Markin et al. 2021). The stability of foam significantly affected the properties of 3D concrete printing, and future work should emphasise further reducing the density and maintaining adequate load-bearing capacity. Wood sawdust and phase change materials were added to enhance the thermal insulation capacity and reduce the density of 3D concrete printing (De Schutter et al. 2018). Carbon-negative biochar was also incorporated to improve the thixotropy of 3D concrete printing, which can provide a new direction for versatile 3D concrete printing (Fig. 5e). The addition of biochar can reduce the density of 3D printing of lightweight concrete and enhance extrudability and buildability. Developing appropriate mixture compositions for high-performance 3D concrete printing has become an opportunity yet a grand challenge. Incorporating biochar into 3D concrete printing has a promising perspective regarding property enhancement and carbon neutrality.

5.6 Biochar phase change materials

Phase change materials (PCMs) with high latent heat storage capacity are novel and sustainable materials for energy storage and conversion (Mohamed et al. 2017). Recent studies have applied PCMs in construction materials to facilitate energy efficiency and decrease the energy consumption of construction (Song et al. 2018; Wi et al. 2021). However, the large-scale applications of PCMs are limited by several drawbacks, such as seepage above the normal melting temperature, insufficient heat transfer performance, inadequate thermal energy absorption and release characteristics. Nano-sized carbon-based materials, including graphite, graphene, porous carbons, and carbon nanotubes, have been applied as supporting materials for improving the performance of PCMs (Atinafu et al. 2018, 2021a; Chen et al. 2020b; Wi et al. 2020). Although these materials can effectively enhance the heat transfer performance of PCMs, their prices are prohibitive for large-scale applications, and the modification processes are often chemically intensive.

Biochar, as a low-cost carbon-negative material, has recently been adopted as a supporting scaffold for enhancing the performance of PCMs (Fig. 5f) (Jeon et al. 2019a; Kim et al. 2021). The biochar-PCMs displayed negligible leakage, good thermal insulation capacity, high chemical compatibility, exudation stability, and shape stability whilst reducing building energy consumption of the referenced building model (up to 531 kWh year⁻¹) (Jeon et al. 2019b). Engineered biochar could significantly prevent the leakage of PCMs, and the corresponding biochar-PCMs achieved fusion enthalpies of 108.3 and 138.1 J g⁻¹, respectively, whilst maintaining the chemical structure and thermal reliability after 1000 heating/cooling cycles (Hekimoğlu et al. 2021). The intermolecular interactions between PCM and biochar (i.e., hydrogen bonding), as well as the characteristics of biochar (e.g., surface functionality, specific surface area, and pore size distribution), were crucial factors influencing the performance of biochar PCMs (Atinafu et al. 2021c). An interconnected network and the high degree of graphitisation of biochar microparticles are essential for enhancing the specific heat capacity and providing nucleation sites to reduce the sub/super-cooling phenomenon of biochar-PCMs (Yang et al. 2019; Yuan et al. 2021). The engineered biochar integrating with multiwalled carbon nanotube exhibited a high loading capacity of PCMs up to 70.2%, exhibiting a high heat storage capacity of 127.4 J g⁻¹ due to the favourable microstructure and interconnected framework of the engineered biochar (Atinafu et al. 2021b). Therefore, engineered biochar with sufficient specific surface areas, pore volumes, and functional groups would be favoured for developing high-performance biochar-PCMs. Overall, the biochar-PCMs with low cost and improved performance can be utilised for diverse thermal energy storage applications, e.g., waste heat recovery and passive cooling of climate-positive design.

5.7 Self-sensing biochar-cement composites

The structural health monitoring of civil engineering infrastructures has received increasing attention nowadays, which involves controlling the functional reliability of infrastructure by using computer software. Underlying this new technique is a self-sensing cement composite combining sensors with electrically conductive additives loaded cement. A substantial amount of research has adopted carbon fibres, carbon black particles, carbon nanotubes, and graphene nanoplatelets as additives for manufacturing self-sensing cement composites (D’Alessandro et al. 2016; Monteiro et al. 2017; Belli et al. 2018). Biochar has been investigated as a potential alternative to graphene nanoplatelets in self-sensing cement composites (Fig. 5g). The addition of biochar at a dosage of 1% (v/v) to cement significantly reduced the electrical resistivity (− 42%) whilst maintaining good mechanical properties (Mobili et al. 2021). Engineered biochar at the dosage of 5% incorporated in self-sensing cement composites, exhibiting a 70% reduction in the water adsorption, a 23% increase in electrical conductivity, and an approximately 45% reduction in embodied carbon footprint (Haque et al. 2021). In future studies, the engineered biochar is recommended to enhance the performance of self-sensing biochar-cement composites.

5.8 Bacteria cargo for self-healing cement composites

Cracks formed in concrete allow aggressive chemicals to penetrate the structure, thereby accelerating the damage to the concrete and jeopardising its durability and service life. Self-healing treatment involves autogenous healing, incorporating polymeric materials and calcium carbonate precipitation by microbial species, a promising concrete crack remediation technique (Vijay et al. 2017). Calcium carbonate production through bio-mineralisation is an efficient and sustainable pathway for sealing cracks, which can hinder the crack development and fill the deep microcracks (Seifan et al. 2016). Biochar has been adopted as a carrier for bacteria spores in cement composite to seal cracks. A crack of 700 μm in maximum can be sealed by combining bacteria-doped biochar with superabsorbent polymers and polypropylene microfibers (Gupta et al. 2018c). Meanwhile, incorporating bacteria-doped biochar could enhance the strength of biochar-cement composites by 38% and reduce the water penetration and absorption by 65% and 70%, respectively, compared with the direct incorporation of spores (Gupta et al. 2018c). This modified system also maintained the crack sealing efficacy after multiple damage cycles.

Biochar as bacteria cargo for self-healing composites can boost the sealing of cracks (Fig. 5h). This provides a relatively low-cost and sustainable solution for solving the crack issue in concrete, which is also beneficial for expanding the life span and durability of concrete. Further study is required to improve the survival rate of bacteria exposed to alkaline conditions of concrete.

6 Future perspectives

• (1) Advancing the scientific understanding of the interfacial reactions in biochar-construction materials with the assistance of cutting-edge technologies such as micro-computed tomography, nanoindentation, and transmission electron microscopy is required to promote the performance development of biochar-construction materials.

• (2) The application of atomistic simulations could further improve the understanding of molecular-level interfacial properties between biochar and asphalt/polymer/cementitious materials under different conditions. The models developed by molecular dynamics simulation also can be adapted to evaluate the bonding strength of biochar/asphalt/polymer/cementitious materials under different chemo-mechanical interactions and form the basis for predicting long-term performance.

• (3) Comprehensive regulation of biochar regarding quality, safety, and properties should be carried out to ensure the proper selection and utilisation of biochar in construction materials. For example, detailed requirements should be defined from raw materials selection to biochar production processes in order to ensure biochar performance. Standardized assessments of biochar construction materials should also be established to safeguard long-term use and human/environmental health. Meanwhile, modifications of biochar would benefit the biochar quality and expand the applications of functional biochar construction materials in respect of technical and environmental performance.

• (4) Environmental and technical advantages of biochar construction materials, such as hygrothermal regulation, electromagnetic shielding, contaminant immobilisation, indoor air quality improvement, self-healing capacity, and acoustic insulation, are still in their infancy and should be further explored to demonstrate their value-added and superior performance.

• (5) A combination of multiple techniques is promising to provide comprehensive information on the advantages and disadvantages of biochar construction materials. For instance, the synchronous use of biochar and CO₂ curing would enhance the performance of the construction materials whilst achieving deeper carbon sequestration compared with single technique utilisation.

• (6) A holistic techno-economic analysis and life cycle assessment should be carried out prior to commercial applications. This would provide an insight into the sustainability of biochar construction materials and their impact on the environment.

7 Conclusions

Biochar is demonstrating its tremendous promise for applications in carbon neutral/negative construction materials and is contributing to the achievement of carbon neutrality targets. The incorporation of biochar derived from waste biomass in construction materials can mitigate CO₂ emissions and natural resource depletion whilst improving the mechanical performance and providing value-added merits for biochar construction materials. Biochar construction materials also offer environmental and technical advantages, such as hygrothermal regulation, electromagnetic shielding, contaminant immobilisation, indoor air quality improvement, self-healing capacity, and acoustic insulation. The versatility of biochar gives biochar construction materials the potential to revolutionise the industrial manufacture of conventional construction materials and become an advocate of sustainable and green development. We also highlight that tailoring the favourable physicochemical properties of engineered biochar based on adequate and scientific designs and advancing the scientific understanding of interfacial reactions in biochar construction materials would facilitate the large-scale development and widespread applications of biochar construction materials.