Biocarbon

(2022) 4:59
Zhang et al. Biochar
https://doi.org/10.1007/s42773-022-00182-x
Biochar
Open Access
REVIEW
Biochar as construction materials
for achieving carbon neutrality
Yuying Zhang1, Mingjing He1, Lei Wang2*, Jianhua Yan2, Bin Ma3, Xiaohong Zhu4, Yong Sik Ok5,
Viktor Mechtcherine6 and Daniel C. W. Tsang1,2,7* 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.
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.
Keywords: Engineered biochar, Biomass waste management, Carbon-negative materials, Carbon neutrality,
Supplementary cementitious materials, Sustainable construction
*Correspondence: [email protected]; [email protected]
1
Department of Civil and Environmental Engineering, The Hong Kong
Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
2
State Key Laboratory of Clean Energy Utilization, Zhejiang University,
Hangzhou 310027, China
Full list of author information is available at the end of the article
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Zhang et al. Biochar
(2022) 4:59
Page 2 of 25
Graphical Abstract
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 C
­ O2-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 P
­ gCO2-eq, of which
1.7–3.7 ­PgCO2-eq (49–59%) is attributed to the C
­ O2
withdrawal from the atmosphere (Lehmann et al. 2021).
Although there is variation in the total C
­ O2 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 C
­ O2 ­yr−1, equivalent to
1.5% of total annual global C
­ O2 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 C
­ O2 emissions of construction materials.
This strategy could also bring additional economic
benefits through carbon trade, incentivising the construction and building industry to curtail overall C
­ O2
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
Zhang et al. Biochar
(2022) 4:59
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
Page 3 of 25
proposed a novel strategy for achieving 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).
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
Fig. 1 Sustainable waste management towards circular economy and carbon neutrality by adopting biochar construction materials
Zhang et al. Biochar
(2022) 4:59
Page 4 of 25
Fig. 2 Pyrolysis and hydrothermal carbonisation of biomass for biochar production
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 coproducts 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,
Zhang et al. Biochar
(2022) 4:59
and surface functionalities of biochar can enhance the
water holding and C
­ O2 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 ­(Kw = ­[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
Page 5 of 25
(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 C
­ O2 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 carbonnegative 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 C
­ O2 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
Zhang et al. Biochar
(2022) 4:59
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 ballmilled 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 C
­ O2 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 C
­ O2 uptake capacity of cementitious materials. In recent studies, for example, incorporating 4 wt% tailored biochar in cement could store an
Page 6 of 25
additional 0.12 kg of C
­ O2 and incorporating 8 wt% ballmilled 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 C
­ O2
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 C
­ O2 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.
Zhang et al. Biochar
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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 C
­ O2 emissions associated with natural aggregates (Zhan et al. 2018, 2020). The
recycled aggregates with porous nature and increased
surface area are suitable for C
­ O2 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 C
­ O2 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 C
­ O2 by 2050 while generating the highest breakeven cost of US$70 per ton of C
­ O2
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 C
­ O2 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
Page 7 of 25
analysis proved that the biochar incorporation significantly reduces the ­
CO2 emissions, whereas biocharenhanced 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
Fig. 3 Upcycling of waste biomass into carbon-negative biochar construction materials
Zhang et al. Biochar
(2022) 4:59
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.
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 C
­ O2 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 C
­ O2 pre-saturated
Page 8 of 25
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 biocharcement 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 biocharcement 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
Zhang et al. Biochar
(2022) 4:59
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 C
­ O2 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/
Page 9 of 25
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 biocharcement 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.
Biochar-rubber
composites
650; 700; 750
Residues miscanthus
n.a
n.a
Ash content
(%)
< 4.5
900
800
Dried distillers’
grains
Lignin
0.5
n.a
n.a
n.a
1.0–4.7
2–10 (diameter); n.a
30–50 (length)
D50 = 35–40
10
Biochar
particle size
(μm)
Arhar stalks and 800
Bael shell
Cellulose; waste 400
cotton fibre
600; 1000
Maple tree
Biochar-resin
composites
Pyrolysis
temperature
(℃)
Feedstock
Products
83.4
n.a
n.a
n.a
n.a
n.a
Specific
surface area
­(m2 ­g−1)
0–40
n.a
2; 4; 6
1; 2; 5; 10
20
1; 2; 4; 20
Filler ratio
(wt.%)
Table 1 Performance and characteristics of biochar and properties of biochar-polymer composites
3.8–9.9
15.6–21.4
30–50
15–28
n.a
~ 23
1.4–2.3
1.3–1.9
n.a
0.8–2.3
n.a
~1
Ref.
Giorcelli et al.
(2019a)
Graphitic structure of biochar
exhibited
hydrophobicity resulting in
a high affinity
with rubber
Jiang et al. (2020)
Lower rolling
Paleri et al. (2021)
resistance and
higher wet skid
resistance; good
for tire applications
Tensile strength Minugu et al.
increased by
(2021)
183%, and flexural strength by
91% compared
with neat epoxy
(4% biochar
added)
Improvement of Bartoli et al.
elongation
(2020)
Electrical
properties of
the composites
increased as
the ­CO2 activated biochar
conductivity
increased
Tensile toughGiorcelli et al.
ness of biochar- (2019b)
resin composite
(2% added)
increased 11
times compared
with pure resin
Tensile
Modulus (GPa) Remarks
strength (MPa)
Zhang et al. Biochar
(2022) 4:59
Page 10 of 25
Biochar-plastic
composites
Products
900
700; 900
n.a
Rice husk
Date palm tree
Mixed hardwood
1000
Coconut shell;
wood pallets
700 (gasification)
n.a
Wood waste
Spruce woodchips
Pyrolysis
temperature
(℃)
Feedstock
Table 1 (continued)
22.9
20–50
150
n.a
0.1–10
~1
Biochar
particle size
(μm)
15
20.57; 21.35
n.a
4.2
12.89; 4.06
2.41
Ash content
(%)
45.4
283.62; 291.11
1.8–297.4
297
375; 137
n.a
Specific
surface area
­(m2 ­g−1)
8, 10, 12
0–15
50
44
0–40
15.2–16
Filler ratio
(wt.%)
n.a
32–35
26.3 (highest)
n.a
~ 15
18–25
n.a
1.12–1.36
1.87 (highest)
n.a
n.a
n.a
Jong et al. (2014)
Peterson and Kim
(2020)
Ref.
PVA/biochar
composite
sensor was
influenced by
thickness and
temperature
Agglomeration
and high ash
content of biochar led to low
conductivity
Nan and DeVallance (2017)
Poulose et al.
(2018)
Stiffness,
Zhang et al.
elasticity, creep (2020a)
resistance, and
stress relaxation
improved
Electrical
Benedetti et al.
conductiv(2021)
ity reached
2 × ­10−3 S ­cm−1
Biochar-polymer interaction
strength was
lower than that
of carbon black
15% biochar
incorporation increased
elongation and
toughness by
31% and 24%,
respectively
Tensile
Modulus (GPa) Remarks
strength (MPa)
Zhang et al. Biochar
(2022) 4:59
Page 11 of 25
Products
Pyrolysis
temperature
(℃)
500; 900
900
400; 450
Feedstock
Perennial
grasses
Pine saw dust
n.a
Table 1 (continued)
425
n.a
16.1; 8.4
Biochar
particle size
(μm)
3.1–8.4
n.a
n.a
Ash content
(%)
1.2–1.6
335
216.3; 8.4
Specific
surface area
­(m2 ­g−1)
6; 12; 18; 24; 30
15; 20; 25
0; 10; 20
Filler ratio
(wt.%)
19–25
~ 25
~ 20
2.8–3.6
~ 3.5
1.2–1.5
Behazin et al.
(2017)
Ref.
Higher flexural Das et al. (2015b)
strength
was found at
24 wt.% biochar
Peak heat
Das et al. (2017a)
release rate and
limiting oxygen
index reached
318 kW ­m−2
and 23%,
respectively
Surface functional groups
elimination and
high specific
surface area
promoted
compatibility
Tensile
Modulus (GPa) Remarks
strength (MPa)
Zhang et al. Biochar
(2022) 4:59
Page 12 of 25
Zhang et al. Biochar
(2022) 4:59
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.
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).
Page 13 of 25
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 biocharrubber 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).
Zhang et al. Biochar
(2022) 4:59
Page 14 of 25
Fig. 4 Biochar as a modifier for asphalt manufacture
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 biocharmodified 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.
Zhang et al. Biochar
(2022) 4:59
Page 15 of 25
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
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 nanotubes) 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
Zhang et al. Biochar
(2022) 4:59
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).
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
Page 16 of 25
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 biocharpervious 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 biocharpervious 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.
5.2 Thermal insulation and noise reduction
Customised biochar possesses 3D porous and 2D flakelike 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 biocharcement 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
Zhang et al. Biochar
(2022) 4:59
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
Page 17 of 25
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
Zhang et al. Biochar
(2022) 4:59
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 nanoclay) 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 carbonbased 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
Page 18 of 25
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 biocharPCMs 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
Zhang et al. Biochar
(2022) 4:59
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 biomineralisation 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.
Page 19 of 25
(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 chemomechanical 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 C
­ O2 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
Zhang et al. Biochar
(2022) 4:59
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, selfhealing 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.
Acknowledgements
The authors appreciate the financial support from the Hong Kong Green
Tech Fund (GTF202020153), Hong Kong Environment and Conservation Fund
(Project 104/2021), and the Alexander von Humboldt Foundation (AvH) for
this study.
Author contributions
YZ: conceptualization, methodology, literature collection and analysis, writing;
MH: literature collection and analysis, writing; LW: conceptualization, methodology, supervision, review and editing; JY: review and editing; BM: methodology, review and editing; XZ: methodology, review and editing; YSO: review
and editing; VM: review and editing; DCWT: conceptualization, methodology,
project administration; supervision, review and editing. All authors read and
approved the final manuscript.
Funding
This work was supported by the Hong Kong Green Tech Fund (GTF202020153)
and Hong Kong Environment and Conservation Fund (Project 104/2021).
Availability of data and materials
Data sharing is not applicable to this article as no datasets were generated or
analyzed during the current study.
Declarations
Competing interests
All authors declare no conflict of interest.
Author details
1
Department of Civil and Environmental Engineering, The Hong Kong
Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. 2 State Key
Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027,
China. 3 Laboratory for Waste Management, Nuclear Energy and Safety, Paul
Scherrer Institute, 5232 Villigen, Switzerland. 4 School of Civil Engineering,
University of Leeds, West Yorkshire, Leeds, UK. 5 Korea Biochar Research Center,
APRU Sustainable Waste Management Program & Division of Environmental
Science and Ecological Engineering, Korea University, Seoul, Korea. 6 Institute
of Construction Materials, Technische Universität Dresden, 01062 Dresden,
Germany. 7 Research Centre for Resources Engineering towards Carbon
Neutrality, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong
Kong, China.
Received: 10 June 2022 Accepted: 23 September 2022
Page 20 of 25
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