Zhang-2022-Biochar As Construction Materials For - (Published Version)
Zhang-2022-Biochar As Construction Materials For - (Published Version)
Zhang-2022-Biochar As Construction Materials For - (Published Version)
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 con-
struction 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 bio-
char-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
Keywords: Engineered biochar, Biomass waste management, Carbon-negative materials, Carbon neutrality,
Supplementary cementitious materials, Sustainable construction
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Zhang et al. Biochar (2022) 4:59 Page 2 of 25
Graphical Abstract
feedstock of biomass and manufacturing technologies proposed a novel strategy for achieving carbon neu-
of biochar would significantly affect the properties of trality by adopting biochar construction materials.
biochar construction materials, which should be criti-
cally investigated. To the best of our knowledge, no 2 Biochar production
literature review has articulated the state-of-the-art Biochar production processes can be divided into three
knowledge about designing different types of engi- categories: pyrolysis, gasification, and hydrothermal car-
neered biochar as construction materials, especially bonisation (HTC). Figure 2 is a schematic diagram of the
for maximising their technical benefits, value-added production processes for different types of biochar, with
functionality, and decarbonisation capacities. Hence, their yields following the order of HTC > pyrolysis > gasi-
in this tutorial review, we firstly introduced different fication. These biochars differ widely in chemical and
processes for biochar production and emphasised the physical properties due to their production conditions and
efficacy of biochar construction materials in achieving feedstock selection. Factors such as pyrolysis temperature,
carbon neutrality. We comprehensively evaluated the heating rate, residence time, pyrolytic atmosphere, gas
environmental impacts of biochar production and cus- pressure, and types of biomass waste play critical roles in
tomisation as well as the decarbonisation capacity of the yield and physicochemical characteristics of biochar for
biochar as filler and aggregate in comparison to con- promoting the performance of biochar construction mate-
ventional materials. Afterwards, we critically reviewed rials (including pore size distribution, specific surface area,
the manufacturing technologies of biochar construc- cation exchange capacity, water retention capacity, etc.) (He
tion materials, including biochar-cement composites, et al. 2022; Leng and Huang 2018; Maljaee et al. 2021).
biochar-asphalt composites, biochar-plastic compos- Biochar is generally a product from partially carbon-
ites, etc. Lastly, we identified the grand challenges in ised biomass to highly refractory carbon with distinctive
employing biochar as supplementary cementitious heterogeneity in physicochemical properties, primar-
materials, fine/coarse aggregate, and functional addi- ily dependent on the feedstocks, production conditions,
tives in the construction materials, and suggested the and various modification methods (Lehmann and Ste-
prospects for future research directions in this review. phen 2015; Wang et al. 2020a, b). As an easily tuneable
As illustrated in Fig. 1, we highlighted sustainable carbon-based material, biochar can be applied for a wide
waste management towards a circular economy and 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
raw materials in construction materials, by engineering pyrolysis conditions, including temperature, duration,
its porous structure, surface functionality, and aromatic/ activation, and modification methods (Cha et al. 2016;
graphitic carbon structure with fit-for-purpose designs Wang and Wang 2019). Pyrolysis temperature plays a
(Chen et al. 2022b; Maljaee et al. 2021; Sajjadi et al. 2019). critical role in the carbonisation process, which affects
Different thermochemical treatments, including conven- the energy value, yield, carbon stability, porous struc-
tional pyrolysis (300–800 °C), gasification (> 700 °C with ture, and the pH value of biochar (He et al. 2021b).
a fast-heating rate of tens of °C min−1), and HTC (180– Increasing pyrolysis temperature could remarkably
250 °C, 2–10 MPa), have been applied to produce bio- increase the pore volume and surface area of biochar
char (or hydrochar from HTC) with the yields following due to the carbon phase change from amorphous to
the order of HTC > pyrolysis > gasification (Fig. 2) (Cao graphitic form and the driving off of pore-blocking
et al. 2021; Wang et al. 2020a, b; You et al. 2018). During substances (Lian and Xing 2017). For instance, by
biochar production, bio-oil and pyrolytic gas are the co- increasing the pyrolysis temperature, the specific sur-
products that can be applied for bioenergy applications face area increased by 3.9-fold for sawdust-derived
or chemical upgrading (Tomczyk et al. 2020; Wang and biochar (400–700 °C; 147.4–572.6 m2 g−1) (Zhu et al.
Wang 2019). Overall, thermochemical technology should 2019), 5.2-fold for rice straw-derived biochar (500–
be customised depending on the nature of biomass waste 700 °C; 22.4–115.5 m2 g−1) (Shen et al. 2019), and
(e.g., moisture content and carbon/mineral composi- 29.5-fold for wood waste-derived biochar (650–950 °C;
tions) such that we can achieve the maximum reduction 10.5–309.2 m2 g−1) (He et al. 2021b). Longer residence
of CO2 emissions and upgrade the technical performance time promotes the repolymerisation process and the
of biochar construction materials. 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
2.1 Pyrolysis biochar
due to the higher economic feasibility and technologi-
Conventional pyrolysis with a slow heating rate
cal maturity (Chen et al. 2019c). Various activation or
(5–10 °C min−1) in an oxygen-limited environment
modification methods, including physical activation
is the most widely adopted thermochemical technol-
(CO2 or steam), chemical activation by acid, alkaline,
ogy to produce biochar owing to its technical simplic-
and oxidising/reducing agents, have been conducted
ity of operation and economic feasibility for upscaling
to augment the surface functionality of engineered bio-
(Tripathi et al. 2016; Yang et al. 2021b). The physico-
char (Sajjadi et al. 2019; Xu et al. 2021; Wan et al. 2021).
chemical properties of pristine biochar are typically
Appropriate specific surface area, pore structures,
regulated by the compositions of feedstocks and the
Zhang et al. Biochar (2022) 4:59 Page 5 of 25
and surface functionalities of biochar can enhance the (Liu et al. 2021). A higher heating rate was also found
water holding and C O2 storage capacity that would fur- to facilitate feedstock decomposition while suppress-
ther promote the performance of biochar construction ing the formation of hydrochar; for instance, Wang et al.
materials via internal curing and accelerated carbona- (2019a) found a decrease in hydrochar yield from 10.3%
tion (Chen et al. 2020a, b; Praneeth et al. 2020). 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
2.2 Gasification biochar nutrients, AAEMs, transition metals, and polycyclic
Gasification with a high reaction temperature (> 700 °C) aromatic hydrocarbons (PAHs) may accumulate in the
in the presence of gasifying agents produces pyrolytic hydrochar, which should be carefully investigated and
gas as the primary products (CO, H2, CO2, and CH4) properly addressed before potential use in construction
and biochar with high aromaticity and porosity by four materials.
reaction stages, including drying (100–200 °C), pyroly-
sis (200–700 °C for fixed bed, 700–910 °C fluidised bed, 3 Carbon neutrality and biochar‑enhanced
and > 1400 °C for entrained flow), combustion (700– construction materials
1500 °C), and reduction (800–1000 °C) (You et al. 2018). Carbon neutrality entails efficiently reducing green-
The production of pyrolytic gas and biochar is typically house gas emissions and sequestering/capturing CO2
controlled by the gasification conditions such as tem- from the atmosphere (Wang et al. 2021a). Biochar as a
perature, properties of feedstocks, gasifying agents (air, soil amendment to curtail C O2 emissions and accomplish
O2-enriched air, O2, CO2, and steam), and the gas pres- carbon sequestration has been intensively investigated
sure (Shayan et al. 2018). High temperature promotes the since its first proposal nearly 15 years ago (He et al. 2022;
release of volatiles with a higher yield of pyrolytic gas at Lehmann et al. 2021). More recently, biochar as a carbon-
the expense of a lower biochar yield, facilitating the for- negative material has been employed in construction
mation of micropores/mesopores with a higher specific materials to enable buildings to become a carbon sink
surface area of biochar (Qi et al. 2021). Compared to the and facilitate the attainment of carbon neutrality targets
pyrolytic biochar, the gasification biochar with abundant (Danish et al. 2021; Maljaee et al. 2021).
pore structures might be a better choice as additives in
construction materials for the sake of internal curing and 3.1 Decarbonisation by biochar production
accelerated carbonation. For the feedstocks with high ash 3.1.1 Decarbonisation by converting biomass waste
content and alkali and alkaline earth metals (AAEMs) into biochar
such as sewage sludge and food waste digestate, the The extent of carbon reduction by biochar is determined
solid–solid interaction between carbon and AAEMs by its production conditions and application environ-
(especially for ion-exchangeable Na+, Mg2+, and Ca2+) ment that would affect the amount of C O2 emissions dur-
could weaken the C–C bond and improve the reactivity ing the entire life cycle (Lehmann et al. 2021; Puettmann
of gasification, hence enhancing the functionalisation of et al. 2020; Yang et al. 2021a, b). Meanwhile, the yield,
biochar (Mafu et al. 2018). carbon content, and stability of biochar, as well as the
energy conversion efficiency of pyrolytic gas and bio-oil
2.3 Hydrothermal carbonisation biochar (hydrochar) as renewable energy, are pivotal factors in determining
HTC exhibits advantages in dealing with wet and bulky the CO2 sequestration potential of thermochemical sys-
biomass such as wet yard waste, food waste, and waste- tems converting biomass into biochar (Yang et al. 2021a,
water sludge, producing carbonised solids (hydrochar) b). The life cycle impacts of biochar systems also encom-
without an energy-intensive drying pretreatment (Cao pass CO2 emissions associated with the transport and
et al. 2021; Chi et al. 2021). The HTC is carried out in storage of biochar, which could be kept minimum as a
a subcritical water system; ~ 1000-fold of ionic prod- marginal contribution to the overall emissions (Matuštík
ucts (Kw = [H+][OH−]) can be dissociated from water et al. 2020). Notably, the large-scale deployment of con-
by increasing temperature from 25 °C to 250 °C, which verting waste biomass to biochar has been considered a
makes water a suitable medium for base- or acid-cat- ready-to-implement NET for achieving carbon neutral-
alysed reactions (Yang et al. 2021b). In the HTC pro- ity targets (Azzi et al. 2019; Yang et al. 2021a, b). The
cess, the carbohydrates, protein, lipid, lignin, and humic pyrolytic gas and bio-oil generated by pyrolysis/gasifica-
substances from the feedstocks would be depolymer- tion/HTC of biomass waste can be used for electricity,
ised to the intermediates by hydrolysis, dehydration, heat generation, and alternative fuels. More importantly,
decarboxylation, deamination, and decomposition pro- biochar/hydrochar can be used in the agriculture and
cesses; then, the intermediates could form hydrochar construction industry to enhance their environmental
by aromatisation, condensation, and polymerisation performance and sequester carbon in the natural or built
Zhang et al. Biochar (2022) 4:59 Page 6 of 25
3.2.2 Decarbonisation of biochar as aggregates in concrete analysis proved that the biochar incorporation signifi-
systems cantly reduces the CO2 emissions, whereas biochar-
It is estimated that approximately 17.5 Gt of aggregates enhanced concrete with 30 wt% biochar can be effectively
are utilised to manufacture concrete each year, whose converted into carbon-negative products (Chen et al.
crushing and transportation induce considerable CO2 2022b). However, the kinetics of CO2 adsorption and
emissions (Miller et al. 2018). The over-extraction of nat- water release of biochar in cementitious materials under
ural aggregates (e.g., sand and gravel) has already caused CO2 curing lack investigations and sufficient understand-
massive environmental damage with cascading effects ing (Chen et al. 2019b; Chen and Gao 2019b). Future
that affect human well-being (Ioannidou et al. 2017; research should intend to tailor the adsorption and des-
Aurora et al. 2017). Crushing the demolition waste into orption kinetics of different engineered biochar to con-
smaller particles for reuse as recycled aggregates is a sus- trol the CO2 diffusion and regulate the crystalline forms
tainable approach to reducing the C O2 emissions associ- of CaCO3 (i.e., calcite, aragonite, and vaterite) for desir-
ated with natural aggregates (Zhan et al. 2018, 2020). The able interfacial chemistry and optimal performance.
recycled aggregates with porous nature and increased
surface area are suitable for C O2 curing that can den- 4 Biochar as construction materials
sify the matrix, enhance mechanical performance, and Due to population growth and infrastructure develop-
increase the lifetime CO2 uptake (Zhan et al. 2019; ment, increasing demand for construction materials is
Habert et al. 2020). Accelerated carbonation is a prom- expected to cause substantial consumption of natural
ising approach for C O2 mineralisation and performance resources and greenhouse gas emissions (China Asso-
enhancement of cementitious materials (Chen and Gao ciation of Building Energy Efficiency 2020). It is crucial
2019a, 2020; Pan et al. 2020). The CO2 utilisation poten- to save the limited natural resources from manufactur-
tial of cementitious construction materials is expected ing construction materials and reduce carbon emissions
to remove 100–1400 tonnes C O2 by 2050 while generat- throughout the entire life cycle. Developing new and sus-
ing the highest breakeven cost of US$70 per ton of C O2 tainable construction materials with low or even negative
utilised (Hepburn et al. 2019). However, the carbonised carbon footprint is an effective way to achieve carbon
areas are concentrated on the surface of cementitious neutrality, which has increased interest in scientific and
materials due to the dense structure and closed pores, industrial communities (Churkina et al. 2020). To allevi-
which under-utilises the decarbonisation potentials. ate the issue of substantial CO2 emissions associated with
When serving as aggregates, biochar with a hierarchi- cement and concrete, several research schemes such as
cal porous structure can improve the pore size distribu- the development of alkali-activated materials, limestone
tion of the matrix and promote the deep carbonation calcined clay cement (LC3) (Scrivener et al. 2018), waste
of cementitious materials. The biochar pores can inter- utilisation for cement replacement (Yin et al. 2018; Mal-
link with those in the cement systems, facilitating the jaee et al. 2021), have been intensively investigated and
diffusion and dissolution of C O2 and enhancing car- practically applied in recent years. In the latest develop-
bonation progress. By incorporating biochar as aggre- ment, biochar as a carbon-negative material has been
gates, the biochar-enhanced concrete can sequester considered a promising candidate for cement and aggre-
59–65 kg CO2 ton−1 whilst delivering a maximum ben- gate substitution in construction materials (Akinyemi
efit of 41.1 USD m−3 (Chen et al. 2022b). The life cycle and Adesina 2020). Figure 3 shows the conversion of
waste biomass into biochar and the proposed biochar biochar (Gupta et al. 2018a, b; Kua et al. 2020). Thus, the
construction materials as a carbon sink. Fine-grained CO2 curing and pre-saturation schemes should be care-
biochar can act as supplementary cementitious mate- fully designed to achieve the required mechanical per-
rial, whilst coarse-grained biochar can partially substitute formance and carbon sequestration capacity. The ability
aggregate in concrete. of biochar for internal curing and maintaining high rela-
tive humidity can also mitigate autogenous shrinkage and
4.1 Biochar‑cement composites dry shrinkage, thus improving the durability of biochar-
4.1.1 Biochar as a filler in cement composites cement composites. By combining biochar with MgO
Biochar can be a promising filler in cement-based com- expansive additive, Mo et al. (2019) solved the autoge-
posites. The roles of biochar in cement composites have nous shrinkage of cement and enhanced the compressive
been investigated regarding rheology, cement hydra- strength. The reduction of autogenous shrinkage could
tion, and mechanical properties. For example, Gupta reach 16.3% at the age of 180 h with an addition of 2 wt%
and Kua (2019) investigated the yield stress and plas- biochar into the cement. Similarly, a 6-week observation
tic viscosity of biochar-cement composites by compar- in another study proved that the autogenous shrinkage
ing coarse (2–100 μm) and fine (0.10–2 μm) biochar as was eliminated using a combination of rice husk biochar
a filler, where macroporous coarse biochar influenced and rice husk ash (Muthukrishnan et al. 2019). It should
the flowability and viscosity of cement paste to a greater be noted that rice husk biochar is rich in active silica,
extent than fine biochar. It was also found that fine bio- which may facilitate pozzolanic reactions and further
char particles exhibited higher early strength (i.e., 1-day relieve the autogenous shrinkage.
and 7-day) and better water tightness than macroporous Benefiting from the abundant micropores/mesopores
biochar (Gupta et al. 2018a). Fine biochar would fill in and high specific surface area of biochar, the water
pores and voids between solid particles in the cement adsorption/retention capacity, thermal insulation, and
system, enhancing the compactness and early strength temperature regulation ability of biochar-cement com-
of the biochar-cement composites. Compared with con- posites can be further enhanced. For instance, Gupta and
ventional fillers, biochar has a large pore volume that can Kua introduced 40 wt% rice husk biochar as a porous
retain the surrounding water. The retained water would micro-filler in cenosphere-containing (10–40 wt%) light-
slowly be released and contribute to internal curing, weight cement mortar, demonstrating 15–20% higher
facilitating the hydration of biochar-cement composites strength retention and 9–25% lower permeability, giv-
(Wang et al. 2021b). Furthermore, the biochar exhib- ing evidence to the significant enhancement of thermal
ited a more pronounced enhancement in the long-term stability of biochar-cement composites at 450 °C (Gupta
strength development via dry curing compared to water and Kua 2020). Meanwhile, the biochar-cement com-
curing (Sirico et al. 2021). Nonetheless, excessive intro- posites with 10–30 wt% silica fume exhibited a 28-day
duction of biochar would increase the overall porosity compressive strength of up to 66 MPa with a density of
and compromise the mechanical strength and workabil- less than 2102 kg m−3. Nevertheless, the relatively high
ity of biochar-cement composites. The effect of different price of cenosphere and silica fume would unavoidably
curing schemes on the long-term mechanical perfor- increase the cost of manufacturing lightweight biochar-
mance of biochar-cement composites deserves further cement composites, and the density for lightweight con-
investigation. crete should be less than 1920 kg m−3 (ACI 213 2003;
Biochar can facilitate CO2 diffusion and regulate mois- ACI 213R-03 2003). Therefore, it is necessary to reduce
ture content in the biochar-cement composites during the consumption of cenosphere and silica fume in future
accelerated carbonation (Praneeth et al. 2020; Wang et al. studies, increase the biochar dosage, and improve the
2020a). Wang et al. (2020a, b) suggested that the combi- properties of engineered biochar for better composite
nation of biochar and C O2 curing can enhance the prop- performance. The biochar-cement composites were also
erties of biochar-cement composites, which is especially tested at 550 °C, showing that the stability was still main-
effective for Mg-based cement. This is because Mg-based tained due to the function of biochar in reducing capil-
cement would expand after hydration and further fill lary porosity and redistribution of water (Gupta and Kua
in the pores of biochar, thus counteracting the adverse 2020). Furthermore, biochar as a hygroscopic filler has
effects of large pores. This novel integration of biochar been applied in pervious concrete to regulate the temper-
with CO2 curing can serve as a promising technique for ature and purify contaminated water, thus contributing
producing sustainable construction materials. How- to developing sponge cities. By incorporating 5% biochar,
ever, CO2 pre-saturated biochar displayed a detrimental the total water adsorption of pervious concrete reached
effect on the development of compressive strength due 117 kg m−3, and the enhancement of water adsorption, in
to weak bonding between cement and C O2 pre-saturated
Zhang et al. Biochar (2022) 4:59 Page 9 of 25
turn, decreased the surface temperature of pervious con- Al-LDHs impregnated biochar has the potential to be
crete by 6 °C (Tan et al. 2021). used as a corrosion control additive in concrete because
Fine biochar (< 125 μm) as an alternative for cement free Cl can be captured in the interlayer of LDHs (Cao
was applied for manufacturing ultra-high performance et al. 2017; Ye 2021). The engineered biochar can also be
concrete (UHPC) with lightweight and high strength combined with Al-rich minerals (e.g., kaolinite) via cation
(Dixit et al. 2019). It was found that biochar with inter- bridging, ligand exchange, and Van der Waals attraction
nal curing and nucleation sites can improve the hydra- (Yang et al. 2018), enhancing the carbon stability of bio-
tion and alleviate the brittle nature of biochar, making it char in the composites. A modification of the electron-
feasible for manufacturing UHPC. Dixit et al. also found egativity of biochar surface may also regulate the cement
that the addition of 2 wt% and 5 wt% biochar reduced hydration process. Therefore, incorporating engineered
the autogenous shrinkage of calcined marine clay-based biochar into cement composites exhibits a good research
UHPC by 21% and 32%, respectively (Dixit et al. 2021). potential and warrants further investigations.
Using 5 wt% biochar in UHPC also contributed to car-
bon sequestration of approximately 115 kg C O2 per m3 of 4.1.2 Biochar as an aggregate in concrete
UHPC (Dixit et al. 2021). Biochar can be used to substitute for aggregate, especially
Replacing cement partially with biochar is a win–win in lightweight concrete. Previous research has investi-
strategy in respect of sustainable waste management gated the application of hollow cenospheres, wood, and
and carbon sequestration. Optimising the biochar dos- fibres (e.g., reed fibres and milled fibres) as low-density
age in the admixture would benefit the enhancement of aggregate in construction sectors for achieving light-
compressive strength, flexural strength, toughness, duc- weight and high performance (Wang et al. 2016a; Shon
tility, and durability of biochar-cement composites. The et al. 2019; Chen et al. 2020a; Lu et al. 2021). Biochar can
optimal dosage of biochar recommended as a filler in the also be incorporated into the concrete as a porous and
biochar-cement composites was 0.5–2 wt% in consid- lightweight fine aggregate. It was found that replacing
eration of the improvement in mechanical performance sand with 20% biochar with an average particle size of
(Maljaee et al. 2021). The incorporated dosage of biochar 26 μm could enhance the flexural strength by 26% while
could be further increased to reduce the CO2 emissions reducing the bulk density by 10% (Praneeth et al. 2021).
associated with construction and buildings, even though Restuccia et al. adopted biochar derived from hazelnut
this would lead to an inevitable strength loss (within an shells and coffee powder as nano-aggregates (10–15 μm),
acceptable range). Biochar with a relatively large particle where the rupture modulus and fracture energy of sam-
size was not recommended as a filler because it could not ples increased by 22% and 61%, respectively (Restuccia
efficiently fill the pores, leading to low strength and high and Ferro 2016). These results indicated that biochar
capillary pores (Akhtar and Sarmah 2018). Meanwhile, could provide a ductile behaviour and strengthen the
the O/C atomic ratio in biochar was strongly associated interfacial transition zones, thus improving the bending
with its hydrophilicity, such that high ratios may ensure strength and fracture energy. Carbon-negative concrete
good water retention capacity and facilitate internal cur- can also be developed by incorporating 30 wt% biochar
ing (Karnati et al. 2020). as aggregates, providing both environmental benefits and
Few studies investigated the customisation of engi- economic profits, and revolutionising the development
neered biochar for improving the performance of bio- of the concrete industry (Chen et al. 2022b). Therefore,
char-cement composites. Engineered biochar can be the application of biochar as an alternative aggregate
tuned to possess hydrophilic functional groups, which and the associated environmental benefits (e.g., net-zero
may be compatible with cement and promote hydra- CO2 emissions, moisture regulation) are worth further
tion reaction. The mineral-rich engineered biochar may substantiation.
be able to facilitate the pozzolanic reactions further. For In the future, large-scale development of biochar-
example, the Si released from Si-enriched biochar could cement composites should be achieved through advanc-
form additional C-S-H with Al and Ca in the cement ing our scientific understanding of the interfacial
system, densifying the structure and enhancing the reactions and further optimising the pore structure and
mechanical performance (Wang et al. 2019b, d; Chen physicochemical properties of engineered biochar. It is
et al. 2022a). The composites of Mg/Al layered double suggested that engineered biochar should be selected
hydroxides (Mg/Al-LDHs) impregnated biochar feature from an appropriate feedstock and can be reinforced
smaller crystallite sizes, larger interlayer spacing, higher with chemical additives or physical approaches to max-
surface area, and more exposed active sites (Peng et al. imise the value-added performance of biochar-cement
2021), which could provide additional nucleation sites composites.
and promote the hydration rate. Meanwhile, the Mg/
Table 1 Performance and characteristics of biochar and properties of biochar-polymer composites
Products Feedstock Pyrolysis Biochar Ash content Specific Filler ratio Tensile Modulus (GPa) Remarks Ref.
temperature particle size (%) surface area (wt.%) strength (MPa)
Zhang et al. Biochar
Biochar-resin Maple tree 600; 1000 10 n.a n.a 1; 2; 4; 20 ~ 23 ~1 Tensile tough- Giorcelli et al.
composites ness of biochar- (2019b)
resin composite
(2022) 4:59
(2% added)
increased 11
times compared
with pure resin
Residues mis- 650; 700; 750 D50 = 35–40 n.a n.a 20 n.a n.a Electrical Giorcelli et al.
canthus properties of (2019a)
the composites
increased as
the CO2 acti-
vated biochar
conductivity
increased
Cellulose; waste 400 2–10 (diameter); n.a n.a 1; 2; 5; 10 15–28 0.8–2.3 Improvement of Bartoli et al.
cotton fibre 30–50 (length) elongation (2020)
Biochar-rubber Arhar stalks and 800 n.a 1.0–4.7 n.a 2; 4; 6 30–50 n.a Tensile strength Minugu et al.
composites Bael shell increased by (2021)
183%, and flex-
ural strength by
91% compared
with neat epoxy
(4% biochar
added)
Dried distillers’ 900 < 4.5 n.a n.a n.a 15.6–21.4 1.3–1.9 Lower rolling Paleri et al. (2021)
grains resistance and
higher wet skid
resistance; good
for tire applica-
tions
Lignin 800 0.5 n.a 83.4 0–40 3.8–9.9 1.4–2.3 Graphitic struc- Jiang et al. (2020)
ture of biochar
exhibited
hydrophobic-
ity resulting in
a high affinity
with rubber
Page 10 of 25
Table 1 (continued)
Zhang et al. Biochar
Products Feedstock Pyrolysis Biochar Ash content Specific Filler ratio Tensile Modulus (GPa) Remarks Ref.
temperature particle size (%) surface area (wt.%) strength (MPa)
(℃) (μm) (m2 g−1)
Wood waste n.a ~1 2.41 n.a 15.2–16 18–25 n.a 15% biochar Peterson and Kim
(2022) 4:59
incorpora- (2020)
tion increased
elongation and
toughness by
31% and 24%,
respectively
Coconut shell; 1000 0.1–10 12.89; 4.06 375; 137 0–40 ~ 15 n.a Biochar-poly- Jong et al. (2014)
wood pallets mer interaction
strength was
lower than that
of carbon black
Biochar-plastic Spruce wood- 700 (gasifica- n.a 4.2 297 44 n.a n.a Electrical Benedetti et al.
composites chips tion) conductiv- (2021)
ity reached
2 × 10−3 S cm−1
Rice husk 900 150 n.a 1.8–297.4 50 26.3 (highest) 1.87 (highest) Stiffness, Zhang et al.
elasticity, creep (2020a)
resistance, and
stress relaxation
improved
Date palm tree 700; 900 20–50 20.57; 21.35 283.62; 291.11 0–15 32–35 1.12–1.36 Agglomeration Poulose et al.
and high ash (2018)
content of bio-
char led to low
conductivity
Mixed hard- n.a 22.9 15 45.4 8, 10, 12 n.a n.a PVA/biochar Nan and DeVal-
wood composite lance (2017)
sensor was
influenced by
thickness and
temperature
Page 11 of 25
Zhang et al. Biochar
(2022) 4:59
Table 1 (continued)
Products Feedstock Pyrolysis Biochar Ash content Specific Filler ratio Tensile Modulus (GPa) Remarks Ref.
temperature particle size (%) surface area (wt.%) strength (MPa)
(℃) (μm) (m2 g−1)
Perennial 500; 900 16.1; 8.4 n.a 216.3; 8.4 0; 10; 20 ~ 20 1.2–1.5 Surface func- Behazin et al.
grasses tional groups (2017)
elimination and
high specific
surface area
promoted
compatibility
Pine saw dust 900 n.a n.a 335 15; 20; 25 ~ 25 ~ 3.5 Peak heat Das et al. (2017a)
release rate and
limiting oxygen
index reached
318 kW m−2
and 23%,
respectively
n.a 400; 450 425 3.1–8.4 1.2–1.6 6; 12; 18; 24; 30 19–25 2.8–3.6 Higher flexural Das et al. (2015b)
strength
was found at
24 wt.% biochar
Page 12 of 25
Zhang et al. Biochar (2022) 4:59 Page 13 of 25
Destructions due to building fire highlight the impor- The electrical conductivity of biochar-plastic compos-
tance of flame-retardant polymer composites. Biochar with ites is also attracting extensive attention regarding their
a stable porous honeycomb structure and no flammable various applications, such as electrostatic dissipation
volatiles is qualified with considerable thermal resistance materials, electromagnetic interference shielding mate-
and can be used as excellent fire-resistance materials (Babu rials, and semiconducting layers to prevent electrical
et al. 2020). The highest thermal stability was observed discharge. Poulose et al. (2018) applied biochar to man-
in the case of WPC with 18 wt% biochar incorporation ufacture biochar polypropylene composites to enhance
(Zhang et al. 2020a). The biochar application into WPC the electrical properties and tensile modulus, but the
would synergistically preserve mechanical properties and agglomeration and the high ash content of biochar would
reduce flammability. Poultry litter biochar was found to hamper the conductivity enhancement. In general, the
impart the optimal tensile and flexural properties of com- integrated properties of the biochar-plastic composites
posites due to the Ca-rich ash in poultry litter biochar (Das are associated with the dispersion of biochar and the net-
et al. 2016a). Besides, biochar addition could save produc- work formation in the polymer matrix (Khushnood et al.
tion cost by approximately 18% as the dosage of coupling 2015).
agent (i.e., maleic anhydride grafted polypropylene) could Other parameters (e.g., characteristics of biochar, poly-
be reduced from 3 to 1 wt% without significant deteriora- mer viscosity, and types of coupling agents) would affect
tion in mechanical performance (Das et al. 2016b). Conven- the integrated properties of the biochar-plastic compos-
tional flame retardants (i.e., ammonium polyphosphate and ites. The addition of coupling agents, wood and biochar
magnesium hydroxide) were introduced into the biochar- was crucial for the tensile and flexural strength of com-
modified WPC to further impede its flammability. Con- posites but had little effect on the flammability (Ikram
sidering both enhancements of resistance to radiative heat et al. 2016). Polar wood biochar exhibited no effect on
and economic benefits, the loading amount of magnesium the melting temperature of high-density polyethene, but
hydroxide was suggested to be 20 wt% (Das et al. 2017a, it promoted the early crystallisation of biochar-plastic
b). A higher dosage of flame retardants (e.g., magnesium composites (Zhang et al. 2019b). Dynamic mechanical
hydroxide at a high loading rate of 50 wt%) may further analysis revealed that biochar incorporation enhanced
strengthen the thermal stability of biochar-modified WPC; the stiffness, elasticity, creep resistance, and stress relax-
however, the excess flame retardants would be trapped in ation of the biochar-plastic composites (Zhang et al.
the biochar pores and obstruct the infiltration of polypro- 2020a). However, other properties of biochar, such as ash
pylene, which consequently reduced the mechanical bond- content, specific surface area, surface functional groups,
ing/interlocking between biochar and polypropylene (Das etc., are not clearly stated regarding the performance of
et al. 2017a). The employment of biochar for enhancing the biochar-plastic composites and require further investi-
flame resistance of WPC is a promising approach concern- gations. In future research, it is also necessary to iden-
ing both environmental sustainability and economics. The tify the optimum levels of various factors to achieve the
effect of biochar on the WPC manufacturing process (e.g., desirable properties of biochar-plastic composites for
extrudability) requires further investigation before indus- either mechanical performance or flammability.
trial applications.
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
as an economic modifier to enhance the properties of and store solar energy and then release the stored energy
asphalt binders and mixtures, such as durability, tem- into the city as sensible heat, contributing to the urban
perature sensitivity, and fatigue performance (Fig. 4). heat island effect (Qin et al. 2018). Pervious concrete
Biochar was more effective in strengthening tempera- features a porous structure that can provide channels
ture susceptibility and rutting resistance of asphalt bind- for heat and moisture transfer in road pavement, ena-
ers than carbon black or carbon fibre (Zhao et al. 2014a, bling evaporative cooling and alleviating the urban heat
b). Biochar with a particle size less than 75 μm could be island effect in hot seasons (Park et al. 2021a). Incorpo-
a favoured asphalt binder modifier for achieving satis- rating biochar into the pervious concrete could reduce
factory rotational viscosity and low-temperature crack albedo and increase water adsorption capacity, mitigating
resistance (Zhang et al. 2018). urban heat island issues (Park et al. 2021b). The biochar-
The ageing of the asphalt binder would lead to crack- pervious concrete could absorb more solar radiation
ing, fatigue, and raving (Nazari et al. 2018). Ageing also than plain pervious concrete, keeping cool through water
contributes to an increase in viscosity, affecting the evaporation and creating an urban microclimate. These
stiffness of the asphalt binders and mixtures (Pasandín findings were validated by Tan et al. (2021) by adopting
et al. 2015). Therefore, more interest has been gained a low-speed straight flow climatic wind tunnel to inves-
in enhancing the ageing resistance and susceptibility of tigate the temperature regulation capacity of biochar-
asphalt (Cong et al. 2014; Kumar et al. 2018; Dong et al. pervious concrete. Their results demonstrated that the
2020). Pyrolysis biochar could primarily improve the age- maximum temperature decreased by 6 °C for 18 h (Tan
ing resistance of asphalt binders by mitigating the oxi- et al. 2021). Therefore, biochar is considered a promis-
dative ageing of asphalt binder components rather than ing hygroscopic filler for manufacturing humidity- and
reducing the volatilisation of lightweight components temperature- regulating construction materials, enhanc-
(Dong et al. 2020). Furthermore, pyrolysis biochar, having ing hygrothermal performance and alleviating the urban
carbon as the primary composition, can shield the sur- heat island issues (Park et al. 2021b). Figure 5a illustrates
face of asphalt from ultraviolet light, prevent photo-oxi- the mechanisms of humidity regulation and urban micro-
dative ageing and improve the high-temperature stability climate of biochar pervious concrete. The hygrothermal
of asphalt (Zhou and Adhikari 2019). Bio-oil, one of the properties of biochar-pervious concrete should be opti-
by-products generated during pyrolysis of biochar, can mised to enhance the capabilities for regulating humidity
also be applied as a rejuvenator for aged asphalt (Zhang and temperature.
et al. 2020b), enabling a combination usage of biochar
and bio-oil for manufacturing sustainable asphalt. 5.2 Thermal insulation and noise reduction
Hydrochar also exhibits good compatibility with Customised biochar possesses 3D porous and 2D flake-
asphalt owing to the micron-sized pits, voids and abun- like structures, which contribute to the formation
dant functional groups on the surface. The high-temper- of additional pathways for heat transfer (Xiong et al.
ature performance of asphalt was significantly improved 2022). When porous biochar is uniformly distributed
by incorporating hydrochar. The optimum dosage of throughout the construction materials, it can induce
hydrochar was 6 wt% with rutting index reaching 76 °C scattered heat propagation, render the heat propaga-
and penetration and softening point reaching 31.7 tion routes multi-directional, and hinder the effect
(0.1 mm) and 54.7 °C, respectively (Hu et al. 2021). How- of unidirectional heat propagation (Jiang et al. 2022;
ever, incorporating hydrochar hindered the workability Wu et al. 2022; Xiong et al. 2022). This phenomenon
of asphalt under low temperatures, which requires fur- effectively slows the expected propagation of heat flow
ther investigation and improvement by adopting tailored and inhibits the heat transfer through solids, enhanc-
hydrochar. ing the thermal insulation potential of biochar-cement
composites (Fig. 5b). It was found that the addition of
5 Environmental and technical advantages biochar decreased the thermal conductivity of biochar-
of biochar‑enhanced construction materials cement composites by 25% (Rodier et al. 2019). Similar
5.1 Humidity regulation and cooling effect results were also obtained in biochar-clay composites,
Excessive artificialisation of the ground (e.g., the exten- where the maximum thermal conductivity decreased
sive application of pavement) caused by the rapid urban by 67% (Lee et al. 2019). The addition of 2 wt% bio-
expansion has disrupted the balance of moisture and char resulted in a low thermal conductivity [0.192 W
heat transition between the ground and the atmosphere, (m·K)−1], and its incorporation also improved the
leading to a series of thermal environment issues in acoustic performance of biochar-cement composites
urban (He et al. 2021a). For example, pavement materi- across the frequency range of 200–2000 Hz (Cuth-
als involving concrete and asphalt mixtures would adsorb bertson et al. 2019). The introduction of biochar can
Zhang et al. Biochar (2022) 4:59 Page 17 of 25
increase the porosity of cement-biochar composites. VOCs, while physical adsorptions typically occurred with
The pores in biochar would break the thermal bridg- pig manure-derived biochar (Zhang et al. 2020b; Zhou
ing within the biochar-cement composites, which is et al. 2020). The adsorption capacity of biochar and bio-
responsible for the low thermal conductivity and ther- char asphalt towards VOCs can be improved by increas-
mal insulation improvement. Therefore, biochar with ing specific surface area, pore volume, and the amount of
high porosity and 3D pore structure is more favoured surface chemical functional groups but decreasing pore
for enhancing the thermal insulation and noise reduc- size (Li et al. 2020). Although biochar exhibits a prom-
tion of construction materials. ising perspective as a VOCs scavenger, few studies have
considered the synergy of both VOCs removal efficiency
5.3 Contaminant immobilisation and indoor air quality and asphalt performance improvement by adopting
improvement biochar.
Biochar has been widely applied for water purifica-
tion and soil remediation due to its peculiar and tune- 5.4 Electromagnetic shielding
able properties, such as high porosity, good stability, Electromagnetic radiation caused by wireless and com-
and high cation exchange capacity. Many studies have munication devices is an increasing public concern. The
focused on incorporating biochar into construction ability to attenuate or hinder electromagnetic interfer-
materials to impart the functional properties (e.g., ences is defined as electromagnetic shielding. The shield-
contaminant immobilisation and indoor air quality ing performance of cement composites is expected to be
improvement) to the composites (Fig. 5c). For instance, increased by incorporating carbon-based materials, such
the water purification process of biochar-pervious as graphene and carbon nanotubes (Chen et al. 2015;
concrete was mainly controlled by adsorption ability Nam et al. 2018). The graphene and carbon nanotubes
and microbial degradation of biochar. Wang et al. also qualified with high specific surface area, low density,
applied the biochar-modified binder for the contami- and high electrical conductivity could enhance the elec-
nated sediment immobilisation, suggesting that biochar tromagnetic shielding efficacy (Zhou et al. 2018). How-
enhanced the immobilisation efficacy of potentially ever, the high cost and agglomeration of graphene and
toxic elements (PTEs) and other organic contaminants carbon nanotubes limit their large-scale applications in
in the sediment (Wang et al. 2019c). The environmen- cement composites. Biochar as a cost-effective material
tal merits of biochar make the sediment products envi- was proposed for improving the electromagnetic inter-
ronmentally acceptable as construction materials, such ference shielding efficacy of cement-based composites
as fill materials and paving blocks. Biochar cement is (Fig. 5d). A maximum increase of shielding effectiveness
a climate-positive and robust binder for immobilising was up to 353% at 1.56 GHz frequency by incorporating
municipal solid waste incineration fly ash, where bio- 0.5 wt% biochar compared to plain cement (Khushnood
char promotes the hydration of cement, resulting in a et al. 2015). The electromagnetic shielding performance
denser matrix for encapsulation of PTEs (Chen et al. of a sustainable lightweight biochar-cement-gypsum
2019a, 2022a). These studies provided new insights into composite was facilitated by increasing biochar content,
adopting climate-positive binders to treat hazardous which became more pronounced at frequencies above
waste. As an economical and highly efficient adsorbent 4 GHz (Natalio et al. 2020). The biochar composite exhib-
for volatile organic compounds (VOCs), biochar also ited high shielding efficacy in the microwave range; the
has the potential to be applied in biochar particleboard mechanisms behind it were not interpreted clearly but
for adsorbing VOCs from interior finishing, contribut- attributed to some conventional reasons, such as natural
ing to indoor air quality improvement (Zou et al. 2019; alignment of carbon ultrastructure (e.g., lignin), dissipa-
Xiang et al. 2020). However, very few studies concern- tion of surface currents and polarisation in the electric
ing incorporating biochar into particleboard to elimi- field.
nate VOCs were performed, requiring further relevant
investigations. 5.5 Biochar‑enhanced 3D concrete printing
Asphalt pavement production would inevitably gener- The three-dimensional (3D) concrete printing involving
ate VOCs, posing health risks to construction labours layer-by-layer concrete deposition by a 3D printer with-
(Cui et al. 2020). To solve this problem, some research- out framework support or vibration processes has gained
ers introduced biochar for removing VOCs in asphalt. tremendous interest in recent years. Construction mate-
Biochar could reduce the VOCs emissions by half, and rials plus 3D printing could reduce construction waste
the adsorption mechanisms depend on the types of bio- by 30–60%, labour expense by 50–80%, and produc-
char (Zhou et al. 2020). For example, chemical adsorp- tion time by 50–70% (Zhang et al. 2019a). However, the
tions occurred between straw/wood-derived biochar and high-efficiency 3D concrete printing requires excellent
Zhang et al. Biochar (2022) 4:59 Page 18 of 25
pumpability, extrudability, and buildability, making con- enhance the heat transfer performance of PCMs, their
ventional cement-based materials difficult to satisfy the prices are prohibitive for large-scale applications, and the
requirements (De Schutter et al. 2018). The appropri- modification processes are often chemically intensive.
ate dosages of polymeric fibres and nano-sized additives Biochar, as a low-cost carbon-negative material,
(e.g., nano-silica, graphene-based materials, and nano- has recently been adopted as a supporting scaffold for
clay) could improve the thixotropy and buildability of 3D enhancing the performance of PCMs (Fig. 5f ) (Jeon et al.
concrete printing (Sikora et al. 2022). For instance, the 2019a; Kim et al. 2021). The biochar-PCMs displayed
nano graphene-based materials at the dosage of 1 wt% negligible leakage, good thermal insulation capacity, high
enhanced the flexural strength of composites by 89% chemical compatibility, exudation stability, and shape sta-
and compressive strength by 28% whilst demonstrating bility whilst reducing building energy consumption of the
excellent shape retention and buildability (Chougan et al. referenced building model (up to 531 kWh year−1) (Jeon
2020). et al. 2019b). Engineered biochar could significantly pre-
Lightweight concrete with a porous structure is a versa- vent the leakage of PCMs, and the corresponding biochar-
tile material applicable for specific structural and insulat- PCMs achieved fusion enthalpies of 108.3 and 138.1 J g−1,
ing purposes. Recently, 3D printing of lightweight foam respectively, whilst maintaining the chemical structure
concrete with a density ranging from 800 to 1200 kg m−3 and thermal reliability after 1000 heating/cooling cycles
was manufactured for exterior wall elements without an (Hekimoğlu et al. 2021). The intermolecular interactions
extra thermal insulation layer (Markin et al. 2021). The between PCM and biochar (i.e., hydrogen bonding), as
stability of foam significantly affected the properties of well as the characteristics of biochar (e.g., surface func-
3D concrete printing, and future work should emphasise tionality, specific surface area, and pore size distribu-
further reducing the density and maintaining adequate tion), were crucial factors influencing the performance of
load-bearing capacity. Wood sawdust and phase change biochar PCMs (Atinafu et al. 2021c). An interconnected
materials were added to enhance the thermal insulation network and the high degree of graphitisation of biochar
capacity and reduce the density of 3D concrete printing microparticles are essential for enhancing the specific
(De Schutter et al. 2018). Carbon-negative biochar was heat capacity and providing nucleation sites to reduce the
also incorporated to improve the thixotropy of 3D con- sub/super-cooling phenomenon of biochar-PCMs (Yang
crete printing, which can provide a new direction for et al. 2019; Yuan et al. 2021). The engineered biochar inte-
versatile 3D concrete printing (Fig. 5e). The addition of grating with multiwalled carbon nanotube exhibited a
biochar can reduce the density of 3D printing of light- high loading capacity of PCMs up to 70.2%, exhibiting a
weight concrete and enhance extrudability and build- high heat storage capacity of 127.4 J g−1 due to the favour-
ability. Developing appropriate mixture compositions for able microstructure and interconnected framework of
high-performance 3D concrete printing has become an the engineered biochar (Atinafu et al. 2021b). Therefore,
opportunity yet a grand challenge. Incorporating biochar engineered biochar with sufficient specific surface areas,
into 3D concrete printing has a promising perspective pore volumes, and functional groups would be favoured
regarding property enhancement and carbon neutrality. for developing high-performance biochar-PCMs. Overall,
the biochar-PCMs with low cost and improved perfor-
5.6 Biochar phase change materials mance can be utilised for diverse thermal energy storage
Phase change materials (PCMs) with high latent heat applications, e.g., waste heat recovery and passive cooling
storage capacity are novel and sustainable materials for of climate-positive design.
energy storage and conversion (Mohamed et al. 2017).
Recent studies have applied PCMs in construction 5.7 Self‑sensing biochar‑cement composites
materials to facilitate energy efficiency and decrease the The structural health monitoring of civil engineering
energy consumption of construction (Song et al. 2018; infrastructures has received increasing attention nowa-
Wi et al. 2021). However, the large-scale applications of days, which involves controlling the functional reli-
PCMs are limited by several drawbacks, such as seepage ability of infrastructure by using computer software.
above the normal melting temperature, insufficient heat Underlying this new technique is a self-sensing cement
transfer performance, inadequate thermal energy absorp- composite combining sensors with electrically conduc-
tion and release characteristics. Nano-sized carbon- tive additives loaded cement. A substantial amount of
based materials, including graphite, graphene, porous research has adopted carbon fibres, carbon black parti-
carbons, and carbon nanotubes, have been applied as cles, carbon nanotubes, and graphene nanoplatelets as
supporting materials for improving the performance additives for manufacturing self-sensing cement compos-
of PCMs (Atinafu et al. 2018, 2021a; Chen et al. 2020b; ites (D’Alessandro et al. 2016; Monteiro et al. 2017; Belli
Wi et al. 2020). Although these materials can effectively et al. 2018). Biochar has been investigated as a potential
Zhang et al. Biochar (2022) 4:59 Page 19 of 25
alternative to graphene nanoplatelets in self-sensing (2) The application of atomistic simulations could fur-
cement composites (Fig. 5g). The addition of biochar at ther improve the understanding of molecular-level
a dosage of 1% (v/v) to cement significantly reduced the interfacial properties between biochar and asphalt/
electrical resistivity (− 42%) whilst maintaining good polymer/cementitious materials under different
mechanical properties (Mobili et al. 2021). Engineered conditions. The models developed by molecular
biochar at the dosage of 5% incorporated in self-sensing dynamics simulation also can be adapted to evalu-
cement composites, exhibiting a 70% reduction in the ate the bonding strength of biochar/asphalt/poly-
water adsorption, a 23% increase in electrical conduc- mer/cementitious materials under different chemo-
tivity, and an approximately 45% reduction in embodied mechanical interactions and form the basis for
carbon footprint (Haque et al. 2021). In future studies, predicting long-term performance.
the engineered biochar is recommended to enhance the (3) Comprehensive regulation of biochar regarding
performance of self-sensing biochar-cement composites. quality, safety, and properties should be carried
out to ensure the proper selection and utilisation
5.8 Bacteria cargo for self‑healing cement composites of biochar in construction materials. For example,
Cracks formed in concrete allow aggressive chemi- detailed requirements should be defined from raw
cals to penetrate the structure, thereby accelerating the materials selection to biochar production processes
damage to the concrete and jeopardising its durability in order to ensure biochar performance. Standard-
and service life. Self-healing treatment involves autog- ized assessments of biochar construction materials
enous healing, incorporating polymeric materials and should also be established to safeguard long-term
calcium carbonate precipitation by microbial species, a use and human/environmental health. Meanwhile,
promising concrete crack remediation technique (Vijay modifications of biochar would benefit the biochar
et al. 2017). Calcium carbonate production through bio- quality and expand the applications of functional
mineralisation is an efficient and sustainable pathway biochar construction materials in respect of techni-
for sealing cracks, which can hinder the crack develop- cal and environmental performance.
ment and fill the deep microcracks (Seifan et al. 2016). (4) Environmental and technical advantages of bio-
Biochar has been adopted as a carrier for bacteria spores char construction materials, such as hygrothermal
in cement composite to seal cracks. A crack of 700 μm regulation, electromagnetic shielding, contaminant
in maximum can be sealed by combining bacteria-doped immobilisation, indoor air quality improvement,
biochar with superabsorbent polymers and polypro- self-healing capacity, and acoustic insulation, are
pylene microfibers (Gupta et al. 2018c). Meanwhile, still in their infancy and should be further explored
incorporating bacteria-doped biochar could enhance to demonstrate their value-added and superior per-
the strength of biochar-cement composites by 38% and formance.
reduce the water penetration and absorption by 65% and (5) A combination of multiple techniques is promis-
70%, respectively, compared with the direct incorpora- ing to provide comprehensive information on the
tion of spores (Gupta et al. 2018c). This modified system advantages and disadvantages of biochar construc-
also maintained the crack sealing efficacy after multiple tion materials. For instance, the synchronous use of
damage cycles. biochar and C O2 curing would enhance the perfor-
Biochar as bacteria cargo for self-healing composites mance of the construction materials whilst achiev-
can boost the sealing of cracks (Fig. 5h). This provides a ing deeper carbon sequestration compared with
relatively low-cost and sustainable solution for solving single technique utilisation.
the crack issue in concrete, which is also beneficial for (6) A holistic techno-economic analysis and life cycle
expanding the life span and durability of concrete. Fur- assessment should be carried out prior to commer-
ther study is required to improve the survival rate of bac- cial applications. This would provide an insight into
teria exposed to alkaline conditions of concrete. the sustainability of biochar construction materials
and their impact on the environment.
6 Future perspectives
(1) Advancing the scientific understanding of the inter-
facial reactions in biochar-construction materials 7 Conclusions
with the assistance of cutting-edge technologies Biochar is demonstrating its tremendous promise for
such as micro-computed tomography, nanoinden- applications in carbon neutral/negative construction
tation, and transmission electron microscopy is materials and is contributing to the achievement of car-
required to promote the performance development bon neutrality targets. The incorporation of biochar
of biochar-construction materials. derived from waste biomass in construction materials
Zhang et al. Biochar (2022) 4:59 Page 20 of 25
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