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Construction and Building Materials 379 (2023) 131221
Available online 5 April 2023
0950-0618/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Carbon sequestration from waste and carbon dioxide mineralisation in 
concrete – A stronger, sustainable and eco-friendly solution to support 
circular economy 
Rajeev Roychand a,*, Jie Li a,*, Shannon Kilmartin-Lynch a, Mohammad Saberian a, 
Jiasheng Zhu a, Osama Youssf b,*, Tuan Ngo c 
a School of Engineering, RMIT University, Melbourne, Australia 
b Structural Engineering Department, Mansoura University, Mansoura, Egypt 
c Department of Infrastructure Engineering, The University of Melbourne, Australia 
A R T I C L E I N F O 
Keywords: 
Carbon sequestration 
Carbon dioxide mineralisation 
Wood biochar 
Concrete 
Sustainability 
A B S T R A C T 
The production of concrete is heavily reliant on the continuous mining of natural resources, with the majority 
component being the natural aggregates which make up about 70–75% of the concrete volume. As the emphasis 
shifts towards promoting sustainability through recycling all types of waste to create a closed-loop circular 
economy, it’s vital to explore alternative waste materials that can replace traditional raw materials in concrete 
production. Biochar derived from different organic waste materials has shown to improve the strength properties 
of concrete. However, the majority of the research has focussed on using biochar as a cement replacement 
material with very low cement replacement levels. Therefore, this research focuses on significantly increasing the 
uptake of the pyrolysed form of organic waste (wood biochar) by using it as a replacement of fine aggregates at 
replacement levels of 10, 20 and 30 vol%. The biochar blended concrete showed an improvement of 63.9, and 
45.6% and a reduction of 9.6% in the 7-day compressive strength results at 10, 20 and 30% replacement levels, 
respectively. At 28 days, the biochar blended concrete samples showed an improvement of 20.1, 22.6, and 16% 
in the compressive strength results at 10, 20 and 30% replacement levels, respectively. Separately, the 10% WBC 
blended concrete was also cured in a CO2 environment for 7 and 28 days. The results showed an improvement of 
24.7 and 37.3 in the 28-day compressive strength results with the respective CO2 curing of 7 and 28 days, 
compared to the control mix. 
1. Introduction 
Globally, the focus is shifting from make, use, and throw policy to-
wards creating a circular economy, where waste generated from one 
source is transformed into a valuable resource for another [1–3]. Gov-
ernments all over the world are supporting various research and 
development initiatives to bring about this positive change [4,5]. One of 
the significant industries that is actively responding to this drive is the 
construction sector. Cement production being one of the largest con-
tributors of greenhouse gas emissions and users of natural resources, is 
attracting extensive research on sustainability [6–10], waste recycling 
[11–16], and the reduction in greenhouse gas emissions [17–20]. 
Various solutions have been developed to cut down its carbon footprint 
and to reduce its reliance on the continuous mining of natural resources, 
like the use of industrial byproducts, such as fly-ash [21,22], slag [23], 
silica fume [24] and biochar derived from different organic wastes 
[25–28] as cement replacement materials. Similarly, several other waste 
materials, like crushed glass [29–31], construction and demolition 
waste [32–34], plastics [35,36], and waste tyre rubber [37–44], have 
been used as a replacement of conventional aggregates to increase sus-
tainability and to support a closed-loop circular economy [45]. 
With the demand for cement and concrete production growing at a 
rapid pace [46], the challenges associated with the existing solutions are 
not being able to meet the growing market demand, thereby delaying 
the transformation of cement concrete into a sustainable and low-carbon 
footprint material. Therefore, this area of research requires multiple 
solutions that can collectively target sustainability and reduction in the 
carbon footprint of concrete. Parallelly, around 13 million tonnes of 
* Corresponding authors. 
E-mail addresses: rajeev.roychand@rmit.edu.au (R. Roychand), jie.li@rmit.edu.au (J. Li), osama.youssf@mymail.unisa.edu.au (O. Youssf). 
Contents lists available at ScienceDirect 
Construction and Building Materials 
journal homepage: www.elsevier.com/locate/conbuildmat 
https://doi.org/10.1016/j.conbuildmat.2023.131221 
Received 26 February 2023; Received in revised form 21 March 2023; Accepted 27 March 2023 
mailto:rajeev.roychand@rmit.edu.au
mailto:jie.li@rmit.edu.au
mailto:osama.youssf@mymail.unisa.edu.au
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CO2-e (carbon dioxide equivalent) is generated as a result of organic 
waste going to landfills, which equates to approximately 3% of Aus-
tralia’s total GHG emissions [47]. The decomposition of organic waste 
can also produce leachate, a toxic liquid that can contaminate ground-
water and soil [48]. This can pose a risk to human health and the 
environment. Furthermore, the disposal of organic waste in landfills 
takes up valuable space. Pyrolysing these organic waste materials not 
only preserves their organic carbon in the form of biochar but also 
produces valuable biofuel, with both having numerous commercial 
applications. 
Recently, the application of biochar derived from different organic 
wastes into concrete has shown great potential to improve the me-
chanical and durability properties [49] in addition to contributing to a 
reduction in its net carbon footprint [50]. Based on the review of 
existing literature [25,27,49,51] on the application of biochar in con-
crete, the majority of the research focuses on using it as a cement 
replacement material which restricts its uptake to a very small quantity. 
Danish et al. [25] carried out a comprehensive review of the application 
of biochar as a cement-replacement material. They concluded that the 
use of biochar in cementitious materials could impact their properties. 
The porosity or water absorption of biochar can increase the yield-stress 
and plastic viscosity of cementitious materials, resulting in reduced 
workability. However, this negative effect can be mitigated by using an 
optimal dosage of superplasticiser, increasing the water/binder ratio, or 
fully saturating the biochar with water. Using coarser biochar may also 
help. Biochar can also reduce the setting time of cementitious materials 
by reducing free water and increasing hydration reaction and heat. By 
replacing 1–2% of cement with biochar, the compressive, splitting ten-
sile, flexural strength, elastic modulus, shrinkage properties, and water 
absorption behaviour of cementitious materials can be improved. This 
improvement is due to pore-refinement, pozzolanic activity, and 
internal-curing effect. 
Another review paper by Gupta and Kua [27] noted that to produce 
biochar suitable for CO2 adsorption, it is essential to properly prepare 
the feedstock and control the pyrolysis process parameters, including 
temperature, pressure, and heating rate. Future research should focus on 
addressing these issues. Biochar can be incorporated into the concrete 
mixture for buildings and civil engineering structures. To capture and 
store CO2, the pore structure of biocharcan be utilized to store the CO2, 
which is subsequently mineralized into calcium carbonate after being 
released into the concrete mixture [52–54]. This approach could lead to 
the sequestration of a significant amount of CO2, a greenhouse gas. 
However, it is crucial to examine the potential effects of biochar con-
taining absorbed/adsorbed CO2, on the durability properties of rein-
forced concrete as a result of carbonation. The mechanism behind CO2 
adsorption by biochar is based on physical and chemical interactions 
between the biochar surface and the CO2 molecules. Physically, the 
porous structure of biochar provides a large surface area for CO2 mol-
ecules to adsorb onto. Chemically, the surface functional groups of 
biochar, such as carboxylic and phenolic groups, can react with CO2 
molecules to form stable carbonates or bicarbonates [55]. Factors that 
can influence CO2 adsorption by biochar include the properties of the 
biochar, such as its surface area, pore size distribution, and surface 
functional groups. Additionally, the concentration and pressure of CO2 
in the environment can also affect the adsorption capacity of biochar. 
Other factors include temperature, humidity, and the presence of other 
gases in the environment [56]. 
Very few studies have looked at the applications of pyrolysed forms 
of different organic wastes as a replacement for conventional aggregates 
[57–59]. Maljaee et al. [57] investigated the effect of olive stone waste 
biochar on the production of lightweight mortar by replacing sand at 25, 
45 and 60% of volume replacement levels. They noted that using biochar 
as a sand replacement material reduced compressive strength by 15%, 
18%, and 22% at replacement levels of 25%, 45%, and 60%, respec-
tively. They ascribed the decline in the compressive strength of biochar 
blended mortar to the porous nature of olive stone wastes biochar. 
Cuthbertson et al. [58] investigated the effect of biochar produced from 
the residual biomass of the bio-ethanol industry on the properties of 
biochar-modified concrete at sand replacement levels of 1, 2 and 3 wt%. 
They noted that the compressive strength of concrete containing 1% 
biochar showed a considerable reduction in the 28-day concrete 
strength, however, 2 and 3% biochar content showed an improvement of 
8.2 and 24.6%. Mrad and Chehab [59] looked at the effect of municipal 
solid waste biochar on the cement biochar composite mortar by 
replacing sand at 10, 25, and 45% by weight of sand. They observed a 
significant reduction in all the mortar mixes containing municipal solid 
waste biochar, with a maximum reduction of 93% at 45% sand 
replacement level with air curing and 97% reduction with water curing 
of samples. 
With the growing concern about the scarcity of natural sand in many 
nations [60], which makes it a major sustainability issue, it becomes 
crucial for the research community to focus on developing alternatives 
to conventional aggregates from different waste streams. Although, M- 
sand [61] and crushed glass [62] have been explored recently as an 
alternative to natural sand, biochar derived from organic wastes provide 
multiple benefits like, improvement in the strength and durability 
properties of concrete, carbon sequestaration, diversion of organic waste 
from going ro landfills which is responsible for high GHG emissions 
[52,56]. Therefore, to address this research gap and the potential 
environmental and sustainability benefits of recycling biochar derived 
from different organic waste in concrete applications, this research 
project looks at utilising pyrolysed forms of natural wood waste at 10, 20 
and 30 vol% replacement of fine aggregates in the biochar blended 
concrete composites. In addition, to capture the recent advancements in 
the utilisation of CO2 capture and storage technology for its minerali-
sation in concrete, the best mix of biochar blended concrete was used for 
7 and 28 days of CO2 curing. Detailed material characterisation tech-
niques like Xray fluorescence (XRF), Xray diffraction (XRD), Carbon, 
Hydrogen, Nitrogen and Sulfur (CHNS) analysis, scanning electron mi-
croscopy (SEM), and laser diffraction particle size analysis were un-
dertaken to ascertain the material properties of the raw material. The 
heat of hydration, SEM analysis, carbonation and compressive-strength 
tests were carried out to identify the performance of the addition of 
wood waste biochar, and CO2 curing on the material properties of bio-
char blended concrete composites. 
2. Materials and mix designs 
2.1. Raw materials 
Ordinary Portland cement, 10 and 7 mm coarse aggregates (Specific 
gravity = 2.72), sand (specific gravity = 2.62), wood-based biochar 
(specific gravity = 0.45) sourced from a local company Green Man Char, 
MasterGlenium SKY 8379 Superplasticiser and water were used as raw 
materials. 
2.2. Material properties 
2.2.1. Particle size distribution of coarse aggregates, sand and biochar 
The particle size distribution of coarse and fine aggregates was car-
ried out using sieve analysis and that of OPC, coarse and fine biochar 
was conducted using “Malvern Mastersizer 3000” laser diffraction 
Table 1 
Particle size distribution of CA, FA, WBC and OPC. 
Material D10 D25 D50 D75 D100 
10 mm (CA) 3.0 mm 5.8 mm 7.4 mm 8.5 mm 10 mm 
7 mm (CA) 2.4 mm 3.5 mm 4.9 mm 5.9 mm 7 mm 
Sand (FA) 137 µm 220 µm 347 µm 515 µm 1420 µm 
WBC 30.2 µm 214 µm 688 µm 1310 µm 3500 µm 
OPC 3.59 µm 4.65 µm 15.5 µm 22.5 µm 40.1 µm 
R. Roychand et al. 
Construction and Building Materials 379 (2023) 131221
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particle size analyser. The particle size distribution of coarse aggregates, 
sand and biochar is provided in Table 1. 
2.2.2. Elemental composition of OPC and wood biochar 
X-ray fluorescence was carried out on the OPC and wood biochar 
powder samples to ascertain their elemental composition using Bruker 
Axs S4 Pioneer Xray fluorescence equipment. The elemental composi-
tion of OPC and wood biochar and biochar is provided in Table 2. 
2.2.3. CHNS analysis of wood biochar 
Since biochar contained a significant amount of carbon content 
which cannot be detected with X-ray fluorescence, CHNS analysis was 
carried out on the wood biochar (WBC) samples using PerkinElmer - 
2400 II CHNS/O Elemental Analyser. Table 3 shows the carbon, 
hydrogen, nitrogen, and sulfur concentrations of the WBC samples (See 
Fig. 1). 
2.2.4. Mineralogical composition of OPC and WBC 
XRD was conducted on the OPC and WBC samples to ascertain their 
mineralogical composition using Bruker AXS D4 Endeavour equipped 
with Cu-Kα radiation source and operated at 40-mA current and 40-kV 
voltage. The samples were tested between 5◦ to 70◦ 2θ angle with 
0.01◦ 2θ step size and 1 s count time per step (See Fig. 1). 
Table 2 
Elemental composition of OPC and WBC. 
Material Na Mg Al Si P S Cl K Ca Ti Mn Fe Cu Zn Sr 
OPC – 0.51 1.66 6.64 0.36 0.97 0.03 0.43 50.57 0.17 0.06 2.33 0.02 0.04 0.07 
WBC 0.17 0.45 0.54 2.26 0.39 0.22 0.72 3.68 10.49 0.12 0.14 1.01 0.03 0.09 0.11 
Table 3 
CHNS analysis of WBC. 
Carbon (%) Hydrogen (%) Nitrogen (%) Sulfur (%) 
70.05 ± 0.99 1.92 ± 0.01 1.49 ± 0.04 0.02 ± 0.01 
Fig. 1. XRD analysis of OPC and WBC. 
R. Roychand et al. 
Construction and Building Materials 379 (2023) 131221
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2.2.5. Microstructure of WBC 
Scanning electron microscopic images of wood biochar were 
collected to ascertain its microstructural properties using FEI Quanta 
SEM at 50x and 500x magnification levels. WBC is non-conductive; 
therefore,it was coated with gold prior to collecting the images. The 
SEM images of WBC show a highly porous microstructure (Fig. 2). 
2.2.6. Mix designs 
The main focus of the experimental program was to replace sand 
with 10, 20, and 30 vol% by wood biochar. The best mix design that 
provides the highest strength enhancement was used for 7 and 28 days of 
CO2 curing at 20 ◦C temperature, 70% relative humidity and 10% CO2 
concentration. The water/cement ratio used for this experimental pro-
gram was 0.5. Wood biochar absorbs water at about 70% of its dry 
weight. The absorption of water by wood biochar reduces its workability 
and the water available for the early hydration of the cement particles. 
Therefore, to compensate for that and to achieve similar workability, 
additional water (AW) was added to the mix designs containing WBC. 
The mix designs are provided in Table 4. 
3. Experimental program 
3.1. Effect of biochar on the properties of biochar blended cement/ 
concrete composites 
In stage 1, the effect of different concentrations of wood biochar used 
as a replacement of fine aggregates on various properties of biochar 
blended cement/concrete composites was ascertained. 
3.1.1. Heat of hydration 
Heat of hydration test was carried out to identify if the addition of 
biochar has any effect on the early (48 hrs) hydration behaviour of 
cement that may influence the compressive-strength properties of the 
blended concrete composites. Since the main aim of this test was just to 
identify the influence of biochar on cement hydration, cement paste 
mixes containing different concentrations of biochar were prepared 
based on the mix designs specified in Table 4, excluding sand and coarse 
aggregates. TAM Air isothermal calorimeter was used to measure the 
heat of hydration. The experiment was carried out for 48 h after the 
casting of cement paste samples. 
3.1.2. Effect of biochar on concrete microstructure 
SEM analysis was undertaken on the blended concrete composites to 
identify the microstructural changes taking place in various samples. To 
conduct SEM analysis, a small portion of the concrete sample was 
extracted from the specimen, cast in epoxy, ground, polished, mounted 
on steel stubs, and finally coated with gold. SEM images were taken on 
these gold-coated samples using FEI Quanta SEM at 500x, 2500x and 
5000x magnification levels. 
3.1.3. Compressive strength 
Compressive Strength tests were undertaken on 3 replicates of 75 
mm Ø × 150 mm cylindrical samples of each mix design. The samples 
were casted and cured in a temperature and humidity control room set at 
23 ◦C temperature and 55% relative humidity. 
3.2. Effect of CO2 curing on 10% wood biochar blended concrete 
composites 
In stage 2 of the experimental program, the blended concrete com-
posite containing the best biochar blended mix was used for CO2 curing. 
The concrete samples were cured in a controlled environment set at 23 
◦C, 70% humidity and 10% CO2 concentration. The samples were cured 
in the molds for 24 h after casting, followed by the CO2 curing for 6 and 
27 days. The first set of six samples that were cured for 6 days in CO2 
chamber were taken out on the seventh day. Three replicates of CO2- 
Fig. 2. SEM images of wood biochar at 50x and 500x magnification levels. 
Table 4 
Concrete mix designs. 
Mix OPC 10 mm 7 mm Sand WBC Water AW SP Curing 
(kg/m3) (L/m3) 
Control 350 550 450 850 – 175 – 1.2 Normal curing 
10WBC 350 550 450 788 15 175 10 1.2 Normal curing 
20WBC 350 550 450 700 30 175 20 1.2 Normal curing 
30WBC 350 550 450 613 45 175 30 1.2 Normal curing 
10WBC_7dCC 350 550 450 788 15 175 10 1.2 7 day CO2 curing 
10WBC_28dCC 350 550 450 788 15 175 10 1.2 28 day CO2 curing 
C = Control, 7d = 7 day, 28d = 28 day, CC = CO2 curing, AW = Additional Water, SP = Superplasticizer. 
R. Roychand et al. 
Construction and Building Materials 379 (2023) 131221
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cured samples were tested at 7 days, and the remaining three samples 
were cured in the temperature and humidity control room (non-CO2 
environment) and tested at 28 days of total curing time. The second set 
of 3 replicates was kept for 27 days of CO2 curing after the first 24 h of 
curing in steel molds. The samples were also tested for the depth of 
carbonation using a phenolphthalein indicator (0.5% Phenolphthalein 
in 50% Ethanol) [44]. 
4. Results and discussion 
4.1. Effect of biochar on the properties of biochar blended cement/ 
concrete composites 
4.1.1. Heat of hydration 
Heat of hydration test was carried out to identify if the addition of 
biochar has any effect on the hydration behavious of cement that may 
influence the compressive strength properties of the blended concrete 
composites. The rate of the heat of hydration graph (Fig. 3a) shows that 
the peak heat decreases with the increase in wood biochar concentra-
tion. The respective peak heat release due to cement hydration of the 
control and biochar blended cement composites were 1.46 mW/g 
(Control) > 1.42 mW/g (10WBC) > 0.76 mW/g (20WBC) > 1.42 mW/g 
(30WBC). Moreover, the peak heat release shifts towards a later curing 
age, i.e., 12.37 h (Control) > 13.34 h (10WBC) > 14.24 h (20WBC) >
15.04 h (30WBC). In addition, the peak width or the duration of heat 
release due to the hydration of cement particles increases with the in-
crease in biochar content. 
Interestingly, the cumulative heat of hydration curve (Fig. 3b) shows 
an increase in the heat of hydration at the early age of curing, i.e., the 
first hour of curing, which increases with the increase in the concen-
tration of wood biochar. This indicates that the biochar content accel-
erates the early-age hydration reaction. The cumulative heat of 
Fig. 3. (a) Rate of heat of hydration and (b) Cumulative heat hydration of control and wood biochar blended cement composites. 
R. Roychand et al. 
Construction and Building Materials 379 (2023) 131221
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hydration shows a sharp increase at around 12 h with a formation of a 
shoulder. This indicates a considerable increase in the degree of cement 
hydration and the formation of more hydration products. This could be 
attributed to the influence of superplasticiser that can accelerate the rate 
of cement hydration and increase the amount of hydration products 
formed. After 24 h of curing, the 10% biochar content shows the highest 
cumulative heat of hydration, indicating the highest degree of hydration 
reaction, which was followed by the mix containing 20% biochar con-
tent. Both the mixes containing 10 and 20% biochar contents showed the 
highest heat of hydration compared to the control sample. However, the 
mix containing 30% biochar content showed a progressive reduction in 
the heat of hydration relative to the control mix after 24 h of curing. 
Fig. 4. SEM images of concrete samples containing different concentrations of wood biochar content. 
Fig. 5. Compressive strength of concrete samples at 7 and 28 days of curing. 
R. Roychand et al. 
Construction and Building Materials 379 (2023) 131221
7
4.1.2. Effect of biochar on concrete microstructure 
Fig. 4 shows the SEM images of concrete samples containing different 
concentrations of wood biochar content. The porous microstructure of 
the wood biochar is clearly evident in the SEM images. Wood biochar 
shows a good bond formationwith the cement matrix in all the different 
biochar blended concrete samples. 
4.1.3. Compressive strength 
Fig. 5 shows the compressive strength results of concrete mixes 
containing different concentrations of wood biochar samples. It can be 
noted that the concrete mixes containing 10% wood biochar (10WBC) 
show a 63.9% and 20.1% increase in the respective 7 and 28-day 
compressive strength results. By increasing the concentration of wood 
biochar to 20% (20WBC), the 7 and 28-day compressive strength results 
showed an improvement of 45.6% and 22.6%, respectively, compared to 
that of the control mix. With the further increase in the wood biochar 
content to 30% (30WBC), though the 7-day compressive strength 
showed a reduction of 9.6%, the 28-day compressive strength showed an 
improvement of 16% compared to the control mix. Based on the 
compressive strength results, the 10% biochar content is can be 
considered optimum if the target is the improvement of 7-day strength, 
however, if the target is the maximum overall increase in strength (i.e., 
28 day strength) 20% is considered the optimum concentration of the 
wood biochar content. 
The improvement in the 7 and 28 day compressive-strength results 
could be attributed to the increase in the hydration reaction, good bio-
char to cement matrix bond formation and internal-curing provided by 
the water stored in the porous microstructure of WBC. The downward 
trend beyond 20% wood biochar content could be attributed to the 
relatively weaker strength of biochar which gets dominant as the con-
centration of the biochar increases. 
Table 5 and 6 provides ANOVA analysis results of Fig. 5. For the 
statistical analysis, ANOVA: Two-Factor with Replication method was 
adopted. It can be seen that the F-Value is greater than the Fcritical-Value. 
Also, the p-value is lower than the alpha value (=0.05). Therefore, it can 
be concluded that the results are set to be highly significant. 
4.2. Effect of CO2 curing on 10% wood biochar blended concrete 
composites 
4.2.1. Concrete carbonation 
Fig. 6 shows the change in the carbonation depths of 10% wood 
biochar concrete (10WBC) samples with no CO2 curing, 7-day CO2 
curing, and 28 days of CO2 curing. Concrete is highly alkaline in nature 
due to the release of calcium hydroxide from the hydration reaction of 
calcium silicate phases (Eq. (1)). This calcium hydrate further reacts 
with the CO2 gas to produce calcium carbonate, thereby lowering the pH 
value of the concrete sample. 
Table 5 
Summary of the two factor ANOVA analysis with replication. 
Control 7-day Strength 28-day Strength Total 
Count 3 3 6 
Sum 57.41868308 92.44407977 149.8628 
Average 19.13956103 30.81469326 24.97713 
Variance 1.164038393 0.967116471 41.74508 
10WBC 
Count 3 3 6 
Sum 94.0245557 110.716686 204.7412 
Average 31.34151857 36.905562 34.12354 
Variance 0.357884768 0.656639331 9.693383 
20WBC 
Count 3 3 6 
Sum 83.59 112.9524531 196.5425 
Average 27.86333333 37.6508177 32.75708 
Variance 0.893333333 1.068861274 29.52333 
30WBC 
Count 3 3 6 
Sum 51.9 107.1789255 159.0789 
Average 17.3 35.72630851 26.51315 
Variance 1.0416 0.615156335 102.5214 
Table 6 
ANOVA analysis results. 
Source of variation Sum of Squares Degrees of freedom Mean Square F-Value p-value Fcritical-Value 
Sample 367.9734052 3 122.6578 145.0578 8.2187E-12 3.23887152 
Columns 774.7396308 1 774.7396 916.2241 1.49017E-15 4.49399848 
Interaction 129.1468503 3 43.04895 50.91063 2.08835E-08 3.23887152 
Within 13.52925981 16 0.845579 
Total 1285.389146 23 
Fig. 6. Carbonation depths of concrete samples (a) no CO2 curing, (b) 7 day CO2 curing, and (c) 28 days of CO2 curing. 
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Construction and Building Materials 379 (2023) 131221
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C2S/C3S + H2O => C-S-H + Ca(OH)2 (1) 
Ca(OH)2 + CO2 => CaCO3 + H2O (2) 
The reduction in the pH value of the concrete samples due to CO2 
curing is clearly evident in Fig. 6. The phenolphthalein indicator turns 
pink in the regions that are highly alkaline due to the presence of cal-
cium hydroxide, whereas the sections that are carbonated do not show 
any change in colour. It can be seen that the carbonation depth of the 
concrete samples increases with the increase in the duration of CO2 
curing. 
4.2.2. Effect of CO2 curing on the compressive strength of 10% wood 
biochar blended concrete samples 
Fig. 7 shows the compressive strength results of control and 10WBC 
samples before and after 7 and 28 days of CO2 curing. It can be seen that 
the compressive strength of the concrete samples increases with the 
increase in CO2 curing, which is also reflected in the increase in the 
carbonation depth of the concrete samples (Fig. 6). This can be attrib-
uted to the conversion of calcium hydroxide to calcium carbonate, 
which produces water that can potentially contribute towards the 
reduction in the internal relative-humidity and the increase in the hy-
dration reaction of cement particles. 
5. Conclusions 
i) Wood biochar shows good bond performance with the cement 
matrix. 
ii) The peak heat release at the early age (≤2 days) hydration re-
duces with the increase in WBC content. In addition, the peak 
heat release shifts towards later curing age (hours) and the peak 
width increases with the increase in WBC content. 
iii) The cumulative heat of hydration increases with the increase in 
biochar content in the first couple of hours of cement hydration. 
However, in the case of 30WBC mix, a transition point was 
observed in the cumulative heat of hydration at around 24 h of 
curing, beyond which the cumulative heat of hydration started 
showing a reduction compared to that of the control mix. This 
rate of reduction in the cumulative heat of hydration compared to 
that of the control mix increased with the increase in the curing 
age. Overall, mix 30WBC shows relatively lower hydration at 48 
h compared to that of the control mix. 
iv) Mix 10WBC shows the highest increase (63.9%) in the 7-day 
compressive strength, which progressively decreases with the 
increase in biochar content. The least 7-day strength was 
observed in 30WBC, which was 9.4% lower than that of the 
control mix. 
v) All the 28-day biochar blended concrete mixes showed higher 
compressive strength compared to that of the control mix. The 
highest compressive strength (37.7 MPa) was achieved with 
20WBC samples, which was 22.4% higher than that of the control 
mix (30.8 MPa). 
vi) CO2 curing shows a positive influence on the compressive 
strength of the biochar blended concrete composite, which in-
creases with the increase in the duration of CO2 curing. The best 
biochar blended (10WBC) and 28-day CO2 cured concrete sample 
showed a 37.3% increase in the 28-day compressive strength 
compared to that of the control mix. 
CRediT authorship contribution statement 
Rajeev Roychand: Conceptualization, Funding acquisition, Meth-
odology, Investigation, Data curation, Formal analysis, Visualization, 
Writing – review & editing. Jie Li: Funding acquisition, Writing – review 
& editing. Shannon Kilmartin-Lynch: Writing – review & editing. 
Mohammad Saberian: Writing – review & editing. Jiasheng Zhu: 
Writing – review & editing. Osama Youssf: Writing – review & editing. 
Tuan Ngo: Writing – review & editing. 
Declaration of Competing Interest 
The authors declare the following financial interests/personal re-
lationships which may be considered as potential competing interests: 
Rajeev Roychand reports financial support was provided by RMIT Uni-
versity’s Strategic Capability Deployment Fund, in collaboration with 
ARUP Australia P/L and Earth systems Pty Ltd. 
Fig. 7. Compressivestrength of concrete samples of control and 10WBC samples before and after 7 and 28 days of CO2 curing. 
R. Roychand et al. 
Construction and Building Materials 379 (2023) 131221
9
Data availability 
Data will be made available on request. 
Acknowledgements 
The authors gratefully acknowledge RMIT University’s Strategic 
Capability Deployment Fund, ARUP Australia P/L and Earth systems P/L 
for their financial and technical support for this project. We also 
acknowledge the RMIT University’s Rheology, X-Ray, and Microscopy & 
Microanalysis Facilities for providing training and access to the material 
characterisation equipment. 
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	Carbon sequestration from waste and carbon dioxide mineralisation in concrete – A stronger, sustainable and eco-friendly so ...
	1 Introduction
	2 Materials and mix designs
	2.1 Raw materials
	2.2 Material properties
	2.2.1 Particle size distribution of coarse aggregates, sand and biochar
	2.2.2 Elemental composition of OPC and wood biochar
	2.2.3 CHNS analysis of wood biochar
	2.2.4 Mineralogical composition of OPC and WBC
	2.2.5 Microstructure of WBC
	2.2.6 Mix designs
	3 Experimental program
	3.1 Effect of biochar on the properties of biochar blended cement/concrete composites
	3.1.1 Heat of hydration
	3.1.2 Effect of biochar on concrete microstructure
	3.1.3 Compressive strength
	3.2 Effect of CO2 curing on 10% wood biochar blended concrete composites
	4 Results and discussion
	4.1 Effect of biochar on the properties of biochar blended cement/concrete composites
	4.1.1 Heat of hydration
	4.1.2 Effect of biochar on concrete microstructure
	4.1.3 Compressive strength
	4.2 Effect of CO2 curing on 10% wood biochar blended concrete composites
	4.2.1 Concrete carbonation
	4.2.2 Effect of CO2 curing on the compressive strength of 10% wood biochar blended concrete samples
	5 Conclusions
	CRediT authorship contribution statement
	Declaration of Competing Interest
	Data availability
	Acknowledgements
	References