Evaluation of the Strength Characteristics of ECC Based on Cement Replacement Ratios
with Biochar
(Sangwoo Kim)
1iD
(Jaewon Gwak)
2
(Sooncheol Choi)
3
(Jinsup Kim)
4†iD
-
Member · Ph.D Candidate · Gyeongsang National University
(kimsangwoo@gnu.ac.kr)
-
Master Course · Gyeongsang National University
(wodnjs0004@naver.com)
-
Master Course · Gyeongsang National University
(csc758@naver.com)
-
Member · Associate Professor · Gyeongsang National University
(Corresponding Author · jinsup.kim@gnu.ac.kr)
Copyright © 2021 by the Korean Society of Civil Engineers
Key words
Biochar, Cement, Engineered cementitious composites(ECC), Strength characteristics, Replacement rate
1. Introduction
As the demands for sustainability and environmental protection in the modern construction
industry increase, the development and application of eco-friendly materials are becoming
increasingly necessary. The domestic cement industry emits approximately 16 million
tons of carbon dioxide annually. Carbon neutrality is emerging as a significant global
issue (Lim et al., 2023), leading to a growing interest in carbon reduction technologies. Research is being
conducted across various fields to reduce emissions of carbon dioxide and greenhouse
gases to zero (Ruziev et al., 2023). The cement industry accounts for 5 % to 8 % of the total carbon dioxide emissions
in the country (Scrivener and Kirkpatrick, 2008), a remarkably high figure. The global annual production of concrete has exceeded
25 Gt, corresponding to more than 3.8 tons per capita per year (Javed et al., 2022). Therefore, efforts to reduce cement consumption are necessary to prevent the greenhouse
effect caused by carbon dioxide (Song et al., 2017). The cement used here is commonly found in everyday structures such as buildings,
bridges, houses, roads, and tunnels. Consequently, there is a need for alternative
materials that are more environmentally friendly and perform better than traditional
cement-based materials to reduce cement consumption.
One promising solution is the incorporation of biochar into cement composites, particularly
engineered cementitious composites (ECC). Biochar, a carbon-rich byproduct generated
from the pyrolysis of organic materials, offers various environmental benefits. The
carbon sequestration and soil health improvement capabilities of biochar are well-documented
in the agricultural sector.
However, its potential to enhance the properties of cement-based materials in the
construction industry is a burgeoning area of research interest.
Including biochar in ECC can yield several beneficial outcomes. Firstly, by partially
replacing cement, biochar can help reduce greenhouse gas emissions, thereby lowering
the overall carbon footprint of the composite. Secondly, the porous structure and
high surface area of biochar can enhance the mechanical properties of ECC, such as
tensile strength, ductility, and durability. These improvements can lead to stronger
infrastructure capable of withstanding greater stress and deformation without catastrophic
failure. Additionally, the intrinsic properties of biochar can positively influence
the hydration process and microstructure of the cement matrix. Biochar's ability to
absorb and retain water can facilitate a more controlled hydration process, resulting
in a denser and more homogeneous microstructure. This, in turn, can improve the longevity
and performance of ECC under various environmental conditions.
The integration of biochar into high-toughness cement composites aligns with the principles
of sustainable development and the circular economy. By utilizing agricultural or
forestry waste products such as biochar, the construction industry can reduce waste,
promote resource efficiency, and create high-performance materials that support eco-friendly
building practices.
Recent research on biochar has highlighted its potential as an eco-friendly material
that can be used for carbon sequestration and greenhouse gas reduction. Biochar is
a carbon-rich solid residue produced through pyrolysis, an eco-friendly and energy-efficient
waste treatment process that thermochemically converts biomass or municipal solid
waste in an oxygen-limited environment (Malkow, 2004). Biochar is one of the waste byproducts being studied for its potential use in cement
and asphalt composites in infrastructure applications (Zhao et al., 2014). During pyrolysis, volatile and organic materials escape from the feedstock, creating
pores of various sizes that result in a honeycomb-like porous structure. This makes
biochar a porous material capable of absorbing and retaining significant amounts of
water, suitable for applications in lightweight aggregates (Mrad and Chehab, 2019). The porosity of biochar can be classified into micropores, mesopores, and macropores,
with larger pores allowing easier penetration of water into the particles (Brewer, 2012).
In domestic research, Choi et al. (2012) confirmed that biochar could be used as a cement substitute to sequester carbon in
cement composites. Additionally, Han and Choi (2023) demonstrated that replacing cement with wood-based biochar is more effective in reducing
the amount of cement than simply adding biochar. In international studies, Mrad and Chehab (2019) found that biochar could act as an internal curing agent, releasing moisture during
the early stages of curing, thereby improving the compressive strength and mechanical
properties of cement composites. Gupta and Kua (2019) confirmed that when biochar is ground to less than 100 microns, it can promote cement
hydration and contribute to early strength development. When ground to less than 20
microns, biochar can have a filler effect in cement composites. Tan et al. (2021) found that biochar less than 125 microns in size acts as a filler in cement composites,
with the presence of pozzolanic reactions being insignificant. Suarez-Riera et al. (2024) confirmed that adding biochar to cement composites could improve the mechanical properties
of the composites. Therefore, in this study, we aimed to determine whether the same
behavior occurs when biochar is added to ordinary mortar and ECC mortar. To achieve
this, we experimentally evaluated and compared the flow, compressive strength, splitting
tensile strength, and flexural strength of composites containing various cement replacement
ratios of biochar.
2. Experimental Program
2.1 Material Properties
The materials used in the mix for biochar-incorporated mortar (hereafter referred
to as BM) were cement, biochar, water, river sand, stone powder, and superplasticizer
(SP). The fine aggregate was used in accordance with KS L ISO 679 (2022). To achieve a particle size distribution similar to standard sand, river sand and
stone powder were mixed in a ratio of 11:4.
The materials used in the mix for biochar-incorporated ECC (hereafter referred to
as BE) were cement, biochar, water, blast furnace slag, fly ash, silica sand, superplasticizer
(SP), thickener (HPMC), defoamer, and polyvinyl alcohol (PVA). Silica sand grades
7 and 8 were used, and Table 1 summarizes the chemical properties of the silica sand. The main characteristics of
the blast furnace slag used are summarized in Table 1. The admixtures used included water reducer, thickener (HPMC), and defoamer.
Table 1. Properties of Silica Sand, Blast Furnace Slag and Biochar
Materials
|
Properties
|
Results
|
Silica Sand
|
SiO2
|
97.2 %
|
Al2O3
|
1.28 %
|
Fe2O3
|
0.59 %
|
Blast Furnace Slag
|
Density
|
2.90 g/cm3
|
Blaine
|
4.460 cm2/g
|
Heat reduction
|
0.45 %
|
Biochar
|
Average particle size
|
10.72 µm
|
Carbon content
|
82.82 %
|
Specific surface area
|
17.50 m2/g
|
Pore volume
|
0.031 cm3/g
|
Pore size
|
7.03 nm
|
Moisture content
|
148.31 %
|
The biochar used to replace the cement was the same as that used by Kim et al. (2024) and underwent the same grinding process. The main properties of the biochar used
are summarized in Table 1. Material analysis of the ground biochar was conducted using HS FE-SEM, HS FE-SEM_EDS,
and PSA. The morphology of the ground biochar was examined with the HS FE-SEM (Heating
Stage Field-Emission Scanning Electron Microscope). Qualitative and quantitative analyses
of the approximate elemental composition were carried out using the HS FE-SEM_EDS.
Additionally, to assess the particle size distribution, a PSA (Particle Size Analyzer)
was used to compare the particle size of the ground biochar with that of the cement
utilized in this study.
2.1.1 Results of HS FE-SEM Analysis
HS FE-SEM images of the ground biochar and cement at a magnification of ×10,000 are
shown in Fig. 1. The morphology observed in Fig. 1 reveals that the ground biochar exhibits a porous structure, attributed to the cellulose
cell structure, in contrast to the cement.
Fig. 1. HS FE-SEM Result of Biochar and Cement, (a) Biochar, (b) Cement
2.1.2 Results of HS FE-SEM_EDS Analysis
The experimental results of HS FE-SEM_EDS are presented in Fig. 2 and Fig. 3. The common elemental compositions, compared with previous studies, are summarized
in Table 2 (Han and Choi, 2023; Gupta et al., 2018). According to Spokas (2010), an Oxygen to Carbon Ratio (O/C) < 0.2 indicates semi-permanent chemical stability.
Biochar used in this experiment has an O/C ratio of 0.19, suggesting that it possesses
chemical stability. Therefore, if biochar is mixed with mortar or concrete as a cement
replacement, it is expected to achieve semi-permanent carbon sequestration without
any changes in composition. Additionally, these results help confirm whether biochar
is chemically stable and verify its suitability as a usable material.
Fig. 2. Mapping Analysis Graph of Biochar
Fig. 3. Element Composition of Biochar, (a) C, (b) O, (c) Na, (d) Mg, (e) Si, (f) Cl, (g) K, (h) Ca, (i) Total Layered
Table 2. HS FE-SEM_EDS of Biochar
Type of Biochar
|
Element (%)
|
Reference
|
C
|
O
|
Mg
|
Na
|
K
|
Ca
|
O/C
|
Biochar
|
82.16
|
15.42
|
0.43
|
0.18
|
0.20
|
1.10
|
0.19
|
This study
|
Wood-based biochar
|
86.92
|
12.86
|
0.03
|
-
|
0.11
|
0.06
|
0.14
|
Han and Choi (2023)
|
Mixed wood saw dust biochar
|
87.13
|
7.21
|
0.51
|
-
|
0.42
|
0.65
|
0.08
|
Gupta et al. (2018)
|
Food waste biochar
|
70.9
|
8.42
|
0.14
|
0.58
|
3.73
|
0.55
|
0.11
|
Gupta et al. (2018)
|
Rice waste biochar
|
66.22
|
13.63
|
3.40
|
1.98
|
2.69
|
0.11
|
0.20
|
Gupta et al. (2018)
|
2.1.3 Particle Size Analysis Results
The particle size analysis (PSA) results for the ground biochar are summarized in
Table 3. The data show that over 90 % of the biochar has a particle size of 17.12 µm, while
cement has a size of 19.01 µm, indicating that both materials are ground to a size
below 20 µm. Fig. 4 presents the particle size distribution curves for the ground biochar and cement.
The average particle size was measured to be 10.72 µm for the ground biochar and 12.38
µm for the cement. Therefore, the ground biochar and cement used in the experiment
have similar particle sizes.
Fig. 4. Particle Size Distribution Curve Comparison of Cement and Biochar, (a) Particle Size Distribution of Biochar, (b) Particle Size Distribution of Cement
Table 3. Comparison of Particle Size of Biochar
Specimen
|
D10 (µm)
|
D50 (µm)
|
D90 (µm)
|
Standard deviation (µm)
|
Average (µm)
|
Biochar
|
5.23
|
9.81
|
17.12
|
5.19
|
10.72
|
Cement
|
6.41
|
11.92
|
19.01
|
4.92
|
12.38
|
Wood-based biochar
(Han, 2022)
|
3.47
|
14.21
|
46.64
|
-
|
20.65
|
Mixed wood saw dust biochar
(Gupta et al., 2018)
|
4
|
75.21
|
250
|
-
|
80
|
2.2 Mix Design and Curing
The mortar mix design was set with variables including the biochar content and the
presence of PVA. The amount of cement was reduced proportionally to the biochar content.
The biochar content was varied at 0 %, 1 %, 2 %, 3 %, 4 %, and 5 % by weight of the
cement, and the PVA content was set at 2 % by volume.
The mix proportions are detailed in Table 4. For the BM (biochar-incorporated mortar) mix, the water-to-binder ratio and the
fine aggregate-to-binder ratio were set at 50 % and 300 %, respectively, by weight,
in accordance with KS L ISO 679 (2022). The superplasticizer was used at 0.17 % of the binder content. For the BE (biochar-incorporated
ECC) mix, the water-to-binder ratio and the water-to-fine aggregate ratio were set
at 45 % and 71 %, respectively, referencing the mix ratios from Jeong et al. (2015). Superplasticizer and thickener were used to maintain the workability of the high-toughness
fiber-reinforced cementitious composites and to prevent material segregation.
The biochar cement composite mix involved the sequential addition of stone powder,
river sand, cement, biochar, and water, as illustrated in Fig. 5. First, crushed stone powder, river sand, cement, and biochar were added and dry
mixed for 1 minute. Following this, water and a superplasticizer were added, and the
mixture was wet mixed for an additional 2 minutes.
Fig. 5. Mixing Procedure of Biochar-incorporated Mortar (BM)
Table 4. Mix Design of Biochar-incorporated Moratr (BM) and Biochar-incorporated ECC (BE)
Specimen
|
Cement
|
Biochar
|
Water
|
W/B*
|
Sand Type
|
Admixture
|
SP
|
HPMC
|
Defoamer
|
PVA
|
River
Sand
|
Stone
Powder
|
Silica
Sand
|
Slag
|
Fly Ash
|
BM0
|
450.00
|
0.00
|
225.00
|
0.500
|
990.00
|
360.00
|
-
|
-
|
-
|
0.765
|
-
|
-
|
-
|
BM1
|
445.50
|
4.50
|
0.505
|
BM2
|
441.00
|
9.00
|
0.510
|
BM3
|
436.50
|
13.50
|
0.515
|
BM4
|
432.00
|
18.00
|
0.521
|
BM5
|
427.50
|
22.50
|
0.526
|
BE0
|
500.50
|
0.00
|
375.00
|
0.450
|
-
|
-
|
561.68
|
166.68
|
166.68
|
1.70
|
1.67
|
0.83
|
39.00
|
BE1
|
495.50
|
5.01
|
0.452
|
BE2
|
490.50
|
10.01
|
0.455
|
BE3
|
485.50
|
15.02
|
0.458
|
BE4
|
480.50
|
20.02
|
0.461
|
BE5
|
475.50
|
25.03
|
0.464
|
* Water to Binder Ratio.
For the BE mix, the components were added in the following order: cement, silica sand,
fly ash, blast furnace slag, biochar, water, and PVA, as shown in Fig. 6. Initially, silica sand, ground granulated blast-furnace slag, fly ash, cement, and
biochar were combined and dry mixed for 1 minute. Then, water, a superplasticizer,
a thickening agent, and a defoamer were added, and the mixture was wet mixed for another
2 minutes. Finally, PVA fibers were introduced, and the mixing continued until the
fibers were fully integrated into the mixture.
For BM, after curing in the air for 24 hours, the form is removed. Then, the specimen
is cured in water for 28 days at a temperature of 20 ± 1 ℃. For BE, after curing in
the air for 48 hours, the form is removed. Then, the specimen is cured in water for
28 days at a temperature of 20 ± 1 ℃.
Fig. 6. Mixing Procedure of Biochar-incorporated ECC (BE)
2.3 Specimen Preparation and Testing Methods
To evaluate the mechanical properties of the cement composites, specimens for compressive
strength, tensile strength, and flexural strength were prepared as shown in Fig. 7 Compressive strength specimens were prepared as cubic samples with dimensions of
50 × 50 × 50 mm3 in accordance with KS L 5105 and were measured using a compression
testing machine. Splitting tensile strength tests were conducted by applying a load
to the center of 50 × 50 × 50 mm3 cubic specimens until failure. Flexural strength
tests were conducted on beam specimens with dimensions of 40 × 40 × 160 mm3, prepared
according to KS L ISO 679, and measured using a three-point bending test setup.
Fig. 7. Experimental Test Setups, (a) Compressive Strength, (b) Spliiting Tensile Strength, (c) Flexural Strength
3. Experimental Results
3.1 Results of HS FE-SEM
The morphology of mortar and ECC containing biochar was captured using HS FE-SEM at
a magnification of ×5,000, as shown in Fig. 8. In Fig. 8, it can be observed that the cement compounds are bonded within the porous structure
of the biochar, and these compounds also show good bonding with the PVA fibers.
Fig. 8. SEM of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE) (×5,000), (a) Biochar-incorporated Mortar (BM), (b) Biochar-incorporated ECC (BE)
3.2 Flow Test
The results of the flow tests for BM and BE with varying biochar contents are summarized
in Table 5. The results from Table 5 are graphically represented in Fig. 9.
In BM, the flow decreased as the biochar content increased. This is because the water
absorption of biochar is relatively higher than that of cement, leading to a shortage
of water as the biochar content increases.
In BE, the flow was not affected even with an increase in biochar content. This is
likely because the water-to-sand ratio in the BE mix is low at 71 %, ensuring that
the amount of water used in the ECC mix is sufficient despite the biochar's water
absorption.
Table 5. Flow Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
Specimen
|
Biochar (%)
|
Flow (mm)
|
BM0
|
0
|
168.62
|
BM1
|
1
|
139.66
|
BM2
|
2
|
123.22
|
BM3
|
3
|
120.81
|
BM4
|
4
|
106.65
|
BM5
|
5
|
109.94
|
BE0
|
0
|
207.35
|
BE1
|
1
|
197.67
|
BE2
|
2
|
208.38
|
BE3
|
3
|
196.05
|
BE4
|
4
|
209.42
|
BE5
|
5
|
209.18
|
Fig. 9. Flow Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
3.3 Compressive Strength Test
The biochar-incorporated mortar (BM) exhibited noticeable cracking and eventual failure,
whereas the biochar-incorporated engineered cementitious composite (BE) showed minimal
cracking as seen in Fig. 10. This can be attributed to the crack-mitigating effect of the polyvinyl alcohol (PVA)
fibers present in BE (Jeong et al., 2015; Sunaga et al., 2020).
The results of the compressive strength tests for BM and BE with varying biochar contents
are summarized in Table 6. The ratio column in Table 6 represents the strength compared to the strength at 0 % biochar content. The results
from Table 6 are graphically represented in Fig. 11.
In BM, the compressive strength of BM1 was approximately 4 % higher than that of BM0.
However, as the biochar content increased beyond 1 %, a decreasing trend in strength
was observed. The initial increase in strength at 1 % biochar content is likely due
to the filler effect of finely ground biochar particles, which can enhance the packing
density of the cement matrix. However, as the biochar content increases further, the
reduction in cement content becomes more significant, leading to a decrease in compressive
strength. Additionally, as the concentration of carbon in biochar increases, more
pores are formed in the plane, because carbon naturally has a porous structure. These
pores can act as weak points for the spread of cracks, which is analyzed to contribute
to the decrease in strength as the biochar content increases.
Fig. 10. Destruction mode of the Compressive Strength Experiment, (a) Biochar-incorporated Mortar (BM), (b) Biochar-incorporated ECC (BE)
Table 6. Compressive Strength Results of Biocahr-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
Specimen
|
Biochar (%)
|
Strength (MPa)
|
Ratio
|
BM0
|
0
|
31.89
|
1.00
|
BM1
|
1
|
33.03
|
1.04
|
BM2
|
2
|
30.64
|
0.96
|
BM3
|
3
|
30.30
|
0.95
|
BM4
|
4
|
30.19
|
0.95
|
BM5
|
5
|
26.39
|
0.83
|
BE0
|
0
|
20.71
|
1.00
|
BE1
|
1
|
21.03
|
1.02
|
BE2
|
2
|
32.57
|
1.57
|
BE3
|
3
|
27.06
|
1.31
|
BE4
|
4
|
26.35
|
1.27
|
BE5
|
5
|
29.18
|
1.41
|
Fig. 11. Compressive Strength Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
In BE, the compressive strength at 2 % biochar content was approximately 57 % higher
than that of BE which had 0 % biochar content. The significant increase in compressive
strength is due to the fine biochar particles smaller than 125 µm filling the voids
between the binder materials, acting as a filler. This can improve the uniformity
and compressive strength of the biochar-cement mixture and plays a crucial role in
forming a denser structure (Mosaberpanah et al., 2024). Additionally, it is analyzed that this is due to the dual effect provided by the
reinforcement of PVA fibers. Further density experiments are deemed necessary to validate
these findings. Although there was a slight decrease in compressive strength as the
biochar content exceeded 2 %, the strength enhancement remained notable compared to
BE0, which contained no biochar. This suggests that the fiber reinforcement helps
maintain structural integrity and compressive performance, allowing BE to effectively
incorporate up to 5 % biochar without significant loss in strength. Thus, BE can maintain
and even improve compressive performance while partially replacing cement with biochar.
3.4 Splitting Tensile Strength Test
The splitting tensile test are shown in Fig. 12. Both BM (biochar-incorporated mortar) and BE (biochar-incorporated ECC) samples
exhibited no cracks before the test. However, after the test, both samples failed
at the center. BE showed less severe cracking due to the crack mitigation effect of
PVA fibers compared to BM (Jeong et al., 2015; Sunaga et al., 2020).
Fig. 12. Destruction mode of Splitting Tensile Strength Experiment, (a) Biochar-incorporated Mortar (BM), (b) Biochar-incorporated ECC (BE)
The results of the splitting tensile strength tests for BM and BE with varying biochar
contents are summarized in Table 7, where the ratio represents the strength compared to the strength at 0 % biochar
content. The results from Table 7 are graphically represented in Fig. 13.
Fig. 13. Splitting Tensile Strength Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
Table 7. Splitting Tensile Strength Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
Specimen
|
Biochar (%)
|
Strength (MPa)
|
Ratio
|
BM0
|
0
|
2.65
|
1.00
|
BM1
|
1
|
2.88
|
1.09
|
BM2
|
2
|
2.86
|
1.08
|
BM3
|
3
|
2.72
|
1.03
|
BM4
|
4
|
2.60
|
0.98
|
BM5
|
5
|
2.48
|
0.94
|
BE0
|
0
|
1.97
|
1.00
|
BE1
|
1
|
1.58
|
0.80
|
BE2
|
2
|
3.08
|
1.56
|
BE3
|
3
|
1.80
|
0.91
|
BE4
|
4
|
1.98
|
1.01
|
BE5
|
5
|
2.54
|
1.29
|
In BM, the tensile strength at 1 % biochar content was approximately 9 % higher than
that of BM0. However, as the biochar content increased beyond 1 %, the tensile strength
showed a decreasing trend. This trend suggests that while the initial biochar content
enhances strength due to its filler role, further increases in biochar content result
in a decrease in tensile strength due to the reduction in cement content. Additionally,
as the carbon content in biochar increases, more pores are formed due to its naturally
porous structure, which can weaken the material and reduce strength.
The test results show some variation due to the non-uniformity and heterogeneity within
the mortar's internal structure, as well as the inconsistent dispersion of PVA during
the specimen preparation process. However, despite these variations, BE maintained
higher tensile strength compared to BE0, which contains 0 % biochar. This is attributed
to the filler role of biochar and the reinforcement effect of PVA fibers, which help
maintain structural integrity and tensile performance. Therefore, BE can sustain sufficient
tensile strength while replacing cement with up to 5 % biochar.
3.5 Flexural Strnegth Test
The flexural tests are shown in Fig. 14. Both BM (biochar-incorporated mortar) and BE (biochar-incorporated ECC) samples
failed at the center. However, BE showed reduced cracking due to the crack-mitigating
effect of PVA fibers (Jeong et al., 2015; Sunaga et al., 2020).
The results of the flexural strength tests for BM and BE with varying biochar contents
are summarized in Table 8, where the ratio represents the strength compared to the strength at 0 % biochar
content (BE0). The results from Table 8 are graphically represented in Fig. 15.
In BM, the flexural strength at 1 % biochar content was approximately 12 % higher
than that of BM0. However, as the biochar content increased beyond 1 %, the flexural
strength showed a decreasing trend. This suggests that while the initial biochar content
enhances strength due to its filler role, further increases in biochar content result
in a decrease in flexural strength due to the reduction in cement content. Additionally,
biochar's naturally porous structure leads to increased pore formation with higher
carbon content, weakening the material and reducing its strength.
In BE, the flexural strength at 3 % biochar content was approximately 2 % higher than
that of BE0, which contains 0 % biochar. Although the flexural strength decreased
slightly as the biochar content exceeded 3 %, the results remained comparable to BE0,
indicating that biochar up to 3 % content does not significantly affect flexural performance.
This suggests that the filler role of biochar and the reinforcement provided by PVA
fibers help maintain structural integrity and flexural strength. Therefore, BE can
effectively maintain sufficient flexural strength while replacing cement with up to
3 % biochar.
Fig. 14. Destruction Mode of the Flexural Strength Experiment, (a) Biochar-incorporated Mortar (BM), (b) Biochar-incorporated ECC (BE)
Fig. 15. Flexural strength Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
Table 8. Flexural Strength Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
Specimen
|
Biochar (%)
|
Strength (MPa)
|
Ratio
|
BM0
|
0
|
7.91
|
1.00
|
BM1
|
1
|
8.87
|
1.12
|
BM2
|
2
|
8.21
|
1.04
|
BM3
|
3
|
7.56
|
0.96
|
BM4
|
4
|
7.41
|
0.94
|
BM5
|
5
|
6.00
|
0.76
|
BE0
|
0
|
12.15
|
1.00
|
BE1
|
1
|
12.20
|
1.00
|
BE2
|
2
|
12.10
|
1.00
|
BE3
|
3
|
12.41
|
1.02
|
BE4
|
4
|
10.84s
|
0.89
|
BE5
|
5
|
10.23
|
0.84
|
3.6 Comparative Analysis of BE and BM
The performance of biochar-incorporated mortar (BM) and biochar-incorporated engineered
cementitious composite (BE) was evaluated through a series of mechanical tests, including
compressive strength, splitting tensile strength, and flexural strength. The following
comparative analysis highlights the key differences and advantages of each material
based on the experimental results presented in sections 3.1, 3.2, and 3.3.
3.6.1 Flow
In BM, the flow showed a noticeable decrease as the biochar content increased. This
result is due to the higher water absorption of biochar compared to cement, leading
to a water shortage as the biochar content increases.
Unlike BM, BE did not show a significant impact on flow with increasing biochar content.
This difference can be attributed to the differences in the mix proportions between
BM and BE. While the water-to-binder ratio is about 5 % higher in BM than in BE, the
water-to-sand ratio is approximately 230 % higher in BM. Therefore, the difference
in flow trends between BM and BE is analyzed to be due to the water-to-sand ratio.
3.6.2 Compressive Strength
BM exhibited an initial increase in compressive strength at a 1 % biochar content,
achieving a strength approximately 4 % higher than BM0. However, as the biochar content
increased beyond 1 %, a clear decreasing trend in compressive strength was observed.
This reduction in strength is primarily due to the dilution of cement content, as
biochar replaces cement in the mixture. The initial increase is likely attributed
to the biochar particles' filler effect, enhancing the matrix's packing density.
In contrast, BE showed a substantial increase in compressive strength at 2 % biochar
content, with strength approximately 57 % higher than BE0, which had 0 % biochar content.
This significant enhancement is due to the combined effects of biochar acting as a
filler and the reinforcement provided by PVA fibers, which help maintain structural
integrity. Even as biochar content increased beyond 2 %, BE continued to show notable
strength compared to BE0. BE could effectively incorporate up to 5 % biochar without
a significant loss in compressive strength, demonstrating its ability to maintain
and even improve compressive performance while partially replacing cement.
3.6.3 Splitting Tensile Strength
For BM, the tensile strength at 1 % biochar content was approximately 9 % higher than
that of BM0. Beyond this content, tensile strength decreased, indicating that while
low levels of biochar enhance strength through its filler effect, higher levels reduce
cement content and consequently weaken the material.
BE showed a significant improvement in tensile strength at 2 % biochar content, with
strength about 56 % higher than BE0. Despite variations due to the non-uniform dispersion
of PVA fibers and internal structural heterogeneity, BE consistently maintained higher
tensile strength compared to BE0. The inclusion of PVA fibers helps mitigate crack
propagation, ensuring structural integrity and tensile performance. BE could maintain
sufficient tensile strength with up to 5 % biochar, highlighting its superior performance
in tensile strength compared to BM.
3.6.4 Flexural Strength
In terms of flexural strength, BM demonstrated an increase at 1 % biochar content,
with strength approximately 12 % higher than BM0. However, as biochar content increased
further, a decline in strength was observed, similar to the compressive and tensile
tests. This trend underscores that while biochar initially improves flexural performance,
excessive biochar content diminishes the amount of cement, leading to decreased structural
performance.
BE, on the other hand, showed improved flexural strength at 3 % biochar content, with
strength about 2 % higher than BE0. The presence of PVA fibers significantly reduced
cracking, contributing to the enhanced flexural performance. The flexural strength
of BE remained relatively stable and comparable to BE0, even as biochar content increased
up to 3 %. This stability suggests that BE can incorporate biochar while maintaining
adequate flexural strength, leveraging the filler role of biochar and the reinforcing
effect of PVA fibers.
4. Conclusion
This study evaluated the mechanical properties of biochar-incorporated mortar (BM)
and biochar-incorporated engineered cementitious composite (BE) through compressive
strength, splitting tensile strength, and flexural strength tests. The key findings
are summarized as follows:
(1) BM showed a slight increase in compressive strength at 1 % biochar content, but
strength decreased with higher biochar contents due to reduced cement content. In
contrast, BE demonstrated a significant increase in compressive strength at 2 % biochar
content, approxi- mately 57 % higher than BE0 (0 % biochar). BE maintained or improved
compressive strength with up to 5 % biochar content, thanks to the combined effects
of biochar as a filler and PVA fibers for reinforcement.
(2) BM's tensile strength increased by 9 % at 1 % biochar content but decreased with
further biochar additions due to reduced cement. BE showed a 56 % increase in tensile
strength at 2 % biochar content compared to BE0. Despite some variation due to non-uniform
PVA dispersion, BE maintained higher tensile strength with up to 5 % biochar content,
indicating effective structural integrity and performance.
(3) BM's flexural strength was 12 % higher at 1 % biochar content but decreased at
higher contents due to reduced cement. BE maintained stable flexural strength up to
3 % biochar content, showing a 2 % improvement over BE0. This indicates that BE can
effectively retain flexural performance with increased biochar, leveraging the reinforcing
effects of PVA fibers.
(4) BE outperformed BM in all tested mechanical properties, showing superior strength
and durability. The results suggest that BE is a viable and sustainable construction
material, capable of incorporating biochar as a cement substitute without compromising
structural integrity.
Acknowledgements
This work was supported by the Ministry of Education of the Republic of Korea
and the National Research Foundation of Korea (RS-2023-00248882).
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