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  1. Member · Ph.D Candidate · Gyeongsang National University (kimsangwoo@gnu.ac.kr)
  2. Master Course · Gyeongsang National University (wodnjs0004@naver.com)
  3. Master Course · Gyeongsang National University (csc758@naver.com)
  4. Member · Associate Professor · Gyeongsang National University (Corresponding Author · jinsup.kim@gnu.ac.kr)



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
../../Resources/KSCE/Ksce.2024.44.5.0615/fig1.png

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
../../Resources/KSCE/Ksce.2024.44.5.0615/fig2.png
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
../../Resources/KSCE/Ksce.2024.44.5.0615/fig3.png
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
../../Resources/KSCE/Ksce.2024.44.5.0615/fig4.png
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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig5.png
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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig6.png

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
../../Resources/KSCE/Ksce.2024.44.5.0615/fig7.png

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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig8.png

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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig9.png

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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig10.png
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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig11.png

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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig12.png

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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig13.png
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)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig14.png
Fig. 15. Flexural strength Results of Biochar-incorporated Mortar (BM) and Biochar-incorporated ECC (BE)
../../Resources/KSCE/Ksce.2024.44.5.0615/fig15.png
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).

References

1 
"Brewer, C. E. (2012). “Biochar characterization and engineering.” Ph.D. Thesis, Iowa State University, Ames, Iowa, USA."URL
2 
"Choi, W. C., Yun, H. D. and Lee, J. Y. (2012). “Mechanical properties of mortar containing bio-char from pyrolysis.” Journal of the Korea Institute for Structural Maintenance and Inspection, Vol. 16, No. 3, pp. 67-74."DOI
3 
"Gupta, S., Kua, H. W. and Koh, H. J. (2018). “Application of biochar from food and wood waste as green admixture for cement mortar.” Science of the Total Environment, Vol. 619-620, pp. 419-435, https://doi.org/10.1016/j.scitotenv.2017.11.044."DOI
4 
"Gupta, S. and Kua, H. W. (2019). “Carbonaceous micro-filler for cement: Effect of Particle size and dosage of biochar on fresh and hardened properties of cement mortar.” Science of the Total Environment, Vol. 662, pp. 952-962, https://doi.org/10.1016/j.scitotenv.2019.01.269."DOI
5 
"Han, S. M. and Choi, W. C. (2023). “Evaluation of the mechanical properties of cement mortar conatining wood-based bio-char.” Journal of the Korea Concrete Institute, Vol. 35, No. 3, pp. 285-292, https://doi.org/10.4334/JKCI.2023.35.3.285 (in Korean)."DOI
6 
"Javed, M. H., Sikandar, M. A., Ahmad, W., Bashir, M. T., Alrowais, R. and Wadud, M. B. (2022). “Effect of various biochars on physical, mechanical, and microstructural characteristics of cement pastes and mortars.” Journal of Building Engineering, Vol. 57, No. 1, 104850, https://doi.org/10.1016/j.jobe.2022.104850."DOI
7 
"Jeong, Y. S., Kwon, M. H. and Seo, H. Y. (2015). “Torsional behavior of beams retrofitted by PVA-ECC.” Journal of the Korean Society for Advanced Composite Structures, Vol. 6, No. 1, pp. 30-37, http://dx.doi.org/10.11004/kosacs.2015.6.1.030."DOI
8 
"Kim, S. W., Jeong, G. H., Hong, Y. J. and Kim, J. S. (2024). “Compressive strength characteristics of concrete and mortar according to wood-based biochar replacement ratio.” Journal of the Korea Society for Advanced Composite Structures, Vol. 15, No. 1, pp. 18-25, https://doi.org/10.11004/kosacs.2024.15.1.018 (in Korean)."DOI
9 
"KS L ISO 679 (2022). “Cement-test methods-determination of strength.” Korea, PA: Korea Agency for Technology and Standards and Korea Standards Association (in Korean)."URL
10 
"KS L 5105 (2022). “Test method for compressive strength of hydraulic cement mortar.” Korea, PA: Korea Agency for Technology and Standards and Korea Standards Association (in Korean)."URL
11 
"Lim, H. S., Choi, Y. S. and Cho, H. K. (2023). “Experiment study of road base materials properties using recycled aggregates with additives type and mix method.” Journal of Korean Society for Advanced Composite Structures, Vol. 14, No. 6, pp. 57-62, https://doi.org/10.11004/kosacs.2023.14.6.057 (in Korean)."DOI
12 
"Malkow, T. (2004). “Novel and innovative pyrolysis and gasification technologies for energy efficient and environmentally sound MSW disposal.” Waste Management, Vol. 24, pp. 53-79, https://doi.org/10.1016/S0956-053X(03)00038-2."DOI
13 
"Mosaberpanah, M. A., Olabimtan, S. B., Balkis, A. P., Rabiu, B. O., Oluwole, B. O. and Ajuonuma, C. S. (2024). “Effect of biochar and sewage sludge ash as partial replacement for cement in cementitious composites: Mechanical, and durability properties.” Sustainability, Vol. 16, No. 4, 1522, https://doi.org/10.3390/su16041522."DOI
14 
"Mrad, R. and Chehab, G. (2019). “Mechanical and microstructure properties of biochar-based mortar: An internal curing agent for PCC.” Sustainability, Vol. 11, No. 9, 2491, https://doi.org/10.3390/su11092491."DOI
15 
"Ruziev, J., Lee, J. Y., Lee, S. J. and Kim, W. S. (2023). “Characteristics of biochar based on its carbonization degree.” Journal of the Korean Society for Advanced Composite Structures, Vol. 14, No. 6, pp. 10-18, https://doi.org/10.11004/kosacs.2023.14.6.010 (in Korean)."DOI
16 
"Scrivener, K. L. and Kirkpatrick, R. J. (2008). “Innovation in use and research on cementitious materials.” Cement and Concrete Research, Vol. 38, pp. 128-136, https://doi.org/10.1016/j.cemconres.2007.09.025."DOI
17 
"Song, J. K., Yang, K. H. and Song, K. I. (2017). “Importance and characteristics of geopolymer concrete technology, magazine of RCR.” Journal of the Korean Recycled Construction Resources Institute, Vol. 12, No. 1, pp. 8-15, https://doi.org/10.14190/MRCR.2017.12.1.008 (in Korean)."DOI
18 
"Spokas, K. A. (2010). “Review of the stability of biochar in soils: predictability of O: C molar ratios.” Carbon Management, Vol. 1, No. 2, pp. 289-303, https://doi.org/10.4155/cmt.10.32."DOI
19 
"Suarez-Riera, D., Lavagna, L., Carvajal, J. F., Tulliani, J., Falliano, D. and Restuccia, L. (2024). “Enhancing cement paste properties with biochar: Mechanical and rheological insights.” Applied Sciences, Vol. 14, No. 6, 2616, https://doi.org/10.3390/app14062616."DOI
20 
"Sunage, D., Namiki, K. and Kanakubo, T. (2020). “Crack width evaluation of fiber-reinforced cementitious composite considering interaction between deformed steel rebar.” Construction and Building Materials, Vol. 261, 119968, https://doi.org/10.1016/j.conbuildmat.2020.119968."DOI
21 
"Tan, K., Qin, Y., Du, T., Li, L., Zhang, L. and Wang, J. (2021). “Biochar from waste biomass as hygroscopic filler for pervious concrete to improve evaporative cooling performance.” Construction and Building Materials, Vol. 287, 123078,https://doi.org/10.1016/j.conbuildmat.2021.123078."DOI
22 
"Zhao, S., Huang, B., Shu, X. and Ye, P. (2014). “Laboratory investigation of biochar-modified asphalt mixture.” Journal of the Transportation Research Board, Vol. 2445, No. 1, pp. 56-64, https://doi.org/10.3141/2445-07."DOI