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.

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)

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

3.1 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.

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)

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)

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)

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.