An Experimental Study on the Thermal Insulation Properties of Concrete Containing Wood-Based Biochar

Abstract

The applicability of biochar as a coarse aggregate substitute in concrete to increase sustainability and multifunctionality was investigated. Biochar, a porous carbon-rich byproduct from biomass pyrolysis, was incorporated at various replacement ratios (5–20%) under four water-to-binder (w/b) conditions (0.25–0.40). The key physical, mechanical, thermal, and microstructural properties, including the unit weight, porosity, compressive strength, flexural strength, and thermal conductivity, were evaluated via SEM and EDS analyses. The results revealed that although increasing the biochar content reduced the mechanical strength, it significantly improved the thermal insulation performance because of the porous structure of the biochar. At low w/b ratios and 5–10% biochar content, sufficient mechanical properties were retained, indicating a viable design range. Higher replacement ratios (>15%) led to excessive porosity, reduced hydration, and impaired durability. This study quantitatively analyzed the interproperty correlations, confirming that the strength and thermal performance are closely linked to the internal matrix density and porosity. These findings suggest that biochar-based concrete has potential for use in thermal energy storage systems, high-temperature insulation, and low-carbon construction. The low-carbon effect is achieved both by sequestering stable carbon within the concrete matrix and by partially replacing cement, thereby reducing CO₂ emissions from cement production. Moreover, the results highlight a strong correlation between increased porosity, enhanced thermal insulation, and reduced strength, thereby offering a solid foundation for sustainable material design. In particular, the term ‘high temperature’ in this context refers to exposure conditions above approximately 200~400 °C, as reported in previous studies. However, this should be considered as a potential application to be validated in future experiments rather than a confirmed outcome of this study.

1. Introduction

Biochar is a carbon-rich material obtained through the pyrolysis of biomass under oxygen-limited conditions at elevated temperatures. Its intrinsic physicochemical features, such as a porous microstructure, large specific surface area, and notable adsorption capacity, are largely governed by feedstock type and pyrolysis parameters including temperature, heating rate, and gas atmosphere. Because of these unique properties, biochar has been extensively studied in diverse fields ranging from soil improvement and environmental remediation to catalysis, adsorption, and advanced energy storage systems such as supercapacitors [1,2]. Recently, growing attention has been directed toward its use in construction materials, where it has shown the potential to reduce density, enhance thermal insulation performance, and promote carbon sequestration, thereby contributing to sustainable infrastructure development [3,4]. This effect arises both from the stable storage of carbon within the concrete matrix and from the partial replacement of cement, which reduces CO₂ emissions from cement production. Recent efforts to decarbonize cement-based materials have expanded beyond bio-based additives to include a range of low-carbon supplementary cementitious materials (SCMs) and binder concepts that dramatically reduce clinker content. Ref. [5] investigated the use and pretreatment of landfilled coal ash as an alternative SCM, showing how mechanical/chemical pretreatments can recover or enhance ash reactivity for cement blends. Similarly, ref. [6] demonstrated routes for transforming landfilled coal ash into ultra-low-clinker binders, highlighting practical pathways to substantially lower clinker fractions in binder formulations. Together, these studies place biochar-based strategies (this work) within a broader toolkit of low-carbon SCM and ultra-low clinker cement approaches for reducing CO₂ emissions in cement production. With the global demand to address climate change and achieve carbon neutrality, the construction industry is increasingly focusing on innovative material technologies that can simultaneously improve functionality and reduce environmental burdens [7,8]. Against this backdrop, biochar has emerged as a promising alternative material capable of reducing carbon emissions and promoting resource circularity [9,10,11], while recent studies have demonstrated its potential to improve thermal performance, regulate microstructure, and enhance durability in cementitious composites [5,6].

Currently, the insulation materials used in buildings primarily include petroleum-based products such as polystyrene (Styrofoam), polyurethane foam, mineral wool, and glass wool. Most conventional insulation materials are derived from petrochemical sources, resulting in significant carbon emissions during the manufacturing process [12]. In addition, as they are composed of composite materials, they are difficult to separate and recycle, thereby imposing a greater environmental burden upon disposal. Some insulation materials are also highly flammable during production or in the event of a fire, exhibiting rapid combustion rates and the release of toxic gases, which can increase the risk to human life. Consequently, owing to the environmental concerns associated with traditional insulation materials, there is a growing demand for sustainable, eco-friendly alternatives [12,13,14,15,16].

Recent studies on the application of biochar in the construction sector have focused on achieving multifunctionality, including improved thermal insulation performance, reduced density, enhanced carbon storage capacity, and mechanical strength enhancement through the partial replacement of cement [17,18,19,20,21,22,23]. For example, the incorporation of biochar derived from hazelnut and peanut shells into cement paste has been reported to increase the compressive strength by up to 78% while also significantly improving the toughness and fracture energy of concrete [24,25,26]. Similarly, substituting 5% cement by weight with biochar produced from hardwood has been shown to increase the compressive strength, which is attributed to the internal curing effect of the biochar and its influence on the water retention of the mortar matrix [27]. In another study, the addition of wood-based biochar to wood–polypropylene composites improved both flexural strength and fire resistance [28]. In [29,30], charcoal produced from rice husk combustion was added to concrete mixtures, resulting in improved flexural and compressive strengths. Furthermore, ref. [31] investigated the effects of replacing 0–2% of cement by weight with biochar as a filler in concrete exposed to high temperatures. The results demonstrated that biochar contributed to the improved compressive strength and durability of thermally exposed concrete and showed potential as a partial replacement for silica fume.

According to [32], the porous structure of biochar can effectively reduce the thermal conductivity of concrete, thereby contributing to the development of lightweight structural components. In [33], it was experimentally demonstrated that partial replacement of cement with biochar can reduce carbon emissions while maintaining mechanical performance. However, ref. [34] reported that when the replacement ratio exceeds 10%, the connectivity of pores and the inhibition of hydration reactions may lead to reduced strength and durability. In contrast, ref. [35] reported that replacing fine aggregate with biochar at 5–10% content can maintain or even improve the mechanical performance of concrete.

Reference [36] systematically analyzed the effects of variables such as biochar particle size, pyrolysis temperature, and replacement ratio on the thermal conductivity, density, and strength of concrete and emphasized the importance of considering the complex interactions among these factors in mix design. Additionally, ref. [37] highlighted that the anion exchange capacity and porous characteristics of biochar are effective for removing environmental pollutants, and these properties may also positively influence the microstructure formation and hydration process of cementitious matrices. However, many of these studies have focused on single-variable experimental conditions or limited physical properties, which pose limitations in comprehensively evaluating material performance under diverse engineering requirements.

Previous studies have indicated that biochar exhibits very low intrinsic thermal conductivity, which significantly affects the heat transfer characteristics of cementitious materials during hydration and hardening. It has also been reported that the thermal conductivity of air remains low only under specific conditions, such as dry air at ambient temperature and atmospheric pressure. In addition, variations in unit weight, porosity, and permeability are closely related to biochar content, influencing both durability and thermal performance. These findings highlight the necessity of a more comprehensive investigation into the mechanical and thermal behavior of biochar-incorporated concrete.

Therefore, this study quantitatively investigated the combined effects of the biochar replacement ratio and the water-to-binder ratio (w/b) on the physical, thermal, and mechanical properties of concrete under multivariable conditions. The objective was to evaluate the feasibility of applying biochar in both structural and insulation concretes. Many previous studies have investigated the application of biochar in cementitious materials; however, most of them have focused on single-variable experimental conditions or limited physical properties, which restricts a comprehensive understanding of material performance under diverse engineering requirements. In contrast, this study provides a systematic and quantitative evaluation of the combined effects of biochar replacement ratio and water-to-binder ratio (w/b) on multiple performance indicators, including compressive strength, flexural strength, porosity, permeability, unit weight, and thermal conductivity. Furthermore, quantitative correlations among these properties were identified, revealing the interdependent behavior between thermal insulation and mechanical performance. These findings underscore the novelty of this work and provide new insights and practical guidelines for the design of sustainable biochar-based concretes.

In this study, biochar was applied as a coarse aggregate replacement, and the effects of varying water-to-binder ratios and replacement levels were comprehensively evaluated in terms of unit weight, porosity, compressive strength, flexural strength, and thermal conductivity. To complement these evaluations, microstructural and compositional changes were further examined using SEM and EDS analyses. The experimental design featured a multivariable matrix reflecting various engineering conditions, and quantitative correlations between performance indicators were analyzed to identify the optimal design range and limitations of biochar-incorporated concrete. The findings of this study establish a foundation for the practical application of biochar-based concrete in areas such as thermal energy storage, high-temperature insulation, and carbon-reducing structural components. In addition, the results are expected to support both academic research and industrial development of sustainable construction materials by providing empirical data for performance-based material design.

2. Experimental Plan

2.1. Variable Settings and Mix Design

To evaluate the performance development of concrete with varying biochar replacement rates, a total of 16 concrete mixtures were prepared. The water-to-binder (w/b) ratios were set at 0.25, 0.30, 0.35, and 0.40, and the biochar replacement levels were set at 5%, 10%, 15%, and 20%. For the w/b ratio of 0.35, which is commonly used in practice, an additional reference mixture without biochar was prepared for comparison.

The biochar used in this study was produced through pyrolysis of wood-based biomass at a target temperature of approximately 600–650 °C, with a heating rate of about 10 °C/min under a nitrogen atmosphere to ensure oxygen-limited conditions. No chemical activator was employed in this process. The resulting material, sourced from a manufacturing facility in South Korea, exhibited a highly porous structure with excellent adsorption capacity and was sieved to a particle size range of 5–10 mm. Most of the particles were uniformly distributed within this range, with a median size of approximately 7 mm. Its physical properties include a bulk density of 0.78 g/cm³, a fineness modulus of 3.09, and a water absorption rate of 9.91%. The elemental composition of the biochar was determined via X-ray fluorescence (XRF) analysis, and the results are presented in

Table 1. Results of XRF analysis.

Component	Result (Mass%)
C	85.27
O	13.38
Ca	0.55
K	0.52
Si	0.16
Mg	0.11

. The analysis revealed that the biochar primarily consists of carbon (C) and oxygen (O), with minor amounts of inorganic elements such as calcium (Ca), potassium (K), silicon (Si), and magnesium (Mg), confirming its classification as an organic carbon-based material.

In this study, biochar was applied as a substitute for coarse aggregate. The mix proportions are presented in

Table 2. Mix proportion (kg/m³).

Mixture	w/b	Water	Cement	Coarse Aggregate	Biochar (%)	Cohesive Agent	Compaction (MPa)
0	0.35	135	386	1411	0	47.3	1.0
1	0.25	135	540	1219	5	47.3	1.0
2	0.30	135	450	1290	5	47.3	1.0
3	0.35	135	386	1340	5	47.3	1.0
4	0.40	135	338	1378	5	47.3	1.0
5	0.25	135	540	1155	10	47.3	1.0
6	0.30	135	450	1222	10	47.3	1.0
7	0.35	135	386	1270	10	47.3	1.0
8	0.40	135	338	1305	10	47.3	1.0
9	0.25	135	540	1091	15	47.3	1.0
10	0.30	135	450	1154	15	47.3	1.0
11	0.35	135	386	1199	15	47.3	1.0
12	0.40	135	338	1233	15	47.3	1.0
13	0.25	135	540	1027	20	47.3	1.0
14	0.30	135	450	1086	20	47.3	1.0
15	0.35	135	386	1129	20	47.3	1.0
16	0.40	135	338	1160	20	47.3	1.0

. Ordinary Portland cement (ASTM type I, conforming to KS L 5201 standard) was used as the binder, and granite-based crushed coarse aggregate with a nominal maximum size of 16 mm, a specific gravity of approximately 2.65, and a water absorption of about 0.8% was employed. Given the substantially lower density of biochar than of conventional aggregates, it was volumetrically substituted. To improve cohesion between aggregate particles, a cellulose-based thickening agent equivalent to 35% of the mixing water by weight was added. Furthermore, to maximize the porosity of the biochar, fine aggregates were excluded by not using sand in the mix design, and the binder content was minimized to reduce pore filling. A compaction molding process was applied to induce strong interparticle bonding.

2.2. Mixing and Specimen Preparation

The procedures for mixing and specimen preparation are illustrated in Figure 1. Since the mix design aimed to maximize the porous characteristics of biochar, there was concern that cohesion between aggregates might be weakened; therefore, a cellulose-based thickening agent was incorporated. Additionally, the high absorption rate of biochar, due to its porous structure, was anticipated to reduce the workability of the fresh concrete; to compensate for this, a compaction molding process was applied.

Compaction molding was conducted uniformly on both cylindrical and flexural test specimens at a constant surface pressure of 1 MPa. During compaction, no material segregation or particle crushing was observed.

The mixing procedure was carried out in sequential steps to ensure homogeneity. First, all dry materials (cement, coarse aggregate, and biochar) were mixed at low speed for 1 min. Water and the thickening agent were then gradually added over approximately 30 s, followed by high-speed mixing for 1 min. After pausing for 30 s to check the workability of the mixture, a final 1 min of high-speed mixing was performed to complete the process.

The biochar used in this study is porous, and visual inspection revealed partial particle fragmentation during the mixing process. The mixing protocol applied in this work (low-speed mixing for 1 min, two high-speed steps totaling 2 min) was fixed for all batches; we did not perform additional tests with stepwise variation in mixing time or energy to quantify the evolution of fragmentation. Consequently, mixing-induced particle crushing is described here as an observed qualitative tendency rather than a quantified effect. Future work should include time-resolved PSD analysis (e.g., laser diffraction or image-based particle analysis) to determine the quantitative relationship between mixing parameters and particle fragmentation.

2.3. Experimental Methods

To evaluate the applicability of biochar-incorporated concrete, a series of experiments were conducted to assess its physical, mechanical, thermal, and microstructural properties. The physical properties included the unit weight, permeability coefficient, and porosity. The mechanical properties were assessed through compressive and flexural strength tests. The thermal performance was evaluated by measuring the thermal conductivity, whereas the microstructural characteristics were analyzed at the microscale by scanning electron microscopy (SEM). All the measured results were analyzed in detail with respect to variations in the w/b ratio and biochar replacement level. The experimental procedures for each test are illustrated in Figure 2.

Unit weight was calculated by measuring the mass of oven-dried cylindrical specimens (diameter 100 mm × height 200 mm) and dividing it by the specimen volume. For each mix condition, three specimens were tested, and the average values were reported. To minimize errors due to surface irregularities, each specimen was carefully prepared based on the same mold to maintain geometric consistency. Diameters and heights were measured at three different locations using a precision caliper (±0.01 mm), and the mean values were used in the volume calculation. The concrete specimens used in this study exhibited porous structures, which may lead to slight deviations from the assumption of an ideal cylindrical volume. However, as all specimens were fabricated using the same mold and tested under identical conditions, the relative comparisons remain valid. Minor geometric deviations were considered within the acceptable experimental error range.

Porosity was determined in accordance with ASTM C1754 (2016) using cylindrical specimens of 100 mm × 200 mm. The saturated mass, oven-dry mass, bulk volume, and water density were measured, and porosity was calculated based on the standard formula. Three specimens were tested for each condition, and the average values were reported.

Permeability coefficient was evaluated following the Korean Environmental Labeling Standard EL.245 [38]. A custom apparatus was prepared with a 150 mm water head difference between the inlet and outlet. To ensure measurement accuracy, the outer chamber was leveled, and rubber packing and perforated plates were installed. Cylindrical specimens were fixed inside the permeameter, and water was gradually introduced until steady-state outflow was achieved. The outflow volume over 30 s was collected at least three times with a mass cylinder, and the average values were used to calculate the permeability coefficient. For each condition, three specimens were tested to ensure reproducibility.

Compressive strength tests were carried out using a Universal Testing Machine (Shimadzu Corporation, Kyoto, Japan; capacity 2000 kN, load accuracy within ±1%) according to ASTM C39 and KS F 2405. Cylindrical specimens (100 mm × 200 mm) were tested at a loading rate of 0.25 MPa/s. For each mix condition, three specimens were tested, and the average compressive strength was reported. All specimens were dried for one day after curing to ensure consistent conditions. Flexural strength tests were conducted with the same UTM following ASTM C78 and KS F 2408 using prismatic specimens (100 mm × 100 mm × 400 mm) in a three-point bending configuration. A loading rate of 0.06 MPa/s was applied. For each condition, three specimens were tested, and the average flexural strength was reported.

Thermal conductivity was measured in accordance with ASTM D7984-16 using the transient plane source (TPS) technique with a TPS-1500 thermal property analyzer (Hot Disk, Gothenburg, Sweden)). For each measurement, dried specimens with flattened surfaces were placed above and below the sensor and fitted together to ensure intimate thermal contact. The instrument was calibrated according to the manufacturer’s instructions prior to testing. Measurements were carried out under controlled laboratory conditions (ambient temperature 20 ± 1 °C and relative humidity 50 ± 5%). Measurement parameters (measurement duration and sensor settings) followed the instrument manufacturer’s recommendations to avoid sample self-heating and to maintain linear response. Each specimen was measured three times, and the arithmetic mean is reported.

Microstructural analysis was performed using a scanning electron microscope (SEM; SU-8220, Hitachi, Japan) at a magnification of approximately 3000×. Energy-dispersive spectroscopy (EDS) was conducted simultaneously to identify the elemental composition of the observed regions. Representative specimens with varying biochar contents were prepared in an oven-dried state prior to imaging. EDS analysis was conducted in area-scan mode, where rectangular regions of interest were selected to obtain the average elemental composition. For clarity, these regions are explicitly marked with colored boxes in the SEM images.

All the test results were presented as average values obtained from multiple specimens or repeated measurements. Repeated testing under controlled conditions ensured the reproducibility and reliability of the experimental results. The measurement variability was effectively controlled within the allowable range of ASTM and KS standards (±2–5%), ensuring the reliability of the data.

3. Experimental Results and Analysis

The test results for all the samples containing biochar are summarized in

Table 3. Test results of all specimens.

No.	w/b	Contents of Biochar (%)	Compressive Strength (MPa)	Flexural Strength (MPa)	Thermal Conductivity (W/m·K)	Permeability Coefficient (mm/s)	Porosity (%)	Unit Weight (kg/m3)
1	0.25	5	28.2	5.82	1.89	0.08	0.15	2348
2	0.25	10	24.5	5.63	1.62	0.09	0.22	2265
3	0.25	15	22.5	5.40	1.59	0.13	0.48	2168
4	0.25	20	21.5	5.31	1.56	0.28	1.30	2089
5	0.30	5	23.0	5.51	1.82	0.14	0.27	2331
6	0.30	10	21.7	5.22	1.57	0.23	0.38	2224
7	0.30	15	20.8	5.01	1.53	0.34	0.89	2145
8	0.30	20	19.7	4.09	1.51	1.49	1.52	2058
9	0.35	0	27.9	5.60	2.23	0.02	0.27	2395
10	0.35	5	20.5	5.06	1.59	0.36	0.37	2315
11	0.35	10	18.2	4.94	1.50	0.59	0.62	2206
12	0.35	15	15.3	4.44	1.40	0.72	1.16	2135
13	0.35	20	14.6	3.61	1.37	1.99	2.10	2045
14	0.40	5	15.9	4.20	1.54	0.58	0.76	2266
15	0.40	10	15.4	3.97	1.37	0.76	0.97	2184
16	0.40	15	13.8	3.42	1.28	0.87	1.39	2084
17	0.40	20	12.7	3.22	1.17	2.25	2.87	2012

Note. The reported values are means of repeated measurements (typically n = 3). Units are provided in the column headers.

. The key performance indicators include compressive strength, flexural strength, thermal conductivity, the permeability coefficient, porosity, and unit weight.

3.1. Mechanical Properties

Figure 3 presents the compressive strength results as a function of the w/b ratio and biochar content. For the reference mixture with a w/b ratio of 0.35 and no biochar, the compressive strength was 27.9 MPa. As the biochar content increased to 5%, 10%, 15%, and 20% at the same w/b ratio, the compressive strength decreased progressively to 20.5, 18.2, 15.3, and 14.6 MPa, respectively. The lowest strength was recorded at 20% biochar content. These findings indicate that the porous nature of biochar negatively influences compressive strength by increasing the number of internal voids and reducing hydration efficiency.

Similar decreasing trends were observed for the other w/b ratios (0.25, 0.30, and 0.40), with 20% biochar always resulting in the lowest compressive strength. However, in the mixture with w/b = 0.25 and 5% biochar, the highest compressive strength of 28.2 MPa was achieved. This suggests that under low-water conditions, the application of compaction molding can maintain workability while allowing for moderate biochar replacement without compromising strength.

For mixtures with the same biochar content, reducing the w/b ratio from 0.40 to 0.25 led to a consistent increase in compressive strength. A lower w/b ratio increases the binder content and reduces excess water, thereby minimizing capillary pores. This denser matrix improves the mechanical performance by promoting better hydration and particle packing.

Figure 4 shows the flexural strength results for the same variable ranges. The overall trends are consistent with those of the compressive strength. For the reference mixture (w/b = 0.35, 0% biochar), the flexural strength was 5.60 MPa. Increasing the biochar content from 5% to 20% led to a gradual reduction in the flexural strength to 5.06, 4.94, 4.44, and 3.61 MPa, respectively.

At w/b = 0.25, mixes with 5% and 10% biochar had flexural strengths of 5.82 and 5.63 MPa, respectively, both of which exceeded those of the reference mixture. This confirms that when proper compaction is achieved, biochar can be used up to a certain threshold without compromising and potentially even improving its mechanical performance.

Across all biochar contents, higher w/b ratios corresponded to lower flexural strength values. This is attributed to the increased water content leading to more pores and a lower hydration efficiency, thus weakening the structural integrity.

3.2. Thermal Conductivity

Thermal conductivity is a critical property for determining whether the porous characteristics of biochar are effectively imparted to concrete. In this study, the thermal conductivity was measured via a TPS-1500 thermal property analyzer. The results are summarized in Figure 5.

For the reference mix with a w/b ratio of 0.35 and no biochar, the thermal conductivity was measured at 2.23 W/m·K. As the biochar content increased to 5%, 10%, 15%, and 20%, the thermal conductivity decreased progressively to 1.59, 1.50, 1.40, and 1.37 W/m·K, respectively. This significant reduction is attributed to the high porosity of the biochar, which interrupts thermal conduction paths and enhances insulation performance.

Similar trends were observed across other w/b ratios. For example, at a 5% biochar content, the thermal conductivity decreased as the w/b ratio increased from 0.25 to 0.40, with values of 1.89, 1.82, 1.59, and 1.54 W/m·K, respectively. This pattern was also consistent at relatively high biochar contents.

The observed decrease in thermal conductivity with an increasing w/b ratio is attributed to the increase in water content, which in turn promotes the formation of more pores during drying. These air filled pores have very low thermal conductivity and thus reduce the overall heat transfer capacity of the concrete. Literature shows that dry biochar–clay composites exhibit thermal conductivities of about 0.06~0.18 W/m·K under nearly dry conditions at ~20 °C [39], while biochar in unsaturated soils has been reported to show 0.057~0.060 W/m·K under low moisture conditions [40]. These low intrinsic values, together with the porosity introduced by biochar, help explain the observed time-varying decrease in thermal conductivity during hydration and hardening. These findings demonstrate that biochar can be used to enhance the thermal insulation properties of cementitious materials and may be especially valuable in applications requiring energy efficiency or heat shielding.

In addition to reducing the conductive heat transfer pathways, the incorporation of biochar can also influence the radiative and convective components of thermal conductivity. The porous and tortuous structure introduced by biochar interrupts radiative transfer within the pore system, thereby lowering its contribution. As for convection, concrete is composed mainly of closed micro-pores where free convection is negligible, and even with the increased porosity induced by biochar, air movement remains minimal. Consequently, the decrease in thermal conductivity observed in biochar concrete is primarily attributed to reduced conduction, with supplementary suppression of radiation and negligible effects on convection.

3.3. Physical Properties

Figure 6 shows the results of the unit weight measurements according to the biochar content. The unit weight is a key physical parameter of concrete and provides indirect insight into matrix compactness and porosity, which are both closely related to thermal performance. The unit weight was calculated according to ASTM C138/C138M [41]:

$$\mathrm{U}\mathrm{n}\mathrm{i}\mathrm{t} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}=\frac{W}{V}={\displaystyle \frac{W}{\frac{\pi \times d^2}{4}\times h}}$$

(1)

where $W$ is the weight of the cylindrical sample (g), $V$ is the volume (mm³), $d$ is the diameter (mm), and $h$ is the height (mm).

For the reference mixture (w/b = 0.35, 0% biochar), the unit weight was the highest among all samples, measured at 2395 kg/m³. This suggests that a denser matrix structure was formed with minimal internal voids. As the biochar content increased from 5% to 20%, the unit weight decreased to 2315, 2206, 2135, and 2045 kg/m³, respectively. This trend was consistent for all w/b ratios.

The reduction in unit weight can be attributed to the highly porous nature of the biochar, which increases the total internal void volume. For a fixed biochar content (e.g., 5%), the unit weight increased with decreasing w/b ratio (from 0.40 to 0.25), yielding values of 2266, 2315, 2331, and 2348 kg/m³. This is because a lower water content limits capillary pore formation and promotes a more compact and cohesive matrix.

These results confirm that the unit weight is governed by both the quantity of porous material (i.e., biochar) and the densification effect of the matrix (governed by the w/b ratio). The use of biochar results in a lighter concrete mix, which may offer advantages for lightweight construction or insulation applications. Figure 6 is provided to clearly illustrate the variation in unit weight with increasing biochar content under different w/b ratios. This figure emphasizes the overall decreasing trend, which can be directly recognized from the experimental data. While more detailed regression analyses may be conducted in future studies, here the focus is on presenting the observed experimental tendencies in a straightforward manner.

The incorporation of biochar into concrete has a direct influence on permeability and porosity. Figure 7 shows how these properties vary depending on the biochar content and water-to-binder ratio. The permeability coefficient was calculated using Darcy’s law (Equation (2)) [42], whereas the porosity was determined following ASTM C642-13 (Equation (3)) [43]:

$$\mathrm{P}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{c}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}={\displaystyle \frac{L}{h}}\times {\displaystyle \frac{Q}{A(t_2-t_1)}}$$

(2)

where $L$ is the specimen thickness (mm), $A$ is the cross-sectional area (mm²), $h$ is the hydraulic head (mm), $t_1$ and $t_2$ are the start and end times (s), and $Q$ is the volume of discharged water (mm³).

$$\mathrm{P}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}=\left\{1-{\displaystyle \frac{(W_2-W_1)}{{\rho }_w\times V}}\right\}\times 100$$

(3)

where $W_1$ is the submerged weight (g), $W_2$ is the oven-dry weight (g), $V$ is the volume of the sample (cm³), and ${\rho }_w$ is the unit weight of water (g/cm³).

As the biochar content increased, both the porosity and permeability clearly increased. This trend was more pronounced at higher w/b ratios. This increase is attributed to the irregular particle morphology and high porosity of the biochar, which lead to more interconnected voids in the cement matrix. In addition, higher w/b ratios generate excess water that forms additional pores during curing.

In particular, mixtures with biochar contents above 15% presented sharp increases in both permeability and porosity. This suggests the onset of a percolation threshold, where interpore connectivity significantly facilitates fluid transport. A clear correlation between the two variables was observed: higher porosity corresponded to higher permeability, indicating that pore volume and pore connectivity governs permeability behavior.

These results imply that while biochar improves thermal insulation by increasing porosity, it may simultaneously reduce durability by allowing for greater fluid ingress. Therefore, for structural applications, limiting the biochar content to 10% or less is recommended, especially under high-w/b conditions. Conversely, low-w/b mixes may help mitigate the adverse effects of porosity and improve overall performance. Figure 7 illustrates the influence of biochar content and w/b ratio on permeability and porosity. The graphical representation highlights the increasing tendencies of both parameters, allowing readers to intuitively grasp the relationship. Although more advanced statistical regression analyses could be pursued in future work, in this study, the figures are intended to demonstrate the observed experimental trends.

Although BET surface area and pore size distribution were not directly measured in this study, the pore-related characteristics of biochar were indirectly evaluated through the porosity and permeability tests as well as SEM observations. The increase in porosity and permeability with higher biochar content consistently reflected the highly porous and interconnected structure of the material, in agreement with the microstructural images. These indirect indicators, together with literature findings, confirm that the pore structure of biochar played a decisive role in governing both the thermal insulation properties and the mechanical performance of the concrete.

3.4. Correlation Between Properties

3.4.1. Compressive Strength vs. Thermal Conductivity

Thermal conductivity is a key indicator of the insulation and thermal response performance of concrete containing biochar, as it is closely associated not only with insulation performance but also with the thermal response characteristics of structural elements. Therefore, in this study, a detailed correlation analysis between the thermal conductivity and mechanical properties was conducted to examine the coupled behavior of the material.

Figure 8 presents the relationship between compressive strength and thermal conductivity, revealing a generally proportional increase in compressive strength with increasing thermal conductivity. This trend suggests that higher thermal conductivity may be attributed to enhanced structural continuity and compactness within the concrete, which are factors contributing to the development of mechanical strength.

Given that the thermal conductivity of air is relatively low (approximately 0.02 W/m·K under dry conditions at 20~25 °C and atmospheric pressure), the porous nature of biochar contributes to increased internal porosity in concrete as the biochar content increases. The increased porosity impedes heat conduction pathways, thereby reducing the thermal conductivity. Simultaneously, this structural looseness decreases the density of the concrete matrix, leading to reduced compressive strength. In particular, air entrapped in pores acts as a barrier to heat transfer at elevated temperatures, which is advantageous from an insulating perspective.

The coefficient of determination (R²) was calculated as 0.834, indicating a strong correlation between compressive strength and thermal conductivity. This suggests that the thermal and mechanical properties are not independent but rather exhibit interdependent behavior, where improvement in one may positively affect the other. However, this correlation should be interpreted as an indirect effect mediated primarily by porosity and unit weight rather than a direct causal link. The observed association reflects the fact that increased porosity simultaneously reduces both unit weight and compressive strength while lowering thermal conductivity. This also emphasizes the necessity of setting an appropriate biochar content to prevent significant degradation in material performance. Although a polynomial fit was used to represent the trend, it should be noted that higher-order regression models provide limited practical guidance and should be interpreted cautiously.

3.4.2. Unit Weight vs. Thermal Conductivity

Figure 9 shows the correlation between the unit weight and thermal conductivity of the biochar-containing concrete. As the thermal conductivity increases, the unit weight also tends to increase. This correlation is attributed primarily to the variation in the material composition and compactness of the concrete matrix.

Biochar has a significantly lower density than conventional aggregates because of its porous structure. Consequently, an increase in biochar content reduces the overall unit weight of concrete. This reduction indicates a rise in internal porosity, with a greater proportion of low-density air or voids within the matrix. Since these voids possess low thermal conductivity, the increase in porosity due to higher biochar content leads to a reduction in overall thermal conductivity.

The coefficient of determination (R²) between the unit weight and thermal conductivity was calculated to be 0.702, indicating a relatively strong correlation. This outcome implies that both properties are influenced by the internal microstructure and density of the concrete. Moreover, changes in biochar content simultaneously and interactively affect both thermal and physical characteristics. As thermal conductivity and unit weight are critical factors influencing insulation performance and structural stability, respectively, their correlation is an essential consideration when optimizing concrete mix designs. This finding reinforces the interpretation that porosity is the dominant parameter governing both mechanical and thermal properties. The strong association between unit weight and thermal conductivity thus provides a more reliable basis for evaluating the thermal performance of biochar-incorporated concretes than compressive strength alone. Similarly, while polynomial regression was applied to visualize the overall trend, the limitations of higher-order fitting must be acknowledged, and the results should be interpreted with caution.

4. Microstructural Analysis

SEM images of specimens with 0%, 10%, and 20% biochar contents were obtained at 3000× magnification (Figure 10). The micrographs clearly reveal increased porosity with higher biochar incorporation. The rectangular boxes in the SEM images indicate the regions where EDS area scans were performed, ensuring a direct correspondence between observed microstructural features and compositional data.

SEM examination revealed distinctive morphologies that corroborate the EDS results. The dominant matrix phase appears as a poorly crystalline, plate-like to fibrillar gel, which we attribute to C–S–H (calcium–silicate–hydrate); these regions exhibit Si and Ca signals in area-scan EDS. Discrete tabular crystals with sharper edges were observed and interpreted as Ca(OH)₂ (portlandite), showing strong Ca signatures in the corresponding scans. Irregular, porous fragments with low backscatter contrast were identified as biochar particles; these fragments display sponge-like surface textures and are carbon-rich in area-scan analyses. Pores and voids appear as dark, signal-deficient regions, commonly located adjacent to biochar inclusions or agglomerates. The EDS area-scan data reported in this manuscript are semi-quantitative and serve to corroborate these morphological identifications.

The reference sample (0% biochar) exhibited a relatively dense platy and granular microstructure with localized microcracks and pores, likely resulting from heterogeneous hydration and internal stresses. EDS analysis for this sample indicated major inorganic elements in the order of O (42.8 wt%), Si (15.5 wt%), and Fe (12.5 wt%). In contrast, samples containing 10% biochar displayed irregular pore networks and loosely packed agglomerates, likely due to the inclusion of biochar and an associated retardation of hydration. The carbon content increased to 23.6 wt%, while the concentrations of inorganic elements such as Si and Ca decreased, suggesting dilution by the organic biochar and partial inhibition of hydration-product formation.

The sample with 20% biochar exhibited the most pronounced structural transformation, with an extremely porous and heterogeneous microstructure evident in the SEM images. This was attributed to severe inhibition of the hydration process due to the high biochar content and poor dispersion of biochar aggregates within the cement matrix. EDS analysis revealed a drastic increase in the carbon content to 72.8 wt%, whereas the oxygen content decreased to 18.8 wt%, the silicon content to 0.4 wt%, and the aluminum content to 0.3 wt%, indicating minimal formation of inorganic hydration products.

Overall, the results confirm that increasing the biochar content leads to a more porous and nonuniform microstructure, along with a rapid shift in chemical composition toward carbon-rich organic matter. While incorporation below 10% may provide advantages in terms of pore regulation and carbon sequestration, excessive contents (e.g., 20%) significantly inhibit hydration and reduce both physical and mechanical performance. Thus, careful control of biochar dosage is essential for maintaining concrete quality.

Although SEM analysis of pure biochar particles was not performed in this study, the pore characteristics of biochar were indirectly assessed through porosity and permeability measurements, unit weight reduction, and the microstructural changes observed in biochar-incorporated concretes. These indirect indicators consistently confirmed the highly porous nature of biochar, which is in good agreement with previously published SEM images of wood-based biochar reported in the literature. This limitation has been acknowledged in the revised manuscript, and the inclusion of direct SEM analysis of biochar will be considered in future work to further strengthen the microstructural evaluation.

5. Conclusions

The potential use of porous biochar in concrete was explored by experimentally evaluating its physical, mechanical, thermal, and microstructural properties. The key findings are as follows:

• Increasing the biochar content led to an overall reduction in the compressive and flexural strengths, with the most significant decrease observed at 20% biochar. This was attributed to the inhibition of hydration and structural discontinuity caused by the porous nature of the biochar. However, at a water-to-binder ratio (w/b) of 0.25, biochar contents ranging from 5 to 10% resulted in greater strength than that of the control, suggesting that sufficient strength can be maintained with proper content selection.
• The thermal conductivity effectively decreased with increasing biochar content. At 20% incorporation, the thermal conductivity decreased to 1.37 W/m·K from 2.23 W/m·K in the control mixture. This reduction, caused by the disruption of heat transfer paths due to porosity, demonstrates enhanced insulation properties suitable for nonstructural elements. The potential use in high-temperature applications requires further investigation and should be considered as a direction for future research rather than a definitive conclusion. Specifically, the ‘high temperature’ mentioned in this study refers to exposure conditions above approximately 200~400 °C, based on the previous literature. As this study did not directly evaluate performance under such conditions, this statement remains a suggestion for future research directions.
• Both porosity and permeability increased with increasing biochar content, especially above 15%, indicating a transition to a percolation threshold where pore connectivity becomes significant. This suggests potential durability issues due to increased fluid permeability; thus, the biochar content should be limited to 10% for structural applications.
• Strong correlations were found between thermal conductivity and both the compressive strength and unit weight, implying that porosity and density jointly influence the thermal and mechanical performance. These interrelations highlight the need for balanced design strategies to optimize both insulation and structural functions.
• SEM and EDS analyses confirmed that higher biochar contents lead to more porous and heterogeneous microstructures, with a marked decline in inorganic hydration products and a surge in the carbon concentration. In particular, at a 20% biochar content, hydration was severely impeded, which could significantly degrade the material performance.

An appropriate biochar dosage is therefore crucial for ensuring functional concrete properties. The findings of this study provide fundamental data for evaluating the applicability of biochar as a construction material and are expected to serve as a valuable reference for the future development of biochar-based sustainable building materials. Overall, this study makes a novel contribution by presenting a systematic and quantitative evaluation of how biochar content and the water-to-binder ratio jointly affect the thermal, physical, and mechanical properties of concrete. By identifying clear correlations among these properties, this research provides new insights into optimizing biochar dosage to balance strength, durability, and insulation performance. These findings extend beyond conventional single-variable studies and offer practical guidelines for the development of multifunctional and sustainable concretes incorporating biochar.