Investigation the effects of adding biochar to concrete mixture experimentally on performance propertiesABSTRACT
The global release of carbon dioxide due to human activity raises concerns about the planet's future. These days, the construction sector accounts for a significant portion of emissions; energy use in the sector accounts for roughly 12% of overall emissions, and the cement sector is responsible for 8% of world CO2 emissions. Alternative elements are now used in cement compositions to reduce their negative effects on the environment. Fly ash, a waste product from companies that burn coal, is one substance that is frequently employed. Nonetheless, the overall emissions from the production of concrete may be significantly reduced by a product that is renewable and sinks carbon emissions. Biochar, a byproduct of the pyrolysis of biomass, is one product that could be appropriate for this. In order to assess the performance attributes, concrete containing biochar ratios of 4, 8, and 12 percent based on the cement weight was examined. In order to test this, 100 mm cubes were cast and their compressive strength was measured after 28 and 56 days in the lab. The qualities of workability, density, microstructure, and chemical composition were also assessed. The outcome demonstrated that concrete mixes containing biochar had much less workability. Additionally, the density dropped as the biochar ratio mixture. The similar thing occurred in terms of compressive strength; after 28 days, the sample containing 12 percent had a compressive strength of 48 MPa whereas the comparative species had a compressive strength of 69 MPa. After an additional 28 days of curing, the sample with 12% biochar showed the greatest gain in compressive strength. The samples with and without the addition of biochar did not exhibit any discernible differences in the FTIR.
Keywords:
Biochar; Concrete mixture; Density; Workability; Microstructure; Compressive strength
1. INTRODUCTION
The construction sector has been identified as a significant source of emissions, accounting for around 40% of worldwide emissions from process and industry. The energy used in building construction is directly responsible for 12% of the total emissions [1]. This includes the production of construction materials, which accounts for the majority of emissions overall, like steel and cement. The increased use of concrete has compelled producers to create more environmentally friendly construction materials [2]. Research studies have been conducted recently to substitute a portion of the binders in concrete with various waste materials, including waste glass, waste husk ash, and trash in general. Recently, research has been conducted in which biochar has been used in place of the binder in concrete [3]. According to earlier research, biochar has been used in place of cement in amounts ranging from 0.09 to 4 percent, and even up to 10 and 18 percent. Even the structural elements of the structure might store CO2 thanks to the inclusion of biochar to the building materials. According to CHOI et al. [4], concrete containing one kilogramme of biochar may retain around 2.5 kilogrammes of CO2 emissions. It has been discovered, nonetheless, that other characteristics than carbon emission retention are valuable for uses in the production of concrete. It has also been discovered that characteristics like high porosity and high specific surface area are appropriate. The majority of earlier research has concentrated on charcoal rather than concrete. As a result, the creation of biochar has taken precedence over the actual manufacture of concrete. This study looks at how biochar affects the performance characteristics of concrete in an effort to address that [5]. The availability of cement is limited, yet it is necessary for all current and upcoming infrastructure project production. A significant amount of cement is consumed. Despite being necessary for the creation of concrete, there is currently no substance that can substitute clinker and cement in the manufacturing process [6]. There is no set amount for the actual emissions associated with one tonne of cement. However, it was said that the production of one tonne of Portland cement resulted in the emissions of 0.91 tonnes of carbon dioxide. YANG et al. [7] provided figures for the amount of CO2 emitted per tonne of cement. According to YANG et al. [7], 0.89 tonnes of CO2 are produced for every tonne of cement. An accurate level of emissions is estimated by taking the mean of these data. The mean value is used to compute the 0.95 tonnes of carbon emissions per tonne of cement. According to a life cycle analysis of biochar produced in the Philippines by BREWER et al. [8], adding biochar to concrete mixes instead of cement could improve the impact on the environment and human health with regard to hazardous waste, ozone formation and depletion, and toxicity in water, soil, and air. Additionally, evidence showing potential decreases in the effects of global warming is given. Additionally, the thesis proposed that fly ash might be substituted with biochar. The materials life cycle, from the point at which it leaves the manufacturer to the point at which it enters the factory gate, was analysed from cradle to gate. The study’s processes include raw material acquisition, material processing, manufacturing, and transportation. Glass, plastic, and leftover materials from building and demolition might all be used in place of cement. Biochar and other renewable biobased products might also be used as a substitute product. Recent studies in the field have examined the use of biochar as a filler or as a partial replacement for cement [9]. This effort aims to assess the unique characteristics of concrete when biochar is added in greater amounts in place of cement. It has been observed that research on higher replacement ratios of biochar in concrete (>10%) is lacking. The objective is to conduct research on the impact of biochar on the mechanical characteristics of concrete when a greater proportion of cement is substituted during the casting process. This is significant from an environmental standpoint since it will increase the sustainability of the concrete when more biochar is used in place of cement. Based mostly on laboratory experiments, the paper examines the mechanical alterations in concrete caused by the addition of biochar.
2. MATERIALS AND METHODS
2.1. Material
Fly ash and limestone are added to “anläggningscement FA” from Cementa, a type of cement in which the clinker ratio has been reduced to 80% for use in the concrete mixture. The cement has a delayed strength development as compared to bas-cement and SH-cement, and a life cycle analysis shows that it emits 22% less carbon dioxide than regular Anläggningscement. Additionally, Skanska’s source in Sunderbyn is used for the 0–10 mm and 10–20 mm aggregate. Master Glenium is the fluid media that is employed. In addition, logging waste biochar is utilised.
2.2. Preparation of the material
With an accuracy of 0.02 kg, the weight of the aggregates needed for the batches was measured using an AE Adam warrior scale. To guarantee uniformity, each specimen was mixed individually in a concrete mixer for a few minutes, allowing the humidity of the two distinct types of aggregate to be ascertained. The aggregate’s relative humidity is determined using
2.3. Mixing and casting of concrete
The fine aggregate was added to the bottom of the mixer before the dry materials, cement, and finally the 10–20 mm aggregate. After the dry components were quickly combined, it was evident that the quantity had shrunk too tiny to fit in the mixer, which prevented the mixture from happening correctly. After one minute of mixing the dry materials, 90% of the fluid medium was added to the water and the mixture was stirred for a further three minutes, during which time the viscosity of the concrete was rather fluid. While the concrete toughened more, it was given some time to rest. After adding the remaining fluid medium, the mixer ran for an additional 35 seconds or so.
2.4. Slump test
The freshly laid concrete underwent a slump test. Since Figure 1 illustrates the self-compacting characteristic of the control sample, the spread diameter is measured at the location with the greatest diameter.
2.5. Cracking
An image of each tested cube after 28 days is included in which shows the cracking resulting from the compressive strength test. The cubes exhibit a comparable pattern of breaking. The form could be more clearly seen once the loose concrete was taken out of the cubes. Each batch’s sample is shown in Figure 2 below.
There were fractures on the top and bottom of a few of the cubes. Figure 3 shows an example of the cracks that developed on the same side that the machine had compressed. Since the fractures might be difficult to discern in some circumstances, it is impossible to evaluate how the 28-day extensional curing period altered the crack pattern.
2.6. Fourier-transform infrared
After the samples were tested, data was gathered, translated to Excel, and shown as graphs. The most intense result from each sample is gathered in Figure 4 below. There is a little variation of around 2900 cm−1. The organic content is most likely to blame for this, as the 2900 cm−1 fits the C-H stretching vibrations. Apart from this, no clear divergencies between the samples were seen, suggesting that the bonding between the concrete with and without the addition of biochar is the same. You can observe the tallest peak at around 950–1000 cm−1.
2.7. Scanning electron microscope
The microstructure of the concrete samples was examined using a Hitachi TM 100 desktop scanning electron microscope (SEM) operating at 22 kV and 10 μA current. Images of the biochar were taken prior to its integration with the concrete. According to the biochar’s SEM pictures, the particles’ average size was around 100 µm, and some of them possessed porous structures, which are characteristic of biochar made from wood. One crucial step that affected the performance characteristics of cement is its hydration. In the compression strength tests, it was shown that the samples’ strength rose in 56 days as opposed to 28 days as the amount of biochar increased. It follows that it is likely that hydration persisted after 28 days. According to the SEM micrographs, the control sample showed variation in the degree of hydration, as shown by the various phases; in contrast, the biochar-containing samples showed less variation, with the 12% sample having a homogeneous matrix [10]. It’s likely that the samples with more biochar held onto water in their pores, allowing the concrete to continue to hydrate and cure over an extended length of time (Figure 5).
3. EXPERIMENTAL PROCEDURE
An experimental investigation is conducted to investigate the consequences of substituting the cement in concrete with biochar. In contrast to earlier research that substituted lesser percentages of cement (0.2–10 percent), the goal of this study is to learn more about the potential effects of adding higher amounts of biochar on the structural characteristics of the concrete. This is predicated on the idea that the product will have lower CO2 emissions the more cement that can be removed from the concrete. However, larger constructions must be able to use the material if biochar-concrete is to truly have an influence on the climatic issues we are now experiencing. The production of concrete has certain specifications. Regulations from the National Board of Housing, Building and Planning state that anybody creating concrete for use in building projects needs to have formal training in the field or experience casting and manufacturing concrete. On the other hand, production needs to be carried out in compliance with European requirements. It is necessary to ascertain the material qualities in order to compute and design the structural element. To apply concrete in constructions, a number of its qualities must be established. Workability, density, compressive strength, chemical characteristics, and microstructure are the attributes that will be determined in this work after a selection process.
4. RESULTS AND ANALYSIS
It is evident from the outcome that the replacement ratio has an impact on the workability, density, and compressive strength of concrete.
4.1. Workability
The addition of biochar has a significant impact on the workability. This most likely has something to do with biochar’s ability to absorb water. Higher replacement ratios of 8 and 12 percent of cement cause the concrete to be regarded as dry due to the impact on workability. Since the strength is correlated with the water to cement ratio, the addition of water to the mixture during this investigation may have contributed to the mixture’s decreased compressive strength. As SIRICO et al. [11] conducted in their experimental test where the super plasticiser was increased with the increased replacement ratio, there may have been an addition of the fluid medium, masterglenium. For batches 1 and 2, the water to cement ratio was 0.40 (0 and 4% ratio), while for batches 3 and 4, the ratio was 0.44 (8 and 12% replacement ratio). The dry bulk density of the biochar utilised in this study was 0.208 g/cm3. The volume of biochar increased significantly in comparison to the volume of cement when the cement was substituted based on its weight. This could also affect the mixture’s workability because it would be expected that a higher percentage of tiny fraction material would result in a thicker consistency. The real water to cement ratio is being somewhat altered by the k factor. Where factor 0.5 is employed in this thesis, adding the k-factor permits us to add a little more water to the biochar.
4.2. Density
As the ratio of biochar rose, a drop in density was seen. As previously indicated, biochar has a density of 1.50 ± 0.50 g/cm3, which is rather low when compared to cement. According to the results of DIXIT et al. [12], which were found to be 0.38 ± 0.02 g/cm3, the density of the cement is 4.1 times that of the biochar. In contrast, the density of the biochar used in this work was 0.145 g/cm3. This thesis’s outcome then differs from the one Dixit et al. (2021) stated. According to the study by DOWNIE et al. [13], the density dropped by almost 5% in the mixture with 12% replacement and by approximately 3.0% in the concrete with a 12% replacement ratio. This might imply that additional elements other than the biochar’s density are crucial to the density dropping. In the case of concrete manufacture, where lower loads should result in less energy consumption from transportation, the drop in density is advantageous. Additionally, buildings with reduced Eigen weight might be constructed to be less heavy (Table 1).
It is evident that adding biochar to a portion of the cement lowers its density. When replacing up to 8% of the original density, the decrease can be viewed as linear. Between eight and twelve percent, there is a negligible shift. Figure 6 below shows a data figure; the numbers have been interpolated to produce a continuous value figure.
As the biochar ratio is raised, the change in density is seen to diminish. The results obtained from 28 to 56 days of curing are consistent with one another, suggesting that the additional curing should not have a significant impact on density. The difference in bulk density between cement and biochar may be caused by the latter’s low bulk density (Sirico et al., 2021). Table 2 below shows the percental change. This number was lower than that of earlier research by GUPTA et al. [14], where a replacement ratio of 12% led to a decreased density of around 6%. The variation may arise from the fact that the density of the biochar varies depending on the pyrolysis procedure and source employed.
4.3. Compressive strength
With the results from the failure load, which are shown in Table 2, and the measurements of the compressed area, the compressive strength is computed using equation [2]. Table 2 presents the computed compressive strength.
For the cube strength, a categorisation might be established. The categorisation cannot be done in accordance with Eurocode as it is unknown which cylindrical strength the samples attain. This Figure 7 shows how the addition of biochar affects compressive strength. In addition, to illustrate the distinction between 28 and 56 days of healing. It should be noted that the ratio decreases as replacement increases, resulting in a greater drop at larger replacement ratios [15].
It is evident from the data that strength decreases as the percentage of biochar increases. It is also evident that the strength is growing rapidly after an extra 28 days. This is seen in Figure 8 down below as well.
It is evident that the combination containing 12 percent biochar showed a 16 percent improvement in compressive strength, compared to a 10 percent rise for the control sample. This is in line with the literature’s assertion that biochar-containing concrete undergoes a second curing process. The chart illustrates that a 3% increase in strength was observed between the 5 and 8 percent replacement ratio, and a 4% rise was observed between the 8 and 12 percent replacement ratio. According to the literature review, the compressive strength decreases as the biochar ratio rises. This thesis did not find a 5% replacement, despite some earlier research suggesting that this may result in a greater strength. Figure 8 illustrates how the change in reduction increases when more biochar is substituted. It’s important to keep in mind that the water to cement ratio was raised between five and eight percent, which is probably why the compressive strength was affected. The findings of biochar extracted from hardwood feedstock show that the values are equivalent when compared to those found in other research (Table 2).
This work did reveal a decrease with the substitution of 5%, however the decline may be regarded as slight. The SEM results support the notion that larger concentrations of biochar improve hydration since they show a more homogeneous matrix in the biochar-containing samples. Because of this, water may be retained in the pore structure of biochar, which prolongs the cement’s curing process. It is challenging to compare the findings of this thesis to the literature on compressive strength since it varies so much throughout research. The results indicated that a decrease in compressive strength was anticipated. Few research have been conducted to investigate the effects of more than 28 days, which raised doubts about the expected outcomes. A second treatment was mentioned in a few research, however those investigations used lesser amounts of biochar [16].
4.4. Crack pattern
It would have been possible to analyse the crack pattern in greater depth. This might have been used to compare the sample with and without biochar to determine whether there were any changes. Also, by comparing more samples, it would have been possible to look at the form of the concrete that remained after compressive testing.
4.5. FTIR
The characteristics of the concrete with the addition of biochar were not affected by any chemical linkages that we were able to identify from the FTIR. In trials with smaller percentages of biochar substitution, this suggests that the bonding that takes occurred should have mechanical qualities. One possible explanation for this might be mechanical interlocking, or the fact that the fracture pattern is shifting and requiring more energy to break [17].
5. CONCLUSIONS
The addition of biochar to the mixture alters the concrete’s workability. When the biochar was added to the mixture, it was evident that the mixture underwent considerable modifications, with the consistency changing from fluid to compact. Concrete’s compressive strength will most certainly decline with the addition of more biochar. This was evident in this thesis’s outcome as well as in the literature. Nonetheless, the research suggests that lesser amounts of biochar may boost compressive strength, with values falling with increasing proportions. When concrete is mixed with biochar and allowed to cure for extended periods of time, the specimen’s compressive strength rises. This is not to be observed, since this thesis found that the compressive strength of the concrete containing biochar was higher than that of the concrete without it. This indicates that, in contrast to concrete without biochar, which grows less quickly after 28 days, the compressive strength is still increasing. Overall, the findings suggest that biochar may eventually be used to substitute cement as a way to mitigate some of the carbon emissions associated with concrete. When the control sample and the 5% ratio sample were analysed, a little change in compressive strength was observed. Additionally, positive outcomes were seen based on the strength growth that continued after 28 days of healing. However, as stiff concrete is less practical in real life, the negative effect of workability might be a problem.
6. FURTHER RESEARCH
A number of tests that were based on Eurocode have been modified for this work. Extended testing should be conducted in order to do further study. For both newly mixed and dried concrete. This is to investigate the effects of applying biochar on the properties of concrete in more detail. To find a coefficient to change the water-to-cement ratio in concrete mixes, it is necessary to assess the water-retaining qualities of biochar. It is also necessary to investigate the long-term effects of biochar concrete under real-world circumstances. The cylindric compressive strength and modulus of elasticity need to be ascertained in order to comprehend how the material will function in structural components.
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Publication Dates
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Publication in this collection
16 June 2025 -
Date of issue
2025
History
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Received
24 Dec 2024 -
Accepted
16 Apr 2025
