Analysis of Biochar–Cement Composites by SEM/EDS: Interfacial Interactions and Effects on Mechanical Strength

Abstract

Portland cement production is one of the main global sources of CO₂ emissions, driving the search for sustainable solutions to reduce its environmental footprint. This study evaluated the use of biochar derived from sugarcane bagasse as a partial cement replacement in cementitious composites, aiming to investigate its effects on mechanical and microstructural properties. Composites were prepared with 0, 2, and 5 (% w w⁻¹) biochar at two water-to-cement (w/c) ratios: 0.28 and 0.35. It was hypothesized that the porous structure and carbon-rich composition of biochar could enhance the microstructure of the cementitious matrix and contribute to strength development. Characterization of the biochar indicated compliance with the European Biochar Certificate (EBC) standard, high thermal stability, and notable water retention capacity. Mechanical testing revealed that incorporating 5% w w⁻¹ biochar increased compressive strength by up to 48% in the 0.35 w/c formulation compared to the control. Microstructural analyses (SEM/EDS) showed good interaction between the biochar and the cementitious matrix, with the formation of hydration products at the interfaces. The results confirm the potential of sugarcane bagasse biochar as a supplementary cementitious material, promoting more sustainable composites with improved mechanical performance and reduced environmental impact.

1. Introduction

Cement production plays a significant role in global carbon dioxide (CO₂) emissions, directly contributing to the intensification of the greenhouse effect [1]. This environmental impact is primarily due to the energy-intensive manufacturing process and the heavy use of raw materials, particularly during the calcination and clinker production stages [2]. These stages release large volumes of CO₂, making the cement industry responsible for approximately 8% of global CO₂ emissions [3]. It is further estimated that each ton of cement produced generates between 800 and 1000 kg of CO₂ [4]. Climate change, driven by greenhouse gas emissions, results in rising sea levels, melting polar ice caps, increasing global temperatures, and more frequent extreme weather events, such as droughts, floods, and wildfires [5].

In pursuit of carbon neutrality by 2050, the construction industry has adopted sustainable alternatives such as supplementary cementitious materials (SCMs) to reduce cement consumption and CO₂ emissions [6]. This initiative directly supports global emission reduction targets, which are essential to address the growing concerns over climate emergencies. Within this context, efforts to reduce the environmental impact of construction have led to the exploration of alternative materials capable of partially replacing cement without compromising mechanical properties. For example, Khan et al. [7] developed three low-carbon concrete formulations, replacing 80%, 85%, and 90% of Portland cement with ground granulated blast furnace slag, fly ash, and silica fume. These blends exhibited satisfactory mechanical performance and a substantial reduction in carbon footprint compared to conventional concrete.

In this regard, biochar has emerged as a promising alternative. Produced via biomass pyrolysis, this carbonaceous material exhibits high porosity, large surface area, and the capacity to capture and store carbon. In addition to reducing CO₂ emissions, biochar improves water retention in cementitious mixtures, enhancing cement hydration and improving specific material properties [8]. Acting also as an SCM, its incorporation into cementitious matrices not only reduces environmental impacts but also promotes innovation in the construction sector [9,10,11]. In this context, biochar–cement composites represent a low-carbon, environmentally sustainable alternative, attracting growing interest for construction applications [12].

The integration of biochar into cementitious composites has shown promising results. Gupta et al. [10] highlighted its potential as a construction material and the parameters that influence its performance, including carbon sequestration. Microstructural characterization enables the understanding of biochar’s effects on mechanical properties, with several studies recommending a maximum replacement of 5% w/w of cement by biochar, although proportions up to 15% may achieve superior strengths depending on the water-to-cement (w/c) ratio. Recently, Lorenzoni et al. [13] investigated this interaction using techniques such as scanning electron microscopy (SEM), revealing that the biochar structure is preserved during mixing and forms channels of varying shapes and diameters—often not filled—indicating the influence of the w/c ratio and the degree of hydration. Isothermal calorimetry analyses confirmed that biochar retains moisture, modifying the effective w/c ratio and the formation of the cementitious matrix.

Biochar production must comply with the European Biochar Certificate (EBC) guidelines, which require the use of approved biomass sources free of contaminants and fossil materials. The regulations also cover the use of additives, traceability of inputs, and pyrolysis conditions, ensuring product safety, sustainability, and quality [14]. In Brazil, the world’s largest sugarcane producer, the residues from this crop create a promising scenario for biochar production. It is estimated that each ton of processed sugarcane generates between 250 and 280 kg of bagasse [15]. Although already widely used for energy generation and other products, converting sugarcane bagasse into biochar represents an additional opportunity aligned with the circular economy.

Despite recent advances, there are still gaps in the detailed understanding of the combined effects of biochar addition and varying w/c ratios on the microstructure and chemical composition at the biochar–cement interface. Little is known about how these variables influence hydration mechanisms and the elemental distribution throughout the cement matrix. Moreover, few studies have focused on biochar–cement composites using biochar derived specifically from sugarcane bagasse, highlighting the need for more targeted investigations. Analytical techniques such as SEM coupled with energy-dispersive X-ray spectroscopy (EDS) can provide complementary evidence of interface formation in biochar–cement composites.

Given the above, this study aimed to evaluate the effect of incorporating sugarcane bagasse-derived biochar as a partial cement replacement in cement pastes, at proportions of 0, 2, and 5% w w⁻¹, considering w/c ratios of 0.28 and 0.35. The mechanical properties of the composites were investigated, as well as the interaction between the biochar and cement matrix, through SEM/EDS analysis. It is hypothesized that sugarcane bagasse biochar, due to its porous structure and carbon-rich composition, can contribute to improvements in the microstructure and mechanical performance of the composites. This work provides novel experimental data on the behavior of sugarcane bagasse biochar in cementitious systems under varying w/c ratios, along with microstructural characterization of the biochar–cement interface. The findings aim to advance the understanding of sustainable cement substitutes, supporting the development of more durable, efficient, and environmentally responsible construction materials.

2. Materials and Methods

2.1. Production and Characterization of Sugarcane Bagasse Biochar

The biochar derived from sugarcane bagasse was produced via pyrolysis in a continuous horizontal reactor, operating at approximately 350 °C in the intermediate zone [16]. The resulting material was ground using a mortar and pestle and sieved through a 75 µm mesh. The material was characterized according to the quality standards established by the European Biochar Certificate [14]. Additionally, a thermal analysis was conducted using a thermogravimetric analyzer (STA 7300, Hitachi, Tokyo, Japan) from 25 °C to 1000 °C at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere.

The morphological characteristics and elemental composition were identified using scanning electron microscopy (Vega 3 LMU, Tescan, Brno-Kohoutovice, Czech Republic) coupled with energy-dispersive X-ray spectroscopy (X-MaxN, Oxford Instruments, Oxford, UK). The sample was mounted on a metal stub with carbon adhesive tape and coated with a thin layer of gold–palladium using a sputtering device (SC7620, Quorum Technologies, Ashford, UK). Images were acquired at an accelerating voltage of 20 kV.

To complete the characterization, the pozzolanic activity of the biochar was evaluated using the electrical conductivity method, which determines pozzolanicity based on the change in conductivity of a saturated calcium hydroxide solution prepared with analytical grade Ca(OH)₂ (purity > 95%) (Synth, São Paulo, Brazil) [17]. For this purpose, 5.0 g of biochar was added to the saturated calcium hydroxide solution maintained at 40 °C. The difference between the electrical conductivity of the saturated solution and the solution after biochar addition was measured after 2 min.

2.2. Preparation of Biochar–Cement Composites

The cementitious composites were cast into cylindrical specimens measuring 5 cm in diameter and 10 cm in height, following NBR 7215 [18], using Portland cement CP V-ARI Premium (Cimentos Liz S.A., Vespasiano, Brazil), as specified by NBR 16697 [19]. Sugarcane bagasse biochar was incorporated as a partial cement replacement at 0, 2, and 5% w w⁻¹. The water-to-cement (w/c) ratio was determined through the normal consistency test, in accordance with NBR 16606 [20], to ensure suitable paste workability. A bench-top mixer was used for blending. Water was first added to the mixing bowl, followed by the cement previously blended with the biochar after 30 s. The mixing process consisted of 30 s at low speed, bowl scraping for 15 s, and an additional 1 min at high speed. Consistency was deemed adequate when the Vicat needle penetration was 6 ± 1 mm, as required by the standard. The resulting w/c ratios were 0.28 for the paste with 0% w w⁻¹ biochar and 0.35 for the composites. After casting, the specimens were cured in lime-saturated water.

2.3. Characterization of Biochar–Cement Composites

The composites, prepared in triplicate, were subjected to compressive strength testing using a universal testing machine (EMIC DL20000, Instron, São José dos Pinhais, Paraná, Brazil). The data were initially analyzed by analysis of variance (ANOVA) and subsequently compared using the Scott–Knott test (p < 0.05). After failure, fragments of the specimens were oven-dried for 24 h. The morphology and composition of these fragments were investigated by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) following the same procedures described for biochar characterization.

3. Results and Discussion

Table 1. Characterization of the biochar sample obtained from sugarcane bagasse.

Analysis	Unit	Result
Bulk density < 3 mm	kg m−3	120.4
pH in CaCl2	-	7.20
Electrical conductivity (E.C.)	mS cm−1	337.9
Salt content	mg KCl L−1	17.8
Water holding capacity (WHC)	% w w−1	17.4
Moisture	% w w−1	6.51
Volatile matter	% w w−1	21.24
Ash	% w w−1	10.56
Fixed carbon	% w w−1	68.21
Carbon (C)	% w w−1	70.81
Hydrogen (H)	% w w−1	2.28
Nitrogen (N)	% w w−1	Nd
Oxygen (O)	% w w−1	16.35
H/C	-	0.38
O/C	-	0.17
Total phosphorus	g t−1	54.2
Iron	g kg−1	3.603
Potassium	g kg−1	2.153
Magnesium	g kg−1	0.421
Calcium	g kg−1	1.724
Boron	g t−1	118.8
Lead	g t−1	<1.0
Cadmium	g t−1	<1.0
Copper	g t−1	5.1
Nickel	g t−1	<1.0
Mercury	g t−1	<10.0
Zinc	g t−1	36.8
Chromium	g t−1	6.8
Arsenic	g t−1	<10.0
Silver	g t−1	<1.0
Manganese	g t−1	56.6
Acenaphthene	g t−1	<0.02
Acenaphthylene	g t−1	<0.02
Anthracene	g t−1	<0.01
Benzo(a)anthracene	g t−1	<0.02
Benzo(a)pyrene	g t−1	<0.02
Benzo(b)fluoranthene	g t−1	<0.02
Benzo(g,h,i)perylene	g t−1	<0.02
Benzo(k)fluoranthene	g t−1	<0.02
Chrysene	g t−1	<0.05
Dibenzo(a,h)anthracene	g t−1	<0.01
Phenanthrene	g t−1	<0.02
Fluoranthene	g t−1	<0.01
Fluorene	g t−1	<0.03
Indeno(1,2,3-c,d)pyrene	g t−1	<0.01
Naphthalene	g t−1	<0.01
Pyrene	g t−1	<0.03
Dioxins and furans	g t−1	<0.01
Polychlorinated biphenyls (PCBs)	g t−1	<0.001

presents the results of the physicochemical and elemental characterization of the biochar derived from sugarcane bagasse. From an environmental standpoint, no significant concentrations of heavy metals or toxic organic compounds such as PAHs, dioxins, furans, and PCBs were detected. All the values were below the limits established by the EBC [14], allowing the material to be classified as EBC-BasicMaterials. This compliance reinforces the feasibility of using biochar in technological applications such as cementitious composites, aligning environmental safety, carbon sequestration, and the valorization of agro-industrial residues.

The biochar exhibited a bulk density of 120.4 kg m⁻³, low moisture content (6.51%), and low volatile matter content (21.24%) (Table 1), characteristics that indicate thermal stability and resistance to microbial degradation. These results demonstrate thermal stability and microbial resistance, as the low moisture content and reduced volatile matter limit the availability of degradable organic fractions and free water, both critical for microbial proliferation [21]. These findings are supported by the thermogravimetric analysis (TGA) (Figure 1), which demonstrates relative thermal stability up to approximately 400 °C. The TGA curve of the biochar exhibits three stages with different weight loss rates. The initial stage (Stage I), from room temperature to 300 °C, involves a gradual decrease in mass (4.1%) attributed to the loss of surface moisture. In the second stage (Stage II), between 300 °C and 650 °C, a more pronounced mass loss (13.0%) is observed, associated with the rapid pyrolysis of residual cellulose, hemicellulose, and lignin in the biochar. The third stage (Stage III), from 650 °C to 1000 °C, is characterized by a slower and continuous mass loss (totaling 27.0% by the end of the analysis), attributed to the degradation of more thermally stable carbon structures [22]. Overall, the high fixed carbon content (68.21%) and atomic H/C ratio of 0.38 indicate a highly aromatic and condensed structure, typical of biochars.

The low electrical conductivity (337.9 mS cm⁻¹) and the reduced content of soluble salts (17.8 mg KCl L⁻¹) of the biochar indicate a low risk of electrochemical interference in cementitious matrices, favoring its incorporation as an additive or partial substitute for aggregates. The near-neutral pH (pH 7.20) is compatible with the stability of the alkaline environments typical of cement-based systems. Furthermore, the material exhibited a water-holding capacity (WHC) of 17.4% (w w⁻¹), which could positively influence the curing process of pastes and mortars by retaining internal moisture for extended periods, thereby promoting cement hydration. This sustained moisture supply mitigates self-desiccation, provides water for unhydrated phases, and contributes to the formation of a denser microstructure [23]. This property can be partly attributed to the porous morphology of the material observed by SEM (Figure 2), which revealed a honeycomb-like structure composed of macropores with diameters ranging approximately from 1 to 70 µm. Gupta et al. [10] reported that biochars with pores between 10 and 30 µm can retain part of the mixing water, thereby reducing the effective free water content in cementitious matrices. This retained water is subsequently released during cement hydration, directly influencing the formation of reaction products.

The ash content of 10.56% w w⁻¹ in the biochar indicates the presence of reactive mineral phases, with potential pozzolanic activity in cementitious composites. The pozzolanic reaction, characterized by the interaction of calcium hydroxide (CH) with amorphous silica (SiO₂) and alumina (Al₂O₃), leads to the formation of compounds such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H), which densify the matrix and improve its properties [24]. Although this reaction occurs more slowly than primary hydration, it results in enhanced mechanical strength, durability, and chemical resistance due to the consumption of free CH and the additional generation of C-S-H.

The elemental analysis by EDS (Figure 3) confirmed that the biochar is predominantly composed of carbon and oxygen, with detectable levels of silicon, potassium, calcium, magnesium, and aluminum. Among these, silicon stands out for its reactivity, promoting C-S-H formation [9], while potassium—present at 2.153 g kg⁻¹ (Table 1)—may act as an alkaline activator, accelerating hydration [25]. Thus, despite its high carbon content, the biochar shows potential to positively influence the development of cementitious microstructure, both through its potential pozzolanic activity and chemical activation effects.

Indeed, the preliminary pozzolanic activity test (Figure 4) showed that the biochar reduced the electrical conductivity of the saturated Ca(OH)₂ solution by 0.96 mS cm⁻¹ after two minutes. This decrease occurs because pozzolanic materials typically react with Ca²⁺ and OH⁻ ions in solution to form poorly soluble calcium silicate hydrate (C-S-H) phases, thereby reducing the ionic concentration and, consequently, the electrical conductivity. According to the criterion proposed by Luxán et al. [17], this behavior classifies the biochar as a material with variable pozzolanicity. However, it is important to note that this test has limitations, as the reduction in conductivity may also result from the physical adsorption of Ca²⁺ and OH⁻ ions onto the surface of the biochar rather than from true pozzolanic reactions. Therefore, complementary analyses—such as compressive strength tests—are essential to confirm the effective pozzolanic contribution of the biochar to the composite material.

In the compression tests (Figure 5), the addition of 2% w w⁻¹ biochar did not result in statistically significant changes in strength for the paste with a water-to-cement ratio (w/c) of 0.28 (p > 0.05), but led to a 26% increase for the w/c ratio of 0.35. When 5% w w⁻¹ biochar was used, the strength gains were even more notable, reaching 29% (w/c = 0.28) and 48% (w/c = 0.35), respectively. As expected, increasing the w/c ratio caused a reduction of approximately 17% in the strength of the control paste, from 39.01 MPa to 32.47 MPa. However, it is noteworthy that the composites containing 5% w w⁻¹ biochar and a w/c ratio of 0.35 surpassed the strength of the reference sample with a w/c of 0.28, highlighting that the biochar not only mitigates the adverse effects of increased porosity but also strengthens the cementitious matrix through pozzolanic mechanisms.

This performance can be attributed to the ability of biochar to provide additional hydration products that fill the spaces between cement grains and densify the microstructure [26]. Accordingly, the results reinforce the hypothesis that biochar acts as a matrix-modifying agent, enhancing mechanical strength even at low concentrations [10]. The improvement is particularly relevant in composites with higher w/c ratios, where the natural tendency toward increased porosity is mitigated by both the physical and chemical action of the biochar. This behavior is mainly attributed to the high specific surface area and porous nature of the biochar, which promote both the partial absorption of mixing water and the filling of capillary pores, contributing significantly to the refinement of the matrix structure during curing [10,27].

Beyond the physical matrix refinement effect, biochar also contributes to self-healing processes. The water retained within its internal structure is gradually released, sustaining the late hydration of cement and supporting the continued formation of C-S-H gel [10]. Zhang et al. [27] demonstrated that even at low dosages, the incorporation of biochar improves compressive strength in ultra-high-performance mortars, with SEM micrographs showing a compact microstructure and the adherence of hydration products to biochar surfaces. Gupta et al. [10] observed the nucleation of C-S-H, ettringite, and calcium hydroxide inside the pores of biochar, as well as the formation of more tortuous fracture paths, confirming the role of the material both as a physical reinforcement and as an agent for enhancing the interfacial transition zone.

In this context, an SEM/EDS analysis is essential to validate these hypotheses, allowing for the visualization of the interface between biochar particles and the cementitious matrix; identification of hydration products formed around the biochar; and mapping of the elemental distribution of calcium, silicon, and aluminum, thereby providing direct evidence of the participation of biochar in hydration processes and in the development of the composite’s microstructure. The SEM micrographs shown in Figure 6 reveal the interaction between biochar particles and the cement matrix in composites with 2% w w⁻¹ (a–c) and 5% w w⁻¹ (d–f) cement replacement, at a w/c ratio of 0.28.

At both concentrations, the porous structure of the biochar is preserved, displaying characteristic interconnected pores, which confirms its potential as a microstructural modulator, contributing to matrix refinement [10,27]. The micrographs (Figure 6c,f) reveal the presence of hydration products adhering to the inner walls of the biochar pores, suggesting that biochar acts as a nucleation site for the formation and growth of cementitious phases such as calcium silicate hydrate (C-S-H) gel and ettringite within its structure [28].

Additionally, the images indicate that biochar particles are not fully consumed during hydration but remain preserved, serving as a physical support for the deposition of hydration products [29]. This nucleation effect not only fosters the formation of a more cohesive interface between the biochar and the matrix but also contributes to the strengthening of the interfacial transition zone, resulting in a denser and stronger composite [27].

The lower-magnification images (Figure 6a,d) highlight the dispersion of biochar particles within the cement matrix, emphasizing their role in filling voids and mitigating capillary porosity. Notably, the higher dosage (5% w w⁻¹) leads to interfaces more densely coated with hydration products (Figure 6f), supporting the observed improvements in compressive strength and suggesting that increasing the biochar content can enhance its functional effect up to a certain threshold. SEM micrographs of the composites with a w/c ratio of 0.35 (Figure 7) reveal similar interaction patterns between the biochar and the cementitious matrix. In the upper-row images (Figure 7a–c), which correspond to the 2% w w⁻¹ biochar content, the particles exhibit a preserved porous morphology and partial filling by hydration products, indicating that biochar acts as a nucleation site even in more porous matrices [10]. In the lower-row images (Figure 7d–f), representing the 5% w w⁻¹ formulation, greater densification of hydration products is observed at the biochar–matrix interface. This suggests a more effective role of biochar in internal water retention and in promoting autogenous healing, which supports prolonged hydration and the development of a denser microstructure over time.

The analysis of microchemical maps (Figure 8) revealed distinct patterns in potassium (K) distribution depending on both the water-to-cement (w/c) ratio and the biochar content. In all the analyzed formulations, typical elements of the hydrated cementitious matrix—carbon (C), oxygen (O), silicon (Si), aluminum (Al), and calcium (Ca)—were identified, along with potassium. In composites with 2% w w⁻¹ biochar and a w/c ratio of 0.28, potassium was predominantly concentrated in regions associated with biochar particles (Figure 9), suggesting that the additive itself is the primary source of this element. As both the biochar content and the w/c ratio increased (to 5% w w⁻¹ and 0.35, respectively), the potassium distribution became more homogeneous and diffuse throughout the cement matrix, indicating enhanced ionic mobility and a greater potential for potassium release into the hydration phase. This behavior can be attributed to the increased matrix porosity associated with a higher biochar content, which facilitates ion transport and the dispersion of K⁺ within the system.

This result highlights that the potassium present in the biochar contributes to the development of the mechanical properties of the biochar–cement composites. Previous studies have indicated that this element acts as an alkaline activating agent, capable of interacting with cement particles through reactions involving potassium salts, thereby accelerating the hydration process. According to Kumar et al. [30], the higher potassium content in the biochar produced at 450 °C was partially responsible for the increase in compressive strength, attributed to the enhancement of hydration promoted by this element. Similar findings were reported by Restuccia and Ferro [25], who observed that biochars derived from coffee waste—rich in potassium—accelerated strength development by acting as alkaline activators during cement hydration. These authors also suggested that carbonized particles may provide reactive silica to the mix, promoting secondary reactions with the free lime released during the hydration of Portland cement. In this context, the more pronounced diffusion of potassium in composites with higher biochar content and higher w/c ratio may indicate not only the greater availability of this element within the matrix but also an effective contribution to the mechanisms of alkaline activation and accelerated hydration, with expected positive impacts on mechanical performance and, potentially, the durability of the composites. This is consistent with the observation that higher w/c ratios tend to reduce the concentration of K⁺ in the pore solution due to dilution and leaching effects, whereas the addition of K-rich biochar can compensate for this reduction by enhancing local alkalinity and sustaining hydration kinetics [31].

Considering the results presented, it is important to highlight some limitations of this study. The pozzolanic activity was assessed through a preliminary conductivity test, which may also reflect ion adsorption effects. The analyses were restricted to cement pastes, without extending to mortars or concretes, which may behave differently due to scale and composition. The mechanical performance was evaluated under controlled curing conditions, without considering environmental exposure. Long-term durability and other performance parameters remain to be investigated. These aspects should be addressed in future studies to expand the understanding of biochar in cement-based materials.

4. Conclusions

The use of biochar in cementitious matrices represents a promising approach for developing more sustainable construction materials. In this study, sugarcane bagasse-derived biochar exhibited high thermal stability, water retention capacity, and compliance with the EBC standard. Its incorporation at 5% (w w⁻¹) increased compressive strength by up to 48% in the 0.35 w/c formulation. The SEM/EDS analysis indicated good interaction between the biochar and the cementitious matrix, with hydration products observed at the interface. These results suggest that biochar can contribute to improved mechanical performance and microstructural development.

In addition to reducing Portland cement clinker content, biochar may act as a stable carbon reservoir, encapsulated within the hardened matrix and protected from degradation over time. This behavior aligns with efforts to lower carbon emissions associated with cement production. The findings support the potential use of sugarcane bagasse biochar as a partial cement replacement in composites with reduced environmental impact. Future studies focusing on long-term durability, curing conditions, and replacement levels may help further understand and expand its applicability in low-carbon construction systems.