Enhancing CO₂ Sequestration Through Corn Stalk Biochar-Enhanced Mortar: A Synergistic Approach with Algal Growth for Carbon Capture Applications

Abstract

This study examines corn stalk biochar (CSB)-enhanced mortar as an innovative material for carbon capture and CO₂ sequestration. CSB, a renewable agricultural byproduct, was incorporated into cement mortar at varying concentrations (2.5% to 75%), and its effects on the mortar’s physicochemical properties, its ability to support algal growth, and the CO₂ absorption capacity of the algae were analyzed. Characterization of CSB showed a high carbon content (62.3%), significant porosity, and a large surface area (680.3 m² g⁻¹), making it ideal for gas capture. At low concentrations (2.5%), CSB slightly improved the mortar’s compressive strength and density. However, higher CSB levels (5% to 75%) led to significant reductions (p < 0.05) in strength and density, while water absorption increased. CO₂ sequestration monitored from algal growth studies revealed that both Chlorella sp. (TISTR 8262) and Scenedesmus sp. (TISTR 9384) thrived on CSB-enhanced mortars. At a 75% CSB concentration, Scenedesmus sp. achieved a 24.2-fold increase in biomass by day 12, outperforming Chlorella sp., which showed a 26.6-fold increase. CO₂ absorption also improved with biochar. Mortars with 75% CSB achieved an 86% CO₂ absorption ratio without algae, while adding algae boosted this to nearly 100%, highlighting the synergistic effect of biochar and algal photosynthesis. Higher CSB levels accelerated CO₂ absorption stabilization, reaching saturation by day 8 at 75% CSB. Scenedesmus sp. showed slightly higher CO₂ absorption efficiency than Chlorella sp., reaching peak absorption earlier and maintaining greater efficiency. Higher CSB concentrations accelerated CO₂ absorption, indicating that biochar–mortar mixtures, particularly when combined with algae, provide a promising solution for enhancing carbon capture and sequestration in green infrastructure.

1. Introduction

Global warming, driven by the anthropogenic emission of greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), has emerged as one of the most critical environmental challenges of our time. Among these gases, CO₂ is recognized as the principal contributor to global warming due to its high atmospheric concentration and long-term persistence [1]. Projections from the Intergovernmental Panel on Climate Change (IPCC) suggest that global temperatures could rise by 1.0 to 3.7 °C over the twenty-first century, depending on future GHG emission trajectories [2]. The ramifications of climate change are already visible, with increased frequency and severity of extreme weather events, rising sea levels, and significant disruptions to ecosystems and human health [3,4]. These alarming trends have spurred a global effort to reduce GHG emissions and enhance carbon sequestration to mitigate the effects of climate change.

One of the most promising strategies for mitigating CO₂ emissions is biological carbon sequestration, which captures and stores atmospheric CO₂ through natural processes. In recent years, biochar has emerged as a sustainable tool in this effort. Biochar is produced through the pyrolysis of biomass under oxygen-limited conditions at moderate temperatures, typically below 700 °C [5,6]. Its highly porous structure and large surface area make biochar particularly effective for improving soil fertility, enhancing microbial activity, and serving as a long-term carbon sink [7,8,9,10]. Given its ability to sequester CO₂ equivalents in terrestrial ecosystems, biochar is increasingly regarded as an eco-friendly material with substantial potential for climate change mitigation [11,12,13,14].

While the agricultural benefits of biochar are well documented, its application in other sectors, such as construction, has gained attention in recent years. Researchers have begun exploring biochar’s integration into cement-based materials as a strategy to reduce the carbon footprint of the construction industry, which is responsible for a significant portion of global CO₂ emissions [15,16]. The global production of cement exceeds 4.1 billion tons annually, making it one of the largest contributors to industrial CO₂ emissions [16]. As concrete remains a cornerstone of modern infrastructure, finding innovative ways to incorporate environmentally friendly materials like biochar into cement formulations is crucial for reducing GHG emissions while maintaining material strength and durability [17,18].

Despite growing interest in biochar-enhanced cement mortar for its CO₂ absorption capabilities and improved mechanical properties [19,20], no studies to date have explored its potential as a medium for algae cultivation. This novel approach not only leverages biochar’s material properties but also integrates biological carbon sequestration through algae cultivation, providing a dual-purpose solution for climate change mitigation. The scientific novelty of this study lies in the synergistic use of biochar-enhanced cement mortar as both a structural material and a biological growth medium for algae. This integration offers dual benefits: improved material properties and enhanced carbon capture through algae photosynthesis. Cultivating algae in such materials presents a unique opportunity to enhance carbon sequestration—algae are highly efficient at capturing CO₂ through photosynthesis, helping to reduce CO₂ levels in the atmosphere. While research in the construction industry has typically focused on preventing algae growth due to concerns over structural degradation and aesthetic damage [21,22], this study explores the innovative use of biochar in cement mortar to actively support algal growth rather than inhibit it. Although previous studies have examined algae cultivation for biochar production [23,24], little research has been conducted on using biochar as a supplement to growth media for algae [25].

This study presents a novel approach by investigating biochar-enhanced mortar as a platform for cultivating algae, specifically Chlorella and Scenedesmus species. These microalgae are not only efficient at CO₂ capture but also hold significant potential for bioenergy production [26,27,28], making them ideal candidates for integration with biochar-based materials. By aligning with several Sustainable Development Goals (SDGs)—such as SDG 13 (Climate Action) and SDG 11 (Sustainable Cities and Communities)—this research contributes to the development of green infrastructure solutions that address global environmental challenges.

The primary objective of this research is to investigate the synergistic potential of biochar-enhanced mortar as both a construction material and a biological growth medium for algae. This approach combines biochar’s physical benefits with the biological process of algae growth, offering a dual-purpose solution for carbon sequestration and sustainable construction practices. By incorporating corn stalk biochar (CSB), a renewable agricultural byproduct, into mortar formulations, this study examines how varying concentrations of biochar influence the mortar’s physicochemical properties, such as compressive strength, porosity, and water absorption. In addition to evaluating the mortar’s structural performance, the research explores its ability to support the cultivation of Chlorella and Scenedesmus species. Furthermore, the study assesses the CO₂ absorption potential of biochar-enhanced mortars, both with and without algal cultivation, to better understand the added value of integrating biological processes with construction materials. This dual-purpose application not only innovates within the construction sector but also contributes to global sustainability efforts through enhanced carbon capture and renewable energy production, paving the way for scalable, eco-friendly solutions.

2. Materials and Methods

2.1. Preparation and Characterization of Corn Stalk Biochar

Corn stalk samples were cut into pieces measuring 5–8 cm in size, thoroughly washed with water, and dried at a temperature of 105 ± 5 °C for 1 h. After drying, the samples were stored in a moisture-free environment to prevent contamination. The biochar synthesis was conducted through pyrolysis at 600 °C for 60 min using a specially assembled pyrolysis furnace with a controlled heating rate of 3 °C min⁻¹. The resulting CSB was ground using a crushing machine (GM/EP-100x60X, Guang Ming, Jiangxi, China) followed by a hammer mill (GM/PC400x200A, Guang Ming, Jiangxi, China) to achieve a fine powder that passed through a No. 325 sieve. The CSB sample was then analyzed to assess its various physical and chemical properties.

For particle size distribution, a laser particle size analyzer (CILAS 1190, CILAS, Orléans, France) was used. Surface morphology of the CSB was examined via scanning electron microscopy (SEM) (JEM2100, Merlin Compact, Carl Zeiss, White Plains, NY, USA) at an accelerating voltage of 5.0 kV, providing detailed structural insights. Functional groups present in the CSB were identified through Fourier-transform infrared (FTIR) spectroscopy (Tensor27, Bruker, Karlsruhe, Germany). The elemental composition of the CSB was analyzed using XRF (Malvern Panalytical Ltd., Malvern, UK) and an organic elemental analyzer (Flash 2000, Thermo Scientific, Waltham, MA, USA). Furthermore, the porosity and surface characteristics of the CSB were determined by nitrogen adsorption–desorption isotherms at 77 K, using a micromeritics instrument (ASAP2460, Micromeritics Instrument Co., Ltd., Norcross, GA, USA). As described by Gotore et al. [29], the Brunauer–Emmett–Teller (BET) method was used to calculate the surface area and pore characteristics of the CSB.

2.2. Mortar Preparation

In this study, ordinary Portland cement (OPC) and coarse aggregate were utilized as the primary components for producing Portland cement (PC) mortar. The chemical composition of OPC was determined through X-ray fluorescence (XRF) analysis (Malvern Panalytical Ltd., Malvern, UK) and is summarized in

Table 1. Elemental composition of ordinary Portland cement (OPC).

Element/Compound	Composition (%) (n = 3)
Calcium oxide (CaO)	63.11 ± 1.20
Silicon dioxide (SiO2)	16.69 ± 0.66
Aluminum oxide (Al2O3)	4.40 ± 0.21
Sulfur trioxide (SO3)	3.51 ± 0.05
Ferric oxide (Fe2O3)	3.13 ± 0.04
Organic matter (CHNO)	5.98 ± 0.08
Magnesium oxide (MgO)	1.60 ± 0.02
Potassium oxide (K2O)	0.842 ± 0.001
Sodium oxide (Na2O)	0.121 ± 0.003
Phosphorus pentoxide (P2O5)	0.120 ± 0.004
Titanium dioxide (TiO2)	0.292 ± 0.005
Strontium oxide (SrO)	0.074 ± 0.011
Chloride (Cl−)	0.032 ± 0.006
Copper oxide (CuO)	0.030 ± 0.004
Zinc oxide (ZnO)	0.030 ± 0.005
Manganese oxide (MnO)	0.024 ± 0.007
Arsenic trioxide (As2O3)	0.011 ± 0.000
Zirconium dioxide (ZrO2)	0.013 ± 0.005
Molybdenum trioxide (MoO3)	0.014 ± 0.004
Rubidium oxide (Rb2O)	0.004 ± 0.000

. The OPC used had a specific gravity of 3.13. River sand, purchased from a local supplier in Songkhla, Thailand, was selected as the fine aggregate, in accordance with the particle size distribution requirements specified by ASTM C33 [30]. Tap water, with a pH range of 6–7, was used for both mixing and curing the mortar specimens to replicate the conditions typically used in real-world cement and mortar production, rather than deionized water commonly used in laboratory settings.

Control mortar mixtures (CM) were prepared using a blend of OPC, sand, and water. CSB was then added to the mixtures at various weight percentages of OPC, specifically at 2.5%, 5%, 10%, 30%, 50%, and 75%, which were designated as CSB2.5, CSB5, CSB10, CSB30, CSB50, and CSB75, respectively. The water-to-cement and cement-to-sand ratios adhered to the standards outlined by ASTM C109/C109M (water–cement–sand = 0.7:1:2.75) [31], which are used to evaluate the compressive strength of hydraulic cement mortars. In the biochar treatments, ‘cement’ refers to the total mass of cementitious material, including both OPC and biochar. The water-to-cement and cement-to-sand ratios were maintained according to ASTM C109/C109M, with the cement mass in the biochar mixtures representing the combined mass of OPC and biochar to ensure consistent ratios across all samples. A detailed summary of the mix proportions can be found in

Table 2. Mix proportions of mortar with and without corn stalk biochar (CSB) used in this study.

Treatment	OPC (g)	Water (ml)	Sand (g)	CSB (g)
CM	250.0	175.0	687.5	-
CSB2.5	243.8	175.0	687.5	6.3
CSB5	237.5	175.0	687.5	12.5
CSB10	225.0	175.0	687.5	25.0
CSB25	187.5	175.0	687.5	62.5
CSB50	125.0	175.0	687.5	125.0
CSB75	62.5	175.0	687.5	187.5

.

The ingredients were carefully measured and mixed using a mortar mixer (ELE International, Milton Keynes, UK), following the ASTM C305 standard [32], to ensure a consistent and uniform mixture. The freshly prepared mortar was then poured into steel molds with dimensions of 5 cm × 5 cm × 5 cm and left to set overnight. After demolding, the specimens were sealed in plastic wrap to preserve moisture and allowed to cure for a period of 28 days. Upon completion of curing, each mix proportion produced 20 specimens that were subjected to a comprehensive series of tests.

2.3. Mortar Tests

Compressive strength testing was performed in full compliance with ASTM C109/C109M [31]. Additionally, six specimens were designated for density measurements using the ASTM C188-17 [33]. Water absorption tests were carried out according to ASTM C1403-15 [34], with water absorption calculated by determining the percentage weight increase of the specimens after immersion in water compared with their dry weight.

The cracked samples, each approximately 1 cm in size, were carefully coated with a thin layer of gold and analyzed using SEM (JEM2100, Merlin Compact, Carl Zeiss, USA) at an accelerating voltage of 5.0 kV to reveal their structural characteristics. In addition, FTIR spectroscopy (Tensor27, Bruker, Karlsruhe, Germany) was performed over a wavenumber range of 500–4000 cm⁻¹, with 64 scans and a spectral resolution of 4 cm⁻¹. Samples were prepared by mixing 10 wt% in KBr and pressed into pellets. All the tests were performed in three replicates.

2.4. Microalgae Cultivation on CSB-Enhanced Mortar Specimens

2.4.1. Initial Cultivation of Algae for Biomass Expansion Before Mortar Transfer

This study aimed to expand the quantity of algae, specifically the Chlorella and Scenedesmus species, both of which are highly effective in capturing CO₂ emissions, and to monitor changes during their cultivation. These species were chosen due to their fast growth rates, high CO₂ fixation capacity, and resilience under varying environmental conditions [35,36]. Additionally, both Chlorella and Scenedesmus have the ability to produce high biomass, making them suitable candidates for biofuel production and other industrial applications.

Chlorella sp. (TISTR 8262) and Scenedesmus sp. (TISTR 9384) were obtained from the Algal Excellence Center (ALEC) in Thailand. A batch experiment was conducted by inoculating Chlorella sp. and Scenedesmus sp. into BG11 liquid medium. The initial algal cell density was determined using a spectrophotometer (Cary 60, Agilent, Waldbronn, Germany) at a wavelength of 560 nm, with the optical density (OD₅₆₀) recorded at approximately 0.8. Subsequently, 14 mL of the inoculated sample was transferred into a 2 L photobioreactor containing 1 L of liquid medium to enhance biomass production. The culture was continuously illuminated with warm white light (LX-73, Digicon Technology Co., Ltd., Taichung, Taiwan) at an intensity of approximately 4000 lux and maintained at a temperature of 25 °C for 24 h. Continuous aeration was provided at a rate of 30 L min⁻¹ using an air pump (AP-10, Shenzhen Yamano Aquariums Co., Ltd., Guangdong, China) to maintain suspension and prevent sedimentation in the culture (Figure 1a).

The algae were cultured until their OD₅₆₀ reached 4.0, after which they were transferred to mortar surfaces for further growth. A 16-day inoculation period was used to monitor algal growth (Figure 1b), with pH levels consistently tracked throughout the experiment. Additionally, aliquots were removed at regular intervals, dried, and weighed to assess biomass accumulation.

2.4.2. Surface Cultivation of Algae on Corn Stalk Biochar-Enhanced Mortar

The process for cultivating both algal species on the surface of mortar blocks involved the following steps: First, the mortar specimens were sanitized using 70% alcohol to reduce potential contaminants and then dried for one hour at a temperature of 60 °C to remove moisture and further inhibit microbial growth, creating a suitable environment for algal cultivation. On the day the algae were ready for cultivation on the CSB-enhanced mortar, 15 mL samples from both algal species were collected and transferred into centrifuge tubes. The samples were centrifuged at 2012× g for 5 min. After centrifugation, 1 g of the algal biomass was extracted and suspended in 1 mL of fresh BG11 culture medium. All mortar specimens were soaked in fresh BG11 medium to maintain sufficient moisture levels. The resuspended algal cells were uniformly applied to the top surfaces of the mortar specimens. The mortar specimens were then placed on Petri dishes, and the medium level was maintained at a constant depth of 2 cm by replenishing it daily throughout the cultivation period. This consistent medium level was necessary to ensure the algae had a stable moisture environment, which is crucial for their growth and metabolic activity. Maintaining adequate moisture prevents desiccation and helps facilitate nutrient exchange between the algae and the medium. Subsequently, the specimens were placed in a sanitized, sealed chamber, maintained at 25 °C, and exposed to a consistent light source with an intensity of 4000 lux. A glass container measuring 30 × 30 × 30 cm³ was used for this setup. The arrangement of the light source and mortar samples is shown in Figure 2a.

2.4.3. Assessment of Algal Growth and Their Carbon Dioxide Absorption Capacity

The quantity of algal growth was indirectly determined by measuring changes in dried biomass during the cultivation period. Biomass was measured by drying the entire mortar samples in an oven at 105 °C for 24 h, and the recorded weights were used to quantify algae growth on the mortar surface. The experiment was conducted in triplicate to ensure the reliability of the results. Visual observations were captured using a Canon EOS 80D digital camera (Tokyo, Japan) equipped with an EF-S 18-135 IS USM lens. The camera settings included ISO 800, a shutter speed of 1/2000, and an aperture of F8.0. The distance between the lens and the surface of the sample was 10 cm. The photos were taken in a dark room with external lighting.

To assess CO₂ absorption efficiency, algae were cultivated on mortar specimens enhanced with different concentrations of CSB, while CSB-enhanced mortars without algae were used as the control for comparison. On the day of maximum growth, as indicated by dried biomass weight, was recorded, the mortar specimens were transferred to a sanitized, sealed chamber maintained at 25 °C with continuous illumination at an intensity of 4000 lux. To maintain moisture levels, each mortar specimen was placed on a Petri dish, ensuring the medium depth remained constant at 2 cm. No additional medium was added during the experiment. Afterward, pure CO₂ (99%) was introduced into the chamber at a controlled flow rate of 0.05 m³ m⁻³ min⁻¹ for 2 min using a gas infusion pump (Watson Marlow Model 502S, WMFTG, Cornwall, UK). CO₂ concentrations were measured at both the inlet (influent CO₂) and outlet (effluent CO₂) using a gas analyzer (Biogas 5000, Geoteck, West Midlands, UK). These measurements were taken every 2 days over a 16-day period to monitor CO₂ alterations. The experiment was performed in triplicate to ensure accuracy and reproducibility.

To calculate the CO₂ absorption ratio, W₀ (g) represents the total mass of CO₂ supplied or injected into the chamber for absorption by the algae/mortar system. The amount of CO₂ absorbed by the system (W₁, g) was then calculated using the following equation, adapted from the study by Yin et al. [37]:

$$W_1={\displaystyle \frac{44(C_0-C_1)}{V}}$$

(1)

where C₀ and C₁ represent the initial and final concentrations of CO₂ in the system (mmol m⁻³), respectively, and V denotes the volume of the container (m³).

The CO₂ absorption ratio (R) was calculated using the following formula:

$$R={\displaystyle \frac{W_1}{W_0}\times 100\%}$$

(2)

It should be noted that this ratio is expressed as a percentage, with values close to 0 indicating minimal CO₂ absorption and values approaching 100 indicating substantial CO₂ absorption.

2.5. Data Analysis

The impact of varying levels of CSB on the physical and mechanical properties of the mortar as well as on the growth of algae was assessed using the General Linear Model (GLM) in SAS software (version 9.4, SAS, 2002). Fisher’s least significant difference (LSD) test was employed to compare the treatment means, with significance set at the 5% probability level.

3. Results and Discussion

3.1. Characterization of Corn-Stalk Biochar

Table 3. Elemental composition and physical properties of corn stalk biochar (CSB).

Parameter	Value (n = 3)
Carbon (C, %)	62.3 ± 0.1
Hydrogen (H, %)	3.5 ± 0.0
Oxygen (O, %)	22.3 ± 0.2
Nitrogen (N, %)	9.3 ± 0.1
Sulfur (S, %)	3.1 ± 0.0
O/C ratio (-)	0.4 ± 0.0
H/C ratio (-)	0.006 ± 0.006
C/H ratio (-)	18.1 ± 0.1
Surface area (m2 g−1)	680.3 ± 2.6
Total pore volume (cm3 g−1)	0.4 ± 0.3
Average pore diameter (nm)	1.9 ± 0.1
CO2 absorption capacity (mmol g−1)	2.9 ± 0.0

summarizes the elemental composition and characteristics of CSB, which contains 62.3% carbon, typical of biochar produced via pyrolysis [38]. The low hydrogen content (3.5%) indicates stability and resistance to decomposition, while the moderate oxygen content (22.3%) suggests the presence of reactive oxygen-containing groups. The relatively high nitrogen content (9.3%) makes CSB suitable for agricultural applications and the adsorption of nitrogenous pollutants. Although the sulfur content is low (3.1%), it still contributes to the material’s overall reactivity. The elemental ratios, including a low O/C ratio and H/C ratio, highlight the high degree of carbonization and structural stability of CSB. Wang et al. [39] noted that the elemental composition of corn biomass can vary naturally at different levels, including between and within species, as well as among different plant components. These variations are influenced by factors such as growth conditions and environmental stress. Furthermore, the large surface area (680.3 m² g⁻¹) and mesoporous structure (average pore diameter of 1.9 nm) suggest a strong potential for efficient pollutant adsorption and gas capture. The CO₂ absorption capacity (2.9 mmol g⁻¹) further emphasizes the material’s potential to reduce atmospheric CO₂ and support carbon sequestration initiatives.

SEM images reveal the morphological characteristics of CSB (Figure 3a,b), showing a mix of large and small pores with diameters ranging from 2 to 10 µm. The pore distribution appeared irregular and uneven, highlighting the natural structural characteristics of the CSB. It is worth noting that the biochar was not subjected to acid- or base-cleaning treatments to remove impurities, a step that could have further refined the pore structure. However, this decision was made as a cost-saving measure, simplifying the production process while still preserving sufficient porosity for practical applications. Despite the absence of additional treatments, the CSB maintains a functional pore structure suitable for various environmental uses, such as pollutant adsorption and gas capture [40].

The BET analysis of the CSB sample, presented in Figure 4a, provides valuable insights into its surface area and adsorption characteristics. The adsorption curve exhibits a rapid increase in gas volume at lower relative pressures (P/P₀), eventually reaching a plateau near 1.0, indicating that the CSB has a high adsorption capacity, particularly at lower pressures. At near-saturation pressures, the maximum adsorption volume is approximately 250 cm³ g⁻¹ STP. This behavior highlights the biochar’s substantial surface area and pore structure, which are crucial for adsorbing gases such as CO₂, making it suitable for carbon sequestration applications [41]. In contrast, the desorption curve shows a gradual decrease in adsorbed gas volume as the relative pressure declines. The observed hysteresis between the adsorption and desorption curves suggests the presence of micropores and mesopores within the biochar. This hysteresis phenomenon is typical of materials that can trap adsorbed gases even when the pressure decreases, thereby enhancing the material’s ability to store and sequester gases such as CO₂ [42]. The unique pore structure of CSB makes it a promising material for CO₂ sequestration and other gas storage applications.

The functional groups present on the surface of the CSB as identified through FTIR analysis are illustrated in Figure 4b. A variety of key functional groups were identified, each contributing to the biochar’s overall chemical properties. The O–H bond, which is indicative of hydroxyl groups, was observed at 3437 cm⁻¹, suggesting the presence of alcohol or phenol groups that could enhance the biochar’s reactivity and adsorption potential [43]. Additionally, the C–H bond corresponding to alkyl hydrocarbons was detected at 2921 cm⁻¹, which is typical of long carbon chains and organic compounds [44,45]. A distinct carbonyl group (C=O bond) was identified at 1627 cm⁻¹, indicating the presence of ketones, aldehydes, or carboxylic acids [46]. The shoulder observed at 1550 cm⁻¹ may indicate the presence of carboxylate groups, which could contribute to the biochar’s ability to bind with metals or enhance its adsorption properties, depending on the pyrolysis conditions [47]. The analysis also revealed an aromatic C=C bond at 1384 cm⁻¹, highlighting the presence of stable aromatic structures that form during pyrolysis [48]. This stability is critical for the biochar’s long-term durability in environmental applications. The C–O stretching vibration was detected at 1262 cm⁻¹, contributing to the variety of functional groups involved in the adsorption and chemical reactivity of the carbon materials [49]. Likewise, the presence of a Si–O bond at 1104 cm⁻¹ suggests the incorporation of silicon oxide and potentially SO₂ compounds, which might be remnants of inorganic material from the corn stalks. Wang et al. [39] indicated that the silicon content in CSB, predominantly found in cereals and crop species, forms a silicate network structure and a silicate skeleton on the external surface of the stalk or stem. This is supported by the appearance of a shoulder at 1050 cm⁻¹, further indicating potential silicon-containing compounds or oxygenated functional groups that contribute to the material’s adsorption potential [49]. Finally, the C–H bending vibration for aromatic structure appeared at 873 cm⁻¹ and 794 cm⁻¹ [38,50]. These diverse functional groups on the surface of CSB play a crucial role in its adsorption capabilities and environmental reactivity.

3.2. Physicochemical Properties of CSB-Enhanced Cement Mortar

3.2.1. Compressive Strength

The compressive strength of cement mortar was notably affected by varying concentrations of CSB, as shown in Figure 5a, with distinct percentage changes observed relative to the CM sample. At a lower concentration of 2.5%, a modest 3.5% increase in compressive strength was recorded. This finding is consistent with the results of studies conducted by Gupta and Kua [51] and Wang et al. [52], which demonstrated that the incorporation of 1% w w⁻¹ biochar into cement mortar enhanced the formation of cement hydrates, leading to compressive strength increases of 16% and 9%, respectively, compared with plain mortar. Similarly, Zhao et al. [53] observed that low biochar dosages (less than 2.5% of binder weight) improved compressive strength by 6% and 7% at 7 and 28 days, respectively. Research by Liu et al. [54] on cement composites containing bamboo biochar also reported an increase in compressive strength and crack resistance, with an optimal biochar content of 1–3% due to the material’s filling and self-curing effects. These improvements in mechanical strength can be attributed to the formation of additional cement hydrates, primarily calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H) gels, which strengthen the overall mortar matrix [55,56].

As the concentration of biochar increases, a significant decrease in compressive strength is observed (p < 0.05). When CSB content reaches 5% or higher, biochar begins to interfere with the cementitious matrix, functioning more as a porous, inert filler rather than contributing to structural reinforcement. This leads to a significant 31.3% reduction in strength at 5% CSB, which further escalates to a 57.3% reduction at 10%. With a CSB concentration of 25%, the strength diminishes by 81.9%, as biochar starts to replace a substantial portion of cement, thereby disrupting hydration and matrix formation. At even higher concentrations, such as 50% and 75%, biochar dominates the mixture, resulting in a compressive strength loss of over 95%, critically weakening the mortar’s structural integrity. These findings are consistent with studies by Praneeth et al. [14] and Akhtar and Sarmah [57], who observed that when biochar is added beyond a certain limit, the volume of hydration products filling the pores becomes insufficient relative to the larger pore sizes of the biochar. This leads to increased porosity instead of a denser matrix, which ultimately results in a reduction in compressive strength across all mixtures.

3.2.2. Density

The incorporation of CSB into cement mortar significantly affects this property (Figure 5b). At a low concentration of 2.5% CSB, the density increases by 4.7% compared with the CM sample. However, as the CSB content increases, a decline in density is observed. For instance, at 5% CSB, the density decreases by 6.4%, and this reduction becomes more pronounced at 10% CSB, where a 13.4% drop is recorded. The decrease continues to intensify at 25% CSB, with a significant 20.6% reduction (p < 0.05). At very high concentrations, such as 50% and 75% CSB, the density sharply declines, with reductions exceeding 40%.

The initial increase in density observed at 2.5% CSB can be explained by the biochar’s ability to fill small voids within the cement matrix. This enhances particle packing, resulting in a denser structure. However, as the biochar content rises beyond 5%, the material becomes lighter and more porous due to the lack of sufficient binding and cohesion between biochar particles and the cement matrix. This leads to a marked reduction in density. Brewer et al. [58] noted that the reduction in bulk density is closely tied to an increase in void spaces within the composite. Since biochar has a lower density than cement and sand, its incorporation reduces the overall bulk density of the mixture. Furthermore, the porous nature of biochar itself contributes to the introduction of additional voids around its particles when integrated into the cement matrix, further diminishing the bulk density [14,59].

3.2.3. Water Absorption

The incorporation of CSB into cement mortar results in a progressive increase in water absorption, with the rise becoming more pronounced as the biochar content increases (Figure 5c). At a low concentration of 2.5% CSB, water absorption increases by 8.0% compared with the control. As the biochar content reaches 5%, water absorption rises to 14.5%, and when the concentration is further increased to 10%, it jumps to 30.7%, as biochar begins to dominate the mortar matrix and creates more voids. With a 25% CSB concentration, water absorption increases significantly, reaching 55.0%. This trend becomes even more pronounced at 50% CSB, where water absorption escalates by 130.4%. At the highest concentration of 75% CSB, water absorption soars to 382.8%, as the biochar fully dominates the mixture, resulting in a highly porous and water-retentive mortar.

A plausible explanation for these results is that the porous structure of biochar particles did not facilitate the creation of a denser surface but rather increased the capillary pores, leading to higher water absorption [57,60]. This observation aligns with Akinyemi and Adesina [61], who found a strong linear relationship between water absorption and the porosity of mortar mixtures incorporating biochar. Additionally, biochar’s high water retention capacity enables it to absorb and retain significant amounts of water, further contributing to the elevated water absorption levels in the composites. The particle size of biochar also plays an important role—larger particles may be less effective at filling voids, reducing the potential for densification and resulting in a greater number of pores, which ultimately increases water absorption [14].

3.3. Algal Growth Dynamics

3.3.1. Increment of Optical Density

The OD measurements for both Chlorella sp. and Scenedesmus sp. exhibited distinct growth patterns over the 16-day cultivation period (Figure 6a). Chlorella sp. began with an OD of 0.80 on day 0 and steadily increased, peaking at 4.1 on day 8—an overall increase of 415%. However, by day 10, the OD had dropped by 6% from its peak, and by day 16, it had declined by 32%. Similarly, Scenedesmus sp. started at an OD of 0.80 on day 0, reaching its maximum of 4.2 on day 8, representing a 423% increase. By day 10, its OD had decreased by 2%, and by day 16, it had declined by 17%. These increases during the first 8 days indicate the exponential growth phase for both species, after which they transitioned into the stationary or decline phases. The growth phase plays a critical role in influencing the photosynthetic activity of both algae species, as highlighted by Oukarroum [62]. As cultures approach the stationary phase, photosynthetic rates tend to decrease, likely due to nutrient depletion and the accumulation of metabolic byproducts within the culture medium. Additionally, as algal biomass increases, light availability can become a limiting factor, further affecting photosynthetic efficiency. Reduced light penetration can diminish electron flow in the photosynthesis process, as noted by Falkowski and Raven [63], which contributes to the overall decline in photosynthetic activity as the cultures age. These combined factors—nutrient limitations, byproduct accumulation, and restricted light availability—create a challenging environment for sustained photosynthesis in older cultures.

The more rapid decline in Chlorella sp. compared with Scenedesmus sp. could be attributed to species-specific metabolic differences, which may cause Chlorella sp. to enter the stationary- or decline-phase sooner. In contrast, Scenedesmus sp. tends to sustain its biomass for a longer period, possibly due to its more efficient nutrient utilization or greater tolerance to environmental stress as the culture ages. This aligns with the findings of Zhang et al. [64], who observed that Scenedesmus exhibited stronger resistance to stress than Chlorella in a co-culture system. They noted that Scenedesmus cells typically form wedge-shaped, four-cell consortia, whereas Chlorella cells remain dispersed and separate in suspension. The aggregation of Scenedesmus may offer an adaptive advantage, enabling it to withstand environmental stressors more effectively. Shaima et al. [65] similarly noted that microalgae growth rates are linked to their adaptability, with higher growth rates reflecting a species’ ability to thrive under specific environmental conditions. These observations are consistent with Sabatini et al. [66], who also highlighted the superior resilience of Scenedesmus under stress conditions.

3.3.2. pH Variations

Throughout the 16-day cultivation period of Chlorella sp. and Scenedesmus sp. in a liquid medium, noticeable changes in pH were observed, with both species displaying similar trends (Figure 6b). In contrast, the control (CT-BG11) maintained a stable pH throughout the experiment. At the beginning of the cultivation, the pH for both Chlorella and Scenedesmus was around 7.2–7.3. During the first two days, the pH dropped slightly to just below 6.9. However, after day 2, the pH began to steadily rise for both species, signifying an increase in photosynthetic activity. By day 8, the pH for Chlorella had reached 7.9, while Scenedesmus reached 7.6, indicating efficient CO₂ absorption. By day 10, both species had peaked at similar pH levels (~8.0), marking their period of maximum photosynthetic activity. After day 10, the pH gradually stabilized around 7.6 by day 16, suggesting that both species had reached a steady state of CO₂ uptake and metabolic equilibrium. In contrast, the control (CT-BG11) exhibited no significant pH fluctuations, remaining around 6.9 for the entire 16-day period. This stability in the control confirms that the pH changes in the algae cultures were primarily driven by their metabolic activities, specifically CO₂ uptake during photosynthesis.

The initial drop in pH is likely due to early metabolic activity, during which the algae absorb CO₂ and release organic acids as metabolic byproducts, increasing the acidity of the medium. This pH decrease is typical in the early stages of algal growth as the cells adjust to their environment and begin utilizing nutrients [67]. As photosynthesis progresses, the pH begins to rise due to the reduction in dissolved CO₂ and carbonic acid [68,69], indicating efficient CO₂ absorption by the algae. This upward trend in pH highlights the effectiveness of both algae species in capturing CO₂ through photosynthesis.

3.3.3. Biomass Accumulation

The comparison of biomass accumulation between Chlorella sp. and Scenedesmus sp. over the 16-day cultivation period reveals distinct differences in their growth dynamics (Figure 6c). At the start of the experiment (day 0), the biomass of both species was nearly identical, with Chlorella sp. at 2.66 g L⁻¹ and Scenedesmus sp. at 2.65 g L⁻¹. By day 4, Scenedesmus sp. exhibited a 12% increase in biomass, slightly exceeding the 10% increase shown by Chlorella sp. This gap widened by day 8, when Scenedesmus sp. achieved a 25% increase in biomass, compared with the 15% growth observed for Chlorella sp. Throughout the cultivation period, Scenedesmus sp. maintained its lead in biomass production, and by day 16, its overall growth reached 25%, while Chlorella sp. exhibited a 23% increase.

The early divergence in growth rates between Scenedesmus sp. and Chlorella sp. can be attributed to several key physiological differences, particularly in their response to environmental conditions like light and nutrient availability. Scenedesmus sp. appears to have a slight advantage in nutrient uptake efficiency and faster adaptation to the culture conditions [62]. During the exponential growth phase around day 8, when cell division and nutrient consumption are at their highest, Scenedesmus sp. significantly outperformed Chlorella sp., suggesting its superior ability to thrive under these conditions. According to Masojídek et al. [70], Scenedesmus sp. exhibits a more effective non-photochemical quenching (NPQ) mechanism, which allows it to handle excess light energy more efficiently. NPQ, regulated by the xanthophyll cycle, helps dissipate excess light energy as heat, protecting the cells from light-induced stress [71]. In contrast, Chlorella sp. has a larger antenna system, making it less efficient in quenching excess light, which can lead to reduced photosynthetic capacity under high light conditions [70]. Additionally, Scenedesmus sp. has physiological traits, such as the ability to form cell aggregates, as noted by Zhang et al. [64]. This aggregation may offer an advantage in nutrient-rich environments, allowing Scenedesmus sp. to sustain higher biomass accumulation over time.

Although OD is commonly used to monitor algal biomass, in this study, the growth trends observed through biomass measurements aligned with the OD data up until day 8 (Figure 6c). Both Chlorella and Scenedesmus exhibited a peak in OD at this point, corresponding to their exponential growth phase. However, despite the subsequent decrease in OD after day 8, the biomass remained stable. This phenomenon may be attributed to factors such as cell aggregation, pigment degradation, or changes in cell size [72,73,74]. Oukarroum et al. [62] explained that when Chlorella and Scenedesmus enter the stationary phase, their maximum photosynthetic performance declines, leading to pigment loss or changes in cell morphology. These alterations reduce light scattering efficiency, causing a decline in OD even though the overall biomass remains stable or continues to increase.

3.4. Algal Growth on Cron Stalk Biochar-Enhanced Mortar and Their Efficiency in CO₂ Sequestration

3.4.1. Growth of Chlorella and Scenedesmus Species

In this study, the growth of algae on CSB-enhanced mortars was tracked by measuring changes in dried biomass over a 16-day period, providing insights into the effects of biochar concentrations on algal cultivation (Figure 7a,b). The growth of both algae on cement mortar, even in the absence of CSB, can be attributed to certain material-related physical factors that enhance the bio-receptivity of the surface. These factors include porosity, surface roughness, and the mineral composition of the mortar [75,76,77]. A rougher surface increases the adherence and retention of microalgae, promoting their growth over time. In addition, the mineral composition of the mortar plays a crucial role in its bio-receptivity. Specific minerals, such as calcite and silica present in the cementitious substrate, can provide essential nutrients that encourage algal colonization [22].

When cultivated on CSB-enhanced mortars, both species of algae exhibited notable growth trends (Figure 7a,b). Under control conditions, Chlorella sp. showed a 2.7-fold increase in biomass by day 12, while Scenedesmus sp. achieved a slightly higher increase of 2.9-fold, indicating that both species thrived in the absence of biochar. At lower biochar concentrations (CSB2.5 + C/S and CSB5 + C/S), the growth of both species remained moderate, with biomass increases ranging from 3.0- to 3.7-fold for Chlorella sp. and 3.5- to 3.7-fold for Scenedesmus sp. by day 16. However, at higher biochar concentrations, algae displayed more pronounced growth, particularly by day 12. Chlorella sp. reached a maximum increase of 19.7-fold at CSB50 + C, while Scenedesmus sp. achieved an even higher 21.6-fold at CSB50 + S, highlighting the positive effect of biochar in promoting algal growth. At the highest biochar concentration (CSB75 + C/S), both species reached their peak growth, with Chlorella sp. showing a 24.2-fold increase and Scenedesmus sp. surpassing it with a 26.6-fold rise by day 12. A visual comparison of algal growth on CSB-enhanced mortar between day 0 and day 16 is presented in Figure 8. The observations confirm that mortar with higher biochar content supports increased algal growth over time.

While a very few studies have explored the use of biochar as a medium for algae cultivation, none have investigated its inclusion in mortar formulations. The enhanced growth of algae observed in CSB-enhanced mortars can be attributed to several factors. For instance, Behl et al. [25] demonstrated that adding biochar to the nutrient medium for cultivating Chlorella pyrenoidosa in an aqueous solution of Direct Red 31 dye significantly enhanced algal growth. This improvement was attributed to the biochar’s richness in essential micronutrients, including manganese (Mn), zinc (Zn), titanium (Ti), iron (Fe), and copper (Cu). However, the specific micronutrient content of biochar can vary depending on the feedstock and pyrolysis conditions. While similar effects may be expected with CSB, further analysis is required to confirm its nutrient composition.

Moreover, when higher doses of biochar are applied, the positive effects on microbial growth become more pronounced. The increased surface area provides additional attachment sites and greater access to nutrients, creating more favorable conditions for microbial proliferation [78,79]. Biochar’s porous structure, as described above, also aids water retention [78], further supporting the growth of algae and other microorganisms. Kholssi et al. [80] suggested that the increase in biomass observed in such conditions is not solely due to the rise in algal cell numbers but also to the production of extracellular polymeric substances (EPS), which include polysaccharides, proteins, lipids, and nucleic acids. These EPS enhance the adhesion between biochar and algae, facilitating the formation of an immobilized complex, as similarly noted by Shen et al. [81]. The composition of EPS, which involves hydrophobic interactions, hydrogen bonding, and ionizable groups [79,82,83], likely strengthens the attachment of algae to biochar. Moreover, physical surface attachment is influenced by several factors, including electrostatic forces and the metal ion content present on biochar surfaces [84]. Hill et al. [79] indicated that biochar’s surface area and adsorption capacity play a primary role in enhancing microbial growth while also contributing to bacterial attachment. However, despite these insights, more detailed research is needed to fully understand the specific physical and chemical interactions between filamentous algae and biochar-enhanced mortar, such as identifying the role of surface functional groups in biochar–algal interactions., which remains an important area for further investigation.

Comparing the two species, Scenedesmus sp. consistently outperformed Chlorella sp., mirroring results observed during flask cultivation (Figure 7a,b). This is likely due to Scenedesmus sp.’s physiological traits, which make it more efficient at utilizing CO₂ and adapting to a broader range of environmental conditions [64]. Its adaptability enables Scenedesmus sp. to take greater advantage of the increased surface area and nutrient availability in biochar-enhanced environments. Additionally, Scenedesmus sp.’s superior ability to retain nutrients and efficiently sequester CO₂ likely contributed to its stronger growth response [62], particularly at higher biochar concentrations, such as CSB50 and CSB75. These combined factors allowed Scenedesmus sp. to achieve greater biomass accumulation, demonstrating its enhanced capacity to thrive in biochar-enriched environments compared with Chlorella sp.

Nevertheless, the reduction in both algae on CSB-enhanced mortars, particularly at higher concentrations like CSB50 and CSB75 after day 12, can be attributed to several factors leading to their earlier transition into the stationary phase. One possible explanation is that the high biochar content may adsorb metabolites, introducing environmental stress in the growth medium and creating competition for nutrients. As Hill et al. [79] suggested, such stressors can significantly impact microbial growth, which may alter the balance of the microbial community. Additionally, the production of EPS may be inhibited at these elevated biochar concentrations due to changes in surface roughness and nutrient availability. These alterations can disrupt the optimal conditions required for continuous growth, particularly after day 12, hindering sustained algal proliferation and causing both species to experience a decline.

3.4.2. Carbon Dioxide Absorption

The variations in CO₂ absorption ratios across CSB-enhanced mortar demonstrate the positive impact of biochar on CO₂ sequestration efficiency (Figure 9a). In the CM sample, the CO₂ absorption ratio (expressed as a percentage) began at 19.7% on day 0 and gradually increased to 48.1% by day 6, remaining constant through day 16, indicating limited sequestration capacity. However, the introduction of low levels of CSB (CSB2.5) resulted in a notable improvement, with the ratio starting at 20.8% on day 0 and reaching 52.3% by day 16, a 9% increase over the control. As biochar concentration increased, CO₂ absorption efficiency continued to improve, with CSB10 reaching 66.7% (a 39% increase), CSB25 reaching 71.8% (a 50% increase), and CSB50 peaking at 78.8% (a 64% improvement). The highest concentration, CSB75, achieved the greatest absorption ratio of 86.0%, marking a 79% increase over the control.

These results reinforce the well-established role of biochar as an effective medium for enhancing carbon sequestration. As biochar concentration increased, so did the absorption efficiency, primarily due to the larger surface area and greater porosity of the mortar, which allowed more CO₂ to be captured. The CO₂ adsorption capacity of biochar, as demonstrated by the BET results in Figure 4a, is influenced by various physicochemical properties. Zhang et al. [85] explained that CO₂ adsorption on biochar surfaces occurs through van der Waals forces between gas molecules and the solid phase, which are strongly associated with the specific surface area, pore size, and pore volume of the biochar. A larger surface area, combined with increased pore volume, provides more active sites for the physical adsorption of CO₂, resulting in a greater overall adsorption capacity [86,87].

Furthermore, the CO₂ absorption capacity of CSB, measured at 2.9 mmol g⁻¹ (Table 3), is largely influenced by its chemical properties, including surface functional groups, alkalinity, and mineral content. Key functional groups, such as hydroxyl (O–H), carboxyl, and carbonyl (C=O), identified through FTIR analysis (Figure 4), enhance CO₂ adsorption by facilitating hydrogen bonding between CO₂ molecules and the biochar surface [88]. Additionally, the presence of aromatic structures (C=C bonds) contributes to the stability of CO₂ retention, preventing rapid desorption and supporting long-term sequestration. This stability, along with biochar’s hydrophobicity and non-polarity, further enhances its capacity to capture and retain CO₂ effectively over time [6]. Though the presence of alkali metals (e.g., Na, K, Ca, Mg, and Li) in biochar can promote the formation of basic sites with a strong affinity for CO2 [6,89], this might not be the case in this study. While these metals are likely present in the unwashed biochar due to residual ash, the pH values in the experiments were not particularly alkaline. The observed increase from 7 to 8 was more likely attributable to photosynthetic processes rather than the presence of alkali metals. It should be also noted that CO₂ adsorption capacity may be reduced in humid environments due to the high affinity of porous materials for water molecules [90,91]. Biochar with hydrophobic and non-polar characteristics could mitigate this issue by limiting the competition from water molecules, thereby maintaining or even enhancing its CO₂ adsorption capacity.

The observation that all treatments reached their peak CO₂ absorption by day 6 suggests that the mortar’s CO₂ sequestration capacity had reached saturation. Once the biochar’s surface area and pores became fully occupied by CO₂, further absorption was restricted, leading to the plateau in absorption across all treatments. This aligns with the findings of Gupta et al. [92], who also noted a saturation effect in similar systems. This indicates that the CO₂ absorption potential is determined not only by the concentration of biochar but also by the physical and chemical limitations of the mortar matrix, which, once saturated, restricts further CO₂ uptake.

The changes in CO₂ absorption ratios of CSB-enhanced mortar at varying concentrations, with Chlorella sp. and Scenedesmus sp. grown on the mortar surfaces, are presented in Figure 9b,c. The presence of algae significantly (p < 0.05) enhanced CO₂ absorption, with CSB-enhanced mortars containing algae exhibiting higher CO₂ sequestration compared with those without algae. Across both species, higher CSB concentrations consistently led to greater CO₂ absorption. For Chlorella sp., in the control condition (CM + C), the CO₂ absorption ratio started at 18.0% on day 0, gradually increasing to 58.7% by day 16. At lower concentrations of biochar (CSB2.5 and CSB5), the CO₂ absorption ratios peaked at around 61.6% and 68.0% by day 12, reflecting moderate growth in absorption efficiency. However, at higher concentrations such as CSB25, CSB50, and CSB75, the CO₂ absorption ratios nearly reached saturation, with values of 99.5%, 99.9%, and 99.9%, respectively, by day 16. This represents a significant enhancement in CO₂ absorption, with Chlorella sp. achieving nearly 100% efficiency at these higher biochar levels.

Similarly, Scenedesmus sp. followed a comparable trend, with the control condition (CM + S) starting at 17.9% and increasing to 57.7% by day 16 (Figure 9b,c). At lower biochar concentrations (CSB2.5 and CSB5), Scenedesmus sp. displayed absorption ratios of 61.3% and 64.6% by day 12, similar to Chlorella sp. However, at higher concentrations (CSB25, CSB50, and CSB75), the absorption ratios again approached complete CO₂ absorption, peaking at 99.4% by day 14 for CSB25 + S and reaching 99.5% and 99.9% by day 16 for CSB50 and CSB75, respectively. This shows that both algae species were highly efficient at CO₂ absorption when enhanced by higher biochar concentrations.

It is evident that the CO₂ absorption ratio stabilized more rapidly at higher concentrations of CSB. For instance, in both Chlorella sp. and Scenedesmus sp. experiments, the CO₂ absorption ratio for CSB75 reached a plateau as early as day 8. This rapid stabilization indicates that higher concentrations of biochar enhance the mortar’s capacity to absorb CO₂ more efficiently. In comparison, lower concentrations of biochar, such as CSB2.5 and CSB5, exhibited a more gradual increase in CO₂ absorption, stabilizing closer to day 12. This suggests that higher CSB levels not only increase CO₂ absorption but also lead to faster stabilization in the absorption process.

These findings highlight the significant role that biochar plays in enhancing CO₂ absorption capacity in mortar, particularly when combined with algae such as Chlorella sp. and Scenedesmus sp. The presence of algae dramatically increased CO₂ sequestration efficiency due to their photosynthetic activity, which utilizes CO₂ as a key substrate for producing oxygen and carbohydrates [93,94]. As biochar concentrations increased, mortars were able to absorb more CO₂, creating favorable conditions for algae growth and more efficient photosynthesis. Additionally, the faster stabilization of CO₂ absorption at higher biochar concentrations suggests that biochar improved both the physical absorption of CO₂ by the mortar and the biological uptake by the algae, allowing the system to reach saturation more quickly. This synergistic relationship between biochar and algae underscores the effectiveness of combining these two components for significantly enhanced CO₂ sequestration in mortar, providing a promising strategy for improving carbon capture in practical applications.

When comparing the two species, Scenedesmus sp. marginally outperformed Chlorella sp. at the highest biochar concentrations, demonstrating a faster increase in absorption ratios and reaching its peak slightly earlier (Figure 9b,c). For example, Scenedesmus sp. reached its maximum absorption ratio by day 14 at CSB25 and maintained a consistent advantage over Chlorella sp. at higher biochar concentrations, such as CSB50 and CSB75. The slight advantage observed for Scenedesmus sp. over Chlorella sp. in CO₂ absorption at higher biochar concentrations can be attributed to the physiological and adaptive differences between the two species. Scenedesmus sp. may have a greater tolerance for the increased surface area and porosity provided by higher biochar levels, allowing it to more effectively utilize CO₂ for photosynthesis. A study by Li et al. [93] demonstrated that the CO₂ fixation potential of Scenedesmus sp. is species-specific, with this species exhibiting superior performance compared with Chlorella sorokiniana GS03 and Chlorella pyrenoidosa SJTU-2 under similar conditions.

Interestingly, the faster rise in CO₂ absorption ratios and earlier stabilization seen in Scenedesmus sp. suggests that it is better suited for environments with higher biochar content. These conditions may create a more favorable environment for its growth and CO₂ sequestration. This efficiency at higher biochar concentrations indicates that Scenedesmus sp. could be more effective in systems where maximum CO₂ absorption is required in a shorter time frame, making it a potentially advantageous species for biochar-enhanced carbon capture applications.

In practical applications, the concentration of CSB in cement mortar must be carefully adjusted to balance structural integrity, carbon sequestration potential, and practical concerns such as water absorption. The primary objective of this research is to explore the use of biochar-enhanced cement mortar as a dual-purpose material—both as a construction medium and as a platform for algae cultivation to enhance carbon capture. This approach aims to contribute not only to the strength of the material but also to its environmental sustainability.

For structural applications, where load-bearing is crucial (e.g., foundations, beams, or columns), lower CSB concentrations (up to 5%) are recommended. This ensures modest CO₂ absorption without compromising the mortar’s strength and durability. This provides an innovative solution by enabling the material to capture carbon while maintaining its structural integrity for load-bearing applications. These materials can be used in building designs that prioritize both environmental benefits and strength. In non-structural applications, such as facades, decorative elements, or paving stones, moderate CSB concentrations (10–25%) offer a balance between adequate strength and enhanced CO₂ sequestration. These applications can tolerate a slight reduction in strength in exchange for significant environmental benefits, specifically increased CO₂ removal through algae photosynthesis. This makes the material ideal for green infrastructure applications, where reducing environmental impact is prioritized over structural performance.

For bioactive facades and other green infrastructure applications, higher CSB concentrations (50–75%) can be used to maximize CO₂ capture. These concentrations are particularly suited for bioactive building surfaces designed to support algae growth for carbon capture or bioremediation. Such applications are especially relevant in urban areas, where bioactive surfaces can play a significant role in sustainability efforts by reducing CO₂ levels in the atmosphere and promoting biodiversity.

However, a key consideration is that both algae and high biochar concentrations can increase water retention in the substrate, potentially leading to issues such as excessive moisture and reduced energy efficiency due to compromised insulation. Additionally, high biochar content reduces the compressive strength and density of the mortar. To address the challenge of low compressive strength in high-CSB mortars, hybrid material systems could be explored. These systems could combine biochar with other strengthening agents or reinforcement strategies, such as fibers or polymers, to improve the mortar’s load-bearing capacity while maintaining its CO₂ sequestration ability. Moreover, reinforcement with materials like steel mesh or carbon fibers could enhance the structural performance of biochar-enhanced mortars, making them more suitable for applications that require load-bearing capacity. These approaches could enable the use of high-CSB mortars in applications that benefit from their carbon-negative properties while also addressing strength limitations.

Therefore, while these formulations show promise for carbon-negative construction and eco-friendly landscaping, they should be limited to non-load-bearing applications. These materials should be designed to manage water retention, structural integrity, and energy efficiency effectively. In this way, they can be optimized for use in non-load-bearing applications such as landscaping, bioactive facades, and decorative elements, where sustainability and environmental impact reduction are the primary goals.

4. Conclusions

This study highlights the novel potential of incorporating CSB into cement mortar, particularly for enhancing carbon sequestration and supporting algae growth. The unique physicochemical properties of CSB, including its high carbon content, porosity, and large surface area, make it an ideal candidate for dual-purpose applications that improve both structural performance and environmental sustainability. The incorporation of biochar significantly increased the mortar’s CO₂ absorption capacity, with concentrations above 25% showing the most notable improvements. At the highest CSB concentration of 75%, the mortar without algae achieved a CO₂ absorption ratio of 86%, while the inclusion of algae increased this ratio to nearly 100%, highlighting the synergistic effect between biochar and algal photosynthesis. Among the two species studied, Scenedesmus sp. outperformed Chlorella sp. in CO₂ absorption, particularly at higher biochar concentrations. This is likely due to Scenedesmus’s greater adaptability to the biochar’s porous structure and increased surface area.

Looking forward, several areas of future research are critical to optimize the use of biochar-enhanced mortar. Long-term durability studies are essential to assess the performance and sustainability of CSB-enhanced mortars, especially in real-world conditions, including exposure to weathering, mechanical stress, and the potential for CO₂ saturation over time. Such studies will be crucial in determining the long-term viability of these materials for infrastructure applications, ensuring that their environmental benefits do not come at the expense of performance.

Hybrid systems that combine algae with other complementary carbon capture technologies may extend the CO₂ absorption capacity of these materials in the long term. Additionally, exploring other biomass sources for biochar production could provide materials with specific characteristics better suited to enhancing both the structural integrity and carbon capture efficiency of the mortar. Scaling up the findings from this study to pilot-scale projects would provide valuable insights into the feasibility of using biochar-enhanced mortar in large-scale construction. Lastly, conducting a life cycle analysis of biochar-enhanced mortar systems will offer a comprehensive view of their environmental impact, including the carbon footprint of biochar production, transportation, and use. Economic assessments, including cost–benefit analysis, will be crucial in determining the long-term financial viability of biochar-enhanced mortar as a sustainable, carbon-negative construction material for the future.