Alkali-Modified Biochar Turns Cement Into a Powerful Carbon Capture Material

By Samudrapom DamReviewed by Bethan DaviesOct 28 2025

A new study reveals how treating biochar with alkali can significantly boost cement’s carbon dioxide (CO₂) absorption and strength.

Study: Investigation of the CO₂ adsorption behavior of alkali-modified biochar components in cement composites. Image Credit: Gulthara/Shutterstock.com

Scientists have discovered that alkali-modified biochar can strengthen cement and boost its carbon-capturing potential, according to a new study in Biochar X.

Background

Cement production is a major source of global CO₂ emissions, significantly contributing to environmental issues like global warming and air pollution. Although total cement output has declined due to economic and technological changes, reducing carbon emissions remains a major challenge.

Although mitigation strategies like cleaner fuels and carbon sequestration have been explored, many face limitations due to high costs and low efficiency. Carbon capture and storage (CCS) has shown promise as an alternative, offering high efficiency, low cost, and minimal harmful by-products.

Among the materials considered for CCS, biochar stands out due to its chemical stability, high specific surface area (SSA), porous structure, and low land requirements for waste disposal. Its CO₂ adsorption performance depends heavily on the type of feedstock used and the pyrolysis conditions, especially temperature. Research shows that rice husk biochar produced at 700?°C offers optimal CO₂ uptake. However, in its unmodified state, biochar’s CO₂ capture capacity still falls short of what's needed for large-scale applications.

To address this, modification techniques such as alkali treatment are used to enhance SSA and refine pore structures, significantly improving CO₂ adsorption.

Integrating modified biochar into cement has shown potential not only to increase carbon sequestration but also to enhance the mechanical strength of the resulting composite. Still, notable gaps remain. Most research treats biochar as a uniform material, often overlooking how its heterogeneous structure impacts CO₂ capture. Additionally, the potential use of CO₂-loaded sedimented particles (SP) within cement composites is an area that has seen little experimental investigation.

The Study

In this work, the research team investigated the alkali-modified biochar components’ CO₂ adsorption behavior in cement composites. The major objectives of the study were to assess the alkali modification’s impact on biochar’s different components; explore the influence of biochar’s different components on CO₂ adsorption performance; and analyze biochar’s effect on the cement composite properties under diverse conditions, including various CO₂ adsorption volumes, addition ratios, and pyrolysis temperatures.

The researchers synthesized the biochar through corn straw pyrolysis at 700, 500, and 300 °C, and obtained SP, the biochar’s key heterogeneous component, through physical separation. Then, they used alkali to treat both the SP and original biochar to examine their CO₂ adsorption performance. CO₂-saturated modified biochar, CO₂-saturated modified SP, original biochar, and SP were incorporated at 1 %, 2 %, 3 %, and 5 % dosages into cement composites to assess their effects on carbon sequestration performance and mechanical properties.

The biochar utilized here was obtained from corn stover and pyrolyzed at different temperatures. The resultant samples were designated as BC700, BC500, and BC300 based on their pyrolysis temperature. The BC700, BC500, and BC300’s median particle sizes were 9, 9.02, and 10.9 μm, respectively. These sizes were smaller compared to the ordinary Portland cement (OPC) particles’ 15.14 μm size. Deionized water and the chemical reagent sodium hydroxide (NaOH) were also used in this study.

The physical decomposition method was used to prepare the precipitated particles. The team prepared the alkali-modified biochar by dispersing biochar in NaOH for 30 minutes, followed by modification, filtration and drying. The obtained samples were designated as modified biochar (MBC)300, MBC500, MBC700, modified SP (MSP)300, MSP500, and MSP700.

Then, alkali-modified biochar’s different components were heated in a nitrogen-filled rotary furnace at 120 °C for 1 hour, cooled to ambient temperature, and exposed to CO₂ for 2 hours to obtain CO₂-saturated forms, including CMBC300, CMBC500, CMBC700, CMSP300, CMSP500, and CMSP700. CMBC and CMSP represent CO₂-saturated modified biochar and SP, respectively.

As CO₂ capture in biochar mainly occurs via physical adsorption with a minor chemical contribution, its morphology and composition remain unchanged, enabling accurate evaluation of its performance in cement composites. CO₂ adsorption tests followed established procedures using thermogravimetric analysis.

Approximately 20 mg of biochar was heated to 120 °C under nitrogen for 1 hour to remove impurities. After cooling to 25 °C, high-purity CO₂ was introduced, and the resulting mass increase indicated the amount of CO₂ adsorbed, neglecting nitrogen adsorption. Blank tests under identical conditions were performed to correct for buoyancy and instrument effects, ensuring accurate determination of CO₂ adsorption capacity.

Study Findings

SP exhibited higher CO₂ adsorption capacity than unmodified biochar, with performance improving as the pyrolysis temperature increased. While alkali modification occasionally led to a slight reduction in total surface area - due to the collapse of mesopores - it refined the microporous structure, resulting in more efficient adsorption. 

The CO₂ capture process was primarily physical, though the Avrami kinetic model suggested some localized chemical adsorption across different regions of the material. Additionally, the presence of biochar initially stimulated the formation of hydration products, but this effect diminished at higher pyrolysis temperatures.

Cement composites with CO₂ -loaded modified SP (CMSP) showed enhanced performance, particularly at a 1 % substitution rate, which yielded higher compressive strength than those containing only SP. Among the tested variants, biochar produced at 500?°C delivered the best overall results, combining strong CO₂  adsorption with improved mechanical performance in the cement matrix. However, excessive CO₂ loading introduced a risk of over-carbonation, which can create additional voids and slightly weaken compressive strength. This underscores the importance of optimizing both biochar dosage and treatment methods.

Importantly, biochar integration reduced the overall carbon footprint of the cement mixtures. That said, emissions from chemical reagents like NaOH, that are used in the modification process, may partially offset these environmental benefits and should be accounted for in life cycle assessments.

In summary, the study highlights that the right combination of biochar type, treatment, and dosage can convert conventional cement into a viable carbon storage medium without sacrificing performance. Still, future research is needed to assess long-term durability under real-world environmental conditions.

Journal Reference

Guo, B. et al. (2025). Investigation of the CO₂ adsorption behavior of alkali-modified biochar components in cement composites. Biochar X, 1(1). DOI: 10.48130/bchax-0025-0004, https://www.maxapress.com/article/doi/10.48130/bchax-0025-0004

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.