New research highlights how modified biochar can reduce emissions and enhance strength in next-generation cement composites. The study was published in Biochar X.
Study: Investigation of the CO2 adsorption behavior of alkali-modified biochar components in cement composites. Image Credit: Gulthara/Shutterstock.com
The Importance of Biochar
Despite recent reductions in output, cement production remains a major source of global carbon emissions, contributing significantly to climate and environmental challenges. While various CO2 reduction technologies are already available, many face limitations when scaled up, primarily due to cost and efficiency concerns. This has pushed carbon capture and storage (CCS) into the spotlight, thanks to its broader applicability and more favorable economics.
Biochar is gaining traction as a carbon capture material due to its chemically stable structure, high specific surface area, and porous nature. It's created through biomass pyrolysis in oxygen-limited (anaerobic) conditions. However, the performance of unmodified biochar in capturing CO2 is still limited. That’s why researchers have been exploring ways to enhance its adsorption capacity through chemical, physical, and biological modifications.
In the cement industry, modified biochar isn’t just about carbon capture - it also offers potential structural benefits. It can improve the mechanical strength of cement composites, making it a dual-purpose material.
When biochar breaks down, it separates into different components: coarse suspended particles (CP), sedimented particles (SP), and soluble fractions containing ultrafine particles (SCUP). Among these, SP stands out. It has the highest specific surface area and most developed pore structure, which means better CO2 adsorption and a notable boost in compressive strength.
However, researchers still know relatively little about how these heterogeneous components behave during CO2 adsorption. Until now, the impact of using CO2-loaded SP in cement composites hasn’t been studied experimentally.
Inside the Study
This research set out to fill those gaps. The team focused on understanding how alkali-modified biochar components behave in terms of CO2 capture and how they influence the properties of cement composites.
To achieve this, they produced biochar from corn stover at three pyrolysis temperatures: 700 °C, 500 °C, and 300 °C. These samples were labeled BC700, BC500, and BC300. Their median particle sizes were 9, 9.02, and 10.9 μm, respectively, all smaller than ordinary Portland cement (OPC), which averaged 15.14 μm.
For the experiments, deionized water and sodium hydroxide were used as base materials. Researchers synthesized precipitated particles (SP) using a physical decomposition method: 3 grams of biochar were dispersed in 100 mL of ultrapure water, ultrasonicated for 30 minutes, and left to settle for 72 hours. The resulting precipitates were labeled SP700, SP500, and SP300, corresponding to their pyrolysis temperatures.
To prepare the alkali-modified samples, 1 gram of each biochar type was mixed with 40 mL of a 1 mol/L sodium hydroxide solution for 30 minutes, then filtered and dried. These were designated MBC700, MBC500, MBC300 for the original biochar and MSP700, MSP500, MSP300 for the SP samples.
Next, the modified samples were heated to 120 °C for one hour in a nitrogen atmosphere, and then cooled and exposed to CO2 for two hours to achieve saturation. These CO2-loaded versions were labeled CMBC700, CMBC500, CMBC300, CMSP700, CMSP500, and CMSP300. Since CO2 capture here was driven by physical adsorption, the biochar’s structure remained unchanged post-saturation.
To measure CO2 uptake, researchers used thermogravimetric analysis. Approximately 20 mg of each sample was heated to 120 °C under nitrogen to remove impurities, then cooled to 25 °C before being introduced to high-purity CO2. Any mass gain was attributed to CO2 adsorption. Blank tests helped correct for buoyancy and equipment variables.
Cement composites were also tested, incorporating 1 %, 2 %, 3 %, and 5 % biochar by weight at a water-to-cement ratio of 0.4. Mixtures were stirred for 10 minutes, poured into 4 × 4 × 4 cm molds, demolded after 24 hours, and cured under standard conditions. Each test was conducted with three replicates. Researchers evaluated both material characteristics and compressive strength.
What the Results Showed
The sedimented particles (SP) consistently showed higher CO2 adsorption capacity compared to unprocessed biochar, and this capacity improved with higher pyrolysis temperatures. Among all samples, the one pyrolyzed at 500 °C delivered the best performance for CO2 uptake.
Alkali modification enhanced the microporous structure of the biochar, boosting its ability to adsorb carbon dioxide. Most of the adsorption occurred through physical mechanisms, with only minimal chemical interaction.
When added to cement, moderate amounts of biochar improved performance through multiple effects - acting as a micro-filler, providing volcanic ash-like reactivity, and promoting secondary hydration. However, using too much increases porosity and reduces strength.
SP, in particular, enhanced the compressive strength more than the original biochar. Even more impressive, CO2-saturated modified SP (CMSP) further boosted performance. That said, oversaturation with CO2 could backfire, causing voids and over-carbonation.
An added bonus: incorporating biochar also reduced the overall carbon footprint of the cement composites.
Takeaway
This study offers valuable insights into how biochar - especially its modified and component forms - can support carbon capture and mechanical performance in cement composites. It opens the door to more sustainable construction materials that actively help mitigate emissions.
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
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