Natural hydraulic lime has been used for centuries in masonry and restoration because it is compatible with traditional building materials and can absorb carbon dioxide as it hardens. Now, researchers have found that adding a small amount of finely ground biochar could strengthen this lime-based material while helping it capture more CO2.
In a study published in Biochar X, researchers examined how different biochar particle sizes and dosages affected the mechanical properties, pore structure, carbonation behavior, and CO2 uptake of natural hydraulic lime.
The best overall results were achieved by adding 2% biochar with a particle size of 325 mesh. Compared with lime containing no biochar, the optimized material showed a 35.7% increase in compressive strength after three days, a 42.1% increase after seven days, and a 10.9% increase after 28 days.
"Our results show that a relatively small amount of finely divided biochar can improve both the early strength and carbon uptake of natural hydraulic lime," said Yue Gu, corresponding author of the study. "This balance is particularly important for developing lower-carbon materials that remain suitable for the repair and conservation of historic buildings."
Natural hydraulic lime is produced at lower temperatures than conventional Portland cement and can absorb atmospheric CO2 through carbonation. During this process, calcium-containing compounds in the lime react with CO2 to form calcium carbonate, which can fill pores and strengthen the material.
Biochar, a carbon-rich porous material produced through the heating of biomass under limited oxygen, can store stable biomass-derived carbon for long periods. Its porous structure can also provide pathways and adsorption sites for gases.
The researchers tested biochar produced from coconut shells at particle sizes of 100, 200, and 325 mesh, and at several dosage levels. They evaluated compressive strength, porosity, pH, mineral composition, microscopic structure, and CO2 uptake over time.
The optimized material containing 2% of the finest biochar absorbed 14.6% more CO2 after six hours and 11.9% more after 24 hours than the control material. Its apparent CO2 uptake rate also increased by 3.2%.
Microscopic and chemical analyses showed that biochar promoted the conversion of calcium hydroxide and other reactive components into calcium carbonate. Quantitative X-ray diffraction indicated that adding 2% biochar increased the calcium carbonate content from approximately 60.2% to 63.9%.
According to the researchers, biochar supports carbonation in three main ways. Its porous structure creates additional pathways for CO2 transport, its large surface area promotes local CO2 enrichment, and its presence increases contact between CO2 and reactive lime components. As calcium carbonate forms, it fills small voids and produces a denser internal structure.
However, more biochar was not always better. Dosages above 2% increased porosity and weakened the continuity of the lime matrix. Although higher dosages captured more CO2, the resulting material did not achieve the same balance between carbon uptake and mechanical strength.
The findings demonstrate that particle size and dosage must be carefully optimized when biochar is incorporated into lime-based construction materials.
The researchers suggest that biochar-modified natural hydraulic lime could provide a low-carbon alternative for historic building restoration, heritage conservation, and environmentally responsible antique-style construction. Future studies will examine how environmental conditions and long-term exposure affect its carbon uptake, durability, and structural performance.
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Journal reference: Zhang H, Qu J, Gu Y, Li Y, Li A, et al. 2026. Influence of biochar dosage and particle size on CO2 uptake and mechanical properties of natural hydraulic lime. Biochar X 2: e017 doi: 10.48130/bchax-0026-0017
https://www.maxapress.com/article/doi/10.48130/bchax-0026-0017
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About the Journal:
Biochar X (e-ISSN: 3070-1686) is an open access, online-only journal aims to transcend traditional disciplinary boundaries by providing a multidisciplinary platform for the exchange of cutting-edge research in both fundamental and applied aspects of biochar. The journal is dedicated to supporting the global biochar research community by offering an innovative, efficient, and professional outlet for sharing new findings and perspectives. Its core focus lies in the discovery of novel insights and the development of emerging applications in the rapidly growing field of biochar science.
