By modifying biochar, researchers have developed cementitious composites that are not only stronger but actively remove carbon dioxide from the atmosphere. The inclusion of biochar modified with K₂SiO₃ achieved a 44% increase in compressive strength and delivered up to a 133% reduction in the global warming potential, successfully creating a high-performance, carbon-negative building material.

In an article published in ACS Sustainable Resource Management, Nishad Ahmed, Adhora Tahsin, Warda Ashraf, Qiangu Yan, and Zhiyong Cai explored how engineered biochar, used as an additive in cementitious composites, can lead to carbon-negative construction materials. Ordinary Portland Cement (OPC) production contributes significantly to global CO_2​ emissions, making the search for sustainable alternatives a critical need. This study investigated eight different biochar types, produced through air exposure for varying durations (1 or 12 weeks) or by incorporating chemical additives like potassium silicate (K_2​SiO_3​ ​) or sodium hydroxide (NaOH) during pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More. These modified biochar samples were added at a high dosage of 30% by weight of the binder, which consisted of a 1:1 mixture of slag cement and OPC.

The addition of all biochar batches to the mortar resulted in a negative carbon footprint, offering a global warming potential (GWP) reduction ranging from 117% to 133% compared to the control batch (which had a high GWP of 277.4 kg of CO_2​ equivalent per m3 of mortar). The unmodified biochar exposed to air for 12 weeks (GB 12wk) showed a GWP of approximately −60 kg CO₂​ equivalent per m3, which was an improvement over the 1-week exposed biochar (GB 1wk). The batches modified with K_2​SiO_3​ ​ exhibited the highest carbon content, ranging from 86.4% to 87.2%, and achieved the maximum negative carbon footprint, ranging from −89.67 to −90.68 kg CO₂​ equivalent per m3. This superior environmental performance is attributed to both the biochar’s high carbon storage capacity and its ability to increase CO₂​ sequestration in the binder matrix itself.

Beyond the environmental benefits, the biochar modifications significantly impacted the mechanical performance of the resulting mortar. In terms of compressive strength, biochar modified with K_2​SiO_3​ for 1-week air exposure (K Si 1wk) performed the best among all biochar-containing batches. This batch achieved a 28-day compressive strength of 33 MPa, which was 44% higher than the strength of the un-modified biochar and was statistically comparable to the control mortar without biochar. This enhancement is likely due to the pozzolanic properties of the silica from the K_2​SiO_3​ ​ addition, which helps to form strength-enhancing calcium silicate hydrate (C-S-H) in the cementitious matrix. Conversely, the benefit of the K_2​SiO_3​ treatment diminished with longer air exposure (K Si 12wk), as the strength was similar to the plain 12-week air-exposed biochar. This reduction is linked to the increased agglomeration of silica on the biochar surface over time.

For the biochar modified with NaOH, the compressive strength showed a dependency on the dosage. The 0.5% Na batch provided the maximum 28-day compressive strength (29 MPa) among the Na-containing batches, offering a 27% increase compared to the air-exposed biochar batch. NaOH acts as an activator for the slag cement in the binder, accelerating the initial cement hydration and leading to higher early-age strength. However, higher dosages of NaOH (1% and 2%) reduced the 28-day strength due to increased capillary porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More within the mortar matrix.

Furthermore, the duration of air exposure for the plain biochar proved important. The biochar exposed to air for 12 weeks improved the 28-day compressive strength by 15% compared to the 1-week exposed batch. This benefit is tied to the longer exposure facilitating more physical adsorption of atmospheric CO_2​ in the biochar’s porous structure. This adsorbed CO_2​ then accelerates the cement hydration process, which was visible in the isothermal calorimeter tests as a faster initial heat release. Overall, this study successfully demonstrated that, by engineering the biochar’s surface properties, it is possible to use a high-dosage of biochar (30% replacement) to create cementitious composites that achieve both a negative carbon footprint and acceptable mechanical strength.

Source: Ahmed, N., Tahsin, A., Ashraf, W., Yan, Q., & Cai, Z. (n.d.). Engineered production of biochar as concrete additives and its role in carbon-negative cementitious composites. ACS Sustainable Resource Management.