Influence of the pore structure of biochar on the load-bearing and filtering behavior of biochar as a concrete admixture

The concrete industry accounts for about 7% of global greenhouse gas emissions, primarily due to calcination and energy-intensive combustion processes used in Portland cement (OPC) production. Reducing emissions by replacing clinker with supplementary cementitious materials (SCMs) is promising but limited. An alternative strategy involves using biochar, a by-product from pyrolysis of agricultural and forestry waste. Due to its high specific surface area and pronounced porosity, biochar can function as a filler or partial cement substitute, optimizing concrete properties and acting as a carbon sink. However, the potential reduction in compressive strength due to biochar incorporation remains a significant research challenge, particularly regarding biochar’s microstructural effects on cementitious materials. This study examines the biochar microstructure and its impact on load-bearing and filter properties in cementitious systems. Initially, synchrotron-based computed tomography (SXCT) characterized the microstructure by quantifying pore size, wall thickness, and porosity. Additionally, chloride ion absorption by biochar was analyzed using ICP-OES. The second study phase assessed biochar’s suitability for filter applications through spatial moisture distribution measured by XCT, moisture profiles determined by ¹H-NMR, and chemical element composition analyzed by LIBS. SXCT analyses revealed significant differences in pore sizes among biochars, whereas wall thicknesses remained relatively uniform. The ¹H-NMR tests demonstrated significantly enhanced moisture absorption in biochar-containing mortar compared to control samples. LIBS analyses indicated higher carbon content in biochar-enhanced mortars but showed no substantial differences in potassium, sodium, calcium, or silicon contents. ICP-OES testing confirmed biochar’s ability to absorb chloride ions, with absorption increasing over prolonged exposure. Results underline biochar’s pore structure as critical to its functional performance within cementitious systems. The interplay between pore size, porosity, and cement paste penetration into biochar pores significantly influences mechanical strength, moisture absorption, and filtration properties. Biochar incorporation effectively enhances moisture retention, demonstrating significant potential for filter applications. However, the concurrent reduction in mechanical strength remains a notable challenge. This research highlights biochar as a promising multifunctional additive for sustainable concrete construction, offering novel strategies to enhance environmental sustainability by reducing CO₂ emissions. Nonetheless, further studies are necessary to fully comprehend the intricate relationships between biochar microstructure, mechanical performance, and filter capabilities. Overall, the findings provide valuable insights into developing sustainable building materials, contributing substantially to emission reductions in the construction sector.