In the global effort to combat climate change, the construction industry faces a significant challenge: reducing its embodied carbon footprint. While operational carbon (emissions from a building’s energy use) has traditionally been the focus, embodied carbon (emissions from material extraction, manufacturing, transport, and disposal) is projected to account for nearly half of new construction’s total carbon emissions by 2050. This necessitates innovative approaches to material design and construction. Nikol Kirova’s PhD thesis, “GRADING CARBON: Architectural design framework for functionally graded biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More cementitious composites towards carbon sequestering building elements,” submitted in April 2025 to Swinburne University of Technology, presents a groundbreaking solution by proposing functionally graded biochar cementitious composites, or “CharCrete,” as a carbon-sequestering building material.

Kirova’s research challenges the conventional homogeneity of concrete, arguing that its uniform composition often leads to overdesign and excessive material use, contributing significantly to embodied carbon. Inspired by nature’s functionally graded materials (FGMs), like human bone and bamboo stems, which exhibit varying compositions and densities to optimize strength with minimal mass, the thesis proposes a “Grading Carbon Design Method”. This method aims to maximize carbon storage within building elements while meeting specific structural requirements by strategically varying the concentration of biochar within the cementitious matrix.

Biochar, a carbon-rich material produced from the 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 of biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More, is central to this innovation due to its negative embodied carbon. The thesis highlights that biochar can sequester up to 3.5 kg of CO2 per kilogram of biochar produced, making it a powerful tool for offsetting the CO2 emitted by cement and sand. Kirova’s research involved extensive material experimentation, developing a “CharCrete” material system with nine grades, varying biochar content from 0% to 70% by volume. These experiments demonstrated that increasing biochar content consistently reduced material density and embodied carbon. Notably, the BCM_B20 grade (20% biochar by weight) achieved a neutral embodied carbon with a compressive strength of 25 MPa, meeting structural requirements for residential construction. Grades with higher biochar content, while having lower mechanical performance, exhibited significantly negative embodied carbon values, making them ideal for non-structural carbon-sink applications.

The “Grading Carbon” design method leverages computational tools, primarily Karamba3D within the Grasshopper environment, to inform the spatial allocation of different CharCrete grades. This computational workflow allows for the optimization of building elements (such as slabs and walls) to maximize carbon sequestration without compromising structural integrity. For instance, studies on 5-meter span flat floor slabs using the CharCrete graded approach achieved a 43% mass reduction and a net negative embodied carbon, comparable to other FGC studies that achieved similar mass reductions but without the carbon sequestration benefit. The method prioritizes embodied carbon reduction as a primary design parameter, setting it apart from traditional topology optimization which often focuses solely on material reduction.

Beyond design, Kirova’s thesis explores practical fabrication strategies for these functionally graded CharCrete elements. Two main methods were investigated: “print and cast” (using 3D printed CharCrete formwork for casting) and “partition and cast” (simultaneous multi-grade casting aided by physical partitions). Experimental results showed that both methods provided good bonding between different CharCrete grades, with the “print and cast” method emerging as more efficient due to lower associated embodied carbon and higher customization potential. This method allows for a seamless transition from digital design to physical realization, effectively bridging the gap between material science, computational design, and architectural fabrication.

In conclusion, the research by Nikol Kirova offers a holistic framework for designing optimized carbon-negative building elements. It redefines architecture’s role in addressing climate change by proposing buildings not merely as shelters but as active carbon sinks, integrating material science with advanced design and fabrication techniques. This work contributes significantly to the development of sustainable construction practices and a more carbon-efficient built environment.

Source: Kirova, N. (2025). Grading Carbon: Architectural design framework for functionally graded biochar cementitious composites towards carbon sequestering building elements. (Doctoral thesis). Swinburne University of Technology. Sources