Carbon-Negative Building Material from Waste and CO₂ Capture

Reducing CO₂ emissions from the building sector is critical, as it consumes 40% of global raw materials and 35% of energy. Yong Ding and Ingo Burgert, Institute for Building Materials, ETH Zürich, Switzerland, and their team tackled this challenge by developing a scalable method to turn demolition waste and biomass byproducts into sustainable building materials while capturing CO₂.

The team extracted calcium ions from recycled concrete aggregates (RCA), which contain calcium silicate hydrates, calcium hydroxide (Ca(OH)₂), and calcium carbonate (CaCO₃). They tested various leaching agents, including deionized water, hydrochloric acid, and ammonium salts. Ammonium acetate proved most effective, yielding 7,857 mg/L of Ca²⁺, 120 times more than water, without releasing stored carbon. The resulting alkaline leachate (pH ~10) was ideal for mineralization.

CO₂ gas was bubbled into the calcium-rich solution, triggering a series of reactions in which CO₂ dissolved in water, formed carbonic acid, which then dissociated into carbonate ions that reacted with Ca²⁺ to precipitate CaCO₃. The process removed over 99.9% of Ca²⁺, and Fourier transform infrared spectroscopy confirmed calcite as the dominant mineral phase.

To create printable ink, the researchers blended the CaCO₃ with kraft lignin, a renewable byproduct from wood pulping, and water to form a mineral binder. Lignin improved dispersion and adhesion to wood. Rheological tests showed the binder was shear-thinning and flowed easily. Mixing in sawdust (10–20 wt%) produced stable inks, with 18 wt% sawdust offering optimal printability. After drying, the printed composite contained 29 wt% CaCO₃, 29 wt% lignin, and 42 wt% sawdust. The team successfully printed modular structures with consistent quality.

Mechanical tests revealed moisture sensitivity: at 25% relative humidity (RH), compressive strength reached 11 MPa; at 95% RH, the material absorbed water and showed sponge-like behavior. A cradle-to-gate life cycle assessment showed a global warming potential (GWP) of −0.85 kg CO₂-equivalent per kg of material, thanks to biogenic carbon storage. Compared to conventional materials like medium-density fiberboard and polyurethane foam, the printed composite had the lowest environmental cost.

By combining CO₂ capture, waste valorization, and additive manufacturing, the researchers demonstrated a closed-loop strategy for decarbonizing construction. Future work could focus on improving moisture resistance and scaling the process for industrial use.