Effect of low-pressure CO₂ injection timing on foam stability and sustainability of foamed concrete

Foamed concrete conventionally relies on chemical or protein-based stabilizers that improve foam stability but significantly increase embodied carbon and material cost. This study introduces low-pressure CO₂ injection as a reactive and sustainable alternative. The central hypothesis is that controlled in-situ carbonation during early hydration enhances foam stability through the simultaneous formation of hydration and carbonation products that densify and reinforce bubble walls. Fresh foamed cement paste (w/c = 0.5) was subjected to CO₂ injection for 10–40 min to identify the optimal reaction window and dosage. Results revealed that nano-calcium carbonate (CaCO₃) formation and accelerated calcium–silicate–hydrate (C–S–H) development refined the pore network and significantly reduced shrinkage by filling capillary voids. The optimal dosage of 2.25% CO₂ by weight of cement (20-minute injection, FC-C20) resulted in 58.6% higher pore sphericity, 40.3% higher compactness, and 27% finer average pore size. This refinement led to 12% higher compressive strength, up to 18% CO₂ uptake, and minimal impact on thermal conductivity. Over-carbonation (CO₂ '2.25 wt%) destabilized the foam structure due to excessive CaCO₃ deposition and film stiffening, while reducing pH to 9.5 and degrading the C-S-H gel. Life-cycle analysis showed that FC-C20 achieved 51–61.5% lower global warming potential (GWP) and up to 56.8% (laboratory scale) and 19.4% (industrial scale) lower total cost compared with conventional stabilizers. Overall, the findings reveal CO₂’s dual role as a foam stabilizer and carbonating agent, resulting in low-carbon, high-performance foamed concrete with direct CO₂ utilization.