A high water-cement ratio aerated concrete using carbon dioxide as the foaming agent: Mix design, pore structure and hydration products

Aerated concrete, due to its porous structure, is an attractive candidate for reducing carbon dioxide (CO₂) emissions. In this study, CO₂ was incorporated into aerated concrete through a chemical foaming method to prepare carbon dioxide aerated concrete (CAC) with targeting density of 600 ± 50 kg/m³. Response surface methodology (RSM) was applied to optimize the mix design and study the significance levels and effect of water-cement ratio (W/C), hydroxypropyl methylcellulose (HPMC) dosage, and foaming agent dosage on 7-day (7d) compressive strength and dry density. Based on the optimized mix proportion, the pore structure and hydration products of CAC with varying W/C were investigated using advanced image processing technique (the open-source segment anything model combined with Image-J software), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Additionally, the environmental and cost benefits of CAC's raw materials were assessed. The results showed that the mix design of CAC using RSM had high accuracy and reliability, the sample with the optimized mix proportion achieved a 7d compressive strength of 3.35 MPa and dry density of 622.34 kg/m³. It was found through microstructure investigation that CO₂ as the foaming agent can form effective pores, and the pores can be evenly distributed in the slurry with the increase of W/C. The porosity and spatial distribution of pores were identified as critical factors correlated with the material properties and should be prioritized in the regulation of pore structure. The introduction of CO₂ altered the morphology of ettringite (AFt) and aluminum hydroxide gel (AH₃), resulting in a more compact internal structure. Environmental and cost assessments showed that CAC exhibited lower embodied CO₂ emission (296.86 kgCO₂-eq/m³) and lower production cost (93.47 USD/m³) compared to conventional autoclaved aerated concrete (AAC) of the same grade (A3.5, B06, GB/T 11968-2020). These findings demonstrated a viable strategy for developing eco-friendly and high-performance aerated concrete materials.