In situ self-constructing superhydrophobic foam concrete driven by CO₂ activation: Towards intelligent self-protective response

The highly porous and hydrophilic nature of foam concrete results in high water absorption and inferior durability, thus restricting its wider applications. Inspired by the self-protective mechanisms observed in organisms, this work developed a foam concrete with a self-constructing superhydrophobic capability in response to CO₂. An inorganic-organic hybrid aqueous foam was engineered to induce micro-roughness hydration products and low surface energy materials on the pore walls. The microstructural regulation mechanisms and in-situ growth characteristics of the superhydrophobic layer on the pore walls were revealed. The modified foam formed a dense layer of fibrous ettringite on the pore walls, creating a micro-scale rough structure. When exposed to CO₂ in the air, the hydrophobic molecules undergo hydrolysis and chemically bond with cement hydrates (i.e., C-S-H and portlandite) to form densely packed hydrophobic gels in the pores. As a result, a robust hydrophobic layer was self-constructed on the pore wall surface, achieving a water contact angle of ∼160° after exposing in air for 8 h (CO₂ ∼0.04 vol%). The superhydrophobic concrete (density ∼830 kg/m3) exhibited low water uptake of ∼1.5 vol% in corrosive solutions (pH 3-11, 3.5 wt% NaCl). After 14 d exposure, the total pore volume of the superhydrophobic surface reduced by ∼25 %, exhibiting excellent chemical robustness, carbonation resistance, and corrosion durability. This study provides a facile solution for designing foam concrete with CO₂-responsive superhydrophobic properties, significantly enhancing the carbonation resistance and chemical durability of porous cementitious materials.