One-pot synthesis of Cu²⁺-doped 4A zeolite from kaolin: Structural characterization and functional potential analysis

• A one-pot hydrothermal method was developed to synthesize 4A/ Cu²⁺ zeolite composites using kaolin as the raw material. • This strategy simplifies synthesis process, ensures uniform dispersion of Cu, and maintains the integrity of the 4A zeolite framework. • Cu²⁺/Cu⁺ redox cycle catalyzed formaldehyde oxidation to harmless CO₂ and H₂O. • Cu²⁺ released from the zeolite formed a stable CuO/Cu₂O passivation film. • The composite exhibited optimal comprehensive performance, featuring low cost, simple synthesis, good compatibility with construction materials. To address critical challenges in the construction field, such as indoor formaldehyde pollution and steel bar corrosion in concrete structures, Cu²⁺-doped 4A zeolite composites (4A/xCu) were synthesized via a one-pot method using kaolin (an abundant and low-cost natural mineral) as the raw material. The crystal structure, morphology, elemental distribution, specific surface area, and functional properties of the composites were systematically characterized via XRD, SEM, TEM, EDX, BET analysis, and XPS. XRD results confirmed that all 4A/xCu composites retained the cubic crystal structure of pure 4A zeolite, with no significant structural destruction even after Cu²⁺ doping—ensuring compatibility with alkaline construction environments (e.g., concrete pore solution, alkaline coatings). SEM and TEM observations showed that pure 4A zeolite exhibited a well-dispersed cubic morphology, while protonation and Cu²⁺ doping modified its surface to an oval shape without compromising crystallinity. This morphological transformation, combined with the porous structure verified by BET analysis, significantly increased the specific surface area of the composites (up to 9.81 m²/g for 4A/80 wt.%Cu, much higher than 1.54 m²/g of pure 4A zeolite)—providing abundant channels and active sites for formaldehyde adsorption and corrosion inhibitor release. EDX and XPS analyses verified the uniform distribution of Cu elements in the composites, where Cu mainly existed as Cu₂O (with a small amount of Cu²⁺). The 4A/60 wt.% Cu has the best corrosion resistance because it has the lowest corrosion voltage (0.92 V), the smallest Tafel slope (21.6 mV dec ‑1 ), and the smallest absolute value of corrosion current (-2.9 × 10 5 μA cm - ²). This study demonstrates that the one-pot-synthesized Cu²⁺-doped 4A zeolite composites exhibit excellent structural stability and dual functional properties (formaldehyde purification and steel corrosion protection) tailored to construction requirements. Especially the 4A/60 wt.%Cu composite, with its low cost, simple synthesis process, and good compatibility with construction materials (e.g., coatings, concrete), holds great potential as a high-performance functional additive in green and durable building materials.