Elevated temperature environments readily induce significant concrete deterioration, manifested through accelerated cracking, exacerbated spalling, and structural integrity degradation resulting from thermal expansion mismatch. To ensure the protective efficacy of concrete structures under high-temperature exposure, functional surface coatings are frequently employed for performance enhancement. Phosphate-based inorganic coatings have garnered considerable attention within high-temperature protection applications due to their exceptional adhesion, superior thermal stability, environmental compatibility, economic viability, and inherent safety. In this study a low-temperature curing aluminum-magnesium phosphate-based inorganic gas barrier coating (designated AHPM) is designed and synthesized to improve application convenience and efficiency for concrete protective coatings. The optimized coating composition comprises: aluminum dihydrogen phosphate binder ((25±2) wt%), alumina reinforcing filler ((48±2) wt.%), modified magnesium oxide curing agent ((2±1) wt.%), supplemented with fumed silica anti-settling agent (1.8wt.%) and silane-based defoamer (0.2wt.%). Application involved uniform manual brushing onto concrete substrates, followed by a brief thermal curing at 200 ℃ for 10 minutes to achieve full polymerization. An innovative dual-medium (coating-concrete) gas permeability model is developed. Leveraging Darcy's law, the intrinsic gas permeability coefficient of the AHPM coating itself (6.89×10-18 m2) is quantitatively determined. This involves linear fitting of gas permeability data obtained from both uncoated concrete specimens and coated concrete composite specimens tested under multiple pressure differentials, followed by computational derivation of the coating's specific permeability. Comprehensive analysis confirms that the formulated AHPM coating exhibits outstanding film-forming characteristics, straightforward manufacturability, rapid surface drying kinetics within minutes, and extended service life under thermal cycling. Scanning Electron Microscopy (SEM) characterization verifies that the AHPM coating cured at 200 ℃ possesses a continuous, crack-free, pore-free, smooth, and planar surface morphology, indicative of excellent film quality. The coating exhibits a densely packed internal microstructure, demonstrates thorough interfacial wetting with the concrete substrate, and achieves remarkably high bonding strength (5 MPa) exceeding typical requirements. FT-IR analysis shows that when the temperature exceeds 100 ℃, a new Al—O peak appears at 748 cm-1, and the intensity approaches its maximum when the temperature rises to 200 ℃. This peak indicates a crosslinking reaction between polyphosphate (P—O—P) and Al2O3 filler, generating aluminum phosphate (P—O—Al). The crosslinking reaction is almost complete, and at this point, the coating has formed a stable three-dimensional network structure crosslinked by P—O—Al bonds. This network tightly combines phosphate, alumina fillers, and SiO2 - encapsulated magnesium oxide curing agents. This crystalline structure provides a fundamental theoretical basis for the coating's high interfacial bond strength and superior resistance to water ingress. Furthermore, the coating exhibits minimal high-temperature mass loss (2.42%), collectively validating its exceptional performance stability and reliable protective functionality under thermal stress. Integrating mechanistic analysis with experimental findings, the curing mechanism of the AHPM coating is elucidated as follows: Thermal energy initiates a dehydration-polycondensation reaction chain. Progressive temperature elevation facilitates the sequential removal of both free water and chemically bound water molecules. The incorporation of MgO elevates the system pH dramatically, thereby accelerating phosphate precipitation and polymerization. Concurrently, Mg²+ cations function as pivotal crosslinking nodes, facilitating the formation of a robust three-dimensional aluminum phosphate (P—O—Al) network structure via ionic and covalent bonding between phosphate anions and metal cations. Completion of the dehydration-polycondensation process culminates in the formation of a solid-state mixed phase comprising crystalline aluminum phosphate and magnesium aluminate compounds. This resultant coherent microstructure critically enhances the coating's adhesive strength, structural impermeability, and thermal resilience.
Key words
phosphate /
gas barrier coating /
mechanism /
low temperature curing /
high temperature protection
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