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Views: 0 Author: Site Editor Publish Time: 2026-08-07 Origin: Site
Aluminum dihydrogen phosphate — also referred to as monoaluminum phosphate, aluminum phosphate monobasic, or simply ADP — is one of the most versatile inorganic binders in high-temperature industrial manufacturing. Its unique ability to form ceramic bonds at elevated temperatures makes it indispensable across refractories, coatings, investment casting, and fireproof construction materials.
Aluminum dihydrogen phosphate is an inorganic phosphate compound with the chemical formula Al(H₂PO₄)₃. It exists in two common commercial forms:
Form | Appearance | Typical Concentration | Primary Use |
Liquid | Colorless, odorless, viscous aqueous solution | 30–50% solids | Refractory castables, coatings, spray-applied linings |
Solid (Powder) | White crystalline or amorphous powder | ≥98% purity | Dry-mix formulations, ramming mixes, specialty cements |
Synonyms:
● Monoaluminum phosphate, Aluminum phosphate monobasic, Aluminum tris(dihydrogen phosphate), ADP
The largest application by volume. Aluminum dihydrogen phosphate serves as the primary binder in:
High-alumina and corundum-based castables
● for steel ladles, tundishes, and cement kilns
Silicon carbide ramming mixes
● for blast furnace troughs and induction furnace linings
Mullite and andalusite castables
● for petrochemical reactor linings
Low-cement and ultra-low-cement castables
● where phosphate provides early strength without compromising hot properties
Typical dosage: 3–8% by weight (as liquid, 50% solids basis) of the total castable formulation. Over-dosing leads to excessive porosity after firing; under-dosing results in insufficient green strength.
Al(H₂PO₄)₃-based coatings are sprayed, brushed, or dipped onto metal and ceramic substrates to provide:
Oxidation protection
● for graphite electrodes and carbon-carbon composites in steelmaking
Molten metal corrosion barriers
● on furnace components and crucibles
Anti-spalling coatings
● on refractory brick surfaces
Thermal barrier coatings
● in aerospace and power generation turbine components
The phosphate binder reacts with alumina or chromia fillers at service temperature to form a dense, gas-impermeable ceramic layer.
In investment casting shell molds, aluminum dihydrogen phosphate is used as a binder for the refractory slurry that coats wax patterns. Advantages over colloidal silica binders include:
● Faster shell build-up (fewer dip coats required)
● Higher mold strength at casting temperatures
● Better compatibility with reactive alloys (titanium, zirconium)
● Excellent surface finish on cast parts
When formulated with intumescent agents (e.g., expandable graphite, pentaerythritol) and fillers, ADP-based coatings provide passive fire protection for structural steel. Upon fire exposure, the phosphate decomposes endothermically and contributes to char formation, which insulates the steel substrate.
Aluminum dihydrogen phosphate reacts with basic oxides (MgO, CaO, ZnO) at ambient temperature to form chemically bonded phosphate ceramics (CBPCs). These find niche applications in:
● Rapid-setting repair mortars for high-temperature equipment
● Nuclear waste encapsulation matrices
● Biologically inert ceramic composites (non-medical, industrial bioceramics for chemical processing)
In fiber-reinforced ceramic matrix composites (CMCs), ADP serves as a precursor for the phosphate bonding phase between ceramic fibers (alumina, silicon carbide) and the matrix, improving interlaminar shear strength at elevated temperatures.
Q: What is the difference between aluminum dihydrogen phosphate and monoaluminum phosphate?
A: They are the same compound — Al(H₂PO₄)₃. "Monoaluminum phosphate" emphasizes the 1:3 aluminum-to-phosphate stoichiometry; "aluminum dihydrogen phosphate" follows IUPAC nomenclature. Both terms are used interchangeably in industry.
Q: Can aluminum dihydrogen phosphate be used with basic refractories like magnesia?
A: Only with careful formulation. The acidic phosphate reacts vigorously with MgO. Use dead-burned magnesite (low surface area) as the hardener at low dosage (1–3%), and expect significant exothermic reaction during mixing.
Q: What causes low strength in ADP-bonded castables after firing?
A: Common causes include:
● Insufficient P/Al ratio (bonding phase insufficient)
● Over-drying too rapidly (cracks before ceramic bond forms)
● Incompatible aggregate chemistry
● Firing at temperatures below 600°C (Al(PO₃)₃ phase not yet formed)
Q: Is aluminum dihydrogen phosphate water-soluble after curing?
A: After ambient-temperature curing (with MgO hardener), the material has limited water resistance. After firing above 500°C, the phosphate bond becomes water-insoluble due to condensation into polyphosphate networks.
Q: How does ADP compare to phosphoric acid alone as a binder?
A: Phosphoric acid (H₃PO₄) alone can also bind refractory aggregates, but it provides lower final strength, creates more porosity, and is more corrosive to mixing equipment. Pre-reacted ADP offers more consistent performance and better handling characteristics.
Aluminum dihydrogen phosphate occupies a unique position in the landscape of high-temperature industrial binders. Its ability to form stable ceramic bonds through the 400–800°C temperature window — precisely where conventional hydraulic cements lose integrity — makes it irreplaceable in demanding refractory, coating, and investment casting applications.
