Phosphate-bonded refractories refer to a category of refractory materials manufactured by mixing phosphoric acid or phosphates (e.g., aluminum dihydrogen phosphate, aluminum phosphate, etc.) as binders with refractory aggregates and powders for shaping, followed by processes such as heat curing.
Their core feature is that the binder achieves high-temp resistance and mechanical strength through chemical reactions (rather than hydraulic or air-hardening properties), making them widely used in medium-to-high temp fields.
I. Classification by Binder Type and Molding Process
The classification of phosphate-bonded refractories mainly centers on “binder form” and “construction/molding method”, which jointly determine the material’s application scenarios and performance.
1. Classification by Binder Type
Different phosphate binders vary significantly in curing temp and strength development rate.
① Phosphoric acid-bonded type: Uses orthophosphoric acid (H2PO4) directly as the binder, requiring aggregates with reactive Al2O3(e.g., high-alumina bauxite, corundum). At room temp, phosphoric acid reacts slowly with Al2O3 in aggregates to form aluminum phosphate (AlPO4); the reaction accelerates at 100-200℃ to develop early strength, and the binder further polymerizes at 300-600℃ for a significant strength increase.
Features: Slow curing, requiring heat curing, excellent high-temp stability.
② Aluminum phosphate-bonded type: Uses aluminum phosphates (e.g., aluminum dihydrogen phosphate Al(H2PO4)3, aluminum phosphate AlPO4) as binders, with a small amount of setting accelerator (e.g., magnesium oxide) added to some products. Compared with pure phosphoric acid, it has higher reactivity at room temp, does not rely on Al2O3 in aggregates, is compatible with siliceous, magnesia and other aggregates, and cures at a lower temp (initial hardening at 80-150℃).
Features: Wide applicability (for multiple aggregates), easy curing; stable medium-temp strength.
③ Composite phosphate-bonded type: Blends two or more phosphates (e.g., aluminum phosphate + magnesium phosphate, aluminum phosphate + zinc phosphate). Performance is optimized by adjusting proportions — e.g., adding magnesium phosphate improves low-temp water resistance, while zinc phosphate enhances corrosion resistance.
Features: High customization, targeted solutions to shortcomings of single binders (e.g., poor water resistance of pure phosphoric acid binders).
2. Classification by Molding / Construction Method
- Phosphoric acid-bonded refractory castables: Binder, aggregates and powders are mixed with a small amount of water, then shaped by casting and vibration.
- Phosphoric acid-bonded refractory ramming mixes: Low fluidity, requiring compaction by manual or mechanical ramming.
- Phosphoric acid-bonded refractory precast components: Pre-pressed in factories (e.g., phosphate-bonded refractory bricks), directly built on-site, high precision and construction efficiency.
II. Phosphate-Bonded Refractories: Outstanding Medium-to-High Temp Performance, Requiring “Curing Activation”
1. Core Advantages of Phosphate-Bonded Refractories
- High medium-to-high temp strength: At room temp, phosphate gel provides early strength. When heated to 600-1200℃, the binder and aggregates undergo sintering to form dense ceramic bonding (e.g., fusion of AlPO4 with corundum aggregates). The strength remains stable above 1200℃, far exceeding that of hydraulic-bonded refractory castables.
- Excellent thermal shock resistance: The phosphate bonding phase has a low thermal expansion coefficient (approx. 5-8×10-6/℃), matching well with aggregates like high-alumina and corundum. It rarely cracks due to thermal stress during heating and cooling.
- Strong corrosion resistance: The dense phosphate glass and ceramic phases formed after curing effectively block penetration and erosion by molten metals (e.g., molten iron) and slags (e.g., blast furnace slag), making it especially suitable for chemically corrosive scenarios.
- Good volume stability: No obvious shrinkage during heating (linear shrinkage rate usually <1%), avoiding structural cracking from volume deformation. Ideal for “integral seamless” linings (e.g., rotary kilns, induction furnaces).
2. Main Limitations of Phosphate-Bonded Refractories
- Low room-temp strength, requiring curing activation: Before heating, phosphate only forms loose gel, with room-temp strength (compressive strength <5MPa) much lower than cement binders. It must undergo heat curing at 80-300℃ (for 12-24 hours) to reach service strength, and cannot be used in room-temp scenarios requiring no heating.
- Poor water resistance (pure phosphoric acid type): Unfully cured phosphate at room temp easily absorbs moisture, leading to strength loss. Thus, it should be put into use soon after curing, or composite phosphate binders with water-resistant additives should be selected.
- High construction requirements: Binder dosage must be strictly controlled (excess causes high-temp softening; insufficiency leads to low compactness). Curing temp needs to be uniform (local overheating causes cracking), demanding high precision in construction processes.
III. Transformation of Phosphate-Bonded Refractories: From “Gel Bonding” to “Ceramic Bonding”
The final structure of phosphate-bonded refractories evolves gradually with increasing temp, eventually forming a dense structure of “aggregate skeleton + ceramic bonding phase”.
| Temp Range | Structure | Strength Source |
| RT-100℃ | Loose structure: Aggregate particles are wrapped in phosphoric acid/phosphate gel; the gel is not fully cured, with numerous micro-pores | Physical bonding of gel (Low strength, easy to absorb water) |
| 100-600℃ | Initial densification: The gel dehydrates and polymerizes, forming aluminum phosphate (AlPO4) crystals that fill some micro-pores. aggregates and binder are initially bonded | Chemical bonding of aluminum phosphate crystals (Strength increases to 15-30MPa) |
| 600+℃ | Ceramic bonding stage: Aluminum phosphate crystals undergo sintering reaction with aggregates (e.g., Al2O3 in corundum), forming a continuous ceramic phase (e.g., Al2O3-AlPO4 solid solution). micro-pores are further reduced, and the structure becomes dense | Metallurgical bonding of ceramic phase (High-temp compressive strength > 50MPa, stability up to 1600-1800℃) |
Final high-temp structure: Refractory aggregates (e.g., corundum, high-alumina particles) serve as the “skeleton”, while the continuous ceramic bonding phase (AlPO4, Al2O3-AlPO4 solid solution) fills the gaps between the skeleton. This forms a dense, interwoven “skeleton-bonding phase” structure, which possesses both high-temp strength and corrosion resistance.
IV. Main Application of Phosphate-Bonded Refractories: Industrial Kilns for “Medium-to-High Temp & High Corrosion”
Based on its performance characteristics, phosphate-bonded refractories are mainly used in industrial environments such as industrial kilns, metallurgical equipment and high-temp reactors, which need to withstand medium-to-high temp (600-1800℃) and chemical corrosion for a long time.
| Industry | Application | Material Type | Core Demand |
| Metallurgy | Blast furnace taphole torpedo ladle lining converter slag stopper | Phosphate-bonded alumina ramming mass corundum precast components | Resistance to molten iron/slag erosion thermal shock (frequent heating/cooling) |
| Building Materials | Cement rotary kiln glass kiln regenerator | Phosphoric acid-bonded high-alumina castable sillimanite precast components | Stable medium-to-high temp strength low volume shrinkage (avoid kiln deformation) |
| Non-Ferrous | Aluminum electrolytic cell copper smelting furnace | Phosphoric acid-bonded silicon carbide castable magnesia-alumina spinel precast components | Resistance to molten electrolyte erosion high temp resistance (1200-1600℃) |
| Others | Induction furnace waste incinerator | Phosphoric acid-bonded corundum castable high-alumina ramming mix | Integral seamless structure flue gas corrosion resistance |
Application Precautions
“Heat curing” must be completed before use (control the heating rate as required by the material, usually 5-10℃/h) to avoid structural cracking at high temp due to insufficient curing.
In humid environments, water-resistant composite phosphate binders should be selected to prevent moisture absorption at room temp.
