In today's article we will explain in detail why phosphate bonded castables are stronger than cement or sodium silicate bonded castables, in terms of bonding mechanism, phase composition, microstructure and temperature adaptability.
The core difference: the nature of the bond is different. That directly affects the compressive strength of refractory castables.
We can imagine the three types of bonding as different “glues”:
Cement bonding castables : like “plaster curing” - physically interwoven by hydration products.
Sodium silicate or say Water-glass bonding castables : like “silicone curing” - adhesion by amorphous gel.
Phosphate bonding castables(Aluminum dihydrogen phosphate, Al(H2PO4)3 ) : like “welding + ceramic sintering” - formation of strong chemical bonds and stabilization of the ceramic phase.
1. Cement bonded castables
Bonding mechanism: relies on the hydration of calcium aluminate cements (CA, CA2, etc.) to produce hydrated calcium aluminate (e.g. CAH₁₀, C₂AH₈, C₃AH₆) and aluminum hydroxide gels. These hydration products encapsulate and intertwine the aggregate and powder to produce strength.
Strength characteristics and limitations:
- Normal temperature strength develops quickly and has good initial strength.
- High temperature strength is its fatal flaws. When the temperature rises above 300°C, the hydration products begin to dehydrate and decompose, and the crystal structure is destroyed, resulting in a sharp drop in strength (called the “mid-temperature strength trough”). At higher temperatures, although ceramic bonding occurs with re-sintering, this process is accompanied by large volume contraction and structural rearrangement, which is prone to cracking, and the recovery of castable compresive strength is limited and unstable.
- Microstructure: The hydration product contains a large amount of crystalline water and gel water, leaving pores after high temperature dehydration, and the structure becomes loose.
2. Sodium silicate bonded castables
- Bonding mechanism mainly rely on the dehydration condensation of water glass (sodium silicate solution) and the reaction with CO₂ in the air to produce amorphous silica gel (SiO₂·nH₂O), which binds the particles together.
Strength characteristics and limitations:
- Room temperature strength: depending on the formation of silica gel, the strength is average and susceptible to environmental humidity.
- High Temperature Strength: Silica gel dehydrates further at high temperatures, shrinks considerably and becomes a brittle glassy phase. This glassy phase decreases in viscosity at high temperatures and is prone to creep, resulting in low strength in the high temperature thermal state.
- Microstructure: the structure of silica gel is not uniform, and a large number of microcracks and pores are produced after dehydration, and the castable bonding force is weak.
3. Phosphate bonded castables (taking the commonly used aluminum dihydrogen phosphate and aluminum-phosphoric acid reaction as examples)
- Bonding mechanism: this is a chemical reaction combined with high temperature ceramization.
- Room temperature hardening: phosphoric acid or phosphate solution and raw materials in the active Al₂O₃ (especially added coagulant promoters, such as magnesium sand, aluminum hydroxide, etc.) acid-base neutralization reaction, the generation of aluminum phosphate gel, the initial production of strength.
- Medium and high temperature strengthening: as the temperature rises (usually above 500°C), the aluminum phosphate gel dehydrates, crystallizes, and undergoes further reaction with oxides such as Al₂O₃ in the raw material to generate various phosphate ceramic phases with high strength, such as aluminum orthophosphate (AlPO₄), aluminum pyrophosphate (Al₄(P₂O ₇)₃) and so on.
The underlying reason for the castable compressive strength advantage:
- Stronger chemical bonding: The aluminum phosphate compounds formed are crystalline structures dominated by strong covalent bonds (P-O-Al) with much higher bonding energies than the hydration products of cement or the amorphous silica of water glass.
- High-temperature phase stability: AlPO₄ is a quartz-like structure of stable crystals with a very high melting temperature (>1500°C), which maintains stable strength and volume over a wide range of temperatures, without the problem of mid-temperature decomposition similar to that of cement.
- Excellent sinterability and ceramic bonding: phosphoric acid plays the role of a “liquid sintering agent” at high temperatures, which can promote mass transfer and rearrangement between particles to form a dense, uniform ceramic network structure, and firmly “weld” the aggregate together.
- Dense microstructure: The whole process of dehydration is gentle, the volume change is small, and the final structure of the ceramic phase has low porosity and tightly interwoven crystals.So the monolithic castable compressive strength is super good.
