What a Corundum Mullite Castable Is
A corundum mullite castable is a low-cement, high-alumina monolithic refractory in which the aggregate system combines tabular or white-fused corundum with mullite grains, bonded by ultrafine reactive alumina and a small addition of calcium aluminate cement. The division of labour is deliberate: the corundum fraction supplies abrasion and slag resistance, while the mullite fraction lowers the thermal expansion of the matrix and raises thermal shock resistance, which is the property that decides whether a lining survives a kiln shutdown.
The material is placed by casting or vibration and is designed to be fired in service rather than fully pre-fired. That is why its strength is quoted three times in a datasheet: after drying at 110 °C, after heat treatment at 1100 °C, and after heat treatment at 1400 °C.
Typical Property Envelope
| Property | Grade 1 | Grade 2 | Grade 3 |
|---|---|---|---|
| Al2O3, % | ≥ 70 | ≥ 75 | ≥ 85 |
| SiO2, % | ≤ 25 | ≤ 17 | ≤ 13 |
| Fe2O3, % | ≤ 1 | ≤ 1 | ≤ 1 |
| Bulk density, g/cm³ | ≥ 2.7 | ≥ 2.8 | ≥ 2.9 |
| Cold crushing strength, MPa (110 °C x 24 h) | ≥ 100 | ≥ 110 | ≥ 120 |
| Cold crushing strength, MPa (1100 °C x 3 h) | ≥ 100 | ≥ 110 | ≥ 120 |
| Cold crushing strength, MPa (1400 °C x 3 h) | ≥ 115 | ≥ 120 | ≥ 125 |
| Modulus of rupture, MPa (1100 °C x 3 h) | ≥ 15 | ≥ 16 | ≥ 17 |
| Refractoriness under load, 0.2 MPa, °C | ≥ 1450 | ≥ 1480 | ≥ 1500 |
| Thermal shock, 900 °C water quench, cycles | ≥ 25 | ≥ 25 | ≥ 25 |
| Maximum service temperature, °C | 1550 | 1550 | 1600 |
| Linear dimensional change after firing, % | ≤ -0.3 | ≤ -0.2 | ≤ -0.2 |
The three grades are not simply a price ladder. Higher alumina buys wear resistance and refractoriness, but it also removes some of the mullite that provides thermal shock tolerance, so the selection has to follow the actual damage mechanism at the point of installation.
Why It Performs in a Cement Kiln
Kiln linings see clinker abrasion, alkali and sulfate vapour attack, and repeated deformation of the shell with each revolution. A castable at 70% to 85% alumina with a service limit of 1550 to 1600 °C holds up in zones where an ordinary alumina-silicate castable would spall or be cut away, typically at the nose ring, the burner area and the transition zone. The low linear dimensional change of -0.2% to -0.3% after firing keeps the lining tight against the shell and reduces open joints, and refractoriness under load of 1450 to 1500 °C at 0.2 MPa provides the creep margin a rotating kiln needs. The same material is used in power station boiler linings and other high-temperature furnace linings with comparable duty.
Installation, Curing and Dry-Out
Water addition should follow the supplier range exactly, typically 5% to 7% for a low-cement castable. Extra water buys workability at the cost of density, strength and abrasion resistance, and the loss is permanent. Mixing should be done in a forced-action mixer with the full batch time observed. Anchors must be laid out with allowance for expansion, and the dry-out schedule should include holds near 110 °C, 300 °C and 600 °C so that steam can escape before the matrix densifies. Skipping a hold is the usual cause of explosive spalling during the first heat-up.
Test Methods and Acceptance Criteria
Cold crushing strength and modulus of rupture are measured to ASTM C133, abrasion resistance to ASTM C704, and reheat change to ASTM C113. Castable classes are assigned from alumina content and density following ASTM C401, and aluminosilicate dense and monolithic products are classified under ISO 10081-1. Bulk density and strength are normally determined on cubes cast from the same batch, cured and dried under the same routine as the field installation, because a sample prepared with a different water addition will not represent the lining. For critical zones it is worth requiring a panel test rather than a cube test, since panel geometry captures the effect of anchors and of one-sided drying.
Frequently Asked Questions
Q: Why is the strength quoted at three different temperatures?
A: A low-cement castable gains strength by hydraulic bonding after drying and by ceramic bonding after firing. Values at 110 °C, 1100 °C and 1400 °C show how the material behaves through that transition.
Q: What does refractoriness under load measure?
A: The temperature at which a test piece deforms by 0.5% under a 0.2 MPa load. It is the best single indicator of creep resistance in a kiln lining.
Q: Can water be added to improve workability?
A: Only within the supplier range. Additional water reduces density, strength and wear resistance, and the loss cannot be recovered by longer curing.
Q: How many thermal shock cycles should be expected?
A: Around 25 cycles in the standard 900 °C water quench test is normal for this class of castable, which is what allows it to survive repeated kiln shutdowns.
Q: Which grade suits a high-wear zone?
A: The higher alumina grade, at 85% Al2O3 and a bulk density of at least 2.9 g/cm³, because abrasion resistance tracks density and corundum content.
Q: How thick should the lining be cast?
A: Thickness follows the thermal and mechanical design of the specific zone; the casting method sets a practical minimum that must be respected to allow proper vibration and anchoring.
