Insulating castables are lightweight refractory mixes in which the pore structure, not the aggregate strength, is the design objective. Prepared from porous lightweight aggregate in a cement bond, they are cast into place like any castable but weigh roughly a third to a half of a dense high...
Insulating castables are lightweight refractory mixes in which the pore structure, not the aggregate strength, is the design objective. Prepared from porous lightweight aggregate in a cement bond, they are cast into place like any castable but weigh roughly a third to a half of a dense high alumina body, and they trade mechanical strength for a large reduction in heat loss through the shell.
XZK Group supplies insulating castables across a density range from about 0.8 to 1.5 g/cm3. In a glass furnace they are always a backing material: they sit behind the dense working lining and are never exposed to the melt.
The body is built from expanded clay or shale, porous fireclay, bubble alumina or a similar lightweight aggregate, bonded with calcium aluminate cement and a fines fraction that holds the porous grains together. Because the aggregate is already hollow or cellular, the pores survive casting rather than being created during firing. Density is set by how much lightweight aggregate is in the mix, and it is density, not cement content, that governs thermal conductivity and strength.
Thermal conductivity falls steeply with density. Halving the bulk density of a castable can cut its conductivity by more than half, which is why an insulating castable is specified to hit a shell-temperature or heat-loss target rather than a strength target. The cost is a proportional drop in cold crushing strength, and an insulating castable should never be treated as load-bearing: it must be supported by the dense lining in front of it and by the steelwork behind it.
Placement follows the same sequence as a dense castable — mix, cast, vibrate, cure, dry out — but the heat-up needs more care because a lightweight, porous body holds water and vents it slowly. The material is best confined to positions where the hot face is protected, the temperature is stable and the mechanical load is carried by something else. Its refractoriness is lower than a dense high alumina castable and is set by the aggregate rather than the binder.
| Index Items | ZKQJ - 0.8 | ZKQJ - 1.0 | ZKQJ - 1.5 | |
| Bulk Density /g/cm³ | 0.8 | 1.0 | 1.3 | |
| Maximum Service Temperature /°C | 1200 | 1250 | 1300 | |
| Thermal Conductivity /W/m·K | 350°C | 0.25 | 0.3 | 0.6 |
| 700°C | 0.30 | 0.4 | 0.7 | |
| Compressive Strength at Normal Temperature /MPa ≥ | 110°C×24h | 2.0 | 4.0 | 15 |
| 1000°C×3h | 2.0 | 5.5 | 20 | |
| Linear Change Rate /% | 1000°C×3h | -1.0 | -1.0 | -0.5 |
Grades are defined by bulk density, most commonly around 0.8, 1.0, 1.2 and 1.5 g/cm3. Lower density gives better insulation but less strength; higher density gives a more robust body that tolerates slightly more mechanical contact while still cutting heat loss. Choose the density that meets the heat-loss target with the least material needed for the load it will actually see.
The insulating castable must also be matched to what it sits against. Its thermal expansion and its refractoriness have to be compatible with the dense lining in front of it, and its service limit must be above the temperature it will actually experience on its hot face. Where the backing position runs hotter than an insulating castable can survive, a lightweight mullite castable provides insulation at a higher temperature; where the position is a dense hot face, a high alumina castable takes over.
In a glass furnace an insulating castable is used behind the working lining, where its job is to cut heat loss without adding weight to the structure:
Manufacturing is controlled from raw material through to finished brick. Four automatic batching lines feed 20 CNC presses ranging from 630 to 2000 tonnes, so density is set by press force rather than operator judgement, and firing runs through four 108-metre fully automatic tunnel kilns and five ultra-high-temperature shuttle kilns for a uniform soak.
Every batch is tested for bulk density, apparent porosity, cold crushing strength and refractoriness under load, and ships with a certificate of analysis. XZK Group holds ISO 9001, ISO 14001, ISO 45001 and ISO 50001 certification and operates the Henan Provincial Engineering Technology Research Center for Sintered Refractories for Glass Furnaces.
Insulating castables are supplied as part of a complete monolithic refractory package together with high alumina castable, lightweight mullite castable, silicon carbide castable and brick insulation such as mullite insulating bricks. See all refractory products or the insulation range.
Al₂O₃:46%-60%;
Fe₂O₃: 0.7%-1.0%;
Operating Temp:1300-1500℃;
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