Refractories Explained — Castables, Fire Brick and Refractory Mortar

The internal walls of hot industrial plant are built not from insulation alone but from refractories. A refractory holds its structural form and performance above 1000°C, handling insulation, fire resistance, abrasion resistance and chemical resistance at once. This guide sets out the types of refractory — castable, fire brick, refractory mortar — and how to choose between them.

1. Castable refractory

A castable is a flowable refractory mixed with water and either poured into formwork or gunned into place. It can be applied to complex shapes and, having no joints, gives excellent tightness.

• Service temperature: 1000–1800°C (by alumina content)
• Alumina (Al₂O₃) content: 35–99% — the higher the content, the better the heat resistance
• Installation: casting, gunning or ramming
• A monolithic structure with no joints — excellent tightness and erosion resistance
• Freely applied to complex furnace geometry and curved surfaces
• Main uses: incinerator, heat-treatment and firing furnace walls, burner blocks, exhaust duct linings

2. Fire brick

Fire brick is a standard-sized brick fired at high temperature. It is simple to install and easy to replace, suiting the regular geometry of blast furnaces, coke ovens and glass melters.

• Types: fireclay (SK-32 to 38), high alumina (Al₂O₃ 45–99%), silica, magnesia
• Service temperature: 1200°C to over 1800°C (by type)
• A standard product — requires jointing, used with refractory mortar
• Easy to replace — only the damaged bricks need changing
• Excellent mechanical strength — suited to heavily loaded structures
• Main uses: walls of blast furnaces, electric arc furnaces, coke ovens and glass melters

3. Refractory mortar

• Made specifically for filling and bonding fire brick joints
• Two types: air-setting and heat-setting
• Service temperature: up to 1700°C (by alumina content)
• Joint thickness: 1–3 mm recommended — thinner gives better strength and tightness
• Main uses: fire brick construction, crack repair

4. Refractory board

• Fibrous refractories such as Cerakwool and silica formed into board
• Light — one-third to one-fifth the weight of castable or fire brick
• Easy to work — cut and drilled
• Service temperature: up to 1430°C
• Main uses: refractory linings for furnace doors and inspection hatches, lightweight insulating panels

Choosing a refractory