Gunning vs Casting Refractories: When to Use Which Method

Two Different Tools for Two Different Jobs

Gunning and casting are both methods of installing monolithic (unshaped) refractories, but they are optimised for fundamentally different scenarios. Treating them as interchangeable leads to poor lining performance and unnecessary cost. Understanding when to use each — and why — is a core skill for any refractory engineer or plant maintenance manager.

Casting: For New Construction and Highest-Quality Linings

Casting (or vibration-cast installation) involves mixing a wet, flowable castable and placing it into a formed void using a mould or formwork, then consolidating it by vibration. The result is a dense, homogeneous monolithic lining with properties approaching the theoretical maximum of the material.

Advantages of Casting

• Maximum density and strength: A well-vibrated cast lining achieves bulk density 5–10% higher than a gunned lining from the same base material. Higher density = lower porosity = better slag resistance and longer life.
• Homogeneous microstructure: No rebound loss means the installed material has exactly the specified chemistry. No binder enrichment or variation across the section.
• Suitable for complex geometries: Castable can be formed into almost any shape with appropriate formwork. Ideal for tundishes, torpedo ladles, complex furnace roofs.
• Thick sections possible: No theoretical thickness limit, though heat-up schedules for thick sections must be more conservative.

Limitations of Casting

• Requires shutdown and cool-down: Cannot be used for hot repairs. The vessel must be empty, accessible, and cool enough to install formwork.
• Longer downtime: Formwork installation, casting, de-moulding, curing (24–48 hours minimum), and controlled heat-up extend the total maintenance window.
• Access requirements: Need access for formwork, vibrators, mixing equipment, and personnel.

Gunning: For Hot Repairs and Rapid Maintenance

Gunning (or shotcrete application) involves projecting dry or semi-dry castable mix through a hose and nozzle using compressed air, with water added at the nozzle or a short distance before. The material is applied directly onto the worn surface at high velocity, forming a bonded layer without formwork.

Advantages of Gunning

• Hot repair capability: Can be applied to a hot lining (surface temperatures up to 1,000 degC for some gunning mixes) without shutting down the entire furnace or vessel. This is the most important advantage — it turns a multi-day rebuild into a 4–8 hour repair.
• Rapid application: An experienced gunner can apply 5–10 tonnes of material per hour. Large areas can be covered quickly.
• No formwork required: Material bonds directly to the existing lining surface via the impact velocity and the binder system. Only access to the nozzle area is needed.
• Repair-specific grades: Gunning mixes are formulated with bonding agents (sodium silicate, cement) that promote adhesion to cold or hot existing refractories.

Limitations of Gunning

• Rebound loss: 10–25% of the material projects off the surface as rebound (particles that don't stick). This material is wasted. The rebound is enriched in coarse particles and some binder, meaning installed chemistry is different from specified chemistry.
• Lower density: Gunned linings are typically 5–15% less dense than cast linings due to trapped air and the rapid application process. Lower density = higher porosity = less slag resistance.
• Thickness limitations: Generally limited to 50–150 mm per pass. Very thick sections require multiple passes with intermediate setting time.
• Skill-dependent: Gunning quality varies significantly with operator skill. Poor nozzle technique causes high rebound and non-uniform coverage.

Property Comparison: Cast vs. Gunned

Property	Cast LCC	Gunned Version of Same Base
Bulk density (g/cm³)	2.85–3.05	2.55–2.80
Apparent porosity (%)	12–16	18–24
Cold Crushing Strength (MPa)	70–100	45–70
Erosion resistance	Excellent	Good
Thermal shock resistance	Good	Good
Bond to existing lining	N/A (not applicable)	Good (when hot surface is properly prepared)
Application rate (t/hr)	1–3	5–10
Requires cool vessel	Yes	No (hot repair capable)

Decision Framework: Cast or Gun?

Use this framework to decide:

1. Is hot repair needed? If the vessel is hot or partially hot and cannot be fully cooled, gunning is the only option. Proceed with gunning.
2. How much downtime is available? Less than 12 hours → gunning. More than 36 hours → casting is feasible and preferable for quality.
3. Is it full reline or patch repair? Full reline = casting. Localized wear repair = gunning almost always.
4. What thickness is needed? Less than 100 mm → gunning is feasible. More than 200 mm → casting preferred for quality.
5. What is the service demand? High slag attack, long campaign life required → cast. Moderate conditions, frequent maintenance tolerated → gunning acceptable.

Best Practice for Hot Gunning Repairs

1. Surface preparation: Remove loose or friable material with a water jet or mechanical scaling. The gunned material bonds only to a sound substrate.
2. Surface temperature targeting: Ideal gunning surface temperature is 200–600 degC. Below 200 degC, bonding is slower. Above 800 degC, the material flash-dries before bonding properly.
3. Nozzle distance and angle: Maintain nozzle 400–600 mm from the surface at 90 degrees to minimize rebound and maximize compaction.
4. Layer thickness: Apply in 50–75 mm lifts. Allow each lift to set before applying the next.
5. Post-repair heat-up: After a gunned repair on a hot vessel, allow gradual heat-up from ambient temperature to service temperature before returning to full production load.

Frequently Asked Questions

Can I cast on top of a gunned lining?

Yes, during a full reline. The existing gunned layer would be demolished, the surface prepared, and fresh casting done. You cannot cast directly over existing gunned material without demolition, as the bond between old and new is unreliable.

What is dry gunning vs. wet gunning?

Dry gunning: dry premix is fed through the hose, water is added at the nozzle. Easier to start/stop; more rebound. Wet gunning (shotcrete): material is premixed wet and pumped to the nozzle, air is added for velocity. Lower rebound; more complex equipment setup. For refractory hot repairs, dry gunning is standard.

How do I reduce rebound loss?

Key techniques: correct water addition at nozzle (too wet = flow-off, too dry = high rebound); proper nozzle distance and angle; consistent air pressure; using a fine mesh screen to capture and dispose of rebound separately. With good technique, rebound can be reduced to 8–12%.