Gunning Mix or Castable: Choosing Under a 24-Hour Shutdown Window

Gunning Mix or Castable: Choosing Under a 24-Hour Shutdown Window

Unplanned shutdowns often leave only 24 to 36 hours for refractory work. Arguing gunning versus casting is pointless at that point — add up the hours and the answer is usually obvious.

The castable time budget

For 20 m² of furnace wall: formwork 4–6 h, pouring 2–3 h, curing after stripping 24 h, bake-out 24–36 h. That is 55–68 h before stripping, cleaning and anchor welding are counted.

A 24-hour window simply does not contain a normal cast installation. Compressing curing and bake-out is paid for later in spalling and explosive spalling, and those failures usually appear at the first cooldown, when the repair cost is far above the time saved.

The gunning time budget

Same 20 m², no formwork, output of 3–5 m²/h at roughly 60 mm thickness: 4–6 h to spray, 6–8 h curing and 8–12 h fast bake-out, or 18–26 h in total — which fits a single maintenance shift.

Two losses to budget in advance

Rebound first. It runs 10–20% for dry gunning and 5–10% for wet. Rebound material cannot be reused: it has segregated by particle size and lost binder, so putting it back buries a weak plane in the lining. Order 1.15–1.25× the theoretical quantity, or the site runs short.

Water addition second. Wet gunning is held at 8–11%, while dry gunning depends on the nozzle water ring and is easily over-watered by feel. Every extra percent of water costs roughly 8–10% cold crushing strength, with nothing visible on site.

When castables are still mandatory

• Regular surfaces and shells above 10 m² favour gunning.
• Irregular areas below 3 m², burner blocks and expansion joints are safer cast.
• Zones in direct contact with hot metal or slag — slag lines, the main trough, ladle working linings — should never be gunned, because sprayed layers cannot match the density of vibration-cast material.

One detail that gets skipped

Keep single gunning passes under 80 mm. Beyond that, work in layers and roughen the interface, or the boundary becomes a spalling path. On rush jobs this is the first requirement dropped, because everyone is chasing the clock.

Three numbers to give the supplier

Available hours, repair area, and whether the zone sees wash or insulation. Let the supplier size the material and process from those three numbers instead of asking for a ton price first. The line usually missing from a quotation — installation and bake-out — is where rush repairs fail.