Gunning refractory (also called gunite or shotcrete refractory) is a castable mix applied pneumatically through a hose and nozzle rather than poured into formwork. The method eliminates formwork construction and allows placement in positions inaccessible to conventional casting — overhead surfaces, irregular cavities, and existing hot linings without full shutdown. The trade-off is density: gunned material typically achieves 85–92% of the bulk density of the equivalent cast grade due to rebound loss (5–30% of material bouncing off the substrate) and the absence of vibration consolidation. For structural primary linings in new construction, this density gap matters. For the four scenarios described in this article — emergency repair, complex geometry, hot-surface application, and partial-thickness face relining — the installation advantages outweigh the density disadvantage and gunning is the correct method. This guide defines when each scenario applies, which mix properties enable hot-face performance, and how to calculate real in-place cost including rebound to compare accurately against casting.
Gunning is used across cement kilns (inlet and outlet zone patches), EAF and ladle repairs, glass furnace crown repairs, and industrial boiler hot-face maintenance. The Zibo refractory manufacturing cluster produces both standard and specialty gunning mixes covering the full temperature range from 1,200℃ to 1,650℃ service, including alkali-resistant and abrasion-resistant formulations for the most demanding repair environments.
The Four Scenarios Where Gunning Wins
Scenario 1: Emergency Hot Repair Without Full Shutdown
When a kiln or furnace develops a localised hot spot — shell temperature above 350℃ on a cement kiln, or visible lining erosion on an EAF — the economic cost of a full shutdown for conventional casting (cool-down, formwork installation, cast, cure, dry-out, heat-up: typically 7–14 days) is often larger than the cost of several months of gunning-maintained operation. Gunning mixes formulated for hot application (substrate temperature 200–600℃) allow repair of the affected zone with a 4–8 hour shutdown window rather than a full relining campaign. The repaired patch operates at reduced thickness (50–100 mm vs 150–200 mm full lining) and will need replacement at the next planned shutdown, but the furnace returns to production within hours.
Operational rule: Hot-repair gunning buys time — it does not replace full relining. Schedule the full repair at the next planned shutdown. Operating a hot-gunned patch beyond 3–4 months without inspection risks accelerated failure because the reduced thickness has lower thermal buffering capacity.
Scenario 2: Geometrically Inaccessible Areas
Formwork casting requires a closed void to fill. Where the geometry prevents formwork construction — curved overhead surfaces, taphole surrounds, tuyere zones, irregularly eroded cavities in existing linings — gunning is the only practical placement method. The material bonds to the substrate under pneumatic force and does not require gravity to fill a mould. Typical applications: EAF roof patches between water-cooled panels, blast furnace taphole surround maintenance, rotary kiln riding ring zones where the kiln support geometry prevents formwork access.
Scenario 3: Partial-Thickness Face Relining (Encapsulation)
When a lining has eroded to 60–70% of original thickness but remains structurally sound (no cracking through the cold face, no shell deformation), gunning a 50–80 mm working face onto the existing lining extends campaign life without full demolition. This is standard practice in glass furnace side walls and steel ladle safety linings. The existing lining provides structural support, which compensates for the lower density of the gunned layer. Adhesion between old and new refractory is critical — the substrate must be cleaned of loose material, slag, and dust before gunning.
Scenario 4: Areas With Interrupted Installation Cycles
Casting requires continuous pouring to avoid cold joints — if a pour is interrupted for more than 45–60 minutes, the partially set material creates a weak plane. In large kilns and furnaces where installation proceeds in sections over multiple shifts, gunning eliminates the cold-joint risk because each pass is a discrete layer that bonds mechanically and thermally to the previous one without requiring wet-to-wet continuity.
When Casting Remains the Better Choice
| Factor | Gunning Advantage | Casting Advantage |
|---|---|---|
| New full-thickness lining, accessible area | — | Higher density, better mechanical properties, lower in-place cost |
| Structural load-bearing zones | — | Full vibration consolidation achieves design density |
| Large monolithic pours (>5 m³) | — | No rebound loss; consistent density throughout |
| Emergency repair, hot substrate | No shutdown required; fast turnaround | — |
| Overhead or inaccessible geometry | No formwork needed; pneumatic placement | — |
| Partial-thickness face repair | Bonds to existing lining; no demolition | — |
| Interrupted installation (multi-shift) | No cold-joint risk between passes | — |
Common mistake: Using gunning as a cost-saving measure on new accessible linings because it avoids formwork labour. The rebound loss (15–30% for dry-process) means the in-place material cost per cubic metre often exceeds casting once rebound is included. Gunning saves installation time and enables hot-surface placement; it does not consistently reduce material cost.
Mix Design: How Gunning Mixes Differ From Castables
A gunning mix is formulated to remain cohesive during pneumatic transport, adhere to the substrate on impact, and set rapidly enough to build thickness without slumping. These requirements differ from castable mix design in three ways:
- Water content: Gunning mixes use 10–14% water vs 6–9% for LCC castables. Higher water enables pumping but reduces final density and strength.
- Accelerator: Rapid-setting accelerators (sodium silicate, calcium aluminate cement additions) are used to achieve 2–4 hour handling strength and prevent sag on overhead applications.
- Aggregate grading: Finer aggregate distribution (top size 6–8 mm vs 10–12 mm for castables) reduces rebound by improving adhesion energy at the substrate surface.
| Property | Gunning Mix (Wet Process) | Gunning Mix (Dry Process) | LCC Castable |
|---|---|---|---|
| Al₂O₃ % | 60–75 | 60–75 | 65–80 |
| Water addition % | 10–14 | Added at nozzle (8–12) | 6–9 |
| Bulk density (installed) kg/m³ | 2,100–2,350 | 1,950–2,250 | 2,400–2,600 |
| Rebound loss % | 5–15 | 15–30 | N/A |
| Min substrate temp ℃ | −5 to +600 | −5 to +800 | 5–40 (ambient cure) |
| Max single-pass thickness mm | 50–80 | 40–70 | No limit (vibrated) |
| Cold crushing strength (110℃) MPa | 25–45 | 20–40 | 50–90 |
In-Place Cost Calculation: Including Rebound
The most common mistake when budgeting gunning vs casting is comparing quoted price per tonne. Rebound loss means the in-place material volume requires more tonnes than the installed volume suggests. Use this calculation:
| Parameter | Dry-Process Gunning | Wet-Process Gunning | LCC Casting |
|---|---|---|---|
| Material price per tonne (index 100) | 85 | 90 | 100 |
| Rebound loss % | 25 | 10 | 0 |
| Tonnes needed per m³ installed (bulk density 2,100 kg/m³) | 2.8 | 2.3 | 2.1 |
| In-place material cost per m³ (index) | 238 | 207 | 210 |
| Labour & formwork per m³ (index) | 60 | 65 | 120 |
| Total installed cost per m³ (index) | 298 | 272 | 330 |
Key insight: Wet-process gunning has the lowest total installed cost per cubic metre in this comparison because formwork savings offset the material premium. Dry-process gunning looks cheap per tonne but rebound loss erases the advantage. When casting is structurally acceptable, it remains competitive on material cost alone; the saving from gunning comes from labour and access, not material price.
Dry-Out Requirements After Gunning
Gunned refractory requires the same staged dry-out as cast refractory — the higher water content in gunning mixes makes dry-out more critical, not less. Rapid heating of a freshly gunned patch causes steam explosion spalling (explosive dehydration) if the temperature ramp exceeds the moisture diffusion rate. Standard dry-out protocol after gunning:
Emergency Repair or Planned Relining?
Share your furnace type, zone dimensions, and substrate temperature. We recommend the right mix and calculate in-place cost with rebound included.
Application Quality: Nozzle Distance and Rebound Control
Rebound loss and in-place density are directly controlled by nozzle technique. Poor technique can double rebound loss and create lamination defects (weak planes between passes where wet material was sprayed onto partially dried previous passes).
- Nozzle distance: 0.6–1.0 m from substrate is optimal. Closer than 0.5 m creates impact-driven delamination; farther than 1.2 m increases rebound and reduces density.
- Nozzle angle: 85–95° perpendicular to surface. Oblique angles increase rebound significantly — a 30° angle can increase rebound by 8–12 percentage points.
- Pass thickness: Apply in 20–30 mm passes. Let each pass stiffen (15–30 minutes) before the next pass to prevent the new material weight causing the previous layer to sag.
- Rebound removal: Sweep fallen rebound material away before it can be buried under subsequent passes. Buried rebound creates weak porous inclusions.
- Cold-substrate pre-wetting: For dry-process gunning on cold substrates (<50℃), light pre-wetting of the substrate surface improves first-layer adhesion and reduces initial rebound.