Refractory Linings: Castables, Bricks, Selection and Dryout

Quick Answer

Refractory lining selection balances four properties that pull against each other: temperature capability, insulating value, mechanical strength and chemical resistance. Dense castables and bricks resist abrasion, slag and mechanical loading but conduct heat; insulating castables and bricks save energy but erode and crush easily — which is why most linings are multi-layer, with a dense hot face and an insulating backup. Conventional castables contain 15-25% calcium aluminate cement and lose strength in the 800-1,100°C intermediate range; low-cement (LCC, 4-8%) and ultra-low-cement (ULCC, 1-4%) castables use fine matrix additions to reach much higher strength and far better thermal shock resistance, and dominate modern petrochemical and steel applications. Brick is classified by alumina content and by ASTM C27 fireclay classes; above about 70% Al₂O₃ performance rises sharply in slag and alkali attack. Dryout is not a formality. A castable lining holds free water plus chemically bound water, and heating too fast generates internal steam pressure that spalls the lining — a properly engineered dryout schedule typically takes 24 to 96 hours with holds at approximately 110-150°C and 300-350°C, and rates in the region of 20-30°C per hour. Skipping or compressing the dryout destroys more new refractory than any service condition does.

A sulphur recovery unit in the Middle East had a new thermal reactor lining installed during a turnaround. The contractor completed the gunning on schedule. The unit was behind, and the commissioning team was under pressure to make up time.

The specified dryout was 72 hours with holds at 120°C and 320°C. They did it in 14 hours.

The lining failed within three weeks. Large sections of the hot face had spalled away, exposing the backup layer and in places the anchors. The reactor shell reached temperatures above its design limit and the unit came down for an unplanned 23-day outage to re-line.

The refractory was correct. The installation was correct. The 58 hours they saved on dryout cost 23 days of a sulphur recovery unit, plus a second lining, plus shell inspection and repair.

Refractory is the only major plant material whose installed quality can be destroyed entirely by the first heat-up. Everything else about a refractory specification matters less than this.

What Refractory Has to Resist

Refractory linings do several jobs at once, and the dominant one determines selection:

Thermal resistance. Protecting the steel shell. Carbon steel loses most of its strength above about 400°C and A516 vessel shells are typically limited to 370-425°C. The lining must keep shell temperature below its design limit — and in sulphur and hydrogen service, often also above the acid or water dew point, which creates a two-sided constraint.

Heat conservation. Reducing fuel consumption. In a fired heater or kiln, the lining is the main thermal barrier and its performance directly determines fuel cost.

Abrasion resistance. Catalyst circulation in an FCC unit, clinker in a cement kiln, coke particles in a gasifier, ash in a boiler. Abrasion is often the life-limiting mechanism, and it demands dense, hard, high-strength material at the hot face regardless of the thermal penalty.

Chemical and slag resistance. Molten slag in steel and non-ferrous furnaces, alkali attack in cement kilns, vanadium and sodium in oil-fired boilers, sulphur compounds, carbon monoxide disintegration in reducing atmospheres, hydrogen attack.

Thermal shock resistance. Cyclic equipment — batch furnaces, ladles, incinerators, equipment subject to emergency trips — cracks and spalls under repeated thermal cycling. Thermal shock resistance correlates with low thermal expansion, high thermal conductivity and a microstructure that arrests crack propagation.

Mechanical load. Arches, roofs, self-supporting walls and hearths carry load at temperature, where creep becomes relevant.

No material maximises all of these. Dense is strong and abrasion-resistant but conducts heat. Insulating saves energy but is weak. The resolution is almost always a multi-layer lining.

Castables (Monolithics)

Castables are refractory aggregates with a binder, mixed with water and installed by casting, gunning, pumping or ramming. They now account for a large majority of refractory tonnage in process industries because they install faster, form complex shapes and avoid the joint network inherent in brickwork.

By cement content

Conventional castable (CC) — 15-25% calcium aluminate cement.

• Easy to mix and place, forgiving of water addition
• Develops strength from cement hydration
• Intermediate-temperature strength loss: between about 800°C and 1,100°C, the hydrate bonds have decomposed but ceramic sintering has not yet developed, and strength falls to a minimum. Linings in this temperature range are vulnerable.
• Higher water demand means higher porosity and lower density
• Uses: back-up linings, low-duty applications, general construction

Low cement castable (LCC) — 4-8% cement.

• Fine matrix additions (microsilica, reactive alumina, dispersants) fill the particle packing and reduce water demand
• Much lower porosity, much higher density, higher strength at all temperatures
• Better thermal shock resistance and much better abrasion resistance
• Needs accurate water addition and good mixing — far less forgiving than conventional castable
• Uses: FCC unit internals, reformer and heater linings, incinerators, transfer lines, most modern process refractory

Ultra-low cement castable (ULCC) — 1-4% cement.

• Highest density, lowest porosity, best corrosion and abrasion resistance
• Very sensitive to water content, mixing energy and placement technique
• Narrow working window; requires experienced installers
• Uses: the most severe abrasion and corrosion service

No cement castable / sol-gel bonded — colloidal silica or alumina bonded, no calcium aluminate cement.

• No hydraulic water, so dryout is dramatically faster and far safer — the main commercial reason for their use
• No lime in the matrix, so better resistance to alkali and to CO disintegration
• Higher material cost offset by shorter outage
• Uses: fast-turnaround repairs, alkali-rich environments

By density and function

Dense castables — bulk density 2,000-2,800 kg/m³, thermal conductivity 1.0-2.0 W/m·K. Hot-face material where abrasion, slag or mechanical load governs.

Medium-weight castables — 1,400-1,900 kg/m³. A compromise where moderate abrasion resistance and moderate insulating value are both needed.

Insulating castables (LWC) — 600-1,400 kg/m³, using lightweight aggregates such as expanded clay, vermiculite, bubble alumina or hollow spheres. Thermal conductivity 0.2-0.6 W/m·K. Backup layers and low-abrasion hot faces.

Erosion-resistant castables — high-alumina or silicon-carbide-containing dense castables specifically formulated for particle erosion, used in FCC cyclones, transfer lines and plenum chambers.

Installation methods

Casting (vibration casting) — poured into formwork and vibrated. Highest density and strength for a given material, best for thick linings and where formwork is practical. Needs form design, release agents and vibration equipment.

Gunning (shotcreting) — pneumatically projected. Dry gunning mixes water at the nozzle: fast, no formwork, but rebound losses of 15-30% and variable water content, so installed properties vary. Wet gunning / shotcreting pumps pre-mixed material and adds accelerator at the nozzle: much lower rebound (5-10%), consistent water content and far more consistent installed properties. Wet shotcreting has displaced dry gunning for quality-critical work.

Pumping (self-flowing castables) — flows and de-airs under its own weight without vibration. Excellent for congested areas and thin sections; requires tight water control.

Ramming (plastics and ramming mixes) — stiff material compacted by pneumatic rammer. Used for hearths, burner blocks, tap-holes and repairs. High density, no shrinkage on drying.

Bricks

Fired shapes, laid with mortar or dry-stacked.

Fireclay brick — 22-45% Al₂O₃. Classified by ASTM C27: low duty, medium duty, high duty and super duty, by pyrometric cone equivalent and reheat change. Economical, widely available, serviceable to about 1,400-1,500°C. Attacked by alkalis and slag.

High-alumina brick — 50-99% Al₂O₃. Performance improves markedly with alumina content, particularly above 70%. Mullite (≈72% Al₂O₃) and corundum (>90%) grades handle high temperature, slag and alkali. Standard for cement kiln transition zones, reheating furnaces, ladles and reformer linings.

Insulating firebrick (IFB) — porous, low density (500-1,100 kg/m³), classified by ASTM C155 Group numbers (Group 20, 23, 26, 28, 30, 32, 33) where the number approximates the maximum service temperature in hundreds of degrees Fahrenheit. Low thermal mass makes them excellent in cyclic furnaces where heat storage is wasted energy. Weak and easily damaged.

Silica brick — above 93% SiO₂. Good load-bearing capability at high temperature and good acid slag resistance, but poor thermal shock resistance and it undergoes crystalline inversions. Coke ovens and glass furnace crowns.

Magnesia and magnesia-chrome — basic refractories for basic slag environments. Steel converters, electric arc furnaces, cement kiln burning zones (magnesia-spinel, since chrome-containing products are being phased out for hexavalent chromium reasons).

Silicon carbide — very high thermal conductivity, excellent abrasion and thermal shock resistance, good in reducing atmospheres. Used where heat transfer through the lining is wanted — muffles, radiant tubes, kiln furniture — and in abrasive duty such as incinerator and gasifier hot faces.

Carbon and graphite — non-wetting to molten metal and slag, extremely good thermal shock resistance. Blast furnace hearths, aluminium cells, submerged components. Oxidise in air above about 400°C, so they require non-oxidising conditions or protection.

Multi-Layer Lining Design

Almost all industrial linings are layered, because no single material does both insulating and hot-face duty.

Typical two-layer construction:

• Hot face: dense castable or brick, selected for the service environment, 50-150 mm
• Backup: insulating castable, insulating firebrick or ceramic fibre board, thickness set by the shell temperature calculation

Typical three-layer construction in high-temperature furnaces:

• Hot face: high-alumina brick or dense castable
• Intermediate: medium-weight insulating brick or castable
• Backup: ceramic fibre board or block against the shell

Design considerations:

Shell temperature calculation. The governing output. The lining thickness is adjusted until calculated shell temperature is comfortably below the design limit at the design hot-face temperature, with an allowance for lining degradation over life. Calculate with worst-case ambient and still air, not with the design wind speed — the sheltered side of the vessel has no wind.

Interface temperature limits. Each layer must be within its own service temperature at the interface. A Group 23 insulating firebrick behind a hot face running at 1,400°C may see 1,100°C at the interface — over its limit — and will shrink and crumble, after which the hot face loses its support.

Dew point constraint in sulphur and acid service. In sulphur recovery units, sour gas service and flue gas ducting, shell temperature must be kept above the acid dew point as well as below the metallurgical limit, or acid condenses on the shell internals and corrodes it from the inside. This two-sided constraint narrows the lining thickness window considerably and sometimes drives selection toward a less insulating backup than energy considerations alone would suggest.

Thermal expansion joints. Refractory expands on heating; a continuous lining with no provision cracks or buckles. Brick linings use mortar joints and designed expansion allowances; castable linings use expansion joints at defined spacing, usually with ceramic fibre fill. Monolithic linings in large vessels typically have joints at 1-2 m spacing in panels.

Vapour and gas venting. Gas permeating the lining, and steam during dryout, must be able to escape. Vent holes in the shell, often with removable plugs, are standard on castable-lined vessels. On large linings their spacing and size are engineering decisions, not site improvisations.

Anchoring

Anchors transfer the weight of the lining to the shell and hold it in place through thermal cycling. Anchor failure is a leading cause of lining collapse, and anchors are routinely under-specified.

Metal anchors

• V-anchors and Y-anchors — formed wire, welded to the shell. Standard for castable linings up to about 1,100°C hot-face temperature at the anchor tip.
• Stud and washer systems — for ceramic fibre modules and board
• Materials: 304 stainless for moderate temperatures, 310 and 316 for higher, and Inconel or other nickel alloys for the most severe duty. Carbon steel anchors are limited to roughly 400°C and are almost never appropriate in a hot face.
• Anchors must be tipped with ceramic fibre or coated with bitumen so they can expand without spalling the surrounding castable — a bare anchor tip expands more than the refractory and cracks it
• Welding must be qualified and inspected; a high proportion of anchor failures are weld failures, and a bend test on a sample of anchors is standard quality control

Anchor spacing. Typically 150-300 mm centres depending on lining thickness, orientation and service. Roofs and overhead surfaces need closer spacing than walls, and vertical surfaces more than floors. Spacing should follow the refractory supplier's design, calculated from lining weight and thermal load.

Hexmesh (hex steel) — a honeycomb of stainless steel strip welded to the shell, filled with a thin dense castable, typically 19-25 mm thick. Standard for severe abrasion in FCC units, cyclones, transfer lines and flue gas ducts. The mesh carries the load and compartmentalises the refractory, so local damage does not propagate.

Ceramic anchors — for temperatures beyond metal anchor capability, typically in high-temperature brick and tile construction.

Dryout and Heat-Up

Everything above is undone by a bad dryout.

Why it matters

A castable lining contains:

1. Free water — physically held in pores, driven off from about 100°C
2. Chemically bound water — in the calcium aluminate hydrates, released roughly 200-400°C
3. Residual hydrate water in some phases, to higher temperatures

Dense LCC and ULCC castables have very low permeability by design. When water turns to steam faster than it can escape through that low-permeability matrix, internal pressure builds. The result is spalling — sheets and chunks of the hot face blowing off — and in severe cases explosive failure of the lining.

The irony is that the better the castable, the more vulnerable it is during dryout, because low porosity and low permeability are exactly what makes it a good refractory and a bad steam vent.

Typical schedule

Schedules are material- and thickness-specific and must come from the supplier, but the shape is consistent:

Total duration commonly 24 to 96 hours depending on thickness and material. Thick linings and ULCC materials sit at the long end.

Points that are regularly got wrong:

• The hold temperatures are not approximate. Holding at 80°C does not drive off free water at a useful rate. Holding at 200°C skips past the free-water stage too fast.
• Measure refractory surface temperature, not gas temperature. A burner raising flue gas to 400°C may have the lining surface at 150°C, or vice versa in a thin lining. Thermocouples on the lining surface, and shell thermocouples, are the control instruments.
• Heat uniformly. A single burner firing into a large vessel heats one area fast and leaves another cold, producing differential expansion and cracking. Dryout burners are sized and positioned for uniform heating, not for capacity.
• Vent holes open throughout. Plugs are removed for dryout and replaced afterward. Blocked vents during dryout is a classic cause of spalling.
• Organic fibre additions help. Many modern castables include polypropylene fibres that melt around 160-170°C, leaving a network of channels that lets steam escape. These materials tolerate faster dryout, and the supplier's schedule will say so. Do not assume the fibres are there — ask.
• Do not place castable in freezing conditions or let it freeze before curing. The water expands and the lining is destroyed before it ever sees heat.
• Curing before dryout. Castables need a curing period — typically 24 hours at above 20°C, kept damp or covered — before any heat is applied. Starting dryout on green castable that has not developed strength causes cracking.

Common Failure Mechanisms

Spalling from dryout — as described. The most common early-life failure.

Thermal shock cracking — repeated rapid temperature change. Dense low-porosity materials are more vulnerable than porous ones; silicon carbide and materials with a crack-arresting microstructure resist it better.

Abrasion and erosion — particle impact wearing the hot face. Predictable from gas velocity and particle loading; mitigated by dense erosion-resistant castables and hexmesh.

Slag and flux attack — molten slag penetrating pores, dissolving the matrix and causing structural loss. Matched by chemistry: acid refractory for acid slag, basic for basic slag.

Alkali attack — potassium and sodium vapour reacting with alumino-silicates to form expansive phases, causing bursting and spalling. Severe in cement kilns and biomass and waste combustion. Mitigated by high alumina content and by low-lime or no-cement castables.

CO disintegration — in reducing atmospheres between about 400°C and 600°C, carbon monoxide disproportionates and deposits carbon catalysed by iron oxide in the refractory, bursting the structure. Specified against by using low-iron refractories.

Hydration of unfired material in storage — castable bags that absorb moisture lose strength permanently. Store on pallets, in dry covered conditions, and respect the shelf life; cement-bonded castables typically have 6-12 months shelf life.

Anchor failure — weld failure, anchor oxidation from using an inadequate alloy, or insufficient spacing. The lining then detaches under its own weight, usually from a roof or overhead surface.

Shell distortion — a hot shell expands and distorts, loading the lining in ways it was not designed for. Often a symptom of a lining already failed somewhere else.

Common Specification Mistakes

1. Compressing or skipping the dryout schedule. The sulphur recovery unit example: 58 hours saved, 23 days lost.

   Prevention: Obtain the supplier's dryout schedule for the specific material and thickness, make it a formal commissioning procedure with recorded temperatures, and require shell and lining surface thermocouples.

2. Carbon steel or 304 anchors in high-temperature hot face. Anchors oxidise, lose strength and the lining detaches.

   Prevention: Calculate the anchor tip temperature and select the anchor alloy against it — 310 or nickel alloy where required — and specify anchor tipping with ceramic fibre or bitumen coating.

3. Backup insulating layer exceeding its service temperature at the interface. It shrinks and crumbles and the hot face loses support.

   Prevention: Calculate the interface temperature and verify each layer against its own rating with margin, not only the shell temperature.

4. Conventional castable in the 800-1,100°C range. Strength falls to a minimum exactly in that band.

   Prevention: Use LCC or ULCC where service temperature falls in the intermediate range, or where the lining will cycle through it.

5. Insulating castable or IFB used as a hot face in abrasive service. Erodes within months.

   Prevention: Determine gas velocity and particle loading, and use dense erosion-resistant castable or hexmesh where abrasion governs, accepting the thermal penalty and compensating with backup thickness.

6. No expansion joints in a large monolithic lining. The lining cracks and buckles on first heat-up.

   Prevention: Specify expansion joint locations and fill material from the supplier's design, typically panel joints at 1-2 m spacing.

7. Shell temperature calculated without considering acid dew point in sulphur or flue gas service. The shell is kept cool enough to condense acid internally.

   Prevention: Calculate the two-sided constraint — below metallurgical limit and above acid dew point — and adjust the lining to satisfy both.

8. Dry gunning used for quality-critical linings. Water content varies with the nozzleman, rebound is 15-30%, and installed properties are inconsistent.

   Prevention: Specify wet shotcreting with accelerator addition at the nozzle for abrasion-critical and process-critical linings, and require installed density and strength testing on panels.

9. Castable stored badly or past shelf life. Moisture absorption permanently reduces strength.

   Prevention: Specify storage requirements, record delivery dates, and reject material past shelf life or with evidence of moisture exposure.

10. No test panels or installed property verification. The lining is accepted on the material certificate rather than on what was actually installed.

    Prevention: Require test panels cast or gunned from the same batches by the same crew, tested for density, cold crushing strength and permanent linear change after firing.

11. No vent holes, or vents blocked during dryout. Steam cannot escape and the hot face spalls.

    Prevention: Specify vent hole size and spacing in the design, and make opening them a signed step in the dryout procedure.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies refractory materials and installation accessories for furnaces, kilns, boilers, reactors and process vessels:

Monolithic refractories

• Conventional, low cement and ultra-low cement castables
• No-cement and sol-gel bonded castables for fast dryout
• Dense, medium-weight and insulating castables
• Erosion-resistant and silicon-carbide-containing castables
• Self-flowing pumpable castables
• Wet shotcreting and dry gunning mixes
• Plastic refractories and ramming mixes
• Mortars, patching compounds and coatings
• Burner block and tap-hole mixes

Bricks and shapes

• Fireclay brick in ASTM C27 duty classes
• High-alumina brick, 50% to 99% Al₂O₃, including mullite and corundum grades
• Insulating firebrick in ASTM C155 Groups 23 to 32
• Silica brick
• Magnesia and magnesia-spinel brick
• Silicon carbide brick, tile and shapes
• Carbon and graphite blocks
• Special shapes, arches, wedges and kiln furniture to drawing

Ceramic fibre products

• Blanket in 96 to 160 kg/m³ densities and 1,260 to 1,600°C grades
• Board, paper, felt, bulk fibre and textiles
• Folded modules with integral anchoring
• Expansion joint and seal materials

Anchors and accessories

• V and Y anchors in 304, 310, 316 and Inconel
• Studs, washers, clips and speed nuts
• Hexmesh and hex steel in stainless grades
• Ceramic anchors and tiles
• Anchor welding consumables and welding studs
• Expansion joint fill, bitumen tipping and ceramic fibre tipping materials
• Needle fibre, stainless steel needles and organic burnout fibre

Associated supply

• Thermal insulation and cladding for the cold face
• Carbon steel and alloy plate for shells and casings
• Process burners and burner components
• Vessel and ductwork fabrication

Engineering support

Send vessel or furnace drawings with operating and design temperatures, atmosphere, gas velocity, particle loading and composition, slag or ash chemistry where relevant, shell material and its temperature limit, cycling frequency, and any dew point constraint. We will return a lining design with layer materials and thicknesses, calculated shell and interface temperatures, anchor type, alloy and spacing, expansion joint layout, and a dryout schedule for the specified materials and thicknesses.

Certification

Material data sheets with chemical analysis, bulk density, cold crushing strength, modulus of rupture, permanent linear change after firing, thermal conductivity versus temperature, and refractoriness under load. Batch certificates, anchor material certificates to EN 10204 3.1, test panel results, and installation procedure documentation.

Logistics

Standard castables, bricks and ceramic fibre generally ship in 3-6 weeks. Special shapes, high-purity products and large tonnages 6-12 weeks. Anchors and hexmesh 3-5 weeks. Shipping from Istanbul by road to Europe, the Caucasus and Iraq, and by sea to Gulf, African and Asian destinations. Castables are moisture sensitive; we ship in sealed palletised packaging with shrink wrap and desiccant where sea transit duration requires it.

Send your vessel drawings and operating conditions and we will return a lining design with dryout schedule within five working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: High-Temperature Equipment Series

• Industrial Process Burners: Types and Selection
• Industrial Insulation Materials: Selection and Thickness
• Carbon Steel Plate: A516 and A36 Grades

Frequently Asked Questions

Q: What is the difference between a conventional and low cement castable?
A: Conventional castables contain 15 to 25% calcium aluminate cement and are easy to mix and place, but they have high water demand, higher porosity and a strength minimum between about 800°C and 1,100°C. Low cement castables use 4 to 8% cement with fine matrix additions such as microsilica and reactive alumina, giving much lower water demand, higher density, better abrasion resistance and much better strength at all temperatures, but they need accurate water addition and good mixing.

Q: Why does refractory need a slow dryout?
A: A castable lining holds both free water in its pores and chemically bound water in the cement hydrates. Dense modern castables have very low permeability, so if heating is too fast the water turns to steam faster than it can escape and internal pressure spalls the hot face off. A typical schedule runs 20 to 30°C per hour with holds of 8 to 24 hours at about 110-150°C and again at 300-350°C, taking 24 to 96 hours in total.

Q: How thick should a refractory lining be?
A: Thickness is set by calculation, not by convention. The lining must keep the steel shell below its design temperature limit — commonly 370 to 425°C for carbon steel vessels — with allowance for lining degradation over life. In sulphur and flue gas service the shell must also stay above the acid dew point, which creates a two-sided constraint. Each layer must additionally remain within its own service temperature at the interface.

Q: What anchors are used for refractory linings?
A: Castable linings use formed wire V or Y anchors welded to the shell, typically at 150 to 300 mm centres, in 304 stainless for moderate temperatures, 310 or 316 for higher, and nickel alloys such as Inconel for severe duty. Anchor tips are coated with bitumen or tipped with ceramic fibre so they can expand without cracking the surrounding castable. Severe abrasion service uses hexmesh filled with thin dense castable instead.

Q: When should hexmesh be used instead of a thick castable lining?
A: Hexmesh is a welded honeycomb of stainless steel strip filled with 19 to 25 mm of dense castable, and it is the standard answer for severe particle abrasion — FCC unit cyclones, transfer lines, plenum chambers and flue gas ducts. The steel honeycomb carries the load and compartmentalises the refractory so local damage does not spread, which a plain monolithic lining in abrasive service cannot do.

Q: What causes refractory to spall?
A: The most common cause in new linings is too fast a dryout, where internal steam pressure blows the hot face off. In service, spalling comes from thermal shock during rapid temperature cycling, from alkali attack forming expansive phases in alumino-silicate refractories, from carbon monoxide disintegration in reducing atmospheres between about 400°C and 600°C, and from slag penetration followed by structural failure at the penetration depth.

Q: How are insulating firebricks classified?
A: Insulating firebricks are classified under ASTM C155 into Groups such as 20, 23, 26, 28, 30, 32 and 33, where the group number approximates the maximum service temperature in hundreds of degrees Fahrenheit — Group 23 is about 1,260°C. They are porous and low density at 500 to 1,100 kg/m³, so they store very little heat, which makes them valuable in cyclic furnaces, but they are weak and erode easily and so are normally used as backup rather than hot face.