Ceramics and refractories

In short

Ceramics are inorganic, non-metallic solids held together by ionic and covalent bonds — hard, chemically inert, high-melting, electrically insulating (usually) and brittle. Refractories are the sub-class of ceramics that keep their strength, shape and chemistry above roughly 1,500 °C, so they line every furnace in which steel, cement, glass, copper and ceramics themselves are made; about 70 % of Indian refractory output goes to iron and steel. The topic is examined in GATE MT (structure, sintering, brittle fracture, refractory properties and selection), in university Materials and Ceramic Technology papers, and in interviews at TRL Krosaki, RHI Magnesita India, Dalmia-OCL, IFGL, Vesuvius India, CUMI, Tata Steel, JSW, SAIL and UltraTech/ACC. Questions almost always combine a property calculation (porosity, bulk density, MOR, thermal shock) with a selection judgement (which brick for which zone, and why).

Explained

1. What a ceramic is: bonding, classification, consequences

• Bonding: mixed ionic–covalent. Ionic fraction rises with electronegativity difference (MgO, Al₂O₃ strongly ionic; SiC, Si₃N₄, BN largely covalent).
• Consequences of that bonding: directional, strong bonds → high melting point, high hardness, high elastic modulus, low thermal expansion; no free electrons → insulating and low thermal conductivity; no easy slip systems (dislocation motion would bring like charges together) → brittle, fails in tension from pre-existing flaws.
• Traditional ceramics: clay-based — bricks, tiles, sanitaryware, porcelain, cement, glass, abrasives, refractories. Raw materials are mined and variable.
• Advanced (engineering/fine) ceramics: made from synthesised powders of controlled purity and size — Al₂O₃, ZrO₂, SiC, Si₃N₄, SiAlON, BaTiO₃, PZT, ferrites, hydroxyapatite.
• By chemistry: oxides (Al₂O₃, MgO, ZrO₂, SiO₂, spinel), non-oxides (carbides, nitrides, borides, silicides), silicates, glasses and glass-ceramics.
• Typical values for orientation: MgO melts ~2,800 °C, Al₂O₃ ~2,054 °C, ZrO₂ ~2,700 °C, SiC decomposes ~2,700 °C; E for alumina ~380 GPa; K_IC for most monolithic ceramics 2–6 MPa·m^0.5 against 50–150 for steels.

2. Crystal structure, silicates and the glassy state

• Pauling's rules / radius ratio: cation coordination follows r_c/r_a — 0.155–0.225 → CN 3, 0.225–0.414 → CN 4 (tetrahedral), 0.414–0.732 → CN 6 (octahedral), 0.732–1.0 → CN 8 (cubic). The structure must also be electrically neutral.
• Key structures: rock salt (MgO, CaO; CN 6), zinc blende/wurtzite (ZnS, SiC, AlN; CN 4), fluorite (ZrO₂, CaF₂, UO₂; CN 8), rutile (TiO₂), corundum (α-Al₂O₃ — HCP oxygen, Al in two-thirds of octahedral sites), spinel AB₂O₄ (MgAl₂O₄), perovskite ABO₃ (BaTiO₃ — ferroelectric below ~120 °C Curie point).
• Silicates are built from the SiO₄⁴⁻ tetrahedron: island (olivine), chain (pyroxene), sheet (kaolinite Al₂Si₂O₅(OH)₄, talc, mica — sheets slide, hence clay plasticity) and framework (quartz, feldspar).
• Clay–water behaviour: plate-like particles with adsorbed water film give plasticity for forming; water is removed on drying (shrinkage, cracking risk) and chemically combined water leaves at 500–600 °C.
• Glass: a supercooled liquid with no long-range order. SiO₂ is the network former; Na₂O/K₂O are network modifiers (break Si–O–Si bridges, lower viscosity and working temperature); B₂O₃ is a second network former, while Al₂O₃, TiO₂, ZnO and PbO are intermediates. Glass transition T_g is where the specific-volume–temperature slope changes; annealing point, softening point and working range follow from viscosity.
• Glass-ceramics: glass given a controlled nucleation-and-crystallisation heat treatment (e.g. Li₂O–Al₂O₃–SiO₂) to give near-zero expansion cookware and hobs.
• Zirconia polymorphs: cubic above ~2,370 °C, tetragonal 1,170–2,370 °C, monoclinic below ~1,170 °C. The t→m transformation on cooling expands ~3–5 % and cracks pure ZrO₂; adding Y₂O₃, MgO or CaO gives PSZ/TZP and the metastable tetragonal phase that transforms at a crack tip — transformation toughening (K_IC up to ~8–12 MPa·m^0.5).
• Al₂O₃–SiO₂ system (the refractory diagram): mullite 3Al₂O₃·2SiO₂ (~72 wt % Al₂O₃) is the only stable intermediate compound, melting near 1,850 °C; the silica-rich eutectic is near 1,587 °C. This is why fireclay bricks soften early (glassy silicate bond) and high-alumina bricks do not.

3. Processing: powder to sintered body

1. Powder preparation: beneficiation/calcination, ball or attrition milling to a controlled particle-size distribution; for refractories a *graded* grain size (coarse grog + fines) gives low shrinkage and high packing density.
2. Batching and additives: binder (PVA, dextrin, lignosulphonate), plasticiser, deflocculant (sodium silicate for slips), lubricant, sintering aid (MgO in alumina pins grain boundaries).
3. Forming:
4. Dry/semi-dry pressing (uniaxial die, 1–3 % moisture) — bricks, tiles, electronic parts; density gradients and die wall friction are the limits.
5. Cold isostatic pressing (CIP) — uniform density, complex shapes.
6. Slip casting — a deflocculated slurry poured into a plaster mould that absorbs water; sanitaryware, crucibles, nozzles.
7. Extrusion — pipes, honeycombs, insulators; injection moulding — small complex precision parts; tape casting — thin substrates and multilayer capacitors.
8. Hand/mechanical pressing of shaped refractory bricks, and monolithic placement (casting, gunning, ramming, shotcreting) with no firing at all.
9. Drying: slow, controlled (constant-rate then falling-rate period) to avoid warping and cracks; drying shrinkage is the main defect source.
10. Firing/sintering: heat to ~0.7–0.9 T_m. Driving force = reduction of total surface free energy. Stages — (i) neck formation between particles, (ii) intermediate stage: open interconnected porosity, most shrinkage and densification, (iii) final stage: isolated closed pores plus grain growth.
11. Mechanisms: surface diffusion and evaporation–condensation coarsen necks but give *no* shrinkage; grain-boundary and lattice diffusion with the grain boundary as source give densification; viscous/liquid flow densifies fastest.
12. Solid-state sintering (pure Al₂O₃), liquid-phase sintering (porcelain, Si₃N₄ with MgO, fireclay — a glassy bond wets grains and pulls them together by capillary force), reaction bonding (RBSN, SiSiC), vitrification (traditional whiteware).
13. Pressure-assisted: hot pressing (uniaxial, graphite dies) and HIP give near-zero porosity and the highest strength, at high cost and simple shapes only.
14. Finishing: diamond grinding, lapping, glazing (whiteware), machining only where unavoidable — it is a major cost and introduces surface flaws.
15. Triaxial porcelain benchmark body: roughly 50 % clay (plasticity) + 25 % feldspar (flux → glass) + 25 % quartz (filler, controls shrinkage).

4. Mechanical, thermal and electrical behaviour

• Brittle fracture: strength is flaw-controlled, not bond-controlled. Theoretical strength ~E/10; measured strength is 100–1,000 times lower because of pores, inclusions, machining damage and large grains.
• Griffith criterion balances released elastic energy against new surface energy; the fracture-mechanics form is K_IC = Yσ√(πa). Strength falls as 1/√a — halving flaw size raises strength by √2.
• Strength is much higher in compression (10–15× tensile) because cracks do not propagate under compressive stress. Ceramics are therefore designed as compression members — a refractory arch, an arch brick lining, a tile bed.
• Measured by bend test, not tension: modulus of rupture (MOR) in 3-point or 4-point bending; gripping and aligning a brittle tensile specimen is impractical.
• Weibull statistics: strength is a distribution, not a number. P_s = exp[−(σ/σ₀)^m]; the Weibull modulus m measures scatter (m ≈ 5–10 for traditional ceramics and refractories, 10–30 for good advanced ceramics, 50–100 for metals). Larger specimens are weaker (more flaws sampled) — the size effect.
• Toughening routes: transformation toughening (ZrO₂), crack deflection and bridging by a second phase (ZTA, SiC-whisker and fibre-reinforced CMCs), microcracking, residual compressive surface stress (glazing, tempering, ion exchange).
• Hardness and wear: Vickers/Knoop; alumina ~15–20 GPa, SiC ~25 GPa, B₄C ~30 GPa — hence grinding media, nozzles, liners, armour.
• Creep above ~0.5 T_m by grain-boundary sliding and diffusional flow; a glassy grain-boundary phase is the usual culprit — the reason low-silica, direct-bonded refractories creep less.
• Thermal shock: a temperature gradient or quench produces σ = EαΔT/(1−ν). Resistance improves with low E, low α, high strength, high thermal conductivity and with controlled porosity/microcracks that blunt cracks. Fused silica survives quenching that destroys alumina; silica bricks are poor below 600 °C because of quartz phase inversions, and magnesia bricks spall readily because α is high.
• Electrical/other functions (brief): insulators and substrates (alumina, steatite, porcelain), dielectrics and piezoelectrics (BaTiO₃, PZT), soft and hard ferrites, ionic conductors (Y-stabilised zirconia in λ-sensors, β-alumina), bioceramics (alumina/zirconia heads, hydroxyapatite coatings), high-T_c superconductors (YBCO).

5. Refractories: definition, classification and the standard tests

• Definition: a material that resists heat, load, thermal shock, abrasion and chemical attack at service temperature without melting, deforming or reacting away. Normal requirement: PCE well above the process temperature, plus chemical compatibility with the slag and atmosphere.
• By chemistry (governs slag compatibility — the single most asked selection rule):
• Acidic: silica, fireclay, zircon — attacked by basic slags.
• Basic: magnesia, dolomite, magnesia-chrome — attacked by acidic slags.
• Neutral: alumina, chromite, carbon, graphite, SiC — tolerate both.
• *Rule:* match the lining to the slag — basic lining for a basic (high-CaO) steelmaking slag, acid lining for an acid slag. A mismatch dissolves the lining.
• By refractoriness: normal (1,580–1,700 °C), high (1,700–1,780 °C), super (above 1,780 °C).
• By form: shaped (standard, arch, wedge, key, special bricks) and unshaped/monolithic (castables — conventional, low-cement LCC and ultra-low-cement, self-flowing; gunning and shotcreting mixes; ramming mass; plastic refractory; mortar; dry vibratable mass for induction furnaces). Monolithics now exceed half of consumption because they need no joints, install fast and repair in place.
• By density: dense (working lining, in contact with heat and slag) and insulating (back-up: insulating firebrick, ceramic fibre blanket and board, calcium silicate board, microporous board) — a two- or three-layer lining puts strength at the hot face and insulation behind it.
• Standard property tests (methods in the IS 1528 series, *Methods of sampling and physical tests for refractory materials*, in several parts; confirm the current part number and edition, and ASTM/ISO equivalents where a customer specifies them):
• PCE (pyrometric cone equivalent): the Seger/Orton cone number whose bending matches the test cone — a softening *range* indicator, not a melting point. Cone 26 ≈ 1,580–1,600 °C, cone 31 ≈ 1,690–1,700 °C, cone 33 ≈ 1,730 °C, cone 35 ≈ 1,770 °C (values vary slightly by cone series).
• RUL (refractoriness under load): subsidence while heated under a constant compressive load (commonly 0.2 MPa). Reported as the temperature for a given % subsidence. It is the *service-relevant* number; PCE is not, because PCE is measured unloaded.
• Creep under load: deformation at constant temperature and load over hours — distinguishes two bricks of equal RUL.
• CCS (cold crushing strength): load/area at failure — an index of bond quality, handling strength and abrasion resistance, not of hot strength.
• HMOR (hot modulus of rupture): bending strength at temperature — the real indicator of hot mechanical behaviour.
• Apparent porosity, bulk density, water absorption, apparent specific gravity: by the boiling/vacuum water-immersion method. Low porosity → better slag and gas penetration resistance; higher porosity → better insulation and thermal-shock tolerance.
• PLC (permanent linear change) / reheat change: irreversible dimension change after soaking at service temperature — governs joint and expansion-allowance design.
• Thermal spalling / thermal shock resistance: cycles survived in a prescribed quench test.
• Slag and corrosion resistance: static crucible (cup) test, rotary slag test; abrasion resistance; CO disintegration test for blast-furnace bricks.
• Thermal conductivity: hot-wire or calorimeter method; also thermal expansion by dilatometer.

6. The main refractory families and where they go in an Indian plant

• Fireclay (Al₂O₃ 25–45 %): cheapest workhorse — ladle back-up, soaking pits, chimneys, boiler and reheating-furnace walls. Grades run from low to super duty (PCE roughly 19 to 33+).
• High alumina (50–99 % Al₂O₃): blast-furnace stack and stoves, lime and cement kilns, ladles, tundish, non-ferrous furnaces. Bauxite/calcined-bauxite based in India, with mullite and corundum phases.
• Silica (SiO₂ ≥ 96 %): coke-oven walls and glass-tank crowns — high RUL close to its melting point, excellent against acid slags, but destroyed by thermal cycling through 200–600 °C.
• Magnesia / magnesia-chrome / dolomite (basic): EAF, converter (BOF), AOD, ladle slag line, cement rotary-kiln burning zone (magnesia-spinel today, since chrome bricks make hexavalent chromium waste). MgO hydrates — storage must be dry.
• Magnesia-carbon and alumina-carbon/alumina-graphite: BOF and EAF slag lines, ladle, continuous-casting nozzles, submerged-entry nozzles, slide-gate plates, stopper rods — carbon is not wetted by slag and conducts heat well, but oxidises (antioxidants Al, Si, B₄C added).
• Carbon/graphite and SiC: blast-furnace hearth and tuyère, aluminium cells and launders, nitride-bonded SiC in kiln furniture and blast-furnace shaft, Si₃N₄/SiAlON risers and thermocouple sheaths.
• Zircon and zirconia: glass contact blocks, slide-gate inserts, nozzles — low wettability, high corrosion resistance.
• Insulating: IFB, calcium silicate board, ceramic (refractory ceramic) fibre blanket — note that RCF is classified as a suspected carcinogen; alkaline-earth silicate (bio-soluble) wool is the substituted product.
• Indicative consumption: ~8–12 kg of refractory per tonne of crude steel in India, trending down with better practice; India is among the world's larger refractory producers, with clusters at Rajgangpur, Belpahar, Katni, Bhiwadi and Visakhapatnam.

7. Standards, codes and Indian practice

• BIS: specification standards exist for moderate- and high-heat-duty fireclay bricks, high-alumina bricks, silica bricks, magnesite and insulating firebricks, with dimensions and tolerances standardised separately; test methods are in the IS 1528 series. Old refractory IS numbers have been revised and some withdrawn — always quote the current number and year from the BIS catalogue rather than from a textbook.
• Cement is covered by IS 269 (OPC 33/43/53 grades in one standard since the 2015 revision) and concrete practice by IS 456; ceramic tiles and sanitaryware follow their own IS specifications and are also under BIS quality-control orders for several product groups.
• Safety and labour: crystalline silica dust (silicosis) is the defining occupational hazard of refractory and ceramic plants; respirable dust control, exhaust ventilation, medical surveillance and PPE are mandatory. The four Labour Codes, in force from 21 November 2025, replaced the Factories Act and related Acts — occupational safety, health and working-condition duties now sit in the OSH&WC Code.
• Environment: kiln emissions, chrome-bearing spent refractory (hexavalent chromium) and used-refractory recycling fall under CPCB/SPCB consent and the hazardous-waste rules; spent refractory recycling into monolithics is now routine industrial practice.
• GST: refractory bricks, blocks and monolithics attract the standard rate; the slabs have been 5 %, 18 % and 40 % since 22 September 2025 — verify the exact HSN-wise rate before quoting it in a costing.

Key points

• Ceramic - inorganic non-metallic solid, ionic/covalent bonded: hard, inert, high-melting, brittle.
• Brittleness cause - few slip systems plus flaw sensitivity, not weak bonds.
• Strength ∝ 1/√a - flaw size, not composition, usually decides the measured strength.
• Weibull modulus m - scatter indicator; low m means a wide strength distribution and a low allowable design stress.
• Compression ≫ tension - design ceramic and refractory structures as arches and compressed linings.
• MOR - bending strength; 3-point gives higher values than 4-point because less volume is at peak stress.
• Sintering - densification driven by surface-energy reduction; only grain-boundary/lattice diffusion and liquid flow densify.
• Liquid-phase sintering - glassy bond densifies fast but lowers hot strength and creep resistance.
• Thermal shock resistance R = σ_f(1−ν)/(Eα) - wants low E, low α, high strength.
• Transformation toughening - metastable tetragonal ZrO₂ → monoclinic at a crack tip; volume expansion closes the crack.
• Mullite 3Al₂O₃·2SiO₂ - the key high-temperature phase in fireclay and high-alumina bricks.
• PCE - unloaded softening indicator (Seger cone); RUL - softening under load, the service-relevant figure.
• Acid/basic/neutral rule - match lining chemistry to slag chemistry or the lining dissolves.
• Apparent porosity - controls slag penetration, gas permeability and insulation; low at the hot face, higher in back-up.
• CCS - cold strength only; it predicts handling and abrasion resistance, never hot behaviour (use HMOR).
• Monolithics - castables, gunning mixes, ramming masses; jointless, fast, repairable, now over half of consumption.
• MgO hydrates, carbon oxidises - the two storage/service weaknesses of basic and carbon-bonded bricks.
• Silicosis - the ceramic industry's defining occupational disease; dust control is a statutory duty under the OSH&WC Code.

Formulas and examples

Example 1 — porosity and density of a fireclay brick. Dry mass D = 2,650 g; after boiling, soaked mass in air W = 2,790 g; suspended mass S = 1,520 g. True density = 2.60 g/cm³.

• Volume of open pores = W − D = 140 g of water = 140 cm³.
• Bulk volume = W − S = 2,790 − 1,520 = 1,270 cm³.
• Apparent porosity = 140/1,270 × 100 = 11.0 %.
• Bulk density = 2,650/1,270 = 2.09 g/cm³.
• Water absorption = 140/2,650 × 100 = 5.3 %; true porosity = (1 − 2.087/2.60) × 100 = 19.7 %.
• Interpretation: true porosity (19.7 %) far exceeds apparent (11.0 %), so nearly half the pore volume is closed — good for insulation, and the open 11 % is what slag can penetrate.

Example 2 — flaw size controls strength. Alumina with K_IC = 4 MPa·m^0.5, Y = 1.

• Surface flaw a = 25 µm: σ_f = 4/√(π × 25×10⁻⁶) = 4/0.00886 = 451 MPa.
• Flaw a = 100 µm: σ_f = 4/√(π × 100×10⁻⁶) = 4/0.01772 = 226 MPa.
• Four times the flaw size halves the strength — which is why grinding damage and a single large pore matter more than composition.

Example 3 — MOR in 3-point bending. Bar b = 10 mm, d = 5 mm, span L = 60 mm, failure load P = 900 N.

• σ = 3 × 900 × 60 / (2 × 10 × 5²) = 162,000/500 = 324 MPa.
• The same bar in 4-point bending (load span 30 mm) would fail at a lower stress because a larger volume carries the peak stress — report the test geometry with the number.

Example 4 — Weibull design stress. A ceramic has m = 8 and a median strength of 200 MPa (P_s = 0.5).

• From exp[−(200/σ₀)⁸] = 0.5: (200/σ₀)⁸ = ln 2 = 0.693 → 200/σ₀ = 0.693^1/8 = 0.955 → σ₀ = 209.4 MPa.
• For 99 % survival: (σ/σ₀)⁸ = −ln 0.99 = 0.01005 → σ/σ₀ = 0.01005^1/8 = 0.563 → σ = 117.9 MPa.
• The safe design stress is only 59 % of the median strength. With m = 20 the same calculation gives σ/σ₀ = 0.01005^1/20 = 0.795, i.e. ~80 % — the payoff of tighter processing.

Example 5 — two-layer furnace lining. Hot face 1,200 °C, outer surface 60 °C. Fireclay 230 mm (k = 1.2 W/m·K) backed by insulating brick 115 mm (k = 0.30 W/m·K).

• ΣL/k = 0.230/1.2 + 0.115/0.30 = 0.19167 + 0.38333 = 0.5750 m²·K/W.
• q = (1,200 − 60)/0.5750 = 1,982.6 W/m².
• Interface temperature = 1,200 − 1,982.6 × 0.19167 = 1,200 − 380.0 = 820 °C — within the insulating brick's limit, so the design is acceptable; without the insulating layer q would be 1,140/0.19167 = 5,948 W/m², three times the loss.

Where it is used

• Iron and steel: blast-furnace hearth and stack, hot-blast stoves, BOF/EAF, ladle and tundish, slide gates and SEN — Tata Steel Jamshedpur/Kalinganagar, JSW Vijayanagar, SAIL Bhilai and Rourkela; refractory suppliers TRL Krosaki (Belpahar), RHI Magnesita India, Dalmia-OCL (Rajgangpur), IFGL (Kalunga), Vesuvius India.
• Cement, lime and glass: magnesia-spinel in the kiln burning zone, high-alumina in preheater and cooler, AZS and silica in glass tanks — UltraTech, ACC, Dalmia Cement, Saint-Gobain and HNG glass plants.
• Non-ferrous, foundry and power: aluminium pots, launders and ladles; induction-furnace dry vibratable linings; CFBC boiler and incinerator linings; ramming mass and castables as the routine consumables.
• Whiteware, tiles and sanitaryware: the Morbi (Gujarat) cluster, Kajaria, Somany, HSIL/Hindware, Cera — slip casting, pressing, glazing, roller-kiln firing.
• Advanced and functional ceramics: CUMI (abrasives, wear liners, alumina ceramics), Aditya Birla Insulators and BHEL (electrical porcelain, substrates), ISRO/DRDO (ceramic thermal protection, radomes), BARC (nuclear fuel ceramics), Jyoti Ceramic (precision alumina/zirconia parts).
• Job roles: refractory engineer at a steel or cement plant (lining design, campaign life, relining shutdowns), process/kiln engineer, quality and testing (porosity, CCS, RUL, PCE labs), R&D in monolithics, technical sales/application engineer, and failure-analysis roles.

Interview questions

• Q: Why are ceramics brittle while metals are ductile? A: Their ionic–covalent bonds are directional and offer few operative slip systems, so dislocations cannot move easily; stress instead concentrates at pre-existing flaws and a crack propagates.
• Q: Why are ceramics far stronger in compression than in tension? A: Cracks only grow under tensile stress; in compression the flaws are held closed, so strength is typically 10–15 times the tensile value.
• Q: Difference between PCE and RUL, and which one would you specify? A: PCE is the softening of an unloaded cone; RUL is subsidence under a compressive load. RUL (and HMOR/creep) is the service-relevant number because a lining always carries load.
• Q: A high-CaO basic slag is eating a lining. What was the selection error? A: An acidic lining (silica or fireclay) was used against a basic slag; it should be a basic or neutral refractory — magnesia, magnesia-carbon or alumina.
• Q: Why is apparent porosity specified rather than total porosity? A: Only open, interconnected pores allow slag, metal and gas to penetrate and cause corrosion, so apparent porosity predicts service life; closed pores mainly affect density and insulation.
• Q: Why do silica bricks spall if cycled through low temperatures? A: Silica's polymorphic inversions (quartz/tridymite/cristobalite) below about 600 °C involve abrupt volume changes that crack the brick, so silica linings are held hot.
• Q: Which sintering mechanisms actually densify a compact? A: Grain-boundary and lattice diffusion with the grain boundary as the vacancy sink, and viscous/liquid flow. Surface diffusion and evaporation–condensation coarsen the necks without shrinkage.
• Q: How does zirconia toughen a ceramic? A: Metastable tetragonal particles transform to monoclinic in the crack-tip stress field; the ~4 % volume expansion puts the crack into compression and absorbs energy.
• Q: Why is bend testing used for ceramics instead of a tensile test? A: Brittle specimens fail at the grips and are intolerant of misalignment; bending is simple and reproducible, though it stresses a smaller volume and so reports higher strength.
• Q: Why have monolithics displaced bricks in so many linings? A: No joints (a joint is the first path for slag), faster and cheaper installation around complex shapes, and in-place repair during a short shutdown.
• Q: Carbon-bearing bricks resist slag well — what is the catch? A: Carbon oxidises above roughly 400–500 °C in an oxidising atmosphere, so antioxidants (Al, Si, B₄C) and a non-oxidising practice are needed; they also conduct more heat to the shell.
• Q: Two bricks have equal RUL but one fails in a month. What would you check? A: Creep under load and HMOR (RUL is a short test), plus apparent porosity, slag-corrosion test results, PLC and thermal-spalling cycles — and whether installation and joint design were right.

Common mistakes

• Treating PCE as a melting point or as proof of load-bearing capability at that temperature.
• Quoting CCS as evidence of hot strength — it is a cold test; hot behaviour needs HMOR, RUL and creep.
• Mixing up apparent, true and total porosity, or using (W − D)/D (water absorption) as porosity.
• Reversing the slag rule — putting an acidic brick against a basic slag, or specifying chrome-bearing brick where hexavalent-chromium waste rules apply.
• Forgetting the test geometry when comparing strengths (3-point vs 4-point, specimen size) and ignoring the Weibull size effect.
• Using a single average strength as a design value instead of a low survival-probability stress.
• Mixing units in porosity and heat-flow sums — cm³ with m³, mm with m, kJ/h with W — and omitting expansion allowance and PLC when designing joints.

Related topics

Polymers and composites · Nanomaterials · Materials characterisation: XRD, SEM and TEM · Non-destructive testing · Phase diagrams and phase transformations · Powder metallurgy and sintering · Heat transfer and furnace design