Question 1

Ideal refractory selection criteria: which characteristic should be high for long service life in most furnaces?

Accepted Answer

Introduction / Context:Refractories are chosen to withstand extreme temperatures and corrosive environments. A key baseline requirement is the ability to resist softening or melting within the service range, which directly relates to the fusion point.Given Data / Assumptions:Higher fusion point correlates with better hot strength potential.Spalling rate should be low, not high.Excessive shrinkage is undesirable as it leads to gaps and cracking.Concept / Approach:The fusion point (or more practically, refractoriness under load) defines the temperature limit where a refractory maintains form and load capacity. While other properties (thermal-shock resistance, corrosion resistance) matter, an adequate melting/softening threshold is foundational.Step-by-Step Solution:Eliminate properties that should be low: spalling rate, excessive shrinkage.Identify the property that should be high for service at temperature: fusion point.Select “Fusion point (melting point).”Verification / Alternative check:Specification sheets for furnace bricks always reference high refractoriness and RUL targets matching the duty temperature.Why Other Options Are Wrong:High spalling rate: accelerates failure.High shrinkage ability: leads to dimensional instability.Common Pitfalls:Focusing only on fusion point; designers must also balance chemistry and shock resistance.Final Answer:Fusion point (melting point)

Question 2

Identify the highest-melting oxide refractory among the options (noting that such materials are selected for ultra-high temperature linings).

Accepted Answer

Introduction / Context:Oxide refractories differ widely in melting points. For extremely high-temperature applications, the highest-melting oxides are especially relevant. This question checks recognition of the relative ranking among common oxide refractories.Given Data / Assumptions:Typical melting points: alumina ~2050 °C, magnesia ~2800+ °C, zirconia ~2700+ °C, thoria ~3300 °C (very high).Variations exist by purity and measurement method but relative order is consistent.Concept / Approach:Among the listed oxides, thoria (ThO2) has one of the highest melting points and is known for exceptional refractoriness. Although use is restricted for practical and regulatory reasons, in pure thermodynamic terms it outranks the others for melting temperature.Step-by-Step Solution:Compare the typical melting points of each candidate.Identify ThO2 as the highest among the options.Select “Thoria (ThO2).”Verification / Alternative check:Reference data confirm the extreme refractoriness of thoria compared with alumina, magnesia, and zirconia.Why Other Options Are Wrong:Al2O3/ZrO2/MgO: high but not as high as ThO2.Common Pitfalls:Memorizing single-point values; focus on the relative ranking for exam purposes.Final Answer:Thoria (ThO2)

Question 3

For standard fireclay (firebrick) compositions, the silica (SiO2) percentage is typically about which of the following ranges?

Accepted Answer

Introduction / Context:Fireclay bricks are alumino-silicate refractories used widely in industry. Knowing their typical SiO2 content aids in matching them to acid/basic environments and predicting corrosion behavior.Given Data / Assumptions:Fireclay bricks usually contain both Al2O3 and SiO2.Typical dense grades cluster around ~55–60% SiO2 and ~30–40% Al2O3.Concept / Approach:The common specification range for standard fireclay places silica around the mid-50s percentage. Lower numbers suggest high-alumina grades; very high numbers (80%+) indicate silica or semi-silica bricks, not standard fireclay.Step-by-Step Solution:Eliminate extremes: >94 or 80–85 represent high-silica categories.35–40 is too low for typical fireclay.Select ~55–60 as representative.Verification / Alternative check:Supplier datasheets and handbooks list fireclay compositions consistent with this band.Why Other Options Are Wrong:Very high or very low SiO2 ranges correspond to different refractory families.Common Pitfalls:Confusing fireclay with either high-alumina or high-silica products.Final Answer:55–60

Question 4

For magnesite (MgO) bricks, the maximum apparent porosity encountered in standard dense products is approximately what percentage?

Accepted Answer

Introduction / Context:Apparent porosity affects corrosion resistance, slag penetration, and strength. For magnesite bricks, designers target a controlled porosity range to balance performance and cost in basic slag environments.Given Data / Assumptions:Question asks for an approximate upper bound for standard dense grades, not insulating MgO products.Very high porosity (40%+) is atypical for dense MgO bricks.Concept / Approach:Dense magnesite bricks commonly exhibit apparent porosity values in the teens to low twenties (%). A reasonable “maximum” figure cited for dense commercial grades is around 24%, beyond which strength and slag resistance degrade rapidly.Step-by-Step Solution:Exclude very low extremes (2–8%) as best-case or specialty.Exclude 44–58% as characteristic of insulating or highly porous structures.Choose 24% as a sensible upper figure for dense MgO.Verification / Alternative check:Typical specification sheets list apparent porosity for dense MgO between ~15% and 22–24% depending on grade.Why Other Options Are Wrong:44/58%: far too high for dense MgO bricks.8%: unusually low and not an upper bound.Common Pitfalls:Confusing dense bricks with insulating grades that intentionally raise porosity.Final Answer:24

Question 5

Identify the correct chemical formula for mullite, the key crystal phase that imparts strength to many alumino-silicate refractories.

Accepted Answer

Introduction / Context:Mullite is the principal high-temperature phase in many alumino-silicate refractories. Its composition controls properties such as creep resistance, thermal shock behavior, and corrosion resistance in service.Given Data / Assumptions:Mullite is an alumina-rich phase within the Al2O3–SiO2 system.We seek the stoichiometric formula most commonly cited for mullite.Concept / Approach:The widely accepted composition for mullite is 3Al2O3 · 2SiO2. Actual mullites can show slight deviations, but the idealized formula remains the standard reference for exam and design contexts.Step-by-Step Solution:Eliminate silica-rich ratios like Al2O3 · 2SiO2 or 2Al2O3 · 3SiO2.Select the alumina-rich stoichiometry: 3Al2O3 · 2SiO2.Verification / Alternative check:Materials science texts consistently list mullite close to the 3:2 alumina-to-silica molar ratio.Why Other Options Are Wrong:They represent different alumino-silicate ratios, not the canonical mullite composition.Common Pitfalls:Confusing mullite with metakaolin or sillimanite-derived phases; mullite is distinctly alumina-rich.Final Answer:3Al2O3 · 2SiO2

Question 6

Operational guidance for carbon/graphite refractories: select the correct statement regarding the preferred furnace atmosphere and behavior.

Accepted Answer

Introduction / Context:Carbon and graphite refractories are widely used where contact with molten iron/steel and slags occurs (e.g., blast-furnace hearths). Their behavior depends strongly on furnace atmosphere and wetting chemistry.Given Data / Assumptions:Carbon oxidizes in strongly oxidising atmospheres at high temperature.Carbon is generally non-wetting to molten iron and many slags.Dimensional stability is good; large shrinkage is not typical.Concept / Approach:Because oxidation damages carbon, these refractories are preferred in neutral or reducing atmospheres. Their low wettability helps resist skull formation and infiltration. Statements implying strong oxidation exposure or high shrinkage are misleading for standard use cases.Step-by-Step Solution:Exclude claims of wetting (carbon is generally non-wetting).Select the neutral/reducing atmosphere guidance as correct.Reject high-shrinkage and universal slag-corrosion claims as inaccurate generalizations.Verification / Alternative check:Design notes for BF hearths and ladles emphasize protecting carbon from oxidation while leveraging its non-wetting behavior.Why Other Options Are Wrong:(a) Opposite of typical behavior; (c) and (d) overstate issues; (e) contradicts best practice.Common Pitfalls:Overlooking atmosphere control; even small oxygen ingress accelerates carbon loss.Final Answer:They should be used in neutral or reducing atmosphere.

Question 7

In refractory testing, a dilatometer is primarily used to determine which property related to dimensional change on heating?

Accepted Answer

Introduction / Context:Accurate measurement of thermal expansion and dimensional changes is critical in refractory quality control. The dilatometer is a common instrument for these evaluations.Given Data / Assumptions:Dilatometry measures length change versus temperature under a specified heating schedule.Permanent linear change (PLC) after firing reflects sintering/shrinkage or expansion.Concept / Approach:While modulus of rupture, RUL, and CO attack resistance require different tests, dilatometry directly captures thermal expansion curves and resultant permanent dimensional changes after a prescribed thermal cycle—reported as PLC, a key acceptance criterion.Step-by-Step Solution:Identify the instrument capability: length change vs temperature.Relate it to reporting PLC after a cycle.Select “Permanent linear change (PLC).”Verification / Alternative check:Standard test methods (e.g., PLC tests) reference dilatometry and post-cycle measurements for specification compliance.Why Other Options Are Wrong:MOR, RUL, CO attack, and conductivity involve different rigs and procedures.Common Pitfalls:Equating thermal expansion coefficient with PLC; PLC is the permanent net change after cycling, not just instantaneous expansion.Final Answer:Permanent linear change (PLC)

Question 8

During firing/heat-up, silica bricks exhibit a maximum linear expansion of approximately what percentage due to phase transformations?

Accepted Answer

Introduction / Context:Silica bricks undergo polymorphic changes (e.g., quartz–tridymite–cristobalite) that cause dimensional changes on heating. Knowing the order of magnitude of linear expansion helps engineers allow for joints and avoid structural stress.Given Data / Assumptions:We consider the net linear expansion during firing/heat-up into service ranges.Exact values depend on grade and schedule, but a typical maximum lies around the lower single-digit percent.Concept / Approach:Silica’s transformation-related expansion is notable but usually around ~1% in linear terms for many practical firing schedules, though some data may report slightly higher figures. For design allowances, the order of magnitude is closest to 1% among the options provided.Step-by-Step Solution:Recall silica exhibits expansion peaks tied to phase changes.Compare plausible percentages: 0.5% too low; 3.5–5% too high for typical linear expansion.Select the closest representative value: 1%.Verification / Alternative check:Handbooks list linear expansions of silica refractories roughly around the 1% range for practical temperature cycles.Why Other Options Are Wrong:2–5% substantially overstate typical linear expansion; 0.5% understates it.Common Pitfalls:Confusing linear expansion with volume expansion; volumetric numbers are roughly three times linear values.Final Answer:1

Question 9

Hot metal mixer (upper portion) linings: mullite bricks are selected primarily to resist which service actions?

Accepted Answer

Introduction / Context:Hot metal mixers store and homogenize molten iron before steelmaking. The upper lining faces combined mechanical washing and chemical corrosion. Mullite bricks provide a balanced resistance profile for these challenges.Given Data / Assumptions:The upper portion contacts splashing hot metal and overlying slag.Requirements include erosion resistance and chemical corrosion resistance.Concept / Approach:Mullite’s alumina-rich composition offers good hot strength and chemical durability against many slags, while microstructure design improves resistance to washing and abrasion. Therefore, mullite linings are chosen to address both dominant degradation modes in the upper mixer regions.Step-by-Step Solution:Identify service threats: washing + slag corrosion.Match mullite’s properties to those threats.Conclude that both effects are mitigated by mullite.Verification / Alternative check:Plant practice and vendor guidance align mullite with zones subject to combined erosion and corrosion.Why Other Options Are Wrong:Choosing only one mechanism underestimates the operational demands.Common Pitfalls:Overlooking mechanical erosion when focusing solely on chemical compatibility.Final Answer:Both (a) & (b)

Question 10

Silica brick bonding: using a higher percentage of lime as the bond tends to reduce which performance attributes?

Accepted Answer

Introduction / Context:Binders and bonds in silica bricks influence microstructure and hot properties. Excess lime can form lower-melting phases, impacting mechanical and wear performance.Given Data / Assumptions:Increasing lime content modifies glassy phases in silica bricks.Lower-melting phases decrease hot strength and wear resistance.Concept / Approach:While lime is necessary for certain bonding reactions, an overly high fraction promotes fusible phases that weaken the bond at temperature and reduce abrasion resistance. Thus both strength and abrasion resistance decline when lime content is excessive.Step-by-Step Solution:Relate lime increase to fusible phase formation.Link fusible phases to reduced hot strength and wear resistance.Select “Both (a) & (b).”Verification / Alternative check:Processing notes for silica bricks emphasize controlling bonding agents to maintain high-temperature mechanical performance.Why Other Options Are Wrong:Single-attribute reductions are incomplete; both properties suffer.Common Pitfalls:Assuming more bond always improves strength; chemistry of the bond matters.Final Answer:Both (a) & (b)

Question 11

Semi-silica bricks compared with silica bricks: which statements about fusion point and thermal-shock (spalling) resistance are correct?

Accepted Answer

Introduction / Context:Semi-silica bricks are engineered to balance refractoriness with improved shock resistance compared with high-silica bricks. This question tests recognition of those trade-offs.Given Data / Assumptions:Silica bricks have very high refractoriness but poor shock resistance.Semi-silica incorporates more alumina-bearing phases (e.g., mullite) improving spalling resistance.Concept / Approach:As alumina increases relative to pure silica, softening temperature can reduce slightly (lower fusion point than pure silica). However, shock resistance typically improves, giving semi-silica an advantage in cycled environments.Step-by-Step Solution:Compare fusion points: semi-silica < silica.Compare shock resistance: semi-silica > silica.Select “Both (a) and (b).”Verification / Alternative check:Material datasheets show semi-silica with better thermal shock indices than silica bricks.Why Other Options Are Wrong:“Neither” contradicts well-known property trends.Common Pitfalls:Equating highest refractoriness with best all-around performance; cyclic service needs shock resistance.Final Answer:Both (a) and (b)

Question 12

Rate of slag attack on refractories typically increases with temperature primarily because of what change in slag behavior?

Accepted Answer

Introduction / Context:Slag corrosion is a major wear mechanism. Understanding why higher temperature accelerates attack helps in choosing compositions and operating windows that extend lining life.Given Data / Assumptions:As temperature rises, slags become more fluid (viscosity decreases).Mass transfer and reaction rates increase with fluidity and temperature.Concept / Approach:Lower viscosity promotes infiltration and dissolution of refractory components by the slag. This, combined with faster reaction kinetics at higher temperature, raises corrosion rates. While thermal conductivity of bricks may change with temperature, it is not the primary reason slag attack increases.Step-by-Step Solution:Relate T ↑ to slag viscosity ↓.Infer increased slag mobility and penetration.Select “Decreased viscosity of slag.”Verification / Alternative check:Corrosion tests show stronger dissolution rates at higher temperatures due to enhanced fluidity and diffusion.Why Other Options Are Wrong:Brick conductivity or oxidising conditions are secondary/context-specific, not the main universal cause.Common Pitfalls:Over-attributing wear to atmosphere alone; slag fluidity is central.Final Answer:Decreased viscosity of slag

Question 13

Chrome–magnesite bricks are NOT used for lining which of the following furnace sections, considering typical industry practice?

Accepted Answer

Introduction / Context:Chrome–magnesite bricks are basic refractories used where basic slags and high temperatures prevail. Some furnace sections, however, traditionally rely on different refractory families such as silica or fireclay.Given Data / Assumptions:Coke oven regenerators typically use silica/fireclay-based checkers and walls.Basic steel and reheating services commonly apply MgO–Cr2O3 bricks.Concept / Approach:Matching refractory chemistry to gas and slag environment is key. Coke ovens favor silica for high-temperature creep resistance and thermal expansion behavior suited to their cycles. Basic furnaces and kiln burning zones often use chrome–magnesite due to compatibility with basic slags and good hot strength.Step-by-Step Solution:List typical applications for MgO–Cr2O3 (basic), excluding acid environments.Identify coke oven regenerators as silica-dominated installations.Select “Coke oven regenerator.”Verification / Alternative check:Industry lining schematics show silica checkerwork in coke ovens versus basic refractories in steel furnace zones.Why Other Options Are Wrong:Soaking pits/reheating/kiln burning zones can and do use basic bricks, including chrome–magnesite.Common Pitfalls:Assuming one brick family fits all high-temperature applications; chemistry matters.Final Answer:Coke oven regenerator

Question 14

To reduce thermal-spalling tendency, a refractory should preferably possess which combination of thermal properties?

Accepted Answer

Introduction / Context:Thermal spalling arises from steep temperature gradients and differential expansion. Material properties that reduce temperature gradients or strain amplitude improve resistance to cracking and flaking under thermal shock.Given Data / Assumptions:Thermal diffusivity = k / (rho * Cp), where k is conductivity and Cp is specific heat.Lower coefficient of thermal expansion reduces stress for a given gradient.Concept / Approach:Higher diffusivity spreads heat faster, lowering internal gradients. Lower Cp also increases diffusivity for a given k and density. Low thermal expansion reduces strain per degree of temperature change. Together, these traits decrease the driving forces for spalling.Step-by-Step Solution:Target properties that minimize gradients and thermal strain.Recognize diffusivity ↑ and expansion ↓ both help.Select “All (a), (b) and (c).”Verification / Alternative check:Thermal shock indices in textbooks correlate improved resistance with lower expansion and higher diffusivity.Why Other Options Are Wrong:Single-property answers omit synergistic effects; high porosity alone is not a universal cure.Common Pitfalls:Confusing conductivity with diffusivity; diffusivity explicitly includes Cp and density.Final Answer:All (a), (b) and (c)

Question 15

Why is grog (pre-fired crushed refractory) added to fireclay mixes during brick manufacture? Select the most complete reason.

Accepted Answer

Introduction / Context:Grog is a staple addition in refractory manufacturing. As inert, pre-fired granules, it shapes firing behavior and final properties of bricks by controlling shrinkage and microstructure.Given Data / Assumptions:Grog is dimensionally stable during firing.It modifies particle packing and minimizes drying/firing shrinkage.It can improve thermal-shock resistance and mechanical strength when well graded.Concept / Approach:Because grog has already undergone firing, it does not shrink further, interrupting continuous plastic matrix and reducing total shrinkage. It adds crack-deflection sites and tailors pore structure, improving spalling resistance. Proper grading and proportioning also raise fired strength via better packing and reduced defect sizes.Step-by-Step Solution:Relate prefired inert grains to shrinkage control.Connect microstructural tailoring to improved shock resistance.Note strength gains from optimized packing and sintering.Choose the comprehensive option: all (a), (b), and (c).Verification / Alternative check:Manufacturing guides consistently recommend grog to stabilize dimensions and enhance service performance.Why Other Options Are Wrong:Single benefits are incomplete; cost reduction alone is not the main technical reason.Common Pitfalls:Overusing coarse grog without fine matrix balance, which can lower strength.Final Answer:All (a), (b) and (c)

Question 16

Basic open-hearth furnace roof linings: which brick family is typically used for the roof (crown) despite the furnace handling basic slags?

Accepted Answer

Introduction / Context:Historically, basic open-hearth furnaces handle basic slags, yet the roof choice does not always mirror hearth chemistry. The roof endures radiant heat and specific thermal gradients, favoring particular materials.Given Data / Assumptions:Roof experiences high temperatures and thermal cycling with limited slag contact.Silica bricks offer excellent creep resistance and integrity in hot crowns.Concept / Approach:Silica bricks are time-tested in crowns/roofs for many furnaces because they hold shape at high temperature and have acceptable expansion behavior for arch stability, even if the bath/slag below is basic. Hearths/slag lines may be basic refractories, but roofs can be silica based on structural performance needs.Step-by-Step Solution:Separate roof requirements from slag-contact requirements.Match silica brick strengths (creep resistance, high-temperature stability) to roofs.Choose “Silica bricks.”Verification / Alternative check:Classical lining schedules for open-hearths report silica crowns with basic hearths/sidewalls.Why Other Options Are Wrong:Basic bricks dominate slag-contact zones, not necessarily roofs.Common Pitfalls:Assuming uniform refractory chemistry throughout the furnace; different zones have different demands.Final Answer:Silica bricks

Question 17

For bricks used in the combustion chamber and dome of blast-furnace stoves, which combination of properties is most important?

Accepted Answer

Introduction / Context:Blast-furnace stoves alternate between heating and blasting cycles. Bricks in the combustion chamber and dome face extreme temperatures, thermal shocks, and mechanical loads, requiring a balanced property set for durability.Given Data / Assumptions:High refractoriness ensures shape retention at peak operating temperatures.Thermal-shock resistance is vital due to cycling.Adequate strength/density confers structural robustness and wear resistance.Concept / Approach:No single property suffices. The material must withstand heat (refractoriness), resist crack generation under rapid temperature swings (spalling resistance), and bear mechanical stresses (strength/density). Therefore, a combination of all three properties is essential for long service life.Step-by-Step Solution:List the dominant degradation modes: high T, cycling, mechanical loads.Map each mode to a key material property.Conclude that all three properties must be high.Verification / Alternative check:Stove lining specifications emphasize refractoriness, shock resistance, and structural strength simultaneously for these zones.Why Other Options Are Wrong:Focusing on only one property invites premature failure via the other mechanisms.Common Pitfalls:Overemphasizing refractoriness and overlooking thermal-shock demands inherent to cycling operation.Final Answer:All (a), (b) and (c)

Question 18

Refractory thermal-shock behavior: the spalling tendency of a brick or castable lining is most effectively reduced by deliberately increasing which material parameter (under controlled, engineered porosity levels)?

Accepted Answer

Introduction / Context:Spalling is the tendency of a refractory lining to crack, flake, or shell off when subjected to rapid heating and cooling. It shortens campaign life and increases maintenance in furnaces, kilns, and ladles. Engineers therefore tailor material parameters to resist thermal shock. One practical lever is the controlled introduction of porosity.Given Data / Assumptions:Lining experiences thermal gradients and cyclic temperature swings.Microstructure (pores, grain size, bonding) governs crack initiation and growth.“Increased porosity” means engineered, well-distributed pores—not random defects.Concept / Approach:Thermal spalling is driven by tensile stresses from expansion differentials. Crack arrest and stress relief are enhanced when a material has compliant pathways. Porosity reduces the effective elastic modulus and introduces crack-deflection sites. As a result, stress intensity at flaws is lowered and catastrophic crack propagation is less likely. This is why insulating firebricks and shock-resistant alumino-silicate bricks often contain significant, well-controlled porosity.Step-by-Step Solution:Identify failure driver: thermal-stress-induced cracking.Relate stress to stiffness and flaws: lower effective modulus reduces peak stress.Controlled pores provide crack-arrest/deflection and strain accommodation.Hence, increased engineered porosity reduces spalling tendency.Verification / Alternative check:Standard refractory design texts show higher thermal-shock resistance indexes for materials with tailored porosity and microcracking. Field practice confirms better cycling endurance for porous shock-resistant grades compared with dense, very stiff bricks.Why Other Options Are Wrong:Specific gravity: density by itself does not guarantee stress relief.Thermal conductivity: may smooth gradients but is less decisive than porosity for crack arrest.Strength / Elastic modulus: very high stiffness can increase spalling risk under shock.Common Pitfalls:Over-increasing porosity, which can hurt abrasion and slag resistance.Confusing hot strength with shock resistance; they are distinct performance metrics.Final Answer:Porosity

Question 19

Appearance recognition in refractories: the typical fired colour range observed for standard fireclay (alumino-silicate) bricks used in furnaces and kilns is best described as:

Accepted Answer

Introduction / Context:Technicians and inspectors often identify brick families in the field by colour and texture. Fireclay bricks (alumino-silicate refractories) display characteristic hues after firing that reflect raw materials, iron content, and kiln atmosphere.Given Data / Assumptions:Standard fireclay compositions fired in oxidizing conditions.Minor iron oxides can warm the tone.Surface contamination from service is ignored; we consider as-fired appearance.Concept / Approach:Fireclay bodies commonly appear pale cream to buff. Slightly higher iron and specific firing profiles can shift the hue toward reddish buff. Dramatic colours such as intense yellow, black, or blue-grey typically indicate other chemistries or service deposits (e.g., carbon pick-up) rather than standard fireclay composition.Step-by-Step Solution:Identify the brick family: alumino-silicate fireclay.Recall fired appearance: light buff spectrum with possible reddish tint.Select the matching descriptor from options.Verification / Alternative check:Manufacturer datasheets routinely list colour as “buff” to “reddish buff” for general-purpose fireclay blocks and shapes, validating this identification.Why Other Options Are Wrong:Yellow: not a standard descriptor for typical fireclay bodies.Black / Blue-grey: usually due to carbon deposition or different chemistries.None of these: incorrect because the listed buff range is standard.Common Pitfalls:Judging colour after long service; slag and soot can mask true hue.Assuming colour alone determines grade; lab analysis is required for specification.Final Answer:Light buff to reddish buff

Question 20

Silica polymorph stability for refractory design: among quartz, tridymite, and cristobalite, which SiO2 phase is thermodynamically stable up to approximately 1470°C under typical kiln conditions?

Accepted Answer

Introduction / Context:Silica (SiO2) exhibits multiple polymorphs with distinct thermal expansion and transformation behaviors, which strongly influence the performance of silica-bearing refractories. Understanding the stability fields is essential for predicting dimensional change and spalling tendencies.Given Data / Assumptions:Equilibrium or near-equilibrium transformations in a kiln.Key transitions: alpha- to beta-quartz near 573°C; tridymite stabilizes at higher temperatures; cristobalite stabilizes above ~1470°C.Pressure effects are negligible in typical furnace service.Concept / Approach:From room temperature to ~870°C, quartz dominates (alpha then beta). From roughly 870°C to about 1470°C, tridymite is the stable polymorph. Above ~1470°C, cristobalite becomes stable until close to the melting region. These fields explain expansion anomalies and why silica bricks are carefully heat-treated.Step-by-Step Solution:Map temperature ranges to polymorphs.Identify the target upper limit 1470°C.Select tridymite as the stable phase up to that temperature.Verification / Alternative check:Silica phase diagrams in refractory references consistently present the sequence: quartz → tridymite → cristobalite with transitions near 870°C and 1470°C, respectively.Why Other Options Are Wrong:Quartz: stable only up to ~870°C.Cristobalite: stable above ~1470°C, not below.Opal / None: contradict established phase equilibria.Common Pitfalls:Confusing kinetic sluggishness with equilibrium stability.Overlooking the sharp expansion at the quartz transition (~573°C) during heat-up.Final Answer:Tridymite

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