Refractory raw materials are the refractory-grade grains, aggregates and fines, natural or synthetic, from which refractory bricks, monolithics and insulation are made; 16 of them form six groups, from tabular alumina to lightweight aggregates. Which grain suits which product depends on chemistry, route and service temperature.
The natural grains are known in the trade as refractory minerals. Refractory raw materials are the input to every form of refractory materials.
Al2O3 runs from 99.5 % or more in tabular alumina down to about 45 % at most in chamotte. ISO 10081-1:2003 names a brick after a raw material making up 50 % or more of it, yet no ISO, EN or ASTM product specification exists for the grains, so buyers rely on producer specifications, test methods and dated USGS unit values. Grain dust falls under the OSHA respirable crystalline silica limit of 50 µg/m³ (29 CFR 1910.1053, effective 2016-06-23).
Every value names its standard and edition, the product data sheet or the dated source it comes from; see our methodology and fact-checking process.
What Are Refractory Raw Materials?#
A refractory raw material is a refractory-grade grain, aggregate or fine powder, such as alumina, bauxite, magnesia, zircon or silicon carbide, that a producer batches into a refractory product. ISO 836:2001 (term 107) defines a refractory as a "non-metallic material or product (but not excluding those containing a proportion of metal) whose chemical and physical properties allow it to be used in a high temperature environment"; the ISO term carries no temperature threshold, while ASTM C71-12(2024) sets the US boundary at 538 °C (1,000 °F).
Refractory grade means a grain processed and sized for that use, not a mineral as mined. Inside a batch, the grains play three size roles: coarse aggregate, matrix fines and submicron reactive fines. The binders that hold them, such as calcium aluminate cement, phosphate or resin, belong to the additives family.
Types of Refractory Raw Materials: 16 Grains in Six Groups#
Refractories use 16 main raw materials in six groups: aluminas, aluminosilicates, silica, basic oxides, special and non-oxide grains, and lightweight aggregates. Table 1 lists each grain with its origin and route, main phase, composition with its basis and main product uses.
| # | Raw material | Group | Origin and route | Main phase | Composition (basis) | Main product uses |
|---|---|---|---|---|---|---|
| 1 | tabular alumina | aluminas | synthetic; Bayer alumina sintered without sintering aids | corundum | Al2O3 ≥ 99.5 % (typical product range) | castables, AMC brick, flow control, precast shapes |
| 2 | calcined and reactive aluminas | aluminas | synthetic; calcined Bayer hydroxide, ground (reactive grades to primary crystals) | corundum | graded by Na2O, fineness and degree of calcination | castable matrix, high-alumina brick, gunning mixes |
| 3 | fused alumina | aluminas | synthetic; electric arc fused (white from Bayer alumina, brown from calcined bauxite with reduction) | corundum (brown: Ti in solid solution) | white ≥ 99.5 %, brown 95 to 97 % Al2O3 (typical product ranges) | castables, high-alumina and AMC brick |
| 4 | refractory-grade calcined bauxite | aluminas | natural; calcined | corundum, mullite, glass | 84 to 91 % Al2O3 (typical product range) | high-alumina brick, castables, mortars |
| 5 | andalusite, kyanite and sillimanite | aluminosilicates | natural; beneficiated, used raw or calcined | Al2SiO5, converts to mullite | 62.9 % Al2O3 (theoretical); 55 to 60 % (typical product range) | high-alumina brick, castables |
| 6 | sintered and fused mullite | aluminosilicates | synthetic; sintered from kaolin, bauxite or blends, or fused from Bayer alumina and silica | mullite 3Al2O3·2SiO2 | 71.8 % Al2O3 (theoretical) | high-alumina brick, castables, flow control |
| 7 | chamotte (grog) | aluminosilicates | calcined fireclay, flint clay or kaolin | mullite, cristobalite, glass | about 45 % Al2O3 at most (typical product range); 45.9 % (theoretical, calcined kaolinite) | firebrick, castables, mortars |
| 8 | fireclay | aluminosilicates | natural kaolinitic clay, used raw as plastic bond | kaolinite | 39.5 % Al2O3, 14.0 % H2O (theoretical kaolinite) | firebrick, mortars, plastics, gunning mixes |
| 9 | fused silica | silica | synthetic; fused high-purity silica | vitreous SiO2 | SiO2 (amorphous) | castables, precast shapes |
| 10 | refractory magnesia | basic | dead-burned from magnesite or caustic calcined magnesia; seawater or brine magnesia; fused | periclase | MgO, with silicate phases set by CaO/SiO2 | magnesia, MgO-C and spinel brick, castables, gunning mixes |
| 11 | chrome ore (chromite) | basic | natural ore, ground; co-sintered or fused with magnesia | chromite spinel (Fe,Mg)(Cr,Al,Fe)2O4 | Cr2O3-bearing spinel | magnesia-chrome brick |
| 12 | zircon and stabilized zirconia | special | zircon: heavy-mineral-sand co-product, milled; zirconia: fused or chemical, stabilized with CaO, MgO or Y2O3 | ZrSiO4; ZrO2 | about 67 % ZrO2 (theoretical zircon) | zircon brick, fused-cast AZS, flow control |
| 13 | silicon carbide grain for refractories | non-oxide | synthetic | alpha-SiC | SiC, with free C and free Si as grade limits | SiC brick, carbon-containing products, antioxidant addition |
| 14 | magnesium aluminate spinel | basic | synthetic only; sintered or fused | MgAl2O4 | 71.8 % Al2O3, 28.2 % MgO (stoichiometric) | magnesia-spinel brick, castables |
| 15 | flake graphite for carbon-containing refractories | non-oxide | natural crystalline flake, floated and purified | graphite | C, graded by purity, ash and flake size | MgO-C and AMC brick |
| 16 | lightweight refractory aggregates | lightweight | bubble alumina: molten alumina atomized into hollow spheres; calcium hexaluminate: synthetic | corundum; CaAl12O19 | Al2O3 (bubble alumina); CaO·6Al2O3 | insulating castables, insulating firebrick |
Typical product ranges are recorded from two or more producers; theoretical values are computed from formula weights; no row is a standard requirement. Chemistry in mass %.
The origin column tells whether a grain is mined or made, and the composition column which basis a figure carries; a theoretical value is a ceiling, never a delivered grade.
Aluminas and Aluminosilicates#
Aluminas and aluminosilicates are the largest group, ordered by Al2O3 from 99.5 % or more in tabular and white fused alumina to about 45 % at most in chamotte (typical product ranges).
Between them sit brown fused alumina at 95 to 97 %, calcined bauxite at 84 to 91 % and the sillimanite minerals at 55 to 60 %. The synthetic aluminas start from Bayer alumina, while calcined bauxite is a low-iron, low-alkali bauxite fired to corundum and mullite. Fireclay enters a batch raw, as a plastic bond; chamotte is the same clay calcined to a dense, low-shrinkage grain.
Silica, Magnesia, Chromite and Spinel#
Fused silica is the one silica grain in the group: vitreous SiO2 made by fusing high-purity silica, which devitrifies to cristobalite at high temperature.
Magnesia (periclase) is the basic workhorse, dead-burned from magnesite, made from seawater or brine, or fused. Chromite is co-sintered or fused with magnesia for magnesia-chrome brick, and magnesium aluminate spinel is synthetic only.
Zircon, Silicon Carbide, Graphite and Lightweight Aggregates#
Zircon (ZrSiO4, about 67 % ZrO2 in theory) and stabilized zirconia supply zirconia to zircon brick, fused-cast AZS and flow-control parts. Zirconia is stabilized with CaO, MgO or Y2O3.
Silicon carbide and flake graphite are the non-oxide grains of carbon-containing and SiC products; graphite adds non-wetting and conductivity. Bubble alumina and calcium hexaluminate are the lightweight grains of insulating castables and firebrick.
How Are Refractory Raw Materials Classified?#
Refractory raw materials are classified three ways: by chemistry (alumina, aluminosilicate, silica, basic, non-oxide), by origin and route (natural raw, calcined, synthetic sintered or fused) and by size role in the batch. The size roles are aggregate, matrix fine and reactive fine.
Slag behavior adds a fourth label. In ISO 836:2001 terms, acid refractories contain substantial silica and react with basic slags, basic ones (magnesia, calcium oxide, chromite, spinel, forsterite) react with acid slags, and neutral ones show no appreciable reaction; see acid, basic and neutral refractories.
Natural, Calcined, Sintered and Fused Grains#
ISO 10081-1:2003 names the state of the raw material as one of five classification criteria: natural raw or calcined, synthetic calcined, or fused. The same standard names a product after its main raw material when that material makes up 50 % or more, else after the two main ones.
Calcination drives off combined water, sintering densifies a grain that keeps small closed pores, and fusion grows large crystals from a melt. Table 2 gives the temperature each source records, from about 1350 °C (2,462 °F) for kyanite to above 1800 °C (3,272 °F) for tabular alumina.
| Route (ISO 10081-1 state) | Raw materials | Temperature (source) | Effect on the grain |
|---|---|---|---|
| natural raw | fireclay, andalusite, raw kyanite | n/a (used as mined and beneficiated) | converts in the product on firing |
| calcined natural | calcined bauxite, chamotte | bauxite 1600 to 1650 °C (2,912 to 3,002 °F) (Nyoka et al. 2013, Journal of the SAIMM) | combined water removed; corundum and mullite form |
| synthetic sintered | tabular alumina, dead-burned magnesia, sintered mullite and spinel | tabular above 1800 °C (3,272 °F) (producer data sheets); dead burning 1500 to 2000 °C (2,732 to 3,632 °F) (UNITECR 2025 conference paper) | dense grain with closed pores |
| calcined and converted | kyanite to mullite | about 1350 to 1380 °C (2,462 to 2,516 °F) (Indian Bureau of Mines 2024) | mullite plus silica-rich glass |
| fused | white and brown fused alumina, fused magnesia, fused silica, fused mullite and spinel | electric arc furnace | large crystals; fused silica stays amorphous |
Theoretical Composition and the ISO 836 Chemistry Boundaries#
Stoichiometry fixes the ceiling of each aluminosilicate: calcined kaolinite holds 45.9 % Al2O3, the Al2SiO5 minerals 62.9 % and 3:2 mullite 71.8 %, computed from formula weights. Magnesium aluminate spinel carries 71.8 % Al2O3 and 28.2 % MgO by the same arithmetic.
These ceilings meet the ISO 836:2001 boundaries: fireclay products hold 30 to below 45 % Al2O3, high-alumina products 45 % and above, silica products 93 % SiO2 or more, magnesia products more than 80 % MgO. Chamotte sits close to the 45 % line, so higher Al2O3 comes from bauxite, sillimanite minerals, mullite or synthetic alumina. The finer groups follow the ISO 10081 classification of dense shaped products.
Which Raw Material Suits Which Temperature and Slag?#
The right raw material is the one whose chemistry matches the slag and atmosphere and whose purity and route carry the service temperature: aluminas and aluminosilicates for acid to neutral service, magnesia, spinel and chromite for basic slags, SiC and graphite where slag meets thermal shock.
Chromite shows how service chemistry can retire a grain. Glass and cement service discontinued chromite refractories because Cr3+ oxidizes to hexavalent chromium with CaO and alkali below about 1022 °C (1,872 °F) (McEwan et al., SAIMM Pyrometallurgy conference paper, 2011). A grain specification should name chemistry with its basis and the test method behind it, not a brand.
Raw Materials by Product Family#
Each product family draws on a different set of grains, as Table 3 shows.
| Product family | Main raw materials |
|---|---|
| refractory bricks | fireclay, chamotte, bauxite, sillimanite minerals, mullite, fused and tabular alumina, magnesia, chromite, spinel, zircon, SiC, graphite |
| monolithic refractories | tabular, calcined, reactive and fused alumina, bauxite, chamotte, fireclay, sillimanite minerals, mullite, fused silica, magnesia, spinel |
| refractory insulation | bubble alumina, calcium hexaluminate |
One grain can feed several families: tabular alumina serves castables, alumina-magnesia-carbon brick, slide gate plates and precast purging plugs.
Aggregate Choice and Castable Service Limits#
The aggregate sets the ceiling of a dense castable, because its impurities and pores decide what melts and what slag enters. Alkali impurities in calcined bauxite can form liquid from about 1100 °C (2,012 °F) and lower creep resistance (Nyoka et al. 2013, Journal of the SAIMM).
Producer papers (Almatis) describe tabular alumina with small closed pores that slag does not infiltrate, against the open macro pores of white fused alumina. The castable's class limit is a test result: ASTM C401-12(2022) caps permanent linear change at 1.5 % after 5 h at the class temperature.
Raw Materials, Binders and Additives in One Mix#
A refractory batch combines raw materials with binders and additives: grains and matrix fines make up the bulk, while calcium aluminate cement, microsilica, phosphate or resin binders, dispersants and antioxidants hold, flow and protect them.
The line between grain and additive follows function: reactive alumina and graphite form the body of the product, while microsilica, hydratable alumina and colloidal silica bond or fill it and sit under refractory additives and binders.
Matrix Aluminas, Microsilica and Mixing Water#
Fine matrix aluminas cut castable water: a producer review (Almatis) reports mixing water of 10 % for a conventional castable, 4.5 % for a low-cement castable and 3.5 % with an optimized matrix and dispersing alumina.
Reactive aluminas are batch-ground to primary crystals, while calcined grades keep porous agglomerates that absorb water and limit sintering. In that matrix concept, reactive alumina replaces microsilica and part of the cement.
Expansion Agents, Spinel Formers and Carbon Carriers#
Some raw materials work as functional additions: raw kyanite expands as it converts to mullite at about 1350 to 1380 °C (2,462 to 2,516 °F) (Indian Bureau of Mines 2024). Spinel grain makes up 10 to 20 % of magnesia-spinel brick (class range, UNITECR 2025 conference paper).
Flake graphite is the carbon carrier: converter grades of magnesia-carbon brick carry 15 to 20 mass % graphite (Nippon Steel Technical Report No. 125, 2020). The same report gives the antioxidant reactions that protect it: 2Al + 3CO = Al2O3 + 3C, Si + C = SiC and B4C + 6CO = 2B2O3 + 7C.
Standards and Test Methods for Refractory Raw Materials#
No ISO, EN or ASTM product specification exists for refractory grains; they are bought on producer specifications and checked with test methods for grain density, chemistry, hydration and ash. Classifications such as ISO 10081-1:2003 and ASTM C27-98(2022) name the raw material of the finished product, not of the grain.
A producer specification states each value as typical, minimum or maximum, and the test method decides whether two offers compare; every method is listed among the refractory standards.
Grain Tests Side by Side: ASTM, ISO and EN#
Six test methods stand behind most raw-material data sheets: grain density, XRF chemistry, wet chemistry, SiC analysis, magnesia hydration and graphite ash. Table 4 pairs ASTM and ISO or EN methods by property; none is declared equivalent.
| Property | ASTM (edition) | ISO / EN (edition) |
|---|---|---|
| grain bulk density | C357-07(2022)e1 (particles ≥ 6.7 mm, 0.265 in) | ISO 8840:2021 / EN ISO 8840:2024 (grains > 2 mm) |
| chemistry by XRF (fused cast bead) | n/a | ISO 12677:2011 / EN ISO 12677:2011 |
| wet chemistry, aluminosilicates | n/a | ISO 21587-1 to -3:2007 |
| wet chemistry, magnesite and dolomite | n/a | ISO 10058-1 to -3:2008 |
| SiC-bearing materials (free C, SiC, free Si) | n/a | ISO 21068-1 to -4:2024 |
| hydration of dead-burned magnesia grain | C544-18(2023) | n/a |
| ash in graphite | C561-23 | n/a |
| sampling of raw materials | n/a | ISO 8656-1:1988 |
| apparent porosity (as quoted on grain data sheets) | C20-00(2022) | ISO 5017:2025 (dense shapes) |
| PCE of fire clay | C24-09(2022) | ISO 528:1983 |
EN ISO 8840:2024 replaces EN 993-18. ASTM C357-07(2022)e1 excludes grains that hydrate in boiling water, which is why dead-burned magnesia has its own test in ASTM C544-18(2023).
ASTM C561-23 for graphite ash belongs to committee D02, not to the refractories committee. Each measured value is explained under properties of refractory materials.
Classifications That Name the Raw Material#
Two product classifications put the raw material into the class name: ISO 10081-1:2003 names it at 50 % or more, and ASTM C27-98(2022) grades high-alumina brick by Al2O3 content, fireclay brick by physical properties.
India's IS 14301:1995 is the one national grade standard recorded for a grain: kyanite Grade 1 needs at least 58 % Al2O3 and Grade 2 at least 54 %, both with Fe2O3 at most 1.50 %, K2O plus Na2O at most 1 % and PCE of at least 36 and 35 (Indian Bureau of Mines, 2024).
Sourcing Refractory Raw Materials#
Sourcing refractory raw materials starts with a specification that states chemistry with its basis, sizing and density minimum, continues with trade codes and dated reference values, and ends with dust rules for handling the grain.
A grain is never installed on its own. Grains reach the lining through a host castable, brick or mix, placed by refractory installation methods.
What a Grain Specification and RFQ Contain#
A grain specification names the material and route, chemistry limits with basis, size fractions, bulk density minimum and the test method behind each value. Six quote factors move a grain offer, listed below.
- Chemistry limits, with basis and test method
- Sizing and fractions, with de-ironing or dedusting
- Bulk density minimum, with the method (ISO 8840:2021 or ASTM C357-07(2022)e1)
- Origin of the grain
- Packaging, bags or bulk
- Incoterm and tariffs
Quantity goes in metric tonnes; a US short ton is 907.18 kg (2,000 lb).
Trade Codes and Dated Unit Values#
US imports declare refractory raw materials under their own tariff lines, for example 2606.00.0030 for calcined refractory-grade bauxite and 2519.90.1000 for dead-burned and fused magnesia (USGS Mineral Commodity Summaries 2026). Table 5 lists the lines with general duty rates as of 2025-12-31.
| HTS line | Description | General duty (2025-12-31) |
|---|---|---|
| 2606.00.0030 | bauxite, calcined, refractory grade | free |
| 2818.10.1000 | crude artificial corundum | free |
| 2818.10.2010 | white, pink and ruby fused alumina grain, Al2O3 > 97.5 % | 1.3 % (plus 7.5 % Section 301 duty on imports from China) |
| 2818.10.2090 | other fused alumina grain | 1.3 % |
| 2508.50.0000 | andalusite, kyanite and sillimanite | free |
| 2508.60.0000 | mullite | free |
| 2519.90.1000 | dead-burned and fused magnesia | free |
| 2610.00.0020, .0040, .0060 | chromium ores by Cr2O3 (≤ 40 %, 40 to 46 %, ≥ 46 %) | free |
Table 6 gives USGS annual unit values for six grain series from 2021 to the 2025 estimate, in USD per metric ton with USD per short ton in parentheses.
| Series (basis, USGS Mineral Commodity Summaries 2026) | 2021 to 2025e range, USD per t (per short ton) | 2025e, USD per t (per short ton) |
|---|---|---|
| crude fused aluminum oxide, US import unit value | 635 to 797 (576 to 723) | 699 (634) |
| ground and refined fused aluminum oxide, US import unit value | 1,290 to 1,560 (1,170 to 1,415) | 1,500 (1,361) |
| premium zircon, CIF China (Fastmarkets data cited by USGS) | 1,530 to 2,300 (1,388 to 2,087) | 1,800 (1,633) |
| crude silicon carbide, US import unit value | 587 to 1,080 (533 to 980) | 829 (752) |
| flake graphite, US import unit value | 1,000 to 1,330 (907 to 1,207) | 1,000 (907) |
| kyanite, US export unit value (FAS) | 369 to 510 (335 to 463) | 510 (463) |
Annual averages reported by USGS; fused alumina and SiC include abrasive grades; not quotes.
Supply is concentrated: China supplied 96 % of US imports of crude fused aluminum oxide in 2025 and 69 % of US imports of dead-burned and fused magnesia in 2021 to 2024, with Brazil at 17 % (USGS Mineral Commodity Summaries 2026).
Handling Grains Before Installation#
Grains need no dry-out of their own; installation and dry-out follow the host castable, brick or mix. Magnesia grain hydrates, the property ASTM C544-18(2023) measures, so it is stored dry; see hydration resistance of basic refractories.
An installation request goes to installers, not grain suppliers.
Dust, Crystalline Silica, Chromium and Fiber Rules#
Grain dust falls under OSHA limits: respirable crystalline silica 50 µg/m³ with an action level of 25 µg/m³, 8-hour TWA (29 CFR 1910.1053 and 1926.1153, effective 2016-06-23). For alpha-alumina, the OSHA PEL is 15 mg/m³ total dust and 5 mg/m³ respirable fraction (NIOSH Pocket Guide entry 0021).
Quartz (crystalline silica, CAS 14808-60-7) carries the line "No Annex VI entry; product-specific, see supplier SDS (ECHA C&L: no majority; not classified 27.5 %, H372 25.2 %, H373 18.8 %, H350 7.7 %)". Cristobalite occurs in chamotte and devitrified fused silica; for cristobalite, see the supplier SDS.
Hexavalent chromium forms from chromite with lime and alkali; OSHA sets a PEL of 5 µg/m³ and an action level of 2.5 µg/m³ (29 CFR 1910.1026, eCFR text of 2026-10-27). Silicon carbide (CAS 409-21-2) carries the line "SiC grain/powder: no Annex VI entry, ECHA C&L not classified (79.8 %); SiC fibres (WHO dimensions): Annex VI 014-048-00-5 Carc. 1B H350i" (Regulation (EU) 2020/1182, in application since 2022-03-01). The "zircon and zirconium industry" is a listed sector for naturally occurring radioactive material in Annex VI of Directive 2013/59/Euratom.
The two owned hazards have their own pages:
- crystalline silica in refractories: cristobalite in calcined grains and the exposure limits
- hexavalent chromium in spent magnesia chrome refractories: Cr(VI) formed from chromite in service
Where Refractory Raw Materials Meet Recycling, Markets and Supply Policy#
Refractory raw materials connect to three neighboring questions: how much spent lining returns as grain, how large the raw-material market is and which grains count as critical for supply.
USGS Mineral Commodity Summaries 2026 reports that up to 30 % of fused aluminum oxide may be recycled and about 5 % of silicon carbide, and names spent zirconia refractories as a recycled source without a share. Spent linings are covered under spent refractory recycling and disposal.
Refractory raw materials start at refractory grade; mining and the ore trade come before that point.
How Much Refractory Raw Material Is Used Each Year?#
Refractories consume about 35 to 40 million tonnes of raw minerals a year, an Almatis estimate presented at the IMFORMED China Refractory Minerals Forum in November 2021.
The same 2021 presentation puts steel at 60 to 70 % of a market estimated at EUR 20 to 25 bn (producer estimate). USGS (2026) reports that 90 % of US kyanite-mullite output goes into refractories, and 60 to 70 % of that use into refractories for iron and steel.
Which Refractory Raw Materials Are EU Critical Raw Materials?#
The 2023 EU list of critical raw materials (Critical Raw Materials Act, European Commission) includes bauxite, natural graphite, magnesium, silicon metal and hafnium, but not magnesite, chromium or zirconium. For refractory buyers, bauxite and natural graphite are the listed grains; magnesium appears as the metal, not as magnesite or dead-burned magnesia.