Refractory mortar is a refractory jointing material, sold as a dry powder or ready-mixed, that bonds and seals the joints of brick linings and sets by one of four bonds: air-setting, heat-setting, phosphate or hydraulic. The bond decides when the joint gains strength: sodium silicate at room temperature, clay on firing, phosphate binders from about 100 °C (212 °F) under producer guidance, and calcium aluminate cement on contact with water.
The main attributes of refractory mortar are summarized in the table below.
| Attribute | Value |
|---|---|
| Role | Bonds and seals the bed and head joints of brick linings |
| Product type | Jointing product, ISO 1927-1:2012 |
| Forms | Dry powder mixed on site, or wet ready-mixed |
| Bonds | Air-setting, heat-setting, phosphate, hydraulic |
| Chemistry | Acid, neutral or basic, matched to the brick |
| Key tests | ASTM C199-22 pier test, ASTM C198-09(2025) cold bonding strength, ISO 13765 series |
| Customs heading | 3816 (refractory cements, mortars, concretes and similar compositions) |
The chemistry of a mortar follows the brick it joins, so fireclay, silica and magnesia joints each take a different product, and the ASTM C199-22 pier test checks whether the mortar flows out of the joints when heated. Mortar is the jointing form of refractory materials, next to bricks and castables among refractory materials.
A US National Bureau of Standards study (1942) found 11 of 17 air-setting mortars weakest after heating to 750 °C (1,382 °F), a trough behind several joint failures.
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 Is Refractory Mortar?#
Refractory mortar is the jointing product among the unshaped refractories: ISO 1927-1:2012 lists jointing products as a product type next to castables, gunning and ramming mixes. It bonds the bed and head joints of every brick lining.
Mortar joins shaped brick, while a castable forms the lining itself; both belong to the unshaped family described under monolithic refractories.
Four Bond Types: Air-Setting, Heat-Setting, Phosphate and Hydraulic#
Refractory mortar sets by one of four bonds: air-setting mortar hardens at room temperature through sodium silicate, heat-setting mortar forms a ceramic bond from clay on firing, phosphate mortar forms aluminum phosphates on heating, and hydraulic mortar sets with water.
The four bond types, their binders and one named product per type are compared in the table below.
| Bond type | Binder | How it sets | Named product (basis) |
|---|---|---|---|
| Air-setting | Sodium silicate (water glass) | Glassy bond at room temperature, through the full joint thickness | HWI SAIRSET wet fireclay mortar, 6.49 % sodium silicate (SDS 2024); Allied BRIKSET 85, 65 and 40 TC (product page) |
| Heat-setting | Plastic clay (kaolin, fireclay) | Ceramic bond on firing | n/a |
| Phosphate-bonded | Phosphoric acid or monoaluminum phosphate | Aluminum phosphates form on heating | Allied DURATAB, 88 % Al2O3; DURATAB C, 79 % Al2O3 with 10 % chrome (product page) |
| Hydraulic | Calcium aluminate cement | Sets with water | n/a |
Air-setting mortar hardens without heat: its sodium silicate dries to a glassy bond through the whole joint, while heat-setting mortar gains its bond only on firing.
Dry, Wet and Ready-Mixed Mortar: What a Refractory Mortar Mix Is#
Refractory mortar is supplied as a dry powder mixed with water or a liquid on site, or as a wet ready-mixed product such as SAIRSET, a wet air-setting fireclay mortar (HWI). A dip consistency exists for dipped joints, ISO 13765-6:2004 measures the moisture of ready-mixed mortar, and the mix ratio is the one on the data sheet.
Trade and Regional Names for Refractory Jointing Mortar#
Five names on bags and data sheets describe refractory jointing mortar.
- Firebrick or fire brick mortar and fireclay mortar, named after the brick or the base clay.
- Refractory jointing mortar, the product type of ISO 1927-1:2012.
- Fire cement, the UK retail name.
- Refractory mud or fire mud, the English used on China-origin bags; Feuerfestmörtel in German and mortier réfractaire in French.
- Fire brick cement, a retail name that can mean a jointing mortar; the binder sense of refractory cement is a different product.
What Is Refractory Mortar Made Of?#
A refractory mortar is a refractory filler plus a binder: the 2024 SDS of HWI SAIRSET lists mullite 30 to 50 %, cristobalite 10 to 25 %, quartz 13.31 %, kaolin 7.95 % and sodium silicate 6.49 %.
The raw materials of refractory mortar and their roles are compared in the table below.
| Material | Role in the mortar | Value and basis | Page |
|---|---|---|---|
| Mullite | Filler | 30 to 50 % in SAIRSET (HWI SDS 2024); base of BRIKSET 65 TC, 65 % Al2O3 (Allied product page) | mullite |
| Calcined bauxite | Filler; Guyanese grade less reactive in acid- and phosphate-bonded mortars, longer shelf life (GNP Graystar brochure 2022) | Base of BRIKSET 85 TC, 80 % Al2O3 (Allied product page) | refractory-grade calcined bauxite |
| Chamotte | Filler in fireclay mortars (Garbers-Craig, Journal of the SAIMM, 2008) | n/a | chamotte (grog) |
| Fireclay and kaolin | Bond clay, workability, water retention | Kaolin 7.95 % in SAIRSET; base of BRIKSET 40 TC, 40 % Al2O3 | fireclay |
| Zircon | Filler in zircon mortars and grouts (Zircon Industry Association) | n/a | zircon and stabilized zirconia |
| Quartz and cristobalite | Crystalline silica phases | Quartz 13.31 %, cristobalite 10 to 25 % in SAIRSET (HWI SDS 2024) | n/a |
Shelf life depends on the filler: a producer brochure (GNP Graystar, 2022) states that Guyanese calcined bauxite is less reactive in acid- and phosphate-bonded mortars, which keeps them usable longer. The grains by chemistry and grade are covered under refractory raw materials.
Binders and Additives in Refractory Mortar#
The binder decides when a refractory mortar gains strength: sodium silicate at room temperature, clay on firing, phosphoric acid or monoaluminum phosphate on heating, and calcium aluminate cement on contact with water. Additives adjust the bond for service: a 10 % chrome addition raises the corrosion resistance of DURATAB C (Allied product page), and barium sulfate suits phosphate-bonded mortars in aluminum contact because it does not react with phosphoric acid (patent US4126474A, role only).
The binders and additives of refractory mortar are compared in the table below.
| Binder or additive | Role | Set or cure behavior | Value and basis | Page |
|---|---|---|---|---|
| Sodium silicate (CAS 1344-09-8, EC 215-687-4) | Air-setting bond | Hardens at room temperature; strength trough before the ceramic bond forms (NBS 1942) | 6.49 % in SAIRSET (HWI SDS 2024) | sodium and potassium silicate binders |
| Phosphoric acid (CAS 7664-38-2, EC 231-633-2) and monoaluminum phosphate (CAS 13530-50-2, EC 236-875-2) | Phosphate bond | High strength from about 100 °C (212 °F); bond insoluble from 300 to 350 °C (572 to 662 °F), producer guidance (Budenheim, via distributor) | Binder of DURATAB (Allied product page) | phosphate binders |
| Calcium aluminate cement (CAS 65997-16-2, EC 266-045-5) | Hydraulic bond | Sets with water | n/a | calcium aluminate cement for refractories |
| Chrome addition | Corrosion resistance | n/a | 10 % in DURATAB C (Allied product page) | n/a |
| Barium sulfate | Non-wetting in aluminum contact; no reaction with phosphoric acid | n/a | Patent US4126474A, role only | anti-wetting additives |
| All binder and additive families | n/a | n/a | n/a | refractory additives and binders |
Refractory Mortar Properties: Test Results Versus Data-Sheet Values#
Refractory mortar is judged by results under named test methods, above all the ASTM C199-22 pier test, while the values on a data sheet describe one named product. The pier test estimates the application temperature limit that the term "high-temperature refractory mortar" refers to.
The properties of refractory mortar, their ASTM and ISO methods and the named product values are compared in the table below.
| Property | ASTM method (edition) | ISO method (edition) | Typical value (named product, basis) |
|---|---|---|---|
| Refractoriness in a joint | C199-22 pier test | n/a | Pass or fail at the test temperature |
| Cold bonding strength | C198-09(2025) | ISO 13765-4:2004, flexural bonding strength (related property, not equivalent) | n/a |
| Consistency | n/a | ISO 13765-1:2004 cone; ISO 13765-2:2004 flow table | Dip consistency offered (Allied product page) |
| permanent linear change on heating | n/a | ISO 13765-7:2021 | n/a |
| refractoriness and pyrometric cone equivalent | C24-09(2022), silica fire clay mortar | n/a | n/a |
| Al2O3 | Chemical analysis | n/a | 88 % DURATAB; 80, 65 and 40 % BRIKSET 85, 65 and 40 TC (Allied product page) |
| Soluble alkali as Na2O | NBS method (1942) | n/a | 0 to 1.90 % across 17 dry air-setting brands (NBS 1942, study range) |
ASTM C24-09(2022) names silica fire clay refractory mortar in its scope next to fireclay and high-alumina brick. Soluble alkali matters because sodium silicate is a flux that lowers the refractoriness of the joint. The wider set of properties of refractory materials sits under properties of refractory materials.
Why Air-Setting Mortar Weakens Between 750 and 1000 °C#
Air-setting mortar loses strength at intermediate temperature because its silicate bond weakens before a ceramic bond forms: in the NBS study by Heindl and Pendergast (1942, RP1461), 11 of 17 neat mortars were weakest after 750 °C (1,382 °F).
The neat bars were weaker after 750 °C than after 1000 or 1350 °C (1,832 or 2,462 °F), and 11 of 17 half-brick assemblages were weaker after 1000 °C (1,832 °F) than after drying or after 750 °C. A joint in a lining zone that stays in this band never reaches its fired strength. These counts are a study finding on 17 brands, not a requirement of any standard.
Which Standards Test and Classify Refractory Mortar?#
Refractory mortar is tested in the US by ASTM C199-22 (pier test) and ASTM C198-09(2025) (cold bonding strength) and internationally by the ISO 13765 series, whose methods run side by side but are not interchangeable. ASTM C199-22 was published on 2022-02-01 and replaced C199-84(2016), withdrawn on 2022-03-03.
The ASTM and ISO documents for refractory mortar are compared in the table below; methods in one row measure related properties, and their results are not interchangeable.
| Purpose | ASTM (edition, status) | ISO (edition, status) |
|---|---|---|
| Refractoriness in a joint | C199-22, published 2022-02-01; C199-84(2016) withdrawn 2022-03-03 | n/a |
| Cold bonding strength | C198-09(2025), reapproved 2025 | ISO 13765-4:2004, flexural bonding strength |
| Classification | C1655-06(2022), Classification of Fireclay and High-Alumina Mortars | n/a |
| Air-setting specification | F1097-17(2022), committee F25 (shipbuilding) | n/a |
| Consistency | n/a | ISO 13765-1:2004 cone; ISO 13765-2:2004 flow table |
| Joint stability | n/a | ISO 13765-3:2004, confirmed |
| Moisture of ready-mixed mortar | n/a | ISO 13765-6:2004 |
| Change in dimensions on heating | n/a | ISO 13765-7:2021 |
| Shear bonding strength | n/a | ISO/PWI 13765-8, preliminary work item |
| Pyrometric cone equivalent | C24-09(2022), see ASTM C24 pyrometric cone equivalent | n/a |
| Product type | n/a | ISO 1927-1:2012, confirmed 2023-09-04 |
A purchase document can cite ASTM C1655-06(2022) for fireclay and high-alumina mortars or ASTM F1097-17(2022), written by the shipbuilding committee F25, for high-temperature air-setting mortar. All ASTM, ISO and EN documents with editions and status are listed among the refractory standards.
Which Mortar for Which Brick: Selection by Lining and Service#
A refractory mortar is chosen to match the chemistry of the brick it joins: fireclay or high-alumina mortar for firebrick, silica mortar for silica brick, basic mortar for magnesia-chrome brick and anhydrous mortar for dolomite brick.
The mortar used with each brick class and its service zones are compared in the table below.
| Brick class | Mortar used | Where | Page |
|---|---|---|---|
| Fireclay brick | Fireclay or high-alumina mortar, air-setting or heat-setting, thin joints | Furnace walls | firebrick (fireclay brick) |
| High-alumina brick | High-alumina air-setting, or heat-setting phosphate mortar | High-alumina linings | high alumina bricks |
| Silica brick | Silica mortar, calculated expansion allowance | Glass crowns, coke ovens | silica bricks |
| Magnesia brick | Magnesia mortar or dry joints | Basic linings | magnesia (magnesite) bricks |
| Magnesia-chrome brick | Basic mortar (HWI CHROMESET MORTAR) | Rotary kilns | magnesia chrome bricks |
| Magnesia-carbon brick | Dry-laid or thin basic mortar | Ladles, converters | magnesia carbon bricks |
| Dolomite brick | Anhydrous (non-aqueous) mortar | Basic linings | dolomite bricks |
| Zircon brick | Zircon or AZS mortar, tight joints in glass contact | Glass furnaces | glass furnace refractories |
| Silicon carbide sidewall blocks | Thin joints of high-conductivity SiC mortar | Aluminum reduction-cell sidewalls | refractories for aluminium |
| Insulating firebrick, super-duty and high-alumina brickwork | BRIKSET air-set range (Allied product page) | Power and heat treatment | insulating firebrick (IFB); boiler refractory |
Dolomite brick takes only anhydrous mortar because its free lime hydrates. Allied positions its BRIKSET air-set range for insulating firebrick, super-duty and high-alumina brickwork in power and heat-treatment furnaces.
How Is Refractory Mortar Applied and Heated Up?#
Refractory mortar is laid in thin joints by rub-and-tap or by dipping the brick, with the chemistry matched to the brick and air-setting mortar kept covered after mixing. Bricklaying is one of several lining methods, next to casting, gunning and ramming, compared under refractory installation methods.
Air-setting mortar hardens as it dries, so a mixed batch stays covered until it goes into the joint.
Fired brick needs no curing, so the mortar, any castable and the backfill govern the heat-up of a brick lining.
Producer schedules are guidance, and their temperatures refer to the hot gas in contact with the lining, not the lining. How long a lining needs is covered under refractory dry-out and heat-up schedules.
Two linked topics complete the installation picture.
- Patching mortars, such as GREENPATCH-421, a wet air-setting patching mortar with 53 % alumina (HWI), are covered under refractory repair and maintenance.
- Expansion allowance in brick linings is covered under refractory lining design.
Why Do Mortar Joints Fail, and How Are They Checked?#
Mortar joints fail in five ways: intermediate-temperature strength loss of silicate bonds, alkali fluxing, washout by slag or metal, chemistry that does not match the brick, and gassing of phosphate mortar.
The five failure modes of mortar joints, their causes and their checks are compared in the table below.
| Failure | Cause | Check |
|---|---|---|
| Strength loss at 750 to 1000 °C (1,382 to 1,832 °F) | Silicate bond weakens before the ceramic bond forms | ASTM C198-09(2025) cold bonding strength after heating |
| Alkali fluxing | Soluble alkali lowers refractoriness; see slag and chemical attack | ASTM C199-22 pier test; ASTM C24-09(2022) PCE |
| Joint washout | Slag or metal attack on the joint | Visual inspection; see refractory failure modes |
| Wrong chemistry | Mortar not matched to the brick | Brick-to-mortar check against the selection table |
| Gassing of phosphate mortar | Phosphoric acid reacting with metallic iron in the aggregate | Inhibited grades; see refractory inspection and quality control |
A test regime for a mortar delivery combines four methods: the ASTM C199-22 pier test, ASTM C198-09(2025) cold bonding strength, ISO 13765-1:2004 or ISO 13765-2:2004 consistency, and ISO 13765-4:2004 bonding strength.
Health and Safety When Handling Refractory Mortar#
The main hazard in alumina-silica refractory mortar is respirable crystalline silica from quartz and cristobalite, regulated in US workplaces at 50 µg/m³ (8-h TWA) with an action level of 25 µg/m³ under 29 CFR 1910.1053 and 1926.1153, effective 2016-06-23. Dust from mixing and cutting carries these phases, which the SAIRSET SDS (HWI, 2024) lists at 10 to 25 % cristobalite and 13.31 % quartz. The table below gives the quartz line verbatim; for cristobalite, see the supplier SDS.
Among the binders only phosphoric acid has an Annex VI entry; sodium silicate, monoaluminum phosphate and calcium aluminate cement depend on the product SDS. Chromium(VI) in spent chrome-bearing linings is a separate topic, linked below.
The classifications, exposure limits and listings relevant to mortar ingredients are compared in the table below.
| Substance or topic | Rule or classification | Instrument and date |
|---|---|---|
| Quartz (crystalline silica), CAS 14808-60-7 | 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 %) | CLP Regulation (EC) No 1272/2008 Annex VI and ECHA C&L notifications, read 2026-10-27 |
| Cristobalite | see the supplier SDS | n/a |
| Respirable crystalline silica, US | PEL 50 µg/m³ (0.05 mg/m³), 8-h TWA; action level 25 µg/m³ | 29 CFR 1910.1053 and 1926.1153, effective 2016-06-23 |
| Respirable crystalline silica, EU | 0.1 mg/m³, respirable fraction, 8-h TWA | Directive (EU) 2017/2398 Annex III (OJ L 345/87, 2017-12-27), transposition by 2020-01-17 |
| Respirable crystalline silica, California | "Silica, crystalline (airborne particles of respirable size)", cancer | Proposition 65 list (OEHHA), listed 1988-10-01, list version 2026-07-31 |
| Crystalline silica (quartz, cristobalite), IARC | Group 1 | IARC Monograph 100C (2012) |
| Sodium silicate (water glass), CAS 1344-09-8 | Per constituent: water glass no Annex VI entry, ECHA C&L notified H315, H318; disodium metasilicate 014-010-00-8: H314, H335; Mg hexafluorosilicate 009-018-00-3: H301; Zn hexafluorosilicate group entry 009-013-00-6: H302. Not on the Candidate List; no Annex XVII entry | ECHA, read 2026-10-27 |
| Phosphoric acid, CAS 7664-38-2 | Harmonised CLP Annex VI (index 015-011-00-6): H314 (Skin Corr. 1B) | CLP Regulation (EC) No 1272/2008 Annex VI, ECHA read 2026-10-27 |
| Monoaluminum phosphate | No Annex VI entry; ECHA C&L notified Eye Dam. 1 H318 (81.15 %, REACH joint submission); see the supplier SDS | ECHA, read 2026-10-27 |
| Calcium aluminate cement, CAS 65997-16-2 | No Annex VI entry; no hazard statement in a majority of ECHA C&L notifications (H315, H318 42.5 % each; H319 38.3 %); classification is product-specific, see the product SDS | ECHA C&L notifications, read 2026-10-22 |
| Chromium(III) oxide | No Annex VI entry, see the supplier SDS; IARC Group 3 | ECHA, read 2026-10-23 |
Two linked pages cover the exposure topics in depth.
- Dust control and monitoring: crystalline silica and cristobalite in refractories.
- Chrome-bearing linings after service: hexavalent chromium in spent magnesia chrome refractories.
Refractory Mortar Grades, Packaging and Sourcing#
Refractory mortar is bought by bond type, Al2O3 level and form: producers list air-setting, heat-setting and phosphate-bonded mortars plus patching mortars, sold as dry mixes or wet ready-mixed products. One producer's range shows the grade ladder: Allied lists air-set BRIKSET grades at 40, 65 and 80 % Al2O3 and the phosphate-bonded DURATAB at 88 % Al2O3 on its product page.
An import quote names the customs heading as well as the grade. Heading 3816 covers "Refractory cements, mortars, concretes and similar compositions, including dolomite ramming mix, other than products of heading 3801" (US HTS, read 2026-10-27), and buying routes by country and product group are covered under refractory manufacturers and suppliers.
Data-Sheet Checklist for Brand-Name Mortars#
Six checks turn any brand-name mortar data sheet into comparable facts.
- Identify the bond type: air-setting, heat-setting, phosphate or hydraulic.
- Read the Al2O3 content together with its basis.
- Find the pier-test result or service limit, and confirm it belongs to the product offered.
- Note which ASTM C1655-06(2022) classification or ASTM F1097-17(2022) specification the product claims, if any.
- Match the mortar chemistry to the brick it will join.
- Read the crystalline silica fractions in section 3 of the SDS.
What Determines the Price of Refractory Mortar?#
Industrial refractory mortar is priced per quote, and the price follows bond type and Al2O3 level, form (dry or wet), quantity and packaging, freight and whether installation is included.
The price factors a buyer states in a mortar request are listed in the table below; industrial refractory mortar is priced per quote.
| Price factor | What the buyer states in the request |
|---|---|
| Bond type | Air-setting, heat-setting, phosphate or hydraulic |
| Al2O3 level and base | Fireclay, mullite or bauxite base |
| Form | Dry or wet ready-mixed |
| Quantity and packaging | Amount in kg or t (lb or short tons) and pack type |
| Freight and delivery country | Customs heading 3816 on imports |
| Scope | Supply only, or installed |
| Price trend | US BLS PPI, clay refractories (WPU1352), 405.818 in September 2026 (1982 = 100); a trend, not a price |
The US BLS producer price index for clay refractories (WPU1352, 1982 = 100) stood at 405.818 in September 2026, a trend, not a price. It never stands in for a quote on a named mortar.
Refractory Mortar Compared With Castables, Cement and Dry Joints#
A refractory mortar bonds brick and cannot form a lining by itself, while a castable is the lining and refractory cement in the binder sense is the calcium aluminate cement that binds castable aggregate. The full verdict on refractory mortar vs refractory cement vs castable is set out in refractory mortar vs refractory cement vs castable.
Retail packs labeled "refractory cement" can hold an air-setting jointing mortar or a sodium-silicate furnace cement paste for sealing joints, so the binder on the label tells which one it is. Fireplace and stove mortars in small retail packs are consumer products that this reference does not cover.
Castable, Plastic and Dry-Joint Alternatives#
Three alternatives replace mortared joints where the lining design allows: a cast lining, a rammed plastic lining or dry-laid brick.
| Alternative | Placement | When it replaces mortared joints | Page |
|---|---|---|---|
| Castable | Cast monolithic lining | No brick joints at all | refractory castables |
| Plastic refractory | Rammed, including phosphate-bonded plastics | Monolithic lining placed by ramming | plastic refractories |
| Dry-laid brick | Basic brick laid without mortar | Magnesia and magnesia-carbon linings | refractory bricks |
| All three compared | n/a | n/a | refractory mortar vs refractory cement vs castable |
Refractory Mortar FAQs#
Can Portland Cement Mortar Replace Refractory Mortar?#
No: a refractory mortar sets by sodium silicate, clay, phosphate binders or calcium aluminate cement, and portland cement building mortar is none of these. A jointing product for a lining proves its temperature limit in refractory mortar tests such as the ASTM C199-22 pier test.
How Long Does Bagged or Wet Refractory Mortar Keep?#
Refractory mortar keeps its strength only in airtight containers: the NBS study (Heindl and Pendergast, 1942) found that air-setting mortar stored in non-airtight containers lost strength. The shelf life of a named product is the one printed on its data sheet.