Refractory Castables: Types, Cement Content, Installation and Selection

A refractory castable is a dry mix of refractory aggregate, fine powders and a binder, in conventional grades calcium aluminate cement, that is mixed with water, cast or pumped into place and sets hydraulically into a monolithic lining. Four cement levels exist, and ISO 836:2001 names them medium-cement (term 086), low-cement (079), ultra-low-cement (135) and no-cement castables (090), ranked by falling CaO content.

ASTM C401-12(2022) uses the same four class names and calls a castable dense when its dried bulk density exceeds 1.68 g/cm³ (105 lb/ft³). Regular dense castables then fall into seven service classes, A to G, tested from 1095 to 1760 °C (2,000 to 3,200 °F). Castables are one of the product forms of refractory materials, next to shaped bricks and insulation.

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.

The key attributes of refractory castables are summarized in the table below.

What Is a Refractory Castable?#

A castable is the monolithic refractory that is placed by casting: ISO 1927-1:2012 lists castables first among unshaped product types (clause 4) and classifies them further by chemical type, main raw material and classification temperature. The word "castable" describes the placement: the mixed material is poured, vibrated or pumped into forms and gains strength by chemical action after tempering with water, the scope wording of ASTM C401-12(2022).

Clause 5 of ISO 1927-1:2012 sorts castables into four chemical types: alumina-silica, basic, special and carbon-containing. Cement-bonded alumina-silica castables form the conventional class, and the other placement families sit with them among the monolithic refractories.

Cement Level: Conventional, Low, Ultra-Low and No-Cement Classes#

Castables are ranked by their CaO content, and ISO 836:2001 names four classes: medium-cement (term 086), low-cement (term 079), ultra-low-cement (term 135) and no-cement castables (term 090). Cement content is a formulation figure, not a class limit: patent WO1996001700A1 describes conventional castables with 10 to 30 % cement by weight and low-cement castables with less than about 10 %.

The three lower classes are deflocculated castables. ISO 836:2001 (term 049) defines a deflocculated castable as one with a hydraulic bond, at least 2 % by weight of ultra-fine particles below 1 µm and at least one deflocculating agent. ASTM C401-12(2022) uses the same four class names, calling the conventional class "regular".

The four cement-level classes are compared in the table below.

Dense and Service-Temperature Classes in ASTM C401#

ASTM C401-12(2022) calls a castable dense when its dried bulk density exceeds 1.68 g/cm³ (105 lb/ft³) and assigns regular castables to classes A to G by the temperature at which permanent linear change stays within 1.5 % after 5 h. The standard, titled "Standard Classification of Alumina and Alumina-Silicate Castable Refractories" and first approved in 1957, covers castables that gain strength by chemical action when tempered with water.

Volume stability, modulus of rupture, bulk density and lime content distinguish the formulations, measured with ASTM C133-24, C113-14(2025), C134-95(2023), C862-16(2020) and C865-22. Insulating castables carry their own classes, N to V. Every class table of the standard is set out on the ASTM C401 classification of castables page.

The seven regular service classes of ASTM C401-12(2022) are listed in the table below.

Regional and Trade Names: Refractory Concrete and Retail Castable Cement#

Five names describe the same product class, and one of them describes a retail product rather than a class. The five names are listed below.

• Refractory concrete and concrete refractory, the construction-trade names of the product.
• Conventional, regular or high-cement castable, with "CC" for conventional castable in India.
• Dense castable, for any castable above the C401-12(2022) threshold of 1.68 g/cm³ (105 lb/ft³).
• Feuerbeton in German and béton réfractaire in French.
• Castable refractory cement, the label of bagged products sold under that name; the trade term castable refractory cement has its own page.

What Are Refractory Castables Made Of?#

A dense castable is graded refractory aggregate held by a fine matrix of powders and cement: the Almatis conventional reference concept with 20 % calcium aluminate cement analyzes 84 % Al2O3 and 5.0 % CaO. The grain in that mix decides the chemistry class, and the cement decides the bond.

The raw materials of alumina-silica castables and their roles are compared in the table below.

The aggregate decides the chemistry class: a chamotte grain gives a fireclay castable, a tabular grain a high-alumina one, and each grain family is described among the refractory raw materials.

Which Binders and Additives Does a Castable Contain?#

A castable's binder system, the cement plus additives dosed at fractions of a percent, decides its water demand, set, dry-out behavior and hot strength. The Almatis water ladder shows the effect: a conventional castable needs 10 % mixing water, a low-cement castable with 5 % silica fume 5 %, a dispersed low-cement castable 4.5 % and an optimized low-cement castable 3.5 to 4.5 %.

Calcium Aluminate Cement and the Hydraulic Bond#

Calcium aluminate cement (CAS 65997-16-2, EC 266-045-5) bonds conventional castables by hydration: the hydrates CAH10, C2AH8, C3AH6 and AH3 give green strength.

The hydrates are lost between about 800 and 1100 °C (1,472 and 2,012 °F) before a ceramic bond forms, which explains the intermediate strength dip of conventional castables. Lower-cement classes cut that hydrate share and the water bound in it. Cement grades, alumina levels and hydration chemistry are covered under calcium aluminate cement for refractories.

Dispersants, Set Control, Microsilica and Fibers#

Five additive groups adjust a cement-bonded castable: dispersants, set accelerators and retarders, microsilica, steel fibers and organic drying fibers.

The five additive groups and their documented dosages are compared in the table below.

Refractory Castable Properties: Standard Requirements and Typical Values#

Castable data split into two kinds: class thresholds that every product of a class must meet, set by ASTM C401-12(2022), and typical values that one tested product or reference mix reached. A typical value describes only the product, mix and firing temperature it was measured on.

The standard requirements and typical values of conventional castables are compared in the table below.

Cement level moves the values as much as firing does: in the same patent WO1996001700A1 example after 816 °C (1,500 °F), a 60 % alumina low-cement castable reaches 20.0 MPa (2,900 psi) modulus of rupture and 75.2 MPa (10,900 psi) crushing strength, against 5.5 MPa (800 psi) and 32.4 MPa (4,700 psi) for the conventional castable. Each property and its methods are explained under properties of refractory materials.

Reading Castable Data: Curing Temperature, Method and Precision#

Six checks turn any producer's castable data sheet into comparable numbers. The six checks are listed below.

1. Confirm the class claim against the class tables of ASTM C401-12(2022), not against a producer's own label.
2. Read the curing, drying or firing temperature behind each value; a dried value and a value after 1500 °C (2,732 °F) are different properties.
3. Note the test method and its edition, for crushing strength ASTM C133 cold crushing strength and modulus of rupture in the C133-24 edition.
4. Allow for test precision: for dense castables, C133 crushing strength repeats within ±7 % in one laboratory and ±23 % between laboratories (precision printed for earlier editions).
5. Read porosity by the vacuum method of ASTM C830-00(2023), preferred over boiling for castables; ASTM C20-00(2022) shows a porosity error on castables heated to 816 °C (1,500 °F) and then water-saturated, rising with cement level.
6. Compare mixing water and the material required per cubic meter of lining, both stated per product.

Which Standards Classify and Test Castables?#

Castables are classified by ASTM C401-12(2022) in the US and by ISO 1927-1:2012 (EN ISO 1927-1:2012 in Europe), and tested with ASTM and ISO methods that run side by side but are not interchangeable. ISO 1927-1:2012 was confirmed on 2023-09-04, and on 2025-12-09 EN ISO 10059-1:2025 and EN ISO 5014:2025 replaced EN 993-5 and EN 993-6 for crushing strength and modulus of rupture.

The classification and test standards for castables are paired in the table below; methods in one row measure the same property, and their results are not interchangeable.

API Std 936, 5th edition (March 2024), governs quality control of monolithic linings in refineries: material and installer qualification, production sampling and dryout. The flow-table method of ASTM C1445-13(2025) repeats within ±25 % in one laboratory and ±40 % between laboratories (precision printed for earlier editions), so a consistency value from one site does not prove a mix at another. The ISO side, from classification in Part 1 to consistency of castables in ISO 1927-4:2012, is described under ISO 1927 monolithic refractory products.

Where Are Refractory Castables Used?#

Conventional dense castables line the doors, piers, lintels and car tops of steel reheating and heat-treatment furnaces, refinery heaters and ducts, and the complex shapes and repairs of general furnaces. Doors, piers and car tops are complex shapes, and a castable forms them in place.

The main industries and zones for castables are compared in the table below.

Low-cement classes line steel ladles and blast furnace runners, according to the Almatis matrix-alumina review. The Hicast Extra data sheet (Morgan Thermal Ceramics) adds reheating hearths, skid protection and cement kiln nose rings and dams for its dense low-cement product.

How Are Castables Placed, Cured and Dried Out?#

A castable reaches its data-sheet values only when it is mixed with the stated water, placed within the working time, cured damp and dried out on a controlled schedule. For one dense low-cement product, the Hicast Extra data sheet (2025) specifies a paddle mixer, 4 to 6 min of mixing after water addition and placement within 20 min. Consistency is checked by flow table to ASTM C1445-13(2025) or ISO 1927-4:2012, and casting, gunning and pumping are compared under refractory installation methods.

Mixing water differs by family, according to the Jenkins 2011 table: conventional castables take 8 to 12 % at 2,082 to 2,723 kg/m³ (130 to 170 lb/ft³), low-cement castables 4 to 8 % and insulating castables 20 to 40 %. A conventional castable needs about 25 % of its added water for hydration, and the rest must leave the lining at first heat.

Producer dry-out schedules are guidance, not a standard, and HWI states that their temperatures refer to the hot gas in contact with the lining, not to the lining itself.

Responsibilities are split, in the Jenkins 2011 account: the castable maker sets the schedule, the installer or designer writes the dry-out plan, the owner approves it and a dry-out contractor executes it. Hold temperatures, heating rates and schedules by lining thickness are set out under refractory dry-out and heat-up schedules.

Two related tools and practice pages for castable placement are listed below.

• refractory anchors: castables are cast over metallic anchors welded to the shell.
• refractory castable quantity calculator: bags from lining volume and the material required per cubic meter.

Why Do Castable Linings Fail, and How Are They Inspected?#

The signature failure of a castable lining is explosive spalling at first heat-up, when steam pressure inside the still-wet lining exceeds its tensile strength. The dry-out schedule prevents spalling; the five other modes in the table have their own checks.

The failure modes of castable linings and their checks are compared in the table below.

In-service inspection of refinery linings follows API RP 982, 1st edition (2023), while installation testing uses C133-24, C134-95(2023), C113-14(2025), C704/C704M-15(2022)e1, C830-00(2023) and the production sampling of API Std 936. Jenkins (2011) records the cost of skipping control: an uncontrolled main-burner dry-out raised the furnace temperature by about 633 °C/h (1,140 °F/h), and the castable separated 25 to 51 mm (1 to 2 in) from the tube sheet.

Health and Safety When Handling Castables#

The main hazard in alumina-silica castables is respirable crystalline silica from quartz and cristobalite in the aggregate, regulated in US workplaces at 50 µg/m³ (8-h TWA) with an action level of 25 µg/m³ under 29 CFR 1910.1053 (effective 2016-06-23). For quartz, the table below gives the ECHA line verbatim; for cristobalite, see the supplier SDS. The EU binding limit for respirable crystalline silica is 0.1 mg/m³ under Directive (EU) 2017/2398 (transposition by 2020-01-17). One named example, the Mizzou Castable Plus SDS (2019), lists cristobalite at 2.5 to 10 % and quartz at 0.1 to 1 % and classifies that product as Carc. 1A and STOT RE 1 under US HazCom.

Calcium aluminate cement has no Annex VI entry (ECHA C&L, read 2026-10-22). Separately, the text of REACH Annex XVII entry 47, as amended by Regulation (EC) No 552/2009, limits soluble Cr(VI) in hydrated cement and cement-containing mixtures to 2 mg/kg (0.0002 %) of the total dry weight of the cement; whether a given castable falls under the entry is not assessed here. Boric acid, used as a set retarder, carries "Harmonised CLP Annex VI: H360FD" (index 005-007-00-2, Repr. 1B, Note 11, ATP 20, in application since 2025-02-01).

The classifications, exposure limits and regulatory texts relevant to castable ingredients are listed in the table below.

Castable Grades, Packaging and Sourcing#

Castables ship as dry bagged mixes, for example 25 kg (55 lb) bags for Hicast Extra and 30 kg (66 lb) bags for Firelite 2600LI G, and buyers compare grades on class, Al2O3 and CaO content, service limit and material required per cubic meter. The material required differs by density: 2,750 kg/m³ (172 lb/ft³) for the dense low-cement product and 1,230 kg/m³ (76.8 lb/ft³) for the insulating one, each from its data sheet.

The five items a castable specification states are listed below.

1. State the class and its standard: an ASTM C401-12(2022) class, an ISO 1927-1:2012 designation or a data-sheet reference.
2. State the chemistry with its basis, Al2O3 and CaO in mass %, calcined or as received.
3. State the service limit together with the product name it comes from.
4. State the mixing water and the working time stated for the product.
5. State the date of manufacture and the storage conditions, since calcium aluminate cement ages in the bag.

Quantity in tonnes or bags, bag size, delivery country and the required date are quote items as well, because packaging and lead time change the offer. Buying routes, from spot orders to full-line service contracts, are described under refractory manufacturers and suppliers.

What Do Refractory Castables Cost?#

Industrial refractory castable is priced per quote, and six factors set that price: class and grade, raw-material origin, quantity and packaging, freight, installation and dry-out, and payment terms.

The six price factors and what a buyer states for each are compared in the table below.

The US BLS producer price index for clay refractories (WPU1352, 1982 = 100, not seasonally adjusted) stood at 405.818 in September 2026. The index tracks the price trend of the group and is not the price of any castable.

Castables Compared With Bricks and Other Monolithics#

A castable competes with shaped brick on joints, shape freedom and installation speed, and with other monolithics on placement method. A fired brick arrives with its ceramic bond already formed, and in the Almatis Table 1 data an 89 % alumina brick shows 18 % open porosity against 26 % for the 20 % CAC conventional castable after more than 1000 °C (1,832 °F). A cast lining has fewer joints and takes complex shapes in place, while a brick lining brings factory-set properties; the full verdict on castable vs refractory brick weighs both.

A refractory mortar bonds brick, while refractory cement in the binder sense is the calcium aluminate cement that binds castable aggregate. Bags sold as refractory cement can contain either product, so the product name and its class, not the bag label, identify what is in the bag.

Gunning, Plastic, Precast and Self-Flow Alternatives#

Four monolithic families replace a cast lining where access, thickness or shape rules casting out. In patent WO1996001700A1, wet spraying of a low-cement castable needs 2.27 to 2.55 m³/min (80 to 90 cfm) of air against 7.08 to 19.82 m³/min (250 to 700 cfm) for gunning, and gunning rebound reaches up to one third.

The alternatives to a cast lining are compared in the table below.

Refractory Castable FAQs#

What Kind of Concrete Is Heat Resistant?#

Refractory castable is the heat-resistant concrete: its calcium aluminate cement bond gives way to a ceramic bond on firing, which portland-cement concrete does not form. ASTM C401-12(2022) classes castables up to 1760 °C (3,200 °F).

How Dense Is a Castable Refractory?#

ASTM C401-12(2022) calls a castable dense above 1.68 g/cm³ (105 lb/ft³) dried, and the Jenkins 2011 table places conventional castables at 2,082 to 2,723 kg/m³ (130 to 170 lb/ft³). Insulating castables sit at 481 to 1,602 kg/m³ (30 to 100 lb/ft³) in the same table.

Is There a Refractory Concrete Mix for Site Batching?#

Refractory castables are supplied as factory dry mixes whose binder, dispersant and mixing water are set by the maker. The water ladder from 10 % for a conventional castable down to 3.5 % for an optimized low-cement castable shows how tightly the mix design controls the water.