Refractory Additives and Binders: Cements, Dispersants, Antioxidants, Fibers and More

Refractory additives and binders are the cements, chemical, carbon and sol binders and the small additions that bond refractory grains or control how a mix flows, sets, dries and resists oxidation or molten metal. They fall into 15 groups, from calcium aluminate cement to temporary pressing binders. How are the groups told apart? By the bond each forms, the product that hosts it and the service it faces.

Buyers who ask for refractory chemicals or castable additives mean the same 15 groups. Calcium aluminate cement under EN 14647:2005 + AC:2006 covers 35 to 58 % Al2O3, while 70 and 80 % refractory cements fall under EAD 150002-00-0301. Additives are inputs, and their outputs are refractory materials.

Boric acid and boron oxide carry a harmonised Repr. 1B H360FD classification, coal tar pitch, high temperature, sits on REACH Annex XIV (entry 41, sunset 2020-10-04, Commission Regulation (EU) 2017/999), and calcium aluminate cement has no Annex VI entry. Consistency, hot strength and residual carbon tests pair ASTM C1445-13(2025), C583-15(2026) and C831-18(2023) with their ISO counterparts.

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 Additives and Binders?#

A refractory binder is the phase that holds refractory grains together by hydration, chemical reaction, gelling, carbonization or a glass bond. A refractory additive is a minor addition that controls flow, setting, drying, oxidation or wetting without forming the main bond.

In castables, calcium aluminate cement (CAC) forms the hydraulic bond of conventional and low-cement grades. Cement-free castables bond through hydratable alumina or a silica sol instead, and chemically bonded mixes use phosphoric acid, monoaluminum phosphate (MAP) or alkali silicates. Carbon-bonded bricks take phenolic resin or coal tar pitch, while silica ramming mass sinters through a boron glass.

Binder Versus Additive: Bond Phase and Small Additions#

Binders split into permanent and temporary types: a permanent binder remains as the bond in service, while a temporary binder gives green strength and burns out. Calcium lignosulfonate, dextrin and polyvinyl alcohol (PVA) are temporary binders that decompose on firing and leave no permanent bond.

Additives act beside the bond: antioxidants such as aluminum, silicon and boron carbide react with oxygen and carbon monoxide before graphite does, and dispersants cut mixing water. Binders let unfired products set without firing, which is what defines monolithic refractories.

Types of Refractory Additives and Binders: 15 Groups#

There are 15 groups: three hydraulic and sol binders, two chemical bonds, three castable matrix additives, two carbon-related groups and five special additions. Table 1 lists each group with its function, main substances, host products and standard or test.

Groups 1 to 3 are hydraulic and sol binders; 4 and 5 chemical bonds; 6 to 8 the castable matrix; 9 and 12 carbon-related; 10, 11, 13, 14 and 15 special additions. No row is a single product.

A row names a group, not a grade: each group page gives the grades, identity numbers and the dosages its sources state.

Calcium Aluminate Cement and Cement-Free Binders#

Calcium aluminate cement (CAS 65997-16-2, EC 266-045-5) is the hydraulic binder of conventional and low-cement castables. EN 14647:2005 + AC:2006 covers grades of 35 to 58 % Al2O3; the 70 and 80 % refractory cements fall outside it and are specified as calcium aluminate based refractory cement under EAD 150002-00-0301.

Hydratable alumina is the cement-free alternative: its rho alumina rehydrates to a boehmite gel and bayerite, and producer statements describe it as more hygroscopic than CAC. Colloidal silica binders are sols of 20 to 40 % solids (UNITECR 2025 conference paper) that gel when small amounts of magnesia, lime or CAC are added. Frost destroys the sol.

Phosphate and Silicate Chemical Bonds#

Phosphate binders bond by acid-base reaction: phosphoric acid reacts with alumina, while MAP solutions of 40 to 60 % carry an Al2O3:P2O5 ratio of about 1:3 (producer brochure). Magnesia sets phosphate mixes at ambient temperature.

Sodium and potassium silicate (water glass) gives an air-setting bond that hardens through the full joint thickness at ambient temperature. A 1942 National Bureau of Standards study documented a strength trough at intermediate temperature, before the ceramic bond forms. Alkali silicates are the chemical bond of basic castables, and their alkali flux lowers refractoriness.

Microsilica, Dispersants and Set Control in the Castable Matrix#

The castable matrix holds three additive groups: microsilica, dispersants and set control. Ultrafines below about 3 µm, used with a dispersant, cut the water a castable needs to flow. On heating, microsilica combines with lime from CAC to form low-melting glasses, which raise creep and lower hot strength and corrosion resistance; low-cement grades limit that lime.

Dispersants fall into four families: polycarboxylate ether comb polymers, phosphates (STPP and SHMP), citric acid and trisodium citrate, and dispersing aluminas. Set control works in two directions in CAC-bonded castables. Lithium carbonate accelerates setting, while citric acid or boric acid retards it, which lengthens working time.

Carbon Binders and Antioxidants for Carbon-Bonded Refractories#

Carbon-bonded refractories take two binders, coal tar pitch and phenolic resin. Pitch cokes to graphitizable carbon with good hot strength but needs hot mixing and releases polyaromatic hydrocarbons; resin mixes cold and gives high green strength but glassy carbon (2015 conference paper, Atlantis Press). Novolac resin needs hexamine to cure, while resol cures itself.

Antioxidants protect the graphite. In a 2012 laboratory study (ISIJ International), additions of Al, Si, SiC and B4C lowered the effective gas diffusion coefficient of the brick, and B4C gave the largest volume expansion on oxidation. Aluminum melts at 660 °C (1,220 °F) (PubChem). Both binder types and antioxidants serve the carbon-containing classes of refractory bricks.

Drying Fibers, Steel Fibers, Sintering Aids, Anti-Wetting Agents and Pressing Binders#

Each of the five special additions does one job. Drying fibers raise permeability so steam escapes; aluminum metal powder or a sol bond in place of cement does the same. Stainless steel fibers of grades 304, 310, 330, 430 and 446 control cracking but oxidize above their alloy limit, and an excess of fiber lowers corrosion resistance.

Boron sintering aids follow a rule stated in an IntechOpen book chapter: a higher furnace operating temperature needs less boric acid or boron oxide, and a boron oxide dose is about half the boric acid dose for the same sintering temperature. Anti-wetting additives (BaSO4, CaF2, AlF3, SiC) resist molten aluminum. Calcium lignosulfonate is the pressing binder of magnesia-spinel cement kiln bricks (2023 peer-reviewed paper).

How Are Refractory Binders Classified by Bond and Cement Level?#

Refractory binders are classified by how the bond forms (hydraulic, sol-gel, chemical, carbon, glass or temporary) and, for castables, by cement level, which ASTM C401-12(2022) expresses through lime (CaO) content.

The bond type sets the temperature range in which a lining is weakest: a hydraulic bond loses its hydrates on first heating, and an air-set silicate bond passes through its strength trough.

Hydraulic, Sol-Gel, Chemical, Carbon, Glass and Temporary Bonds#

Six bond types exist, each forming in its own temperature range, as Table 2 compares.

Sol bonds change phase in service: the silica crystallizes to cristobalite above about 1100 °C (2,012 °F) and forms mullite with alumina from about 1200 °C (2,192 °F). The borosilicate glass range of about 500 to 700 °C (932 to 1,292 °F) is a supplier statement by U.S. Borax, not a standard value.

Cement Level and the CaO Classes of Castables#

ASTM C401-12(2022) separates regular, low-cement, ultra-low-cement and no-cement castables partly by lime (CaO) content, read together with volume stability, strength and bulk density. The full class structure is set out under ASTM C401 classification of castables.

The cement itself has its own requirements. EN 14647:2005 + AC:2006 sets a standard requirement of at least 18 MPa (2,611 psi) compressive strength at 6 h, at least 40 MPa (5,802 psi) at 24 h and an initial set of at least 90 min. ISO 1927-1:2012, confirmed 2023-09-04, classifies monolithic products on the ISO side and replaces ISO 1927:1984.

Which Binder or Additive Suits Which Refractory and Service?#

The right binder follows the host product and its service: CAC or a cement-free binder for castables, phosphate or silicate bonds for plastics and mortars, and resin with antioxidants for carbon-bonded brick. Boron sintering aids serve silica ramming mass, and anti-wetting additions serve linings that touch molten aluminum.

A specification should name the binder system and its standard, not a brand. The atmosphere then decides whether carbon needs antioxidants, and the metal decides whether anti-wetting additions are needed.

Binders and Additives by Product Class and Industry#

Nine product classes carry most of the 15 groups, as Table 3 shows with one main industry each.

Coal tar pitch remains the reference binder of taphole clay for injectability and hearth protection, and low-PAH alternatives were benchmarked against it in 2023 (de Oliveira, Ironmaking and Steelmaking, 2024).

Steel, Aluminum and Induction Furnace Service#

Steel service puts antioxidants under the closest scrutiny. In one trial reported in Open Ceramics (2023), antioxidants caused Al, Si and C pick-up in the steel and gave no slag-corrosion benefit at an oxygen partial pressure of 10⁻¹⁵ atm and 1600 °C (2,912 °F).

Aluminum service calls for non-wetting phases. Celsian (BaAl2Si2O8), which forms from BaSO4 in aluminosilicate castables, is described in a research paper as a non-wetting compound that raises corrosion resistance.

Induction furnace service uses boron only in acidic (silica) masses, not in neutral masses. Boric acid releases water vapor during sintering that can harm coil insulation, while boron oxide contains no water.

Water, Hydrates and Burnout Fibers During First Heat-Up#

Hydrates and fibers decide how a castable releases water on first heat-up. CAC cured below 10 °C (50 °F) forms CAH10 and alumina gel, which raise the risk of explosive spalling (Almatis technical paper). One research paper on CAC-bonded castables places hydrate dehydration in four ranges between 100 and 360 °C (212 and 680 °F).

Drying fibers open the escape routes: polyethylene melts at 100 to 120 °C (212 to 248 °F) and polypropylene above 150 °C (302 °F) (Bezerra et al., Ceramics International 46, 2020). Holds and heating rates belong to the lining's schedule, covered under refractory dry-out and heat-up schedules.

Identity, Hazard Lines and REACH Status of Additive Substances#

Several refractory additives carry a harmonised CLP Annex VI classification, among them phosphoric acid, citric acid, boric acid, boron oxide, coal tar pitch, hexamine and aluminum powder. Calcium aluminate cement and colloidal silica have no Annex VI entry, so the product SDS decides.

Table 4 gives one row per substance with the hazard line as recorded from ECHA.

Hazard lines from the EU CLP Annex VI list and the ECHA C&L records as read on 2026-10-27; mixtures are classified by their SDS.

Silica identity splits by form: microsilica is CAS 69012-64-2, while colloidal and fumed silica share CAS 7631-86-9. For silicon carbide, Annex VI entry 014-048-00-5 (Carc. 1B H350i, in application since 2022-03-01) covers only silicon carbide fibers with a diameter below 3 µm, a length above 5 µm and an aspect ratio of at least 3; silicon carbide powder and grain have no entry.

Pitch, Boron and Cement Rules That Apply to Additives#

Coal tar pitch, high temperature, is REACH Annex XIV entry 41, with a sunset date of 2020-10-04 under Commission Regulation (EU) 2017/999, so its use in the EU after that date needs an authorization.

Boric acid has carried Repr. 1B H360FD since 2025-02-01 (ATP 20) and has been on the Candidate List since 2010-06-18; Annex XVII entry 30 bars its supply to the general public at or above the classification limit. Separately, the text of REACH Annex XVII entry 47 (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 product falls under the entry is not assessed here. EAD 150002-00-0301 requires refractory cements to meet that limit. Fired sol bonds contain cristobalite, so crystalline silica rules apply to spent linings.

Each hazard has its own page, listed below.

• coal tar pitch in carbon-bonded refractories
• crystalline silica in refractories
• refractory health and safety

Standards and Test Methods for Refractory Binders and Additives#

Binders and additives fall under three kinds of standard: cement specifications, castable classifications (ASTM C401-12(2022), ISO 1927-1:2012) and test methods for consistency, hot strength and residual carbon. Each designation with its edition is listed under refractory standards.

Five groups have no product standard for the additive itself: microsilica, steel fibers, sintering aids, anti-wetting additives and temporary binders. Their specification cites the host product's standard.

Cement and Castable Classification Standards Side by Side#

ASTM and ISO/EN standards cover the same subjects in Table 5, but their requirements are not declared equivalent.

EAD 150002-00-0301 limits composition rather than strength: clinker at least 70 %, alumina-rich material (Al2O3 above 35 %) at most 30 % and additives at most 0.2 %. ISO 10081-3:2003, confirmed 2025-06-10, classifies basic products with 7 to < 50 % residual carbon, and ASTM C199-22 tests refractory mortars in a brick pier.

Consistency, Hot Strength and Residual Carbon Tests#

Four test families measure what additives change: flow, self-flow, hot strength and residual carbon, as Table 6 lists.

What each value means is covered under properties of refractory materials.

ASTM C1446-19(2025) measures the consistency of self-flowing castables, not setting time, and C860-15(2025) covers C401 castables including metal-fiber grades but not gunning, pumping or self-flow mixes. ISO 1927-4:2012 has been at the stage "to be revised" since 2026-09-15, with no new edition published. The combustion procedure of C831-18(2023) does not separate carbide carbon, so SiC and B4C antioxidants count toward the result.

Sourcing Refractory Additives and Binders#

Additives are bought by substance, grade and identity rather than by refractory class, and refractory dry mixes made with them ship under HS heading 3816 (US HTS, checked 2026-10-27).

A specification names the CAS number, particle size or solids content, packaging and date of manufacture. Placing the finished mix on site is a separate scope, described under refractory installation methods.

What an Additive Specification and RFQ Contain#

An additive RFQ contains six items, listed below.

• substance and grade
• CAS number
• d50 and BET surface for powders, or solids content for liquids
• packaging and date of manufacture
• SDS and REACH status
• quantity in metric tonnes, and HS heading 3816 for dry mixes

The date of manufacture matters because CAC ages in its bags; a US short ton equals 907.2 kg (2,000 lb). Quote factors are grade, raw-material origin, quantity, packaging and freight; marketplace listings are offers, not market prices.

Handling Additive-Bearing Mixes Before Placement#

Three additive groups need care before placement: metal powders, phosphate binders and sols. Metal powders carry a dust-explosion risk during mixing, and phosphoric acid reacts with metallic iron in aggregates and releases gas, which is why inhibited grades exist.

Sols must be kept from freezing, and the reaction of magnesia with MAP is strongly exothermic and very fast.

How Do Additives Differ From Refractory Cement and Raw Materials?#

Refractory additives are inputs, refractory cement is a trade term for finished mixes, and raw materials are the grains that binders hold together.

Calcium aluminate cement is one binder inside products sold under the name refractory cement. What the trade term covers is explained under refractory cement.

Dispersing and reactive aluminas sit on the border between the two lists: matrix additives made as alumina grades.

Which Additives Also Serve as Refractory Grains?#

Silicon carbide and alumina serve both as grains and as additives. Silicon carbide is a refractory grain, an antioxidant and an anti-wetting component, depending on the product.

Alumina (CAS 1344-28-1, EC 215-691-6; "No Annex VI entry; ECHA C&L notified: not classified (93.7 %)") is an aggregate and the base of hydratable alumina. Graphite is a carbon source in carbon-bonded brick, not a binder. Grains of both kinds are described under refractory raw materials.