GGBS, short for ground granulated blast-furnace slag, replaces 20 to 80 % of the total cementitious material in concrete by mass, substituted 1:1 for portland cement (Slag Cement Association Info Sheet 2). The same powder is called GGBFS or slag cement in the US and ground granulated blastfurnace slag in UK standards, and US slag cement shipments passed 4 million t in 2025 (Slag Cement Association).
Three specifications define it side by side: ASTM C989/C989M-26 with Grades 80, 100 and 120, EN 15167-1:2006 in Europe and IS 16714:2018 in India. That reach places slag cement among the concrete additives that producers batch and dose worldwide.
Why does one blast-furnace slag harden in concrete while another stays inert? Glass content decides it, because only rapidly quenched slag is glassy enough to react.
This page follows slag cement from manufacture and reaction through replacement levels, standards, admixture interactions, tests, durability, safety, cost and supply.
The table below summarizes slag cement as a concrete constituent.
| Attribute | Value |
|---|---|
| Material | Glassy calcium aluminosilicate from water-quenched iron blast-furnace slag, ground to a powder |
| Replacement | 20 to 80 % by mass of total cementitious material, 1:1 by mass (SCA Info Sheet 2) |
| Specific gravity | 2.85 to 2.94, against 3.15 for portland cement (SCA Info Sheet 2) |
| Standards | ASTM C989/C989M-26 (US), EN 15167-1:2006 (EU), IS 16714:2018 + Amd 1 (India) |
| Identity | Slags, ferrous metal, blast furnace (CAS 65996-69-2, EC 266-002-0), REACH registered |
| Hazard line | No Annex VI entry; ECHA C&L notified classification: not classified (100 %) |
Each value is traced to its source: standards and codes to their issuing body, product values to the data sheet named with them, prices to the dated source beside them (methodology and fact-checking).
What Is Slag Cement (GGBS)?#
Slag cement (GGBS) is the glassy by-product of iron blast furnaces, quenched and ground into a powder that replaces portland cement 1:1 by mass and hydrates once cement activates it. It is a constituent of concrete, not a cement on its own.
ACI 233R-17, "Guide to the Use of Slag Cement in Concrete and Mortar", is the American Concrete Institute document for its use, and ASTM C989/C989M-26 refers to ACI 233R in its appendices on use.
Slag cement shares the mix design role of fly ash, silica fume, metakaolin and calcined clay, the family grouped as supplementary cementitious materials.
From molten slag to ground powder#
Slag cement is made in two steps: rapid water quenching of molten blast-furnace slag into glassy granules, then grinding of those granules to a fine powder. EN 15167-1:2006 defines granulated blast furnace slag as vitrified material made by rapid cooling of a slag melt from smelting iron ore in a blast furnace, at least two thirds glassy and hydraulic when suitably activated.
Cooling rate decides the outcome. Air-cooled slag is crystalline and not cementitious, so only the quenched glass becomes slag cement.
The oxide composition of slag cement spans SiO2 32 to 42 %, Al2O3 7 to 16 %, CaO 32 to 45 %, MgO 5 to 15 %, sulfur 0.7 to 2.2 %, Fe2O3 0.1 to 1.5 % and MnO 0.2 to 1.0 % by mass (ACI 233R-17, as reported by the National Academies, 2025). Lime and silica together make up most of the glass.
Grinding sets the reactivity. EN 15167-1:2006 requires a specific surface of at least 275 m²/kg and moisture of at most 1.0 %, while ASTM C989/C989M allows at most 20 % retained on the 45 µm (No. 325) sieve.
Regional names: slag cement, GGBS, GGBFS and addition S#
Regional names differ by standard: slag cement or GGBFS in the US (ASTM C989/C989M, AASHTO M 302), GGBS in the UK and Ireland, GGBS or constituent S in the EU, and GGBS in India (IS 16714:2018). British concrete practice to BS 8500 writes slag combinations as CIIIA and CIIIB, and the EU treats GGBS as a type II addition under EN 15167-1 and EN 206.
All these names cover one registered substance: Slags, ferrous metal, blast furnace (CAS 65996-69-2, EC 266-002-0), REACH registered (ECHA, checked 2026-10-23).
In India "slag cement" can also mean Portland slag cement (IS 455); slag interground in Type IS, CEM III or IS 455 cement is a cement product and is not covered here.
How Does Slag Cement React in Concrete?#
Slag cement reacts as a latent hydraulic glass: the alkalis and calcium hydroxide released by portland cement activate it, so it hydrates to calcium silicate hydrate (C-S-H) and also consumes calcium hydroxide. The ADOT SPR-780 report (2025) classes slag as latent hydraulic plus pozzolanic, while Class F fly ash is only pozzolanic.
That double reaction explains the hardened properties. Slag hydrates fill pores, so the paste becomes denser and less permeable than plain portland cement paste at later ages.
Which constituents of portland cement does slag dilute? Slag contains negligible tricalcium aluminate (C3A), the phase that feeds sulfate attack. It also binds alkalis in its hydrates, lowering the hydroxide concentration of the pore solution beyond the effect of simple dilution, and it lowers the temperature rise from hydration.
What Does Slag Cement Change in Fresh and Hardened Concrete?#
Slag cement changes three things in concrete: workability and water demand, setting and strength gain with lower heat, and color. Each change scales with the replacement level and the concrete temperature, and the three H3 sections below follow that order.
Workability, water demand and air content#
Slag cement lowers water demand by up to 5 % for a given slump (SCA Info Sheet 2). Its lower specific gravity also adds paste volume per kilogram, which the worked example below quantifies.
Air content stays stable. Slag contains no carbon, so it does not cause the air content fluctuations that unburned carbon in fly ash causes, and SCA Info Sheet 2 states that slag should not cause air fluctuations.
Setting, strength gain and heat#
Slag concrete sets later below 29 °C (85 °F), and replacements below 30 % have no significant effect on set (SCA Info Sheet 3). Above that level and in cold weather, setting time grows with slag content.
Strength follows the same pattern. Slag concrete has lower strength through about 7 to 14 days and higher later-age strength; at equal cementitious content it reaches higher compressive strength, with the optimum at 40 to 50 % slag (SCA FAQ).
Heat rise falls in proportion to the slag fraction, and low-heat mixes need 50 to 80 % slag (SCA FAQ). The PCA rule of thumb in EB547 (Gajda, 2006) puts adiabatic rise at 0.13 °C per kg/m³ (0.14 °F per lb/yd³) of equivalent cement, counting slag as 0.9 of cement at 50 % replacement and 0.8 at 75 %.
Color and greening#
Slag cement is white, so it lightens hardened concrete (SCA FAQ). A temporary green-blue tint, called greening, forms when sulfides in the slag oxidize.
Greening fades on exposure to air and sunlight, but it can persist in constantly wet uses such as pools. The SCA FAQ names two remedies: waiting for oxidation, or applying hydrogen peroxide to speed it.
Slag Cement Replacement Levels by Application#
Slag cement replaces 20 to 80 % of total cementitious material by mass, and the application sets the range, from 20 to 50 % in precast and pavers to 50 to 80 % in mass concrete (SCA Info Sheet 2).
Replacement ranges and their basis#
Replacement ranges are percentages by mass of total cementitious material, taken from Table 1 of SCA Info Sheet 2. The table lists them by application.
| Application | Slag cement, % by mass of total cementitious material | Source |
|---|---|---|
| Paving, flatwork, walls, tilt-up, high-strength concrete | 25 to 50 | SCA Info Sheet 2 Table 1 |
| Footings | 30 to 65 | SCA Info Sheet 2 Table 1 |
| Precast, prestressed, block, pavers | 20 to 50 | SCA Info Sheet 2 Table 1 |
| ASR mitigation | 25 to 70 | SCA Info Sheet 2 Table 1 |
| Sulfate resistance, Type II equivalence | 25 to 50 | SCA Info Sheet 2 Table 1 |
| Sulfate resistance, Type V equivalence | 50 to 65 | SCA Info Sheet 2 Table 1 |
| Lower permeability | 25 to 65 | SCA Info Sheet 2 Table 1 |
| Mass concrete | 50 to 80 (thickest placements 65 to 80) | SCA Info Sheet 2 Table 1; SCA FAQ |
Maximum strength and lowest permeability fall at 40 to 50 % slag (SCA Info Sheet 2).
Worked example, calculated from the SCA 1:1 mass rule and specific gravities: a mix with an assumed 400 kg/m³ (674 lb/yd³) of cementitious material at an assumed 40 % slag holds 160 kg/m³ (270 lb/yd³) of slag cement and 240 kg/m³ (405 lb/yd³) of portland cement. At a specific gravity of 2.85 to 2.94, the 160 kg of slag occupies 54.4 to 56.1 L/m³ (11.0 to 11.3 gal/yd³), against 50.8 L/m³ (10.3 gal/yd³) for the same mass of portland cement at 3.15. The 3.6 to 5.3 L/m³ of extra paste volume is taken out of the aggregate in the yield calculation.
Batching: silo, weighing and w/cm#
Slag cement is batched as a dry bulk powder alongside cement. The four batching steps are listed below.
- Store slag cement in its own silo, separate from portland cement.
- Weigh it as cementitious material, replacing cement 1:1 by mass.
- Count it in the water to cementitious materials ratio (w/cm).
- Adjust the yield for its lower specific gravity and larger volume per kilogram.
Which Standards Classify Slag Cement and GGBS?#
Three specifications classify slag cement: ASTM C989/C989M-26 in the US, EN 15167-1:2006 in Europe and IS 16714:2018 in India. The table lists edition, status and scope.
| Region | Standard (edition, status) | What it sets | Page |
|---|---|---|---|
| US | ASTM C989/C989M-26, active (ASTM page updated 2026-09-11) | Grades 80, 100 and 120 by slag activity index; fineness; sulfide sulfur; no additions | ASTM C989 slag cement grades |
| US | AASHTO M 302 | Same requirements as ASTM C989 (Caltrans 2021) | This page |
| EU | EN 15167-1:2006, harmonized under CPR 305/2011, AVCP 1+ | Composition, glass content, fineness, setting and activity index | EN 15167-1 GGBS specification |
| EU | EN 15167-2:2024 | Assessment and verification of constancy of performance | This page |
| IN | IS 16714:2018, active, Amendment 1 (December 2019) | GGBS for use in cement, mortar and concrete | This page |
Grades 80, 100 and 120 under the US specification#
ASTM C989/C989M-26 grades slag cement by its 28-day average slag activity index, not by the number in the grade name: Grade 80 needs at least 75 %, Grade 100 at least 95 % and Grade 120 at least 115 %.
The other requirements, as reported for C989-18 by the National Academies (2025) and Caltrans (2021), cap fineness at 20 % retained on the 45 µm (No. 325) sieve and sulfide sulfur at 2.5 %, and they allow no additions. AASHTO M 302 carries the same requirements, and Caltrans requires Grade 100 or 120. In the ADOT SPR-780 survey (2025), 22 of 34 responding state transportation agencies (65 %) allow slag.
Type II addition under the European standard#
EN 15167-1:2006 classifies GGBS as a type II addition and is harmonized under the Construction Products Regulation 305/2011 with AVCP system 1+, applicable from 2008-01-01. Its requirements are listed below.
- Composition: glass at least two thirds by mass; CaO + MgO + SiO2 at least two thirds; (CaO + MgO)/SiO2 above 1.0.
- Chemical limits: MgO at most 18 %; sulfide at most 2.0 %; sulfate at most 2.5 %; loss on ignition at most 3.0 %; chloride at most 0.10 % unless declared; moisture at most 1.0 %.
- Fineness and set: specific surface at least 275 m²/kg (EN 196-6); initial set of a 50/50 blend at most twice that of the test cement.
- Activity index: at least 45 % at 7 days and 70 % at 28 days for a 50/50 blend at w/b 0.50.
Part 2, revised as EN 15167-2:2024, covers assessment and verification of constancy of performance.
IS 16714 in India#
India specifies GGBS for use in cement, mortar and concrete through IS 16714:2018, listed as active with one amendment (December 2019) on the BIS standards portal. The requirement values sit in the standard text, so a specifier reads them from the current BIS edition with its amendment.
How Does Slag Cement Interact with Admixtures and Other SCMs?#
Slag cement is compatible with chemical admixtures and with fly ash and silica fume in ternary mixes, and its interactions depend on concrete temperature and replacement level. Cold concrete at high slag content sets slowly and gains early strength slowly.
Binary and ternary combinations#
Binary mixes pair slag with portland cement alone; ternary mixes add a second SCM to offset a slag weakness or to cut carbon further. The two common partners are listed below.
- Fly ash: the NRMCA industry-wide EPD v4 (2026) lists a 20 % fly ash plus 30 % slag mix at 215.07 kg CO2e/m³, close to 50 % slag alone; see fly ash in concrete.
- Silica fume: offsets the early-strength loss of slag mixes, and both 50 % slag and 25 % slag with 3.8 to 4 % silica fume were effective against ASR (Hooton et al., 2013, via the National Academies); see silica fume (microsilica).
Temperature, accelerators and air-entraining dose#
Three offsets correct slow setting in slag concrete: accelerating admixtures, heated materials or a lower slag content (SCA Info Sheets 2 and 3).
Air-entraining admixture demand rises with fine slag (FHWA-HIF-20-085 and HIF-17-009), so the dose is confirmed by air tests when a finer slag arrives.
When to Choose Slag Cement, and When Not To#
Choose slag cement for durability, low heat and lower global warming potential; limit it in cold weather, on hand-finished flatwork exposed to deicers and in constantly wet service where color matters. The NRMCA industry-wide EPD v4 (2026) shows the carbon case for a 27.6 MPa (4,000 psi) non-air-entrained mix: 330.17 kg CO2e/m³ (252.4 per yd³) with no SCM against 216.12 kg CO2e/m³ (165.2 per yd³) with 50 % slag.
The SCA FAQ adds that replacing one ton of portland cement with slag cement eliminates about one ton of CO2 and uses nearly 90 % less energy per ton.
The four limits that argue for a lower slag share are listed below.
- Cold weather: later set below 29 °C (85 °F) and lower strength through about 7 to 14 days.
- Deicer-exposed hand finishing: FHWA-HIF-17-008, citing ACI 201.2R-16, limits slag to 50 % on hand-finished surfaces, not on formed or machine-finished ones.
- Constantly wet service: greening can persist, as in pools.
- Mass concrete: some slags do not lower heat enough at 50 to 65 %, so the slag source is checked by trial.
Which Tests Prove a Slag Cement Mix?#
Slag cement mixes are proven in two stages: material tests on the slag itself, then concrete tests on the project mix. Material tests are the ASTM C989/C989M slag activity index or the EN 15167-1 activity index, with fineness and chemistry from the mill test report.
ASTM C1897-20 (R3 reactivity by isothermal calorimetry or bound water at 40 °C (104 °F), 3 and 7 days) screens reactivity, but it does not distinguish hydraulic from pozzolanic reactivity and is not a substitute for concrete tests.
Concrete tests then cover the exposure: ASTM C1012/C1012M-24a mortar bars for sulfate resistance, ASTM C1293/C1293M-23ae1 concrete prisms for ASR and ASTM C1202-25 for chloride penetration. Age matters for the last one, because FHWA-HRT-05-056 (2006) found 56 days too early for concretes with pozzolans and slag and recommends later test ages where the schedule permits.
How Does Slag Cement Improve Durability?#
Slag cement improves durability through three mechanisms, sulfate resistance, ASR mitigation and lower permeability to chlorides, with one trade-off: faster carbonation. Each mechanism needs its own replacement range.
Sulfate resistance#
Slag cement gives moderate sulfate resistance at 15 to 50 % and high sulfate resistance at 35 to 50 %, measured by ASTM C1012/C1012M-24a (SCA Info Sheet 7). In the SCA example, a Type IL cement reached moderate resistance at 15 % slag and high resistance at 35 %.
Slag adds negligible C3A, consumes calcium hydroxide and lowers permeability. For comparison, ASTM C150/C150M-24 limits C3A to 8 % in Type II and 5 % in Type V cement.
ACI CODE-318-25 Table 19.3.2.1 allows exposure class S3 by Option 1: Type V or HS cement plus pozzolan or slag cement, w/cm at most 0.45 and 31.0 MPa (4,500 psi). Table 26.4.2.2(c) qualifies that combination at 0.10 % C1012 expansion at 18 months.
Reactive aggregates and alkali loading#
Slag cement mitigates alkali-silica reaction at 25 to 70 % by mass of total cementitious material, depending on aggregate reactivity and cement alkali (SCA Info Sheet 8). At a sufficient dose it keeps 2-year ASTM C1293 prism expansion below the 0.04 % limit with a cement of 1.25 % Na2Oeq.
AASHTO PP 65-11, as reproduced in FHWA-HIF-13-019 (2013), sets minimum slag levels by prevention level: 25 % for W, 35 % for X, 50 % for Y and 65 % for Z, for slag with Na2Oe at most 1.0 %.
ASTM C1778-25 does not count slag alkalis in the alkali loading of the concrete (SCA Info Sheet 8).
Permeability, chlorides and carbonation#
Chloride diffusion in slag concrete falls with age. In the Life-365 v2.2.3 example at w/cm 0.40, all mixes start at 79 x10⁻¹³ m²/s, but the 25-year diffusion coefficient reaches 25 x10⁻¹³ m²/s for portland cement against 9.3 x10⁻¹³ m²/s with 30 % slag.
Carbonation runs the other way. RILEM TC 281-CCC reports a carbonation coefficient about 1.3 times that of portland cement at 25 % slag, so cover and curing matter more.
Freeze-thaw caps have changed. ACI 318-19 Table 26.4.2.2(b) capped slag at 50 % of total cementitious material in exposure class F3, and ACI CODE-318-25 removed class F3 and its SCM maximums (NRMCA, 2026).
Slag Cement Safety and Handling#
Slags, ferrous metal, blast furnace (CAS 65996-69-2, EC 266-002-0) has no Annex VI entry; ECHA C&L notified classification: not classified (100 % of notifications, REACH joint submission; ECHA checked 2026-10-23). That line covers the dry substance, not the wet mix.
Wet paste containing slag cement and portland cement is alkaline, so product hazard statements, skin and eye protection and dust controls come from the supplier SDS. Dust handling at silos and transfer points follows the same SDS.
What Does Slag Cement Cost?#
The USGS Mineral Commodity Summaries 2026 put some US GGBFS at about USD 140 per metric ton or more in 2025, the upper end of the iron and steel slag market. The all-slag average of USD 40 per metric ton FOB plant (2025 estimate) covers every iron and steel slag and is not a GGBFS price.
An older reference point is the FHWA cost table of July 2003, which listed slag at USD 28 to 39 per tonne (USD 25 to 35 per ton), a historic value only. Haul distance and terminal logistics dominate delivered cost, and grinding adds processing cost.
GGBS Grades, Supply and Sourcing#
GGBS reaches concrete plants as a dry powder delivered in bulk, and buyers compare offers on grade, test report and source of the granulate before price.
The sourcing practice for bulk SCMs follows the one used by concrete admixture manufacturers and suppliers.
US supply depends on few granulators and on imports. The USGS (2026) counts four domestic granulators at three operations in 2025, against two at the end of 2015, with new grinding plants in Texas (2024, 2025) and announced in Indiana and Florida; imports reached 2.1 million t in 2025 (estimate), from Japan (60 %), China (22 %), Brazil (6 %) and Mexico (6 %) in 2021 to 2024.
Slag Cement Association members supply over 95 % of US slag cement, which is widely available east of the Rocky Mountains (SCA FAQ).
The five items to compare in every offer are listed below.
- Grade and standard: ASTM C989/C989M-26 Grade 100 or 120, EN 15167-1:2006 or IS 16714:2018.
- Test report: slag activity index or activity index, fineness and sulfide sulfur.
- Source: domestic or imported granulate, and the grinding plant.
- Safety data sheet: the current SDS for the delivered product.
- Delivery: bulk delivery to a dedicated silo, offered by slag cement suppliers.
Slag Cement Among the Supplementary Cementitious Materials#
Slag cement is the latent hydraulic member of the SCM family, while Class F fly ash, silica fume and natural pozzolans are pozzolanic and react only with calcium hydroxide.
Class C fly ash sits between them as partly hydraulic, the point compared in fly ash vs slag cement.
Two further SCMs with their own pages are listed below.
- Calcined clay: a manufactured, alumina-rich pozzolan; see calcined clay and LC3.
- Volcanic ash and pumice: raw or calcined; see natural pozzolans.
GGBS FAQs#
What is the density of GGBS?#
GGBS has a specific gravity of 2.85 to 2.94, so its particle density is about 2,850 to 2,940 kg/m³ (about 178 to 184 lb/ft³), against 3.15 for portland cement (SCA Info Sheet 2).
How many types of slag are there?#
Blast-furnace slag comes in two types by cooling: granulated slag, quenched to glass and ground into slag cement, and air-cooled slag, crystalline and not cementitious. Granulated slag was under 30 % of US slag tonnage but about 80 % of its value in 2025 (USGS, 2026).
How to make GGBS?#
GGBS is made by quenching molten blast-furnace slag with water into glassy granules, then drying and grinding them to at least 275 m²/kg under EN 15167-1:2006.
What is the cost of GGBS?#
Some US GGBFS cost about USD 140 per metric ton or more in 2025, according to the USGS Mineral Commodity Summaries 2026.