Codes referenced
Fineness (≥ 320 m²/kg), glass content (≥ 85%), slag activity index (≥ 60 % at 7 days and ≥ 75 % at 28 days), chemical composition. The current Indian specification for ground slag.
Permits GGBS as a part replacement of cement with uniform blending, and states no percentage. IS 10262:2019 Table 9 recommends 25–50 % by mass of total cementitious materials, and IS 456 Table 5 Note 1 allows the slag to be counted towards the cement content only up to the limit in IS 455.
Fly ash gives 1.5-2x chloride reduction vs 3-5x for GGBS. GGBS is preferred where chloride is the limiting factor.
EN 15167 covers European GGBS; ASTM C989 grades slag by 28-day activity index (Grade 80 / 100 / 120).
GGBS chemistry — glassy phase, latent hydraulicity
GGBS is a by-product of iron manufacture. Molten slag from the blast furnace is rapidly quenched with water, producing a glassy granular material. Ground to a fineness of at least 320 m²/kg, the resulting powder has latent hydraulic activity: the glassy phase dissolves in the alkaline pore solution of hydrating OPC and reacts with calcium hydroxide (portlandite) released during cement hydration. The reaction is similar to pozzolanic in fly ash, but slag also contributes independent hydraulic products — making GGBS a "latent hydraulic" rather than purely pozzolanic material. The denser hydration products and refined pore structure are the source of the chloride-resistance dividend.
IS 16714:2018 — fineness, glass content, chemistry
| Parameter | IS 16714:2018 limit | Significance |
|---|---|---|
| Specific surface (Blaine) | ≥ 320 m²/kg | Reactivity — finer is better |
| Glass content | ≥ 85% | Active glassy phase — crystalline slag is inert |
| Slag activity index, 7 days | ≥ 60% of the control OPC 43 mortar cube | Early strength of a 50:50 GGBS–OPC blend |
| Slag activity index, 28 days | ≥ 75% of the control OPC 43 mortar cube | 28-day strength of a 50:50 GGBS–OPC blend |
| CaO+MgO+⅓Al₂O₃ over SiO₂+⅔Al₂O₃ | ≥ 1.0 | Active oxide modulus |
| MgO | ≤ 17.0% | Soundness |
| Sulphide sulphur (S) | ≤ 2.0% | Expansive sulphides |
| Sulphate (as SO₃) | ≤ 3.0% | Sulphate attack risk |
| Insoluble residue | ≤ 3.0% | Unreactive fraction |
| Chloride content | ≤ 0.1% | Embedded-steel corrosion risk |
| LOI | ≤ 3.0% | Lower than fly ash — slag has no carbon |
| Moisture | ≤ 1% | Storage stability |
Replacement levels — where the limits come from
IS 456 Cl 5.2 permits GGBS as a part replacement of cement and states no percentage, so the working numbers come from three other places:
- General exposure — 40-50% GGBS as partial cement replacement, with project specification confirming limits.
- Chloride exposure — GGBS ≥ 50% (or PSC with slag ≥ 50%). For severe marine exposure many specs mandate GGBS ≥ 60%.
- Sulphate exposure — GGBS ≥ 50% combined with C₃A-limited OPC (≤ 5% for severe, ≤ 8% for very severe).
- Mass concrete — up to 80% GGBS to manage heat of hydration.
Beyond 70%, the 28-day strength drops significantly and the 90-day crossover may not compensate for thin sections with strict stripping schedules. Always re-validate the mix design at the actual replacement level.
Performance — chloride, sulphate, heat, colour
GGBS at 50-70% reduces chloride diffusion coefficient by 3-5x (e.g. from 4.5 × 10⁻¹² m²/s OPC to 1.2 × 10⁻¹² m²/s GGBS 70%). Critical for marine piles and coastal structures.
Refined pore structure and lower Ca(OH)₂ content reduce sulphate attack. Combined with C₃A-limited OPC, GGBS is the IS 456-aligned choice for sulphate soils.
Lower than OPC and slightly lower than fly ash at equivalent replacement. Good for mass pours — dams, thick pile caps, raft foundations.
Lighter colour than OPC (off-white / pale grey). Useful for architectural concrete and lighter floor slabs.
Recommended dosage: GGBS 25–50 %
IS 456 Cl 5.2 permits GGBS as a part replacement of cement and states no percentage, so the level is a project decision, taken on the chloride performance the structure needs and the mix design that delivers it. IS 10262:2019 Table 9 recommends 25–50 % by mass of total cementitious materials; a marine project specification normally asks for 50–70 %. The mechanism is the denser C-S-H gel and refined pore structure that physically reduces chloride ingress, combined with chemical binding of chloride ions in the hydration products. Always pair with adequate cover per IS 456 Cl 26 and crack-width control per Cl 15.
Source: IS 10262:2019 Table 9 (recommended dosages for high-strength mixes). IS 456 Cl 5.2 permits the material and states no percentage; IS 456 Table 5 Note 1 allows the slag to be counted towards the cement content only up to the limit in IS 455 (25–70 % slag in PSC). IS 16714:2018 is the material specification.
Worked example — M40 marine pile, 55% GGBS
| Parameter | OPC control | 55% GGBS mix |
|---|---|---|
| Target 28-d strength | 40 MPa (M40) | 40 MPa (M40) |
| Total cementitious | 400 kg / m³ | 400 kg / m³ (180 OPC + 220 GGBS) |
| w/c ratio | 0.40 | 0.38 |
| 7-d strength | 30 MPa | 22 MPa (slower set) |
| 28-d strength | 44 MPa | 42 MPa |
| 90-d strength | 48 MPa | 52 MPa — exceeds OPC |
| Cl⁻ diffusion coeff (10⁻¹² m²/s) | 4.5 | 1.2 (~75% reduction) |
| RCPT (ASTM C1202, coulombs) | ~3000 (moderate) | ~900 (low) |
| Heat of hydration | Baseline | ~30% lower |
| Embodied CO₂ (binder only) | ~360 kg / m³ | ~200 kg / m³ (~45% lower) |
Common deviations seen on Indian sites
Spec calls for PSC (factory-blended); site team switches to OPC + GGBS at the plant to "optimise cost" but doesn't pass the saving to the client or revalidate the mix. Cost variation orders should be documented and the new mix tested for chloride performance.
GGBS slowly absorbs moisture and CO₂ from air, reducing reactivity. IS 16714:2018 caps the moisture content of GGBS at 1 % by mass, so stored material has to be checked and kept dry. The re-test interval itself is set by the project quality plan, not by the standard. Sites routinely skip the check; the 28-day cube strength falls short and the contractor blames the lab.
Two cements dosed separately into the mixer without pre-blending in a silo or paddle mixer. Particle size difference (OPC ~325 m²/kg, GGBS ~325-400 m²/kg) causes segregation in the mixer. Pre-blend or dose at the same point.
GGBS slows setting in cool weather (good for long hauls) but the effect is less pronounced in hot weather. Retarder dosage designed for an OPC mix may over-retard the GGBS mix in winter. Always re-tune the admixture package for the binder and ambient.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a slag decision is made, verified and defended on a marine pile.
Questionnaire — ask before the slag goes into the silo
- What is the exposure class for this member, and what fixes the replacement level — IS 456 or the project specification? IS 456 Cl 5.2 permits GGBS but states no percentage; IS 10262:2019 Table 9 recommends 25–50% by mass of total cementitious materials, and a marine project normally specifies the level it wants, often 50–70%.
- Does the slag meet IS 16714 — fineness at least 320 m²/kg, glass content at least 85%, slag activity index at least 60% at 7 days and 75% at 28 days, MgO at most 17.0%, sulphate (as SO₃) at most 3.0%, moisture at most 1% — on a test certificate tied to the batch being delivered?
- How long has the slag been in store, and has it been re-tested if it has crossed the interval the quality plan sets? Slag picks up moisture and CO₂ from the air and loses reactivity doing it; IS 16714 caps the moisture content at 1% by mass for that reason.
- Is the slag pre-blended with the OPC in a silo or paddle mixer, or are two cements being dosed separately into the mixer?
- Has the admixture package been re-tuned for the slag blend and the ambient temperature? A retarder dosed for an OPC mix over-retards a GGBS mix in cool weather.
- Is the mix re-validated at the actual replacement level, with 7-day, 28-day and 90-day results, against the stripping schedule and the strength the structure needs?
- If the specification calls for PSC, has anyone switched the plant to OPC plus GGBS, and is the cost variation and the chloride performance both documented?
Inspector's checklist — what to verify at the plant and on site
- IS 12089 test certificate for the batch in use, showing Blaine fineness, glass content, the active oxide sum, MgO, SO₃, LOI and moisture.
- Store-age record with the date of delivery, the date of the last re-test, and the moisture result that says whether the slag is still usable.
- Batch sheet showing the actual replacement percentage batch by batch, against the approved mix design, and the binder source recorded per batch.
- Pre-blending confirmation — silo or paddle mixer in use, or a written procedure for the two-cement dosing with the segregation risk acknowledged.
- Admixture re-tuning record for the new binder and the current ambient temperature, with the dosages actually dosed.
- Strength series at the design replacement level: the 7-day, 28-day and 90-day results, checked against the stripping time the programme assumes.
- Chloride performance evidence for the blend: the diffusion coefficient or RCPT result, if the design relies on it.
- Variation order on file wherever the delivered cement differs from the specification, with the mix re-validated and the cost position settled.
What happens if… three worked examples
What happens. The mix was validated at 55% GGBS, where the chloride diffusion coefficient comes in at 1.2 × 10⁻² m²/s against 4.5 × 10⁻² m²/s for the OPC control. At 30% replacement the design has lost most of that advantage, and the chloride resistance the design paid for is simply not in the concrete, because no clause of IS 456 was ever going to catch it — that was the project specification's job. The strength will still look acceptable; the durability will not.
Likely outcome. The pile fails the specification on chloride grounds, and the strength records will not catch it, because the mix at 30% still delivers usable 28-day strength. That is the dangerous part: the cube register looks fine while the element is short of the durability the design paid for.
Preventive correction. Tie the replacement percentage in the batch sheet to the approved mix design and reconcile it monthly against the delivery records. On a marine project this is the one number that cannot be allowed to drift.
What happens. Slag absorbs moisture and CO₂ from the air over time, so the glassy phase that drives the latent hydraulic reaction is partly spent before it reaches the mixer. The 7-day strength, which the OPC control mix reached at 30 MPa, comes in at 22 MPa on the slag mix. On site this gets blamed on the laboratory, because the cubes were cast and cured properly.
Likely outcome. A dispute between the lab, the plant and the contractor, over concrete that was never bad. IS 16714 caps the moisture content of GGBS at 1% by mass, and the project quality plan sets the re-test interval; if the moisture check was skipped, the material is the cause. The 90-day crossover to 52 MPa against the OPC control's 48 MPa still happens, but by then the stripping schedule has already been missed.
Preventive correction. Store-age is a record, not a memory. Date every delivery, re-test at three months, and quarantine anything past it until a fresh certificate is in hand.
What happens. The concrete is chemically sound. A PSC is a factory blend whose granulated slag constituent runs from 25% to 70% by mass under IS 455:2015, and OPC plus GGBS dosed separately can reach the same slag content with better control of the replacement level. What breaks is the paperwork: the contract calls for PSC, the plant supplies something else, and neither the client nor the structural designer has been told.
Likely outcome. Two separate problems. The cost saving has not been passed on, so the client has paid PSC rates for a cheaper product. And the chloride performance has never been validated for the blend actually used, because the validation was done on the PSC. Neither is fixable from the batch sheet after the event.
Preventive correction. Pre-blend properly and treat the switch as a formal substitution: a variation order, a re-validated mix design, and a chloride performance result for the blend as batched. The separate-dosing route also carries its own risk, since the fineness difference between OPC and GGBS can cause segregation in the mixer.
Frequently Asked Questions
What is the difference between GGBS and PSC?
Why is GGBS better than fly ash for chloride exposure?
Can GGBS and fly ash be combined in the same mix?
Related articles
Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For GGBS mix design, marine concrete specification or chloride audit, contact amit@concreteinfo.in.
Technical updates, not sales emails
One email when a new article goes out on concrete technology, QA/QC and construction quality. No promotions, and your address is never sold or passed to advertising networks. Subscribing is separate from making an enquiry.