Codes & standards referenced
Cl. 5.2 lists the permitted mineral admixtures and requires uniform blending — it states no replacement percentage. Table 5 Note 1 says how much fly ash and slag may be counted toward cement content. Cl. 8.2.4.2 is the maximum cement content, 450 kg/m³ excluding fly ash and GGBS. Confirm against the current BIS controlled copy before tendering; amendments issued between reprints change specific clauses.
Pt 1: For use as pozzolana. Pt 2: For use as admixture in cement, cement mortar and concrete. Chemical & physical limits. Reaffirmed 2022.
Current standard for GGBS. Supersedes IS 12089:1987. Specifies chemical requirements (CaO, SiO₂, Al₂O₃, MgO, SO₃, glass content), Blaine fineness ≥ 275 m²/kg, and grades 80/100/120 by Slag Activity Index.
Specification for microsilica for use in cementitious systems. Min SiO₂ content, max LOI, specific surface.
Chemical and physical requirements for metakaolin as a pozzolanic SCM. Calcined kaolin-grade product with high SiO₂ + Al₂O₃ reactivity.
Distinct from IS 16714. Blaine fineness ≥ 320 m²/kg (commercial product typically 600–1300 m²/kg). For high-strength and high-durability concrete.
No dedicated Indian Standard product specification exists for RHA. Reference IS 1727 for testing methodology; ASTM C618 (US) classifies RHA as a pozzolanic material for international tendering.
India's first BIS product standard for UFFA. Average particle size 3–5 µm (BIS, PIB release ID 2097250, 29 Jan 2025). Recommended 5–10% cement replacement for enhanced strength and lower permeability. Confirmed at the BIS Madurai Manak Manthan meeting on 28 Jan 2025.
Why use SCMs at all?
OPC production emits ~0.9 t CO₂ per tonne of clinker. Every 100 kg of clinker replaced by fly ash avoids ~90 kg of CO₂. But sustainability is the second-order reason — the first-order reasons are engineering:
Mass pours (raft foundations, large piers) crack from thermal stress. SCMs spread heat release over weeks instead of days.
Pozzolanic reaction continues for months, filling capillary pores that water and chlorides would otherwise penetrate.
Fly ash and GGBS are ~30–60% the cost of OPC per kg. Silica fume is more expensive but enables strength grades impossible with OPC alone.
w/c, w/cm, w/p — three ratios, three different questions
SCMs make the water-to-binder ratio less straightforward. Three terms are in common use, and they are not interchangeable. Read this section before quoting a w/c, w/cm or w/p number from any source.
The mass of mix water divided by the mass of OPC. Excludes mineral admixtures. Not the term IS 456 uses for compliance; the IS 456 Table 5 ceilings (0.55, 0.50, 0.45, 0.45, 0.40) are water / (cement + permitted mineral admixture), not w/c in the strict sense.
The IS 456 term: mix water divided by the mass of OPC + fly ash + GGBS + silica fume + other permitted SCMs. This is the number that has to clear the IS 456 Table 5 ceiling for the classified exposure. When this post quotes a w/c number, it is the w/cm value in the IS 456 sense.
Mix water divided by the total powder content (cement + SCMs + inert filler such as limestone powder or quarry fines). Used in SCC and high-powder mix design where limestone or quartz filler is a deliberate third component. Always larger than w/cm.
Practical rule: when reading a w/c figure on a mix-design sheet, identify the denominator before comparing it against IS 456 Table 5. Numbers quoted against w/c (cement only) are not the numbers IS 456 audits against.
1 — Fly ash (IS 3812)
Pulverized fuel ash (PFA) is the fine residue from coal-fired thermal power plants. In India, ~220 MT/year is generated — and only ~60% finds productive use. Concrete can absorb much more if specifications are met.
From anthracite / bituminous coal. SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 70%. Predominantly pozzolanic. Default choice in India.
From lignite. CaO ≥ 10%. Both pozzolanic and cementitious — can self-harden. Variable in India; check IS 3812 Part 1.
Where fly ash earns its place
- · Mass concrete (raft, pile-cap, dam): 25–35% replacement reduces peak temperature by 10–15 °C.
- · Roller-compacted concrete (RCC pavements): 30–50% replacement is normal.
- · Coastal structures: fly ash improves sulphate and chloride resistance dramatically.
- · PPC (IS 1489): factory-blended cement already contains 15–35% fly ash — convenient for sites without batching control.
2 — GGBS (IS 16714:2018)
Ground granulated blast furnace slag is the glassy by-product of iron production, quenched and ground to cement fineness. Hydraulic (not just pozzolanic) — it reacts with water and CH from OPC hydration, generating C-S-H gel without needing portlandite activation the way fly ash does.
| Property | IS 16714:2018 limit | Why it matters |
|---|---|---|
| CaO + MgO content | ≥ 40% | Hydraulic reactivity |
| Glass content | ≥ 85% | Crystalline slag is inert |
| Fineness (specific surface) | ≥ 275 m²/kg | Higher = more reactive |
| 28-day activity index | ≥ 75% of OPC control | Hardness rate benchmark |
GGBS gives whiter concrete, better sulphate resistance (often the deciding factor in foundation work), and lower chloride permeability. The flip side: very pale colour that some architects dislike, and slower early strength gain. Typical Indian replacement: 30–50% for M30+ mixes; higher replacement for mass concrete is a project-specific choice and is not a default.
3 — Silica fume (IS 15388)
Microsilica is an ultra-fine (specific surface 15,000–25,000 m²/kg) by-product of silicon metal and ferrosilicon alloy production. Its particles are ~100× finer than cement. Pozzolanic reactivity is exceptional — 5–8% replacement can raise strength 15–25 MPa.
Typical applications
- · High-strength concrete (M60+, HPC): 5–10% replacement, combined with low w/c and HRWR.
- · UHPC (ultra-high-performance concrete): 20–30% replacement.
- · Shotcrete (IS 9012 / ACI 506R): rebound reduction and density boost.
- · Marine and chemical-resistant concrete: impermeability improvements.
Caveats: silica fume is expensive (₹30–50/kg vs ~₹7/kg for OPC in 2026), difficult to handle (dust control, slurry form required for dry addition), and highly sensitive to water — gets sticky fast. Always add with HRWR and a slow, thorough mix.
4 — Metakaolin (IS 16354:2015)
Metakaolin is produced by calcining kaolin clay at 650–850 °C — just hot enough to drive off the chemically bound water without fusing the structure. The resulting amorphous SiO₂ + Al₂O₃ powder is one of the most reactive pozzolans available, with a specific surface comparable to silica fume (10,000–20,000 m²/kg) but with different chemistry: high Al₂O₃ content gives metakaolin its characteristic chloride-binding capability.
- · SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 85%
- · LOI ≤ 1.0% (low — calcined product)
- · Pozzolanic activity index ≥ 80% at 28 days
- · Specific surface ≥ 10,000 m²/kg
- · High-performance / HPC: 5–10% replacement, paired with silica fume for densest mixes
- · Marine exposure: chloride binding via Friedel's salt formation
- · Architectural white concrete: brighter than OPC, no silica-fume darkening
- · LC3 (limestone calcined clay cement): IS 18189:2023 emerging category
Typical replacement: 5–15% by mass of binder. Above 15%, water demand rises sharply and the mix becomes sticky, similar to silica fume. Metakaolin is significantly lighter than OPC (bulk density ~600 kg/m³) — adjust batching by mass, not volume.
5 — Rice Husk Ash (RHA)
BIS does not currently publish a product specification for rice husk ash. For testing methodology, apply IS 1727:1967 (Methods of Test for Pozzolanic Materials). For international tendering, ASTM C618 classifies RHA as a Class N pozzolanic material. Quality control on every lot is critical — RHA properties swing widely based on combustion temperature and residence time.
RHA is the ash from burning rice husks (a major agricultural waste in Punjab, Andhra Pradesh, Tamil Nadu and West Bengal). When produced by controlled combustion below 700 °C, the silica remains amorphous and highly pozzolanic — SiO₂ content can exceed 90% with very low LOI. Burned above 800 °C, the silica crystallises (cristobalite/tridymite) and reactivity collapses.
- · SiO₂ ≥ 85% (preferably ≥ 90% for high-reactivity grade)
- · LOI < 5% (unburnt carbon is the most common quality issue)
- · Reactive SiO₂ ≥ 80% of total SiO₂
- · Specific surface 20,000–50,000 m²/kg (ground product)
- · Pozzolanic activity index ≥ 75% at 28 days (IS 1727 test)
- · Low-cost rural housing: 10–20% replacement, good 90-day strength
- · Agro-industrial projects in rice-producing states
- · Supplementary binder in PPC-equivalent blends
- · Research / sustainability showcase projects
Typical replacement: 10–20% by mass of binder. The grinding step is critical — unground RHA has minimal reactivity. Verify XRD/amorphous content and LOI for every lot. Without controlled combustion and grinding, RHA acts as inert filler rather than a pozzolanic SCM.
6 — Ultrafine GGBS (IS 16715:2018)
Ultrafine GGBS is regular GGBS ground to a higher Blaine fineness — minimum 320 m²/kg per IS 16715, with commercial products typically in the 600–1,300 m²/kg range (vs ≥ 275 m²/kg for regular GGBS per IS 16714). Higher fineness means faster and more complete hydraulic reaction at early ages, narrowing the early-strength gap with OPC while keeping the long-term durability benefits.
- · Specific surface (Blaine) ≥ 320 m²/kg
- · Glass content ≥ 85% (same as IS 16714)
- · 7-day activity index ≥ 75% of OPC control
- · 28-day activity index ≥ 95% of OPC control
- · Particle size: 90% < 10 µm (typical commercial product)
- · High-strength concrete (M60–M80): 20–40% replacement, low w/b
- · Pre-stressed concrete: early strength gain matches OPC
- · Marine / chloride environments: ultrafine particles densify ITZ
- · Lower-carbon concrete targets: cement reduction without 28-day strength penalty
Typical replacement: 10–30% by mass of binder for structural concrete, up to 50% in mass pours. Higher cost than regular GGBS (typically 1.3–1.8×) — the premium is justified when early strength or reduced cement content is critical. The first BIS product licence was granted to M/s Counto Microfine Products Pvt Ltd; check supplier certification per IS 16715.
7 — Ultrafine Fly Ash (IS 19058:2024)
BIS published IS 19058:2024 — Ultrafine Fly Ash — Specification in December 2024, ending the "commercial classification only" status of UFFA in Indian concrete practice. Source: PIB release ID 2097250, 29 Jan 2025, and the standardsbis.bsbedge.com entry. Subject of public discussion at the BIS Madurai Manak Manthan meeting on 28 Jan 2025; comments are routed through the BIS CARE App. Specifying engineers should still demand mill certificates per lot showing particle size distribution (laser diffraction) and activity index.
Ultrafine fly ash is regular fly ash classified or processed to a finer particle size — typically < 10 µm median particle diameter, with specific surface 800–1,500 m²/kg (vs 300–500 m²/kg for regular fly ash). The reduction in particle size increases pozzolanic reactivity and accelerates early-age strength gain, closing the gap with OPC-only mixes.
- · Average particle size 3–5 µm per IS 19058 (PIB release ID 2097250)
- · SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 70% (Class F chemistry)
- · LOI < 3% (carbonaceous particles interfere with air entrainment)
- · 28-day activity index ≥ 80% of OPC control
- · Mill test certificate per lot, with particle-size distribution curve
- · HPC / UHPC: 10–25% replacement for densification
- · High-durability bridges / marine structures
- · Mass concrete with early-strength targets
- · Sustainability-driven mix designs needing lower cement content
Typical replacement: 10–25% by mass of binder. Cost is typically 1.5–2.5× that of regular fly ash. The price premium is justified only when the project needs the combination of (a) high replacement levels, (b) low w/b, and (c) acceptable 7-day strength — which neither regular fly ash nor GGBS alone can deliver.
The strength-gain timeline
SCMs shift strength gain to later ages. A 30% fly ash M30 mix that hits 30 MPa at 28 days may be at only 17 MPa at 7 days. This is normal pozzolanic behaviour, not a quality failure.
| Binder blend (target M30) | 3-day | 7-day | 28-day | 90-day |
|---|---|---|---|---|
| 100% OPC (control) | 16 | 22 | 32 | 36 |
| 70% OPC + 30% fly ash | 11 | 17 | 30 | 41 |
| 50% OPC + 50% GGBS | 10 | 18 | 34 | 45 |
| 90% OPC + 10% silica fume | 18 | 26 | 42 | 50 |
Indicative MPa values for M30 target, w/cm 0.45, 100 mm slump, 20 mm crushed aggregate. Each project-specific trial must confirm these indicative values against the actual inputs.
Which SCM when?
- · Mass concrete (heat control critical)
- · Coastal / marine exposure (chloride resistance)
- · Cost is a primary driver
- · 56- or 90-day design strength acceptable
- · PPC (IS 1489) is acceptable per spec
- · Sulphate-bearing soil or water
- · Foundation / substructure durability critical
- · Lighter colour desired (architectural)
- · Lower chloride permeability required
- · Steel-mill slag is locally available (cost)
- · Strength grade ≥ M60 required
- · Very low permeability essential (water tanks, tunnels)
- · Shotcrete application
- · Abrasive / chemical exposure
- · Budget accommodates ₹30+/kg admixture cost
- · HPC with low w/b needs chloride-binding pozzolan
- · Architectural white or coloured concrete
- · Moderate early strength needed (better than fly ash at 7d)
- · LC3 / low-carbon cement blends being explored
- · M60–M80 strength grade required with ≥ 25% replacement
- · Pre-stressed concrete (early strength matters)
- · Marine / chloride exposure plus low w/b
- · Sustainability targets demand lower cement content
- · Agro-based projects (rice-producing regions)
- · Rural / low-cost housing with 56-day acceptance
- · Sustainability showcase (agricultural waste valorisation)
- · Amorphous (low-temp-burned) RHA is locally available
- · HPC / UHPC needs high replacement without early-strength penalty
- · Particle size verification (laser diffraction) is in the QA scope
- · Project can reference IS 19058:2024 (UFFA, Dec 2024)
- · Budget accommodates 1.5–2.5× regular fly-ash premium
- · Combines fly ash workability + GGBS strength
- · Common in ready-mix plants for cost + performance balance
- · Common ratios: 30% FA + 30% GGBS + 40% OPC
- · Requires tight quality control on both SCMs
Pitfalls on Indian sites
Source-to-source LOI variation can swing 0.5–12% — high-LOI fly ash kills air entrainment and slows strength. Test every new lot per IS 3812.
Slow-cooled or improperly quenched slag is partially crystalline — chemically inert. Glass content < 85% = reactivity < 75% = weak concrete.
A 30% fly ash mix will fail the 7-day cube comfortably. Acceptance at 28 days is correct, but supervisors must understand the timeline.
SCMs need longer wet curing — minimum 7 days, ideally 14. Premature drying kills the pozzolanic reaction and leaves capillaries open.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how an SCM blend is chosen against the IS 10262 dosage envelopes and the project specification, verified lot by lot, and read against the strength timeline it actually produces.
Questionnaire — ask when the binder blend is fixed on the mixing plant
- Which limits is the blend being judged against? IS 456 Cl 5.2 admits the materials and requires uniform blending — it states no percentage. The working numbers are the IS 10262:2019 Table 9 recommended dosages (fly ash 15–30%, GGBS 25–50%, metakaolin 5–15%, silica fume 5–10%), the maxima in the cement standards IS 456 Table 5 Note 1 points to (IS 1489 Part 1 for pozzolana, IS 455 for slag), and the project specification, which is what an audit actually reads.
- Which of the seven SCMs is going in, and under which product standard? IS 3812 for fly ash, IS 16714:2018 for GGBS (it supersedes IS 12089:1987), IS 15388 for silica fume, IS 16354:2015 for metakaolin, IS 16715:2018 for ultrafine GGBS, IS 19058:2024 for ultrafine fly ash. Rice husk ash still has no BIS product specification, so IS 1727 test methods apply.
- Is the ratio on the mix sheet a w/c or a w/cm? The IS 456 Table 5 ceilings of 0.55, 0.50, 0.45, 0.45 and 0.40 are water divided by cement plus permitted mineral admixture. A figure quoted against cement alone is not the number IS 456 audits.
- What age does the acceptance and the programme assume? The blend shifts strength to later ages, so a 28-day acceptance and a 56-day acceptance are different decisions about the same pour.
- What is the curing period behind the pour? Minimum 7 days wet, ideally 14, because premature drying kills the pozzolanic reaction and leaves the capillaries open. That is the control on the durability the blend was bought for.
- For GGBS, is this lot certified at or above 85% glass content and 275 m²/kg Blaine fineness under IS 16714:2018? Slow-cooled or improperly quenched slag is partly crystalline and chemically inert, and no amount of blending will fix it.
- For ultrafine GGBS, is the lot at or above 320 m²/kg with a 7-day activity index at or above 75% and a 28-day index at or above 95% of the OPC control? That is the whole reason for paying 1.3–1.8× the price of regular slag, so it needs a certificate, not a brand name.
- For rice husk ash, has the combustion been controlled below 700 °C and the ash ground, with per-lot LOI and reactive SiO₂ results? Burned above 800 °C the silica crystallises and the reactivity collapses, and unground ash acts as inert filler.
Inspector's checklist — what to verify on site
- Per-lot certificates for every SCM in the blend, each against the standard that governs that material, with the delivery batch tied to the certificate.
- The binder blend sheet, showing the replacement percentage of each SCM against IS 10262:2019 Table 9 and the project specification, and the cement content against the IS 456 Cl 8.2.4.2 limit of 450 kg/m³ excluding fly ash and GGBS.
- The w/cm computed on cementitious material and checked against the IS 456 Table 5 ceiling for the exposure class.
- Strength series at 3, 7, 28 and 90 days from the trial mix on the actual blend, not a neighbouring blend.
- Curing record: method, start time and duration, with the minimum at 7 days and the target at 14 for SCM-rich mixes.
- Lot register: LOI for fly ash, where source-to-source variation can swing 0.5–12%; glass content and activity index for GGBS; SiO₂ for silica fume; LOI at or under 1.0% for metakaolin.
- For rice husk ash, the XRD or amorphous-content verification per lot together with the grinding record.
- For ultrafine fly ash, the mill certificate with the particle-size distribution curve from laser diffraction, against the 3–5 µm average particle size of IS 19058:2024.
What happens if… three worked examples
What happens. Against the 100% OPC control, which reads 16, 22, 32 and 36 MPa at 3, 7, 28 and 90 days, the 70% OPC plus 30% fly ash blend reads 11, 17, 30 and 41 MPa. At 7 days it is 5 MPa behind, 17 against 22, a deficit of about 23%. At 28 days it is 2 MPa behind, 30 against 32. At 90 days it is 5 MPa ahead, 41 against 36.
Likely outcome. The 7-day failure is normal pozzolanic behaviour and not a quality failure, so acceptance at 28 days is correct and the supervisor needs to understand the timeline rather than chase the 7-day result. Where the programme genuinely needs strength at 7 days, a post-tensioning transfer or a stacking yard, the blend has to change. The cubes do not.
What happens. Thirty per cent fly ash plus 30% GGBS plus 40% OPC is quoted as a common ratio in ready-mix plants. Every material sits inside its own recommended band — fly ash at 30% is at the top of the IS 10262:2019 Table 9 range of 15–30%, GGBS at 30% is inside 25–50%, and both are within the pozzolana and slag maxima IS 456 Table 5 Note 1 points to. But the combined replacement is 30 + 30 = 60% of binder, and nothing in IS 456 caps it: Cl 5.2 states no percentage, and Cl 8.2.4.2 is a ceiling on cement content, not on admixture. Add silica fume at 10% and the total reaches 70%, and the blend now has to be defended on strength, heat, durability and supply grounds instead of on a clause.
Likely outcome. A mix that is defensible in the laboratory and rejected on site, because the project specification — not IS 456 — is what the audit reads. IS 10262 Cl 5.4.2 puts the fly ash and GGBS percentage on the project requirement and the quality of the materials, so the specification decides, not a code clause. Check the blend against that document before the first batch, not after the first audit.
What happens. IS 16714:2018 requires glass content at or above 85%, fineness at or above 275 m²/kg and a 28-day activity index at or above 75% of the OPC control. A compliant 50% OPC plus 50% GGBS blend delivers: 10 MPa at 3 days, 18 MPa at 7 days, 34 MPa at 28 days and 45 MPa at 90 days, against the control's 16, 22, 32 and 36. At 28 days the blend is already 2 MPa ahead and at 90 days it is 9 MPa ahead. That table only holds for slag that passes IS 16714. Below 85% glass content the slag is partly crystalline and inert, and the arithmetic is unforgiving: glass content below 85% gives reactivity below 75%, which gives weak concrete.
Likely outcome. The strength timeline the design was built on quietly stops applying, and nobody notices until the 90-day cubes come in short. Test every new lot before it reaches the mixer, and hold the delivery until the certificate is in hand.
Frequently Asked Questions
Related articles
Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in concrete technology, RMC plant operations and SCM-blend optimisation. Has specified fly ash, GGBS and silica fume blends for thermal power, coastal and high-rise projects across India.
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.