Codes referenced
Source, chemical composition, fineness, moisture limits for raw PFA from thermal power stations.
Physical requirements (fineness, lime reactivity, soundness) for fly ash blended in concrete.
Cl 5.2 permits mineral admixtures as part replacement of cement. Cl 5.2.1.1 allows fly ash conforming to IS 3812 (Part 1) with uniform blending and states no percentage; the replacement limits come from IS 1489 (Part 1) and IS 455.
International comparison: ASTM C618 defines Class F (low-CaO) and Class C (high-CaO); EN 450 covers European fly ash.
Fly ash chemistry — Class F vs Class C
Fly ash is a by-product of coal combustion in thermal power stations. The classification by ASTM C618 (and followed informally in IS 3812) splits fly ash into two chemistry families:
Sum of SiO₂ + Al₂O₃ + Fe₂O₃ > 70% and CaO < 10%. Produced from bituminous or anthracite coal. Indian thermal coal is predominantly Class F. Has only pozzolanic activity — needs OPC portlandite to react. Slower strength gain but lowest heat and best durability.
CaO > 10% and similar Si + Al + Fe sum. Produced from lignite or sub-bituminous coal. Self-cementing — sets and hardens with water alone because of free lime and reactive calcium aluminates. Faster strength gain, but higher sulphate risk.
IS 3812 Part 2 limits — physical & chemical
| Parameter | IS 3812 Pt 2 limit | Significance |
|---|---|---|
| SiO₂ + Al₂O₃ + Fe₂O₃ | ≥ 70% | Pozzolanic activity indicator |
| MgO | ≤ 5% | Soundness (periclase expansion) |
| SO₃ | ≤ 3% | Sulphate attack risk |
| LOI (loss on ignition) | ≤ 5% (project specs often ≤ 3%) | Unburnt carbon — absorbs admixture |
| Fineness (retained on 45 µm) | ≤ 10% | Reactivity — finer is better |
| Lime reactivity (MPa) | ≥ 4.5 | Strength activity with lime |
| Soundness (autoclave) | ≤ 0.8% | Long-term volume stability |
Replacement levels — IS 1489 (Part 1) and IS 455
IS 456 Cl 5.2 permits mineral admixtures as part replacement of cement, but Cl 5.2.1.1 states no percentage — it simply allows fly ash conforming to IS 3812 (Part 1), with the blending requirement of Cl 5.2 met. The ceiling on replacement comes from the cement standard: IS 1489 (Part 1) for fly ash and IS 455 for slag, which is the route IS 456 Table 5 Note 1 takes. The headline limits are 35% by mass of cementitious material for OPC and 40% for PSC. Higher replacements (up to 50%) are permitted for mass concrete (dams, large pile caps, raft foundations) with a documented trial mix and engineer approval. Always confirm the project specification — many infra projects (metro, nuclear) impose stricter 25-30% caps for durability predictability.
Performance — strength, permeability, heat
The fly ash trade-off is well documented: 28-day strength is typically 5-10% lower than the OPC control, but 56-90 day strength crosses over and equals or exceeds OPC because of the slower pozzolanic reaction continuing past the cement hydration window. Permeability drops by 25-40% at 90 days because the additional C-S-H gel fills capillary pores. Heat of hydration drops by 25-40%, which is the reason mass concrete specifications routinely require fly ash. Colour is lighter than OPC, which aids finish quality.
Quality control — the LOI trap
The single most important QC parameter for fly ash is LOI (loss on ignition), which measures unburnt carbon. High-LOI fly ash is a poison for PCE superplasticizers — the porous carbon particles adsorb the polymer, the mix loses slump within 30 minutes, and site engineers respond by overdosing admixture (which delays setting) or adding water (which drops strength). IS 3812 caps LOI at 5%; many project specs require under 3%. Always demand a lot-wise LOI certificate from the supplier and re-test on receipt.
Chemical & physical requirements summary
The headline physical requirements: fineness ≤ 10% retained on 45 µm IS sieve, lime reactivity ≥ 4.5 MPa, soundness ≤ 0.8% by autoclave. Chemical: SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 70%, MgO ≤ 5%, SO₃ ≤ 3%, LOI ≤ 5%, moisture ≤ 2%. The fineness and lime-reactivity tests are the two most useful acceptance checks on a job site — chemistry is supplier-controlled and rarely re-tested in real time.
Source: IS 3812 (Part 2):2013 Table 1 — Physical and chemical requirements for fly ash for use as pozzolanic material.
Worked example — M30, 30% fly ash replacement
| Parameter | OPC control | 30% PFA mix |
|---|---|---|
| Target 28-d strength | 30 MPa | 30 MPa |
| Total cementitious | 350 kg / m³ | 350 kg / m³ (245 OPC + 105 PFA) |
| w/c ratio | 0.40 | 0.40 |
| Water content | 140 kg / m³ | 140 kg / m³ |
| Slump | 100 mm | 100 mm (slightly more cohesive) |
| 7-d strength | 22 MPa | 18 MPa |
| 28-d strength | 32 MPa | 30 MPa |
| 90-d strength (crossover) | 36 MPa | 38 MPa — exceeds OPC |
| Chloride permeability (90 d) | Baseline | ~30% lower |
| Cost (cementitious only) | Baseline | ~5-8% lower |
Common deviations seen on Indian sites
Site accepts fly ash on mill test certificate only, with no lot-wise LOI. Within a month, superplasticizer demand doubles and slump loss becomes unmanageable. Insist on LOI ≤ 5% (better ≤ 3%) per batch, and re-test on receipt.
Lab trial mix uses 40% replacement on a mass pour, then the same number is applied to a 150 mm slab. 28-day strength falls short; the slab can't be stripped. Cap replacement per member thickness and curing regime.
Spec changes from OPC to PPC / fly ash blend mid-project without a fresh trial mix. First pour cubes fail at 7 days. Always re-validate the mix design when binder composition changes.
Fly ash silo or bag stockpile exposed to rain. Pre-hydration begins, reactivity drops, lumps form. IS 3812 caps moisture at 2%; reject wet or lumpy fly ash at receipt.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how a fly ash decision is made at the plant gate, verified on receipt and defended at 90 days.
Questionnaire — ask before the fly ash is unloaded at the plant gate
- Which cement standard governs the binder in this pour? IS 456 Cl 5.2.1.1 lets fly ash conforming to Grade 1 of IS 3812 into the mix with uniform blending, but states no percentage at all. The 35% ceiling for OPC and 40% for PSC come from IS 1489 (Part 1) and IS 455. Confirm which one applies to this delivery, and whether the project specification is tighter, because many infra projects cap replacement at 25–30% for durability predictability.
- What is the lot-wise LOI on the certificate? IS 3812 Pt 2 caps it at 5%, and many project specifications want it under 3%. This is the one parameter that decides whether your PCE dosage still works.
- Do the two physical acceptance checks hold on this lot? Fineness at or under 10% retained on the 45 µm sieve, and lime reactivity at or above 4.5 MPa. These are the two checks you can run yourself; the chemistry is supplier-controlled and rarely re-tested in real time.
- What is the member thickness at this pour, and does the replacement level match it? A 40% replacement validated on a mass pour is not a number you carry over to a 150 mm slab. This exact carry-over is on the list of deviations seen on Indian sites.
- Has the mix been re-validated at the actual replacement level, with 7, 28 and 90-day results? The whole case for fly ash rests on the 90-day crossover, so a mix approved on 28-day data alone has never been measured for the property that justifies it.
- Is the material stored dry, and for how long? IS 3812 caps moisture at 2%. Pre-hydration in a wet silo or a rain-exposed bag stockpile begins before the mixer ever sees it.
- Is the ash pre-blended into PPC, or dosed separately, and has the PCE dosage been re-tuned against the carbon in this lot? High-LOI material destroys the admixture, and the failure arrives as slump loss on site, not as a certificate rejection.
Inspector's checklist — what to verify on site
- Lot-wise LOI certificate with the batch number tied to the delivery in hand, at or under 5% per IS 3812 Pt 2, or under 3% where the project specification says so.
- Re-test on receipt, not the mill test certificate alone. A site that accepts fly ash on the mill certificate alone finds superplasticizer demand doubling within a month.
- Fineness and lime reactivity entries on the same certificate: at or under 10% retained on 45 µm, and at or above 4.5 MPa.
- Condition check at unloading: moisture at or under 2%, no lumps, no wet bag stockpile. Reject on the spot, because a wetted bag has already lost reactivity by the time you find it.
- Batch sheet showing the actual replacement percentage batch by batch against the approved mix design.
- Trial-mix strength series at the replacement level in use: 7, 28 and 90 days, with the 90-day crossover against the OPC control recorded, not assumed.
- Superplasticizer dosage record for the current lot, re-tuned when the LOI has moved from the lot used in the trial.
- Storage register: silo condition, date of delivery, and the date of the last re-test.
What happens if… three worked examples
What happens. The mix is 350 kg/m³ of total cementitious material at 30% replacement, so 350 × 30% = 105 kg/m³ of PFA and 350 − 105 = 245 kg/m³ of OPC, with water held at 140 kg/m³ and the ratio at 0.40 for both mixes. At 7 days the fly ash mix reads 18 MPa against the OPC control's 22 MPa, a 4 MPa gap. At 28 days it reads 30 against 32, a 2 MPa gap, which is the 5–10% drop at 28 days. At 90 days it reads 38 against 36, so the fly ash mix overtakes the control by 2 MPa.
Likely outcome. The 7-day cube fails against an OPC schedule and nothing is wrong with the concrete. Acceptance has to be written at 28 days, because that is where the pozzolanic reaction has taken you. If the programme needs the strength at 7 days, the blend is wrong, not the cubes.
What happens. IS 1489 (Part 1) caps OPC at 35% and PSC at 40%, and IS 456 Cl 5.2.1.1 states no percentage of its own. Higher replacement, up to 50%, is permitted for mass concrete with a documented trial mix and engineer approval. The raft is mass concrete. A 150 mm slab is neither, so the 40% figure moves onto a member it was never validated for, and on OPC it is above the 35% ceiling anyway.
Likely outcome. 28-day strength falls short and the slab cannot be stripped. The trial mix that justified 40% was run on a different element, with a different section and a different heat regime, so it says nothing about this one. The fix is a trial on the slab's own geometry, not a concession on the slab's own cube results.
What happens. LOI measures unburnt carbon, and IS 3812 Pt 2 caps it at 5%. High-LOI fly ash is a poison for PCE superplasticizers: the porous carbon adsorbs the polymer, the mix loses slump within 30 minutes, and the site's two usual responses are to overdose the admixture, which delays setting, or to add water, which drops strength. Overdosing delays the setting and the added water drops the strength, so the pour is slower and the concrete is weaker at the same time.
Likely outcome. A month of unpredictable slump, a strength loss nobody can trace, and a durability dividend spent on a w/c that was quietly raised at the mixer gate. The control is dull and cheap: a lot-wise LOI certificate on every delivery, a re-test on receipt, and a dosage record that changes when the carbon content changes.
Frequently Asked Questions
When does fly ash contribute to 28-day strength rather than only at later ages?
What is the difference between Class F and Class C fly ash?
Why is high LOI bad in fly ash?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For fly-ash mix-design review or LOI audit, contact amit@concreteinfo.in.
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