IS 3812 Part 2 : 2023Pulverized fuel ash - Specification - Part 2: For use as admixture in cement mortar and concrete

IS 3812 (Part 2) specifies pulverised fuel ash (fly ash) for use as admixture in cement mortar and concrete — the supplementary cementitious material (SCM) generated as a by-product of coal-fired thermal power plants. Fly ash is one of the most widely used SCMs in Indian construction; addressed by IS 3812 in two parts.

Use IS 3812 Part 2 fly ash when: - Concrete mix design includes fly ash as Portland cement replacement (typical 15-30 % by mass) - Sustainability / carbon-footprint reduction targets (fly ash replaces cement → lower clinker → lower CO₂) - Mass concrete pours where heat reduction matters (fly ash slows hydration, reduces thermal cracking) - High-durability concrete for marine / aggressive environment - Cost optimisation (fly ash is cheaper than cement)

The two Parts of IS 3812: - Part 1: For use in cement, cement mortar and concrete — the pozzolan in PPC (IS 1489 Part 1:2015) - Part 2 (this code): For use as admixture in cement mortar and concrete — fly ash added directly at concrete batching plant (separately from cement)

The difference matters: - IS 3812 Part 1 fly ash is pre-blended with cement at cement plant → PPC - IS 3812 Part 2 fly ash is added to OPC concrete at the batching plant → on-site optimisation

Class C (high CaO) vs Class F (low CaO) terminology from ASTM is sometimes used in India alongside IS 3812 grade designations.

Fly ash's value in concrete: - Lower water demand (spherical particles improve workability) - Lower heat of hydration (slower cement reaction) - Higher long-term strength (pozzolanic gain over months) - Better durability (lower permeability, better chloride resistance) - Lower carbon footprint (cement displaced) - Cheaper than cement

IS 3812:2023 acceptance criteria (Part 2 fly ash for direct use):

| Property | Min/max | |---|---| | SiO₂ + Al₂O₃ + Fe₂O₃ (sum) | ≥ 70 % | | SiO₂ alone | ≥ 35 % | | Reactive SiO₂ | ≥ 20 % | | MgO (max) | 5.0 % | | SO₃ (max) | 3.0 % | | Cl (max) | 0.10 % | | Loss on ignition (max) | 5.0 % | | Total alkalis (Na₂O eq, max) | 1.5 % | | Specific gravity | 1.9-2.6 | | Fineness (passing 45 µm sieve, min) | 70 % | | Specific surface (Blaine, min) | 320 m²/kg | | Pozzolanic activity index at 28 days, min | 75 % | | Drying shrinkage of mortar | ≤ 0.15 % | | Soundness (autoclave, max) | 0.8 % |

Fly ash dose in concrete: - Routine concrete (M15-M30): 15-25 % cement replacement - Higher-grade concrete (M30-M50): 20-30 % - Mass concrete / dam: 30-40 % (low heat priority) - Marine / aggressive: 25-35 % (durability priority) - Pumped concrete: 15-20 % (workability)

Mix design implications: - Water-cement ratio: maintain or slightly reduce vs OPC alone - Total cementitious content: cement + fly ash combined ~ same as OPC alone (or slightly more for high-strength) - Workability: improved (less water for same slump) - Setting time: extended 30-60 minutes - Early strength (3-day): lower than OPC by 20-30 % - 7-day strength: lower by 10-15 % - 28-day strength: similar or slightly lower than OPC alone - 90-day strength: typically 5-15 % higher than OPC alone

Cost benefit: - Fly ash: ₹2-4 per kg (much cheaper than cement at ₹8-9 per kg) - 25 % fly ash in M30 mix: cement reduced by ~80 kg/m³, fly ash added ~80 kg/m³ - Net cost saving: ~₹400-500 per m³ - Carbon footprint reduction: ~80 kg CO₂ per m³

Source qualification: - Confirmed from coal-fired thermal power plant - Pre-classified electrostatic precipitator output (vs bottom-ash which is coarser) - Periodic test certificate from plant - Storage: dry; bagged for site use; bulk for RMC

• IS 3812 Part 1:2013 — for use in cement, cement mortar and concrete (pre-blended in PPC).
• IS 1489 Part 1:2015 — Portland-Pozzolana Cement (with IS 3812 fly ash).
• IS 1489 Part 2:2015 — PPC Calcined Clay (alternative pozzolan).
• IS 8112:1989 / IS 12269:2013 — OPC standards.
• IS 456:2000 — RCC code (durability provisions accommodate fly ash).
• IS 10262:2019 — concrete mix design (Annex on fly ash).
• IS 9103:1999 — admixtures (HRWR pairs with fly ash mix).
• IS 383:2016 — aggregates.
• IS 12894:2002 — fly ash-clay bricks (alternative use of fly ash).
• IS 15388:2003 — silica fume (often combined with fly ash for ultra-high performance).
• IS 455:2015 — Portland Slag Cement (alternative SCM cement).
• ASTM C618 — international counterpart.
• BS EN 450 (Part 1, 2) — European standard for fly ash in concrete.
• MoEFCC Notification on Fly Ash — mandates fly ash use in construction within 100 km of thermal power plants (encourages waste utilisation).
• IRC SP 70:2005 — cement-treated and recycled materials (fly ash for soil stabilisation).

1. Fly ash with high LOI (> 5 %). Indicates unburnt carbon; reduces pozzolanic activity + adsorbs admixture. Verify per IS 3812 acceptance. 2. Fly ash from petcoke / non-coal source. Not all 'fly ash' is coal-derived; petcoke ash has different chemistry. Source qualification mandatory. 3. Sub-classified bottom ash mistakenly used. Bottom ash is coarser, less reactive than fly ash. Verify by sieve test (≥ 70 % through 45 µm). 4. Wet fly ash storage. Hygroscopic; absorbs moisture; lump formation; reactivity loss. Store dry, indoors. 5. Inadequate trial mix verification. Different fly ash sources have different reactivity; same dose gives different result. Always trial mix per project. 6. Higher early-strength mix needed for fast turnaround. Fly ash slows early strength; not suitable for precast / fast-cycle. Use lower fly ash dose or no fly ash. 7. Hot weather + high fly ash + retarder. Set may be excessively delayed. Balance. 8. Fly ash in high-strength concrete (M70+). Beyond 25 % replacement, target strength may not be achievable. Trial verification. 9. Carbon footprint / sustainability claim without supplier EPD. For green building rating, need Environmental Product Declaration (EPD). Demand from supplier. 10. Fly ash in marine concrete without verification. Despite expected durability gain, verify by RCPT or IS 9013:1978 water permeability test on actual mix. 11. Mixing pre-blended PPC with separate fly ash. Compounding fly ash (PPC already has it) leads to over-replacement; strength drops. 12. Cure inadequate for fly ash mix. Pozzolanic reaction continues for weeks; without good curing, long-term strength gain lost. Extended cure (14-28 days) recommended.

Sustainable concrete options in India:

1. OPC + IS 3812 Part 2 fly ash on-site (this code): customisable; cost saving + sustainability 2. PPC fly ash (IS 1489 Part 1:2015): pre-blended; standardised; convenient 3. PSC (IS 455:2015): GGBS-based blended cement 4. Composite cements: multi-pozzolan blends 5. Combined fly ash + silica fume: ultra-high performance

Project workflow:

1. Sustainability target — carbon, cost, durability targets. 2. Mix design (IS 10262:2019) — include fly ash dose; trial verify. 3. Source qualification — fly ash supplier IS 3812 compliant; classified ESP fly ash. 4. Procurement — bulk for RMC; bagged for site. 5. Storage — dry, FIFO. 6. Plant calibration — fly ash silo, weighing system. 7. Production — fly ash dosed at batch as per design. 8. Quality control — strength tests; durability tests; certification of carbon savings. 9. Documentation — fly ash supply records for sustainability rating (GRIHA, IGBC, LEED).

Indian context: - 200+ million tonnes of fly ash generated annually from thermal power plants - Fly ash utilisation in concrete: 30-50 % of generated (rest landfilled) - MoEFCC Fly Ash Notification mandates fly ash use within 100 km of thermal plants (encourages utilisation) - Major Indian concrete producers (UltraTech, ACC, Ambuja) supply PPC with fly ash; on-site fly ash use grew with sustainability awareness

IS 3812 Part 2 enables flexible, cost-effective, sustainable concrete on Indian projects. With careful mix design + quality control, fly ash + OPC mix delivers concrete that's lower-carbon, lower-cost, and equal-or-better long-term performance vs OPC alone.