IS 15388 : 2003Silica Fume - Specification

IS 15388 is the specification for silica fume — an ultra-fine pozzolanic material (~100× finer than cement) used as a supplementary cementitious material (SCM) to enhance concrete properties. Silica fume is a by-product of silicon / ferrosilicon manufacture, captured from furnace exhaust. Despite cost premium, silica fume enables high-strength + high-durability concrete that conventional OPC + aggregate alone cannot achieve.

Use IS 15388 silica fume in: - High-strength concrete (M60-M100+) for tall buildings, columns, prestressed beams - High-performance concrete for marine / aggressive exposure - Self-compacting concrete (SCC) — silica fume improves rheology + cohesion - Repair / strengthening overlays (high bond, low permeability) - Underwater concreting (low bleed, anti-washout) - Concrete with reactive aggregates (silica fume mitigates AAR) - Bridge decks (durability + reduced permeability) - Industrial floors with chemical exposure - Hydraulic structures (tunnels, dams, water-retaining)

Silica fume enhances concrete via two mechanisms: 1. Pozzolanic reaction — reacts with calcium hydroxide from cement hydration to form additional CSH gel 2. Filler effect — ultra-fine particles fill gaps between cement grains; reduces porosity

Dosage typically 5-15 % by mass of cement (Portland cement reduced by similar amount). Higher doses cause excessive water demand.

IS 15388:2003 acceptance criteria:

| Property | Acceptable value | |---|---| | SiO₂ content (min) | 85 % | | Specific surface (min) | 15,000 m²/kg | | Particle size | 95 % < 1 µm; 50 % ~ 0.15 µm | | Loss on ignition (max) | 6.0 % | | Pozzolanic activity index at 7 days (min, vs control mortar) | 105 % | | Available alkalies (Na₂O eq, max) | 1.5 % | | Density | 2.20-2.40 g/cm³ | | Bulk density (loose) | 200-300 kg/m³ (very low — like flour) |

Form: - Densified silica fume — bulk density 500-700 kg/m³; pellet-like, ships and handles efficiently - Slurry silica fume — pre-mixed with water at 50 % solids; ready-to-use; popular for RMC plants - Undensified powder — original form, very low bulk density; rarely used (handling difficult)

Mix design impact (8-10 % silica fume by mass of cement):

| Property | Without SF | With 8-10 % SF | |---|---|---| | 28-d compressive strength | M40 | M55-M80 (higher) | | Water demand (constant slump) | baseline | +5-10 % (need superplasticizer) | | Permeability (chloride) | 2000-4000 coulombs (RCPT) | < 1000 coulombs (very low — durable) | | Bleed water | normal | minimal (no bleed) | | Cohesion / mix appearance | normal | sticky, cohesive | | Setting time | normal | similar (slight extension) | | Heat of hydration | normal | similar |

Cost: - Densified silica fume: ₹70-120 per kg (much higher than OPC at ₹8-9 per kg) - 8 % silica fume in M60 concrete adds ~₹3000-4000/m³ to concrete cost - Used judiciously where benefits justify cost — high-strength columns, marine exposure, etc.

Trial mix essential: - Silica fume's effect highly mix-dependent - Fixed dose for similar mix may not work for new aggregates / cement source - Trial mix at 5%, 8%, 10% to find optimum + verify performance

• IS 456:2000 — RCC code (durability provisions; SCM use addressed).
• IS 10262:2019 — concrete mix design (silica fume input methodology).
• IS 9103:1999 — admixtures (HRWR essential for silica-fume mix).
• IS 8112:1989 / IS 12269:2013 — cement standards (paired with silica fume).
• IS 1489 Part 1:2015 — PPC fly-ash (alternative SCM; silica fume often used along with PPC for ultra-high performance).
• IS 383:2016 — aggregates.
• IS 516 Part 1:2021 — compressive strength testing.
• IS 5816:1999 — split tensile strength.
• IS 3812 (Part 1 / 2) — fly ash specification.
• IS 12089 — granulated slag (for cement grinding).
• ASTM C1240 — international counterpart for silica fume.
• ACI 234R — guide for use of silica fume in concrete (international reference).
• IS 16700:2017 — tall building design (frequent user of silica-fume concrete).
• IRC:112:2020 — concrete bridge code (high-durability mixes).
• IS 13311 Part 1:1992 — UPV / rebound test (in-situ verification).
• IS 2911 Parts 1-4 — pile foundations (high-strength piles use silica fume).

1. Specifying silica fume without trial mix verification. Each cement + aggregate combination behaves differently; trial mix essential. 2. Inadequate superplasticizer. Silica fume increases water demand significantly; without HRWR, slump drops to zero. Use Type F or G IS 9103:1999 at 0.8-1.5 % cement mass. 3. Mixing without prolonged time. Silica fume agglomerates need to disperse; mix at high speed for ≥ 5 minutes or use slurry form. 4. Insufficient cure. Silica fume dramatically reduces permeability — but only if cured properly. 28-day moist cure essential. 5. Powder form spilled / lost. Bulk density 200-300 kg/m³; powder escapes via plant exhaust; loss + worker exposure. Use densified form or slurry; install dust-control filters. 6. Worker exposure to silica dust. Silica is hazardous (silicosis if inhaled). Mandatory respiratory PPE + dust extraction. 7. Wrong silica fume product (not BIS-licensed). Local 'silica fume' may be ground sand (no pozzolanic activity). Demand IS 15388 + ISI mark. 8. Concrete laid in hot weather without retarder. Silica-fume mix's early-age set + low bleed makes it sensitive to plastic shrinkage cracking. Add retarder; mist surface. 9. Plastic shrinkage cracks (no bleed water). Conventional concrete bleeds; silica-fume doesn't; surface evaporation > water rise; cracking. Cover with wet hessian + fog spray immediately after lay-down. 10. Cost-effectiveness analysis missing. Silica fume adds ₹3000-4000/m³; design must justify (e.g., M60 vs M40 = 30 % less cement weight + smaller column section, may pay back through structure savings). 11. Old / damp silica fume. Hydrates in storage; loses pozzolanic activity. Use within 6 months; store dry. 12. Mixing silica fume in cement at supplier vs at RMC. Pre-blended with cement (composite cement) gives consistency; site-blending gives variability.

Concrete-grade hierarchy with SCMs:

| Grade range | Typical mix | |---|---| | M15-M30 | OPC alone or PPC fly-ash | | M30-M40 | OPC + admixture | | M40-M50 | OPC + admixture + (optional) fly ash | | M50-M70 | OPC + admixture + silica fume (5-10 %) + (optional) fly ash | | M70+ | OPC + admixture + silica fume (8-15 %) + (optional) fly ash + special aggregate | | Ultra-high (M100+) | Engineered: high-strength cement + silica fume + ultra-fine aggregate (UHPC) |

Project-side application:

1. Design — high-strength / high-durability / SCC / marine exposure → consider silica fume. 2. Mix design (IS 10262:2019): - Dose silica fume 5-15 % by cement mass - Reduce cement by similar mass - Increase HRWR to maintain workability - Trial mix at 3 doses to optimise 3. Performance verification: - 28-d strength target - Permeability (RCPT — Rapid Chloride Penetration Test) - Durability (chloride, sulphate exposure) 4. Procurement: - ISI marked, IS 15388 compliant - Densified or slurry form per supplier convenience - Source qualification certificate 5. Plant + transport: - Compatibility check with batching system - Weighing accuracy ±3 % (more critical than for cement) 6. Lay-down: - Hot-weather precautions (cooling water, retarder) - Curing immediately + continuously 7. Quality acceptance: - Strength tests per IS 516:2021 - Permeability test on cores (RCPT) - Durability monitoring over 1-2 years

Silica fume is a high-cost, high-leverage material. Properly used (in M60+ structural concrete or marine exposure), it pays back via reduced section size + extended service life. Improperly used (in routine M30 concrete), it adds cost without commensurate benefit.