Clause 5 extends the mix design procedure of Clause 4 to incorporate mineral admixtures such as fly ash (pulverised fuel ash conforming to IS 3812 Part 1), ground granulated blast-furnace slag (GGBS conforming to IS 12089), silica fume (IS 15388), and metakaolin. The key concept is the efficiency factor (k), which converts the mass of mineral admixture to an equivalent cement mass. The total cementitious content replaces cement in the mix design calculations, with the effective binder = cement + k × admixture.
Key Requirements
•Fly ash: maximum replacement level 35% of total cementitious material, efficiency factor k = 0.7
•GGBS: maximum replacement level 65% of total cementitious material, efficiency factor k = 0.7
•Silica fume: maximum replacement level 10% of total cementitious material, efficiency factor k = 2.0
•Metakaolin: maximum replacement level 15% of total cementitious material, efficiency factor k = 2.0 (when available)
•Minimum OPC content shall not fall below 160 kg/m³ when fly ash is used (IS 456 provision)
•Total cementitious material shall meet the minimum cement content requirements of IS 456 Table 5
•W/C ratio becomes water/(cement + k × admixture) for strength calculation
•Maximum total cementitious content should not exceed 500 kg/m³
Reference Tables
Efficiency Factors (k-values) for Mineral Admixtures (Clause 5)
Mineral Admixture
Conforming IS
Max Replacement (%)
Efficiency Factor (k)
Typical Dosage (%)
Fly ash (PFA)
IS 3812 (Part 1)
35
0.7
20-35
GGBS
IS 12089
65
0.7
30-50
Silica fume
IS 15388
10
2
5-10
Metakaolin
—
15
2
8-12
k-values are as per IS 10262:2019. Higher k means more efficient as a binder. Silica fume is twice as effective as OPC per unit mass.
Example — M30 Mix with 25% Fly Ash Replacement
Parameter
Without Fly Ash
With 25% Fly Ash
fck
30 MPa
30 MPa
f'ck = fck + 1.65 × 5.0
38.25 MPa
38.25 MPa
W/C ratio
0.45
0.45 (W/effective binder)
Water content (20 mm MSA)
186 kg/m³
186 kg/m³
Total cementitious content
413 kg/m³ (OPC only)
413 kg/m³ total
OPC content
413 kg/m³
310 kg/m³ (75%)
Fly ash content
—
103 kg/m³ (25%)
Effective binder
413 kg/m³
310 + 0.7 × 103 = 382 kg/m³
Effective W/B ratio
0.45
186/382 = 0.487
Check: min OPC 160 kg/m³
—
310 > 160 ✓
With k = 0.7, 103 kg of fly ash is equivalent to only 72 kg of OPC. The effective W/B ratio is higher, so additional total cementitious material may be needed to meet the target strength.
Formulas
Effective binder = C + k × P
Effective cementitious content with mineral admixture
C = Mass of OPC in kg/m³k = Efficiency factor of the mineral admixtureP = Mass of mineral admixture in kg/m³
W/(C + k × P) ≤ max W/C from IS 456
Effective water-binder ratio check for durability
W = Free water content in kg/m³C = Mass of OPC in kg/m³k = Efficiency factorP = Mass of mineral admixture in kg/m³
P = replacement% × (C + P) / 100
Mass of mineral admixture based on percentage replacement of total cementitious material
P = Mass of mineral admixture in kg/m³replacement% = Percentage replacement of total cementitious materialC + P = Total cementitious content in kg/m³
Practical Notes
✓Fly ash at 20-30% replacement is the most common mineral admixture in Indian concrete. It reduces heat of hydration (critical for mass concrete), improves long-term strength, and reduces cost by 5-10%.
✓GGBS at 50% replacement is standard for marine structures and underground works — it provides excellent sulphate resistance and low permeability.
✓Silica fume at 5-8% is used for M50+ grades and special applications (water tanks, nuclear structures). It dramatically reduces permeability but needs careful handling (increased water demand, sticky mix).
✓The k-value approach is conservative — in practice, 28-day strength with fly ash at k = 0.7 is close to OPC-only mixes, and 56/90-day strength often exceeds them. Many engineers use k = 0.8-0.9 based on local data.
✓PPC (IS 1489 Part 1) already contains 15-35% fly ash. Do not add fly ash on top of PPC and count both — the total fly ash should not exceed 35% of total cementitious material.
Common Mistakes
⚠Exceeding 35% fly ash or 65% GGBS replacement, which can delay setting and reduce early strength significantly.
⚠Applying fly ash efficiency factor to PPC which already contains fly ash — this results in double-counting.
⚠Ignoring the minimum OPC content of 160 kg/m³ when using fly ash — going below this compromises early strength and setting.
⚠Not adjusting superplasticizer dosage for fly ash mixes — fly ash reduces water demand, so admixture dosage may need reduction.
⚠Using silica fume without adequate mixing time and equipment — it needs high-shear mixing for proper dispersion.