by Concrete mix design is no longer limited to cement, water and aggregates. Many modern concrete mixes use supplementary cementitious materials such as:
- fly ash;
- ground granulated blast-furnace slag or GGBS; and
- silica fume.
These materials can partially replace ordinary Portland cement and influence workability, strength development, heat generation, permeability, durability and chemical-admixture requirement.
However, there is no single replacement percentage that is correct for every concrete mix.
The appropriate proportion depends on:
- type and quality of the SCM;
- cement type;
- concrete grade;
- required early-age strength;
- later-age strength;
- exposure condition;
- durability requirement;
- water-cementitious material ratio;
- required workability;
- curing conditions;
- concrete temperature;
- chemical-admixture compatibility; and
- laboratory trial results.
IS 10262:2019 therefore needs to be used together with the applicable concrete, material and project specifications when SCMs are included in a mix.
For the complete mix-design sequence, see Concrete Mix Design Procedure as per IS 10262:2019.
Page Contents
What Are Supplementary Cementitious Materials?
Supplementary cementitious materials, commonly called SCMs, are materials used together with cement as part of the cementitious system of concrete.
Depending on the material, they may have:
- pozzolanic properties;
- latent hydraulic properties; or
- very high fineness and filler/reactive effects.
The three common SCMs discussed in this article are:
Fly Ash
GGBS
Silica Fume
These materials should not simply be added to an existing concrete mix without recalculating the complete cementitious and volume balance.
Applicable Indian Standards
For Indian concrete practice, the material itself should conform to the applicable Indian Standard.
Fly ash used as pozzolana for part replacement of cement is covered by IS 3812 (Part 1):2013. BIS currently lists this standard and describes its scope as pulverized fuel ash for use as pozzolana in cement, cement mortar and concrete. Bureau of Indian Standards
GGBS for use in cement, mortar and concrete is covered by IS 16714:2018. BIS lists this standard as reviewed in 2023.
Silica fume is covered by IS 15388:2003, which specifies its chemical and physical requirements for use in concrete and other hydraulic-cement systems. BIS was still issuing certification documentation against IS 15388:2003 in 2025. Bureau of Indian Standards
The applicable editions, amendments, project specifications and contract requirements should always be checked before final mix approval.
Difference Between Cement Content and Cementitious Material Content
This distinction is essential when SCMs are used.
Suppose:
Cement = 300 kg/m³
GGBS = 100 kg/m³
Then:
Cement content = 300 kg/m³
but:
Total cementitious material content = 400 kg/m³
Therefore:
Total Cementitious Material = Cement + SCMs
If two SCMs are used:
Total Cementitious Material = Cement + SCM 1 + SCM 2
For detailed calculation of the total binder quantity, see Cementitious Material Content Calculation in Concrete Mix Design.
What Does SCM Replacement Percentage Mean?
The replacement percentage should always be clearly defined.
For mix-design calculations, if an SCM is specified as a percentage of total cementitious material:
SCM Quantity = Total Cementitious Material × SCM Percentage
and:
Cement Quantity = Total Cementitious Material − SCM Quantity
Example
Suppose:
Total cementitious material = 400 kg/m³
GGBS = 30% of total cementitious material
Then:
GGBS
= 400 × 30/100
= 120 kg/m³
Cement:
= 400 − 120
= 280 kg/m³
Therefore:
| Material | Quantity |
|---|---|
| Cement | 280 kg/m³ |
| GGBS | 120 kg/m³ |
| Total Cementitious Material | 400 kg/m³ |
Always state whether a percentage is based on:
- cement mass;
- total cementitious-material mass; or
- another specified reference quantity.
Otherwise, the calculation can become ambiguous.
Fly Ash in Concrete Mix Design
Fly ash is a fine material produced from coal combustion and can be used as a pozzolanic component of the cementitious system when it meets the applicable material requirements.
Its effect on concrete depends strongly on the source and properties of the fly ash.
Fly ash may influence:
- fresh concrete workability;
- water demand;
- setting behaviour;
- early-age strength;
- later-age strength;
- heat development;
- permeability; and
- long-term durability.
Fly ash particles are often relatively smooth and spherical, which can improve particle movement and workability in some mixes.
However, this should not be interpreted as a guaranteed fixed reduction in water demand.
Actual performance should be established through trials.
Fly Ash Quantity Calculation
Suppose:
Total cementitious material = 420 kg/m³
Selected fly ash proportion = 25%
Then:
Fly ash:
= 420 × 0.25
= 105 kg/m³
Cement:
= 420 − 105
= 315 kg/m³
Therefore:
Cement = 315 kg/m³
Fly Ash = 105 kg/m³
Total Binder = 420 kg/m³
This is a calculation example only.
The selected 25% should not be treated as a universal recommendation.
GGBS in Concrete Mix Design
Ground granulated blast-furnace slag is obtained by grinding granulated blast-furnace slag to the required fineness.
IS 16714:2018 specifies GGBS for use in cement, mortar and concrete. Bureau of Indian Standards
GGBS may influence:
- workability;
- heat development;
- early strength;
- later-age strength;
- permeability;
- chloride resistance;
- sulphate-related performance; and
- colour and finishing characteristics.
Its actual effect depends on:
- slag properties;
- replacement level;
- cement characteristics;
- water-cementitious ratio;
- curing;
- temperature; and
- concrete age.
Higher replacement levels can make early-strength development more dependent on temperature and curing.
Therefore, required formwork-removal or early-loading strength should be checked during trials.
GGBS Quantity Calculation
Suppose:
Total cementitious material = 450 kg/m³
Adopted GGBS proportion = 40%
Then:
GGBS:
= 450 × 0.40
= 180 kg/m³
Cement:
= 450 − 180
= 270 kg/m³
Therefore:
| Material | Quantity |
|---|---|
| Cement | 270 kg/m³ |
| GGBS | 180 kg/m³ |
| Total Cementitious Material | 450 kg/m³ |
Again, 40% is only an example calculation.
The actual proportion must satisfy the applicable code, specification and performance requirements.
For a practical grade-specific example, see M60 Concrete Mix Design with OPC Cement and GGBS.
Silica Fume in Concrete Mix Design
Silica fume is an extremely fine reactive material.
Because of its very fine particle size and high surface area, even relatively small quantities can significantly affect fresh and hardened concrete behaviour.
Silica fume may increase:
- cohesiveness;
- paste viscosity;
- chemical-admixture demand; and
- sensitivity to proper mixing and curing.
When correctly designed and cured, it can also contribute to a dense cementitious matrix and reduced permeability.
However, silica-fume concrete should not simply be designed by replacing cement while leaving every other mix parameter unchanged.
The effects on:
- water demand;
- superplasticizer dosage;
- workability;
- finishing;
- curing; and
- total paste volume
must also be considered.
IS 15388:2003 remains the Indian specification for silica fume. Bureau of Indian Standards
Silica Fume Quantity Calculation
Suppose:
Total cementitious material = 450 kg/m³
Selected silica fume = 7%
Then:
Silica fume:
= 450 × 0.07
= 31.5 kg/m³
Cement:
= 450 − 31.5
= 418.5 kg/m³
Therefore:
Silica Fume = 31.5 kg/m³
Cement = 418.5 kg/m³
This example only demonstrates the calculation.
The final percentage should be selected from the applicable specification and verified through trials.
Using More Than One SCM
Concrete may also contain more than one SCM.
For example:
- cement + fly ash + silica fume;
- cement + GGBS + silica fume; or
- where permitted, cement + fly ash + GGBS.
The total percentage must be calculated carefully.
Suppose:
Total cementitious material = 450 kg/m³
Fly ash = 20%
Silica fume = 7%
Fly Ash
450 × 20%
= 90 kg/m³
Silica Fume
450 × 7%
= 31.5 kg/m³
Cement
= 450 − 90 − 31.5
= 328.5 kg/m³
Therefore:
| Material | Quantity |
|---|---|
| Cement | 328.5 kg/m³ |
| Fly Ash | 90 kg/m³ |
| Silica Fume | 31.5 kg/m³ |
| Total | 450 kg/m³ |
The arithmetic is simple.
The important part is confirming that the proposed combination is permitted by the applicable code and project specification.
Current IS 456 Checks for SCM Proportions
Current IS 456 provisions should be checked in addition to the mix-proportioning calculations.
Amendment No. 6 to IS 456:2000 introduced specific provisions for combinations of mineral admixtures. Among other requirements, it states that when GGBS alone is used as a mineral admixture, slag should not exceed 50% of total cementitious material, and when fly ash alone is used, fly ash should not exceed 35% of total cementitious material. It also addresses combined fly ash/GGBS systems and combinations involving ultrafine materials such as silica fume. Bureau of Indian Standards
For a combination of fly ash and GGBS with OPC, the amendment specifies a minimum OPC content of 55% and maximum fly ash content of 20%, subject to its other requirements. Bureau of Indian Standards
Where ultrafine materials such as silica fume are combined with fly ash/PPC or GGBS/PSC, separate combined limits apply. Therefore, a designer should not simply add independently selected percentages for several SCMs without performing the applicable IS 456 check. Bureau of Indian Standards
Important
Do not interpret a code maximum as the recommended optimum percentage.
For example:
Permitted maximum ≠ optimum mix-design percentage
The optimum proportion is the proportion that satisfies the complete project requirements through testing and trials.
How SCM Percentage Affects Water-Cementitious Ratio
When approved SCMs form part of the cementitious system, the relevant cementitious quantity must be used consistently when calculating the adopted water-to-cementitious-material ratio, subject to the applicable code provisions.
Suppose:
Cement = 300 kg/m³
GGBS = 100 kg/m³
Total cementitious material:
= 400 kg/m³
Free water:
= 160 kg/m³
Then:
Water-to-cementitious-material ratio:
= 160 ÷ 400
= 0.40
Do not calculate:
160 ÷ 300
= 0.533
and treat that as the same ratio.
The definitions and code basis being used must remain consistent throughout the design.
For more details, see Water-Cement Ratio in Concrete.
SCMs and Water Content
Adding an SCM can change the water requirement of a concrete mix.
However, the direction and magnitude of the change cannot be predicted from replacement percentage alone.
It depends on:
- particle shape;
- fineness;
- cement chemistry;
- aggregate characteristics;
- selected slump;
- chemical admixture;
- temperature; and
- total powder content.
Therefore, first determine the preliminary water content using the applicable procedure.
Then verify the actual water demand through trial mixes.
See Water Content Calculation in Concrete Mix Design.
SCMs and Superplasticizer Dosage
SCM changes can also change superplasticizer performance.
A dosage that performs well with an OPC-only mix may not provide the same:
- initial slump;
- water reduction;
- slump retention; or
- setting behaviour
after fly ash, GGBS or silica fume is introduced.
Silica fume in particular can significantly change the rheology of the mix because of its high fineness.
Therefore, chemical-admixture dosage should be established for the complete cementitious system, rather than copied from another mix.
See Superplasticizer in Concrete Mix Design.
Absolute Volume of SCMs
SCMs must also be included correctly in the absolute-volume calculation.
The general formula is:
Absolute Volume = Mass ÷ (Specific Gravity × 1000)
Each cementitious material should normally be calculated separately because its specific gravity may be different.
Example
Suppose:
Cement = 300 kg/m³
Specific gravity of cement = 3.15
Then:
Cement volume:
= 300 ÷ (3.15 × 1000)
= 0.0952 m³
Suppose:
GGBS = 100 kg/m³
Specific gravity of GGBS = 2.90
GGBS volume:
= 100 ÷ (2.90 × 1000)
= 0.0345 m³
Therefore, total cementitious volume:
= 0.0952 + 0.0345
= 0.1297 m³
Do not calculate the combined 400 kg using the cement specific gravity of 3.15.
For the complete calculation, see Absolute Volume Method for Concrete Mix Design.
Why Replacing Cement Changes Aggregate Quantity
Suppose cement is partially replaced with an SCM having a lower specific gravity.
For the same total cementitious mass, the SCM may occupy a larger absolute volume.
This means the volume left for aggregates may decrease.
Therefore, changing an SCM percentage can also change:
- fine aggregate quantity;
- coarse aggregate quantity;
- total paste volume; and
- concrete yield.
This is why the entire mix should be recalculated after changing the cementitious-material split.
Example of Volume Difference
Consider:
100 kg cement with SG = 3.15
Absolute volume:
= 100 ÷ 3150
= 0.03175 m³
Now consider:
100 kg GGBS with SG = 2.90
Absolute volume:
= 100 ÷ 2900
= 0.03448 m³
The masses are equal, but their occupied volumes are different.
Therefore, direct kilogram-for-kilogram replacement changes the internal volume balance.
SCMs and Fine/Coarse Aggregate Calculation
After calculating:
- cement volume;
- SCM volume;
- water volume;
- admixture volume; and
- air volume,
the remaining volume becomes the total aggregate volume.
That aggregate volume is then divided into:
- fine aggregate; and
- coarse aggregate.
Therefore, if the SCM system changes, the final aggregate masses should also be recalculated.
See Fine and Coarse Aggregate Proportioning in Concrete Mix Design.
Effect of Fly Ash on Concrete
Depending on the material and mix, fly ash may:
- improve workability;
- reduce bleeding in a suitably proportioned mix;
- modify setting time;
- reduce early heat development;
- reduce early-age strength relative to an otherwise comparable OPC mix;
- contribute to later-age strength; and
- improve certain durability characteristics.
These effects are not identical for every fly ash.
Representative material testing is therefore important.
Effect of GGBS on Concrete
GGBS may:
- improve workability;
- reduce heat development;
- contribute to later-age strength;
- reduce permeability;
- influence finishing and setting;
- improve resistance in some aggressive environments; and
- alter early-age strength development.
The effect becomes especially important when:
- replacement is high;
- concrete temperature is low; or
- early stripping strength is required.
Effect of Silica Fume on Concrete
Silica fume may:
- greatly increase cohesiveness;
- reduce bleeding;
- reduce permeability;
- contribute to high strength;
- increase paste stickiness;
- increase superplasticizer demand; and
- increase sensitivity to curing quality.
Very fine mineral admixtures can make concrete sticky and more difficult to finish if the complete mix is not properly proportioned.
Therefore, silica fume should be evaluated together with water content and chemical admixture dosage rather than in isolation.
SCMs and Early Strength
One common mistake is to compare only 28-day strength.
Some projects require strength at:
- 1 day;
- 3 days;
- 7 days;
- prestressing age;
- formwork removal; or
- loading age.
SCM content can influence strength development at these ages.
Therefore, the laboratory trial programme should follow the actual construction requirement.
SCMs and Later-Age Strength
Concrete containing suitable SCMs may continue developing significant strength beyond 28 days.
Where the project specification permits evaluation at later ages, additional tests such as:
- 56-day; or
- 90-day
strength may provide useful information.
However, the specified acceptance age should not be changed merely because an SCM has been used.
Follow the approved specification.
SCMs and Heat of Hydration
SCMs are often considered for large concrete sections where temperature rise is important.
Examples include:
- raft foundations;
- thick pile caps;
- large retaining structures;
- dams;
- large machine foundations; and
- other mass-concrete elements.
The selection should be supported by the relevant thermal and structural requirements.
Do not select a very high SCM replacement only because “lower heat is better.”
Required early strength, curing and construction schedule must also be considered.
SCMs and Durability
SCMs can significantly affect transport properties and durability.
But durability cannot be judged from SCM percentage alone.
It also depends on:
- water-cementitious ratio;
- concrete cover;
- compaction;
- curing;
- cracking;
- material quality;
- permeability;
- exposure; and
- workmanship.
A concrete containing SCMs but having poor curing or excessive free water can still perform badly.
SCMs and Curing
Proper curing is particularly important for SCM-containing concrete.
Curing supports:
- hydration;
- pozzolanic reaction;
- strength development;
- surface quality; and
- durability.
A theoretically good SCM mix can underperform if curing is inadequate.
Therefore, curing requirements should be considered during mix approval and site execution.
SCMs and Aggregate Moisture Correction
SCM selection does not remove the need for aggregate moisture correction.
After the SSD-based design quantities have been established, actual site aggregate moisture should be measured.
Then correct:
- wet fine aggregate mass;
- wet coarse aggregate mass; and
- water added at the mixer.
See Moisture Correction in Concrete Mix Design.
Practical Step-by-Step SCM Selection Procedure
A practical sequence is:
Step 1 — Establish project requirements
Identify:
- grade;
- exposure;
- workability;
- strength ages;
- durability;
- heat restrictions;
- placement method; and
- construction programme.
Step 2 — Confirm SCM quality
Verify that fly ash, GGBS or silica fume complies with the applicable specification.
Step 3 — Determine preliminary total cementitious content
Calculate the total cementitious-material requirement.
Step 4 — Select a preliminary SCM percentage
Use:
- applicable code limits;
- project specification;
- previous experience;
- material characteristics; and
- trial programme.
Step 5 — Calculate SCM and cement masses
Use:
SCM Mass = Total Cementitious Material × SCM Fraction
and:
Cement Mass = Total Cementitious Material − Total SCM Mass
Step 6 — Check current code limits
Check both individual and combined SCM limits.
Step 7 — Recalculate specific-gravity volumes
Calculate cement and every SCM separately.
Step 8 — Calculate remaining aggregate volume
Complete the absolute-volume calculation.
Step 9 — Determine aggregate quantities
Apply the selected fine/coarse aggregate proportions.
Step 10 — Select preliminary chemical-admixture dosage
Use the manufacturer’s technical information and trial data.
Step 11 — Produce laboratory trial mixes
Check fresh and hardened concrete.
Step 12 — Adjust and recalculate
If water, SCM, cement or admixture is changed, update the complete volume balance.
Step 13 — Finalize the approved mix
Only after satisfactory trial results should the production mix be finalized.
Example of Complete SCM Split
Suppose preliminary design gives:
Total cementitious material = 420 kg/m³
Free water = 168 kg/m³
Adopted w/cm:
= 168 ÷ 420
= 0.40
Suppose a trial mix uses:
GGBS = 35%
Then:
GGBS:
= 420 × 0.35
= 147 kg/m³
Cement:
= 420 − 147
= 273 kg/m³
Final preliminary cementitious quantities:
| Material | Quantity |
|---|---|
| Cement | 273 kg/m³ |
| GGBS | 147 kg/m³ |
| Total Cementitious Material | 420 kg/m³ |
| Free Water | 168 kg/m³ |
| w/cm | 0.40 |
The next steps are to:
- calculate individual absolute volumes;
- calculate aggregate quantities;
- select admixture dosage;
- produce a trial mix; and
- verify performance.
The table does not mean that 35% GGBS is automatically appropriate for every project.
Common Mistakes When Using SCMs
Using a Universal Replacement Percentage
There is no universal rule such as:
“Always use 30% fly ash”
or:
“Always use 50% GGBS.”
Select the percentage for the actual project.
Confusing Cement With Total Cementitious Material
For example:
300 kg cement + 100 kg GGBS
does not mean cement content is 400 kg/m³.
Cement is 300 kg/m³.
Total cementitious material is 400 kg/m³.
Ignoring Current IS 456 Limits
A percentage that works mathematically may still not satisfy the applicable code.
Copying SCM Percentages From Another Grade
The same SCM percentage need not be optimum for M30, M40, M50 and M60 concrete.
Ignoring Early Strength
A mix may achieve excellent later strength but fail the required stripping or loading schedule.
Ignoring Specific Gravity
Changing SCM percentage changes the absolute-volume balance.
Using the Same Superplasticizer Dosage
Changing cementitious composition can change chemical-admixture demand.
Ignoring Curing
SCM-containing concrete requires effective curing to develop the intended performance.
Adding SCM Without Recalculating Aggregate
The aggregate quantities should be recalculated because the binder volume has changed.
Assuming Maximum Allowed Means Best
Code limits are not optimisation targets.
Quick Calculation Formulas
| Requirement | Formula |
|---|---|
| SCM mass | Total Cementitious Material × SCM Fraction |
| Cement mass | Total Cementitious Material − Total SCM Mass |
| SCM percentage | SCM Mass ÷ Total Cementitious Material × 100 |
| w/cm | Free Water ÷ Total Applicable Cementitious Material |
| SCM absolute volume | SCM Mass ÷ (SCM SG × 1000) |
| Cement absolute volume | Cement Mass ÷ (Cement SG × 1000) |
Recommended Mix-Design Sequence
For a concrete containing SCMs, the calculation sequence should normally be understood as:
Target Mean Strength
↓
Water-Cementitious Ratio
↓
Water Content
↓
Total Cementitious Material
↓
Select Fly Ash/GGBS/Silica Fume Proportion
↓
Calculate Cement + SCM Masses
↓
Check IS 456 / Project Limits
↓
Calculate Separate Absolute Volumes
↓
Calculate Total Aggregate Volume
↓
Fine/Coarse Aggregate Proportioning
↓
Select Chemical Admixture Dosage
↓
Moisture Correction
↓
Laboratory Trial Mix
↓
Adjust and Recalculate
↓
Final Approved Mix
Frequently Asked Questions
What is an SCM in concrete?
SCM means supplementary cementitious material. Common examples include fly ash, GGBS and silica fume.
Can fly ash replace cement in concrete?
Yes, suitable fly ash conforming to the applicable specification can be used as part replacement of cement, subject to code limits, project requirements and trial verification.
What is the maximum fly ash percentage?
Do not use a universal percentage. Current IS 456 provisions include limits depending on whether fly ash is used alone or together with other cementitious materials. For fly ash alone as a mineral admixture, Amendment No. 6 states a limit of 35% of total cementitious material. Bureau of Indian Standards
What is the maximum GGBS percentage?
Where GGBS alone is used as the mineral admixture, current IS 456 Amendment No. 6 states that slag should not exceed 50% of total cementitious material. Other combinations have separate requirements. Bureau of Indian Standards
Can fly ash and GGBS be used together?
Current IS 456 provisions permit specified combined use subject to conditions, including a minimum OPC proportion and limit on fly ash. The exact applicable requirements should be checked before selecting the mix. Bureau of Indian Standards
Can silica fume be used with fly ash or GGBS?
Yes, subject to the applicable combined limits, material specification and satisfactory trials.
Is silica fume the same as fly ash?
No. They differ substantially in source, particle characteristics, fineness and behaviour in concrete.
Does replacing cement with GGBS change aggregate quantity?
It can. If the specific gravities differ, the cementitious materials occupy different absolute volumes, changing the volume available for aggregates.
Should GGBS and cement use the same specific gravity?
No. Use representative specific gravity for each material.
Does SCM reduce the water requirement?
Sometimes, but not always. The effect depends on the SCM, fineness, particle shape, cement, aggregate, admixture and required workability.
Should the same superplasticizer dosage be used after adding SCM?
Not automatically. The complete binder-admixture combination should be verified through trials.
Is the calculated SCM percentage the final production percentage?
No. It is a preliminary selection that must satisfy applicable limits and laboratory or project trial requirements.
Conclusion
Fly ash, GGBS and silica fume can be important parts of modern concrete mix design, but they should not be treated as simple additions to an OPC mix.
The correct sequence is:
Determine Total Cementitious Material → Select SCM Type → Select Preliminary Percentage → Calculate Cement and SCM Quantities → Check Code Limits → Recalculate Absolute Volumes → Calculate Aggregate Quantities → Adjust Admixture → Conduct Trial Mix → Verify Strength and Workability → Finalize Mix
Remember these key points:
- there is no universal SCM percentage;
- distinguish cement content from total cementitious-material content;
- use material complying with the applicable Indian Standard;
- check current IS 456 limits;
- calculate each cementitious material separately in the absolute-volume method;
- reconsider water and chemical-admixture requirements;
- check early as well as later-age strength;
- ensure adequate curing; and
- finalize proportions only after satisfactory trials.
Continue the complete learning sequence through the Concrete Mix Design Hub.
You can also use the Concrete Mix Design Calculator as per IS 10262:2019 for preliminary calculation of cement, SCM, water and aggregate quantities.
Engineering Note: This article is intended for educational and preliminary concrete mix-design guidance. Final mix proportions should comply with the latest applicable Indian Standards and amendments, approved project specifications, representative material-test results and laboratory or field trial performance.
