Pozzolan Replacement Calculator
Calculate how much fly ash, slag or another approved supplementary cementitious material replaces cement at a chosen percentage. Choose mass replacement for a straightforward binder split or volume replacement when density differences must be preserved.
Set the replacement
Replacement mix
Mass replacement versus volume replacement
In a simple mass replacement, an SCM percentage is applied to the original total binder mass. For example, 20% replacement of 400 kg total cementitious binder gives 80 kg SCM and 320 kg cement.
But materials can have different specific gravities. A volume-based replacement can therefore require a different SCM mass to replace the same absolute volume.
Volume replacement: preserve cement volume replaced by SCM volume
Worked mass-replacement example
For 320 kg original cement at 20% mass replacement, the total binder basis is treated as 320 kg before replacement:
Remaining cement = 320 − 64 = 256 kg
The combined cementitious mass remains 320 kg in this simple mass-replacement scenario.
Why SCM selection matters
Fly ash, slag, natural pozzolans and other supplementary cementitious materials are not interchangeable. Their chemistry, fineness, reactivity and classification can influence setting, strength development, workability and durability.
ASTM's supplementary-cementitious-material work covers multiple material classes and emphasizes evaluation of performance rather than assuming one material behaves like another.
Technical context: ASTM Subcommittee C09.24.
Replacement percentage is not a recommendation
This calculator answers the arithmetic question “what quantity corresponds to this percentage?” It does not answer “what percentage should this project use?”
The suitable replacement level depends on the approved mix, material qualification, performance requirements, early-age behavior, exposure and project specifications.
How density changes volume replacement
If cement has specific gravity 3.15 and an SCM has specific gravity 2.30, equal masses do not occupy equal absolute volumes. This can matter when the mixture is being reconciled by absolute-volume proportioning.
Use measured or specified density data for the actual materials rather than generic defaults for final design work.
Common mistakes
Calling 20% mass replacement equal to 20% volume replacement: density differences can make them different.
Changing water arbitrarily: SCM replacement does not create a universal water adjustment rule.
Using the calculator as a performance prediction: replacement quantity is not a strength prediction.
Ignoring material qualification: use approved SCM data.
Limitations
This tool performs binder replacement arithmetic. It does not validate SCM chemistry, strength development, durability, workability or project compliance.
Frequently Asked Questions
How do I calculate a 20% cement replacement?
On a mass basis, multiply the original binder basis by 20% to obtain SCM mass.
Can replacement be based on volume?
Yes, if the mixture method calls for absolute-volume equivalence and appropriate specific gravities are available.
What materials can be replaced?
Approved supplementary cementitious materials such as fly ash, slag or other qualifying materials.
Does replacement change w/c?
It can change w/c and w/cm differently depending on the new binder split.
Does this calculator recommend a replacement level?
No. The project mixture and material qualification determine the appropriate level.