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Fly Ash / Silica Fume SCM Replacement Calculator

Split a concrete mix's cementitious content by mass into portland cement and an SCM, fly ash or silica fume, at a target replacement percentage.

  • Answers as you type
  • Every formula cited
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The total cement plus SCM, as mass per unit volume of concrete.

This is the mix design's total binder content, quoted per cubic metre or per cubic yard of concrete rather than as a plain weight — check your mix design sheet or the Concrete Mix Ratio Calculator.

The type of SCM being used.

Fly ash and silica fume have different typical replacement ranges and effects on the mix.

The percentage of total cementitious mass to replace with the SCM.

Stay within the typical range for your chosen SCM type unless your mix design engineer specifies otherwise.

Portland cement required

17.48 pcf

High confidence
SCM mass required
4.37 pcf
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Supplementary cementitious materials commonly replace 15-30% of portland cement by mass for fly ash (ACI 232.2R) and 5-10% for silica fume (ACI 234R), reducing embodied carbon and, at these replacement levels, generally improving long-term strength and durability

Inputs used

Total Cementitious Material Content
589.94 lb/yd³
SCM Type
Fly Ash (Class F, typical 15-30% replacement)
Replacement Percentage
20

Intermediate steps

SCM mass required
4.37 pcf
Final result17.48 pcf

What this calculation does not cover

  • This is a mass split and nothing else. It divides the binder you type at the percentage you type; it does not check that the resulting concrete meets its specified strength, water-cementitious ratio, air content or slump. Replacing cement changes water demand and admixture dosage, and none of that is recalculated here.
  • It does not check your project specification. Specs, exposure classes and durability provisions routinely cap how much fly ash or silica fume is permitted, and some structural and cold-weather applications restrict it further. The confidence note only compares your percentage against a published typical band, never against the job you are batching for.
  • Nothing about the SCM material itself is modelled: fly ash class and loss on ignition, whether silica fume arrives densified or as slurry, alkali and sulfate content, or batch-to-batch variability. It assumes a conforming, tested material (ASTM C618 for fly ash, C1240 for silica fume) and treats every unit of SCM as the same mass of cement removed.
  • Early strength gain is not adjusted. Fly ash mixes build strength more slowly at early ages, and this calculation gives you no revised age for formwork striking, post-tensioning, sawcutting or opening to traffic. Those come from trial mixes or in-place maturity testing, not from a mass split.
  • One SCM at a time, and no carbon number. Ternary blends of cement plus fly ash plus silica fume are not computed in a single pass, and the calculator reports no embodied-carbon saving — the mass split alone does not produce a kgCO2e figure.

Add the equipment this sizes

This result is a specification — 17.48 pcf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.1.1

Regulatory standards & verification citations1
  1. Supplementary cementitious materials commonly replace 15-30% of portland cement by mass for fly ash (ACI 232.2R) and 5-10% for silica fume (ACI 234R), reducing embodied carbon and, at these replacement levels, generally improving long-term strength and durability

Which documents these citations point at

Standards referenced: ACI 232.2R, ACI 234R (American Concrete Institute, United States).

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for.

Called something else where you work? Cement vs concrete — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Lower-Carbon Concrete vs Changing the Material — the factors that actually differ, with no invented prices.

How to calculate fly ash / silica fume SCM replacement in 4 steps

  1. Total Cementitious Material ContentThe total cement plus SCM, as mass per unit volume of concrete.
  2. SCM TypeThe type of SCM being used.
  3. Replacement PercentageThe percentage of total cementitious mass to replace with the SCM.
  4. Portland cement requiredThe tool computes the portland cement required from those figures and shows the formula, its sources, and a confidence rating alongside it.

Portland cement required by total cementitious material content

Page defaults, not your figures above.

Total Cementitious Material ContentPortland cement required (pcf)
400 lb/yd³11.9
600 lb/yd³17.8
800 lb/yd³23.7
1,000 lb/yd³29.6

Frequently asked questions

Why replace portland cement with an SCM?
Portland cement production is a major source of embodied carbon; fly ash and silica fume are industrial byproducts that reduce cement demand while typically improving long-term strength, reducing permeability, and often improving workability.
Does using an SCM slow down strength gain?
Usually yes for fly ash, especially at early ages (the first 7-28 days) — the mix design and curing schedule should account for this, particularly on fast-track construction schedules.
Can I combine fly ash and silica fume in the same mix?
Yes, ternary blends (portland cement + fly ash + silica fume) are common in high-performance concrete — this calculator handles one SCM type at a time; run it twice and subtract for a ternary blend.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.