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Fly Ash / Silica Fume SCM Replacement Calculator

Split a concrete mix's cementitious material into portland cement and a supplementary cementitious material (SCM) by mass, at a target replacement percentage.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.1.0

Market
Imperial · sales tax

Portland cement required

17.48 pcf

High confidence
SCM mass required
4.37 pcf

Running these inputs gives 280 kg per m³ as the portland cement required. Scm mass required carries the most weight in this calculation, at 70 kg per m³. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

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.

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 — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 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 while can improving long-term strength and durability

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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.

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.