Fly Ash and Slag Cement in Concrete
US mix designs commonly replace part of the portland cement with a supplementary cementitious material (SCM) — fly ash or slag cement. This changes how the water ratio is expressed and named, and it interacts with ACI 318's durability limits.
What counts as an SCM
Fly ash, covered by ASTM C618 (Class F or Class C, distinguished mainly by calcium content and the source coal), and slag cement, covered by ASTM C989, are the two SCMs most commonly used in US ready-mix concrete. Both are byproducts — fly ash from coal combustion, slag cement from iron blast-furnace slag — that react with the products of cement hydration and contribute to strength gain and long-term durability. Silica fume is a third, less commonly encountered SCM, typically used in smaller percentages for high-strength or low-permeability mixes.
Why "cementitious," not just "cement"
When an SCM is part of the mix, the portland cement and the SCM together are called the cementitious material. Every ratio and limit that used to be expressed against cement alone — the water ratio, the minimum content requirement — is instead expressed against the total cementitious material: w/cm, water divided by (cement + SCM), rather than w/c. This is not a cosmetic relabeling: on a mix with a meaningful percentage of SCM, w/c and w/cm can differ enough to matter, so using the right term keeps the ratio meaningful. Because SCM use is so common in US ready-mix practice, "w/cm" is the standard term used throughout current ACI 211.1 and ACI 318 documents, and this site uses it the same way.
SCM percentage and exposure limits
The proportion of SCM in the total cementitious material is a mix design choice, generally expressed as a percentage of the total. Beyond the strength and durability benefits, ACI 318 places its own limits on how much of the cementitious material can be SCM for concrete assigned to the most severe exposure categories — notably F3, severe freeze-thaw exposure combined with exposure to deicing chemicals. These limits exist specifically because very high SCM replacement levels can slow early-age strength gain and affect the concrete's resistance to scaling under freeze-thaw and deicing cycles, and they are stricter for F3 than for milder exposure categories. The exact percentage limits are set in the current ACI 318 edition and are not something to estimate from memory — check the table that applies to your project's assigned exposure categories.
Frequently asked questions
Why does w/cm matter instead of w/c?
Once fly ash or slag is part of the cementitious material, the ratio that controls strength and durability is water divided by the total cementitious material (cement plus SCM), not water divided by portland cement alone. Using w/c instead of w/cm on an SCM mix understates the water-to-binder ratio that actually governs performance.
Do fly ash and slag replace cement one-for-one?
Not necessarily by mass in every mix design method — the replacement percentage is a mix design decision based on the SCM class, the strength and durability targets, and trial batch results, not a fixed universal ratio.
Are there limits on how much SCM I can use?
Yes. ACI 318 places limits on the SCM proportion of total cementitious material for concrete assigned to the more severe exposure categories, such as F3 (severe freeze-thaw exposure with deicing chemicals). These limits are stricter for F3 than for milder categories — check the current ACI 318 edition for the exact percentages that apply to your project.