fly ash Class F vs Class C
Fly Ash Class F vs Class C: Which One Does Your Concrete Mix Need?
Class F vs Class C fly ash under ASTM C618: chemistry, pozzolanic vs cementitious behaviour, typical uses and storage tips for Mindanao batching plants.
"Fly ash" on a purchase order is not a specification. ASTM C618 splits coal fly ash into Class F and Class C, and the two behave differently in the mixer, in the silo and in the finished structure. This guide explains the difference in plain terms so a batching plant or contractor in Region XII can order the right class for the right job, and know which documents to ask a source for.
What ASTM C618 actually says
ASTM C618 covers coal fly ash and raw or calcined natural pozzolan for use in concrete. AASHTO M 295 is the closely related highway specification; the two are aligned but not identical in every limit. C618 defines three classes:
- Class N, natural pozzolans such as volcanic ash and calcined clay.
- Class F, fly ash that is mainly pozzolanic, traditionally from anthracite or bituminous coal and low in calcium.
- Class C, fly ash that is pozzolanic and cementitious, traditionally from lignite or sub-bituminous coal and high in calcium.
The dividing line is chemistry, and it moved. Editions of C618 before 2019 set the class by the sum of silica, alumina and iron oxide (SiO2 + Al2O3 + Fe2O3): at least 70% for Class F and at least 50% for Class C. From the 2019 revision, that sum must be at least 50% for both classes and the class is set by calcium oxide: Class F has 18% CaO or less, Class C has more than 18%. Check which edition your project specification cites, and ask the source whether its certificate of analysis reports both the oxide sum and the CaO, so the class can be checked either way. FHWA's guidance describes the same divide in practical terms: Class F ashes typically contain less than 10% CaO, Class C ashes more than 20%.
Both classes must meet the same physical and chemical limits. The ones that matter most on a certificate of analysis, as set in ASTM C618:
| Requirement | Limit in ASTM C618 |
|---|---|
| Sulfur trioxide (SO3) | max 5.0% |
| Moisture content | max 3.0% |
| Loss on ignition (LOI) | max 6.0% (AASHTO M 295 is stricter at 5.0%) |
| Fineness, retained on the 45 µm (No. 325) sieve | max 34% |
| Strength activity index at 7 or 28 days | min 75% of control |
| Water requirement | max 105% of control |
| Autoclave expansion or contraction | max 0.8% |
Two of these deserve a buyer's attention. LOI is a measure of unburned carbon: FHWA notes that the higher the carbon content, the harder it is to control the air content of the concrete. Fineness drives reactivity: coarse ash reacts slowly and contributes less strength.
Pozzolanic vs cementitious: why the chemistry matters
When portland cement hydrates it produces calcium silicate hydrate (the "glue") and calcium hydroxide (a weak, soluble by-product). A pozzolan does nothing on its own; it needs that calcium hydroxide. Class F fly ash reacts with it slowly, converting a weak by-product into more binding gel, giving a denser concrete that keeps gaining strength for months.
A cementitious material sets by itself when mixed with water. Class C carries enough reactive lime to do this, which is why wet Class C hardens in the bag.
That single difference explains most of the practical behaviour, and FHWA's summary of fly ash in concrete lists the same effects:
- Early strength. Fly ash mixes typically give lower strength at early ages; Class F, which relies entirely on the pozzolanic reaction, slows early strength the most. Mixes designed for equivalent early strength ultimately exceed straight-cement mixes.
- Heat of hydration. Replacing cement with fly ash reduces the heat of hydration, which is why Class F is the usual choice for mass pours.
- Water demand. At about 20% of the cementitious material, fly ash reduces water demand by roughly 10%.
- Durability. Fly ash improves sulfate resistance and resistance to alkali–silica reaction (ASR). Class F is the standard choice for both; ACI 232.2R reports that Class C performance on sulfate resistance is variable and must be proven by testing.
- Setting time. Fly ash usually extends setting time, but some Class C ashes set quickly, so trial batches with the actual cement and admixtures are essential.
Class F: where it fits
Class F is the default for durability-driven work:
- Mass concrete such as bridge piers, thick foundations and spillway sections, where heat must be controlled.
- Marine and coastal structures exposed to seawater, such as port and fish-landing facilities around Sarangani Bay.
- Concrete in sulfate-bearing soils or groundwater.
- Mixes using aggregates suspected of ASR, with the dose confirmed by testing the actual combination of cementitious materials and aggregate under ASTM C1567.
FHWA gives typical replacement rates of 15 to 30% of the portland cement, with higher percentages used for mass concrete. High-volume fly ash mixes go further under a properly designed and tested mix (see ACI 232.2R).
Class C: where it fits
Class C is chosen when early strength or self-cementing behaviour is the goal:
- Slabs on grade, pavements and precast elements on a fast-track schedule.
- Flowable fill, also called controlled low-strength material (CLSM), for utility trenches and backfill. FHWA notes that with high-CaO Class C ashes no cement may be required.
- Stabilized base courses and soil stabilization, where FHWA describes Class C as usable as a stand-alone stabilizer, while Class F needs lime or cement blended with it.
- General structural concrete where trial mixes have confirmed setting time and strength.
Because it sets on its own, Class C is less forgiving of moisture in storage and of long delays between mixing and placing.
Side-by-side comparison
| Property | Class F | Class C |
|---|---|---|
| Reaction type | Pozzolanic | Pozzolanic + cementitious |
| Calcium oxide (ASTM C618, 2019 onward) | 18% or less | More than 18% |
| Early strength | Slower | Faster |
| Long-term strength | Continues to gain | Moderate gain |
| Heat of hydration | Lower | Reduced less |
| Sulfate resistance | Generally improves | Variable; test |
| ASR mitigation | Effective at an adequate, tested dose | Variable; test |
| Sensitivity to moisture in storage | Cakes and lumps | Hardens; can set solid |
| Typical dose | 15–30% of cementitious, higher in mass concrete | Similar; confirm by trial batches |
Choosing the class for typical Region XII jobs
Fly ash comes from coal-fired power plants, and the class depends on the coal being burned at the time. A plant that changes coal supply can produce ash that moves across the Class F/Class C boundary, so ask for current test data covering the lot you are buying rather than last year's, and settle with the source what it can issue before you commit.
Practical matches for work around General Santos City, Koronadal, Kidapawan and Tacurong:
- Bridge substructure or large footing pour on a hot afternoon: Class F to reduce peak temperature and thermal cracking risk.
- Wharf or seawall along Sarangani Bay: Class F for chloride and sulfate exposure, with a low water–cementitious ratio.
- Warehouse floor slab that must carry traffic within a week: Class C, or a low dose of Class F proven in trial batches.
- DPWH road project: confirm which mineral admixtures the contract's DPWH items allow and which specification they cite (ASTM C618 or AASHTO M 295). The mix design still needs approval with the specific ash.
Whatever the choice, run trial batches with the actual cement, ash, admixtures and aggregates the plant will use; FHWA recommends performance evaluation with local materials. Two ashes with similar chemistry can still set and gain strength differently.
Storage and moisture control
Fly ash must stay dry: ASTM C618 limits moisture to 3.0%, and wet Class C sets solid. Use a dedicated, labelled silo that is never cross-filled with cement, or covered pallet storage for bags, and read the full guide on fly ash storage, handling and moisture control before the first delivery arrives.
How Stone Depot Trading helps
Stone Depot Trading coordinates B2B sourcing and supply of fly ash and other bulk materials through trade partner companies; it does not run a power plant or a laboratory Tell us the class your specification cites and the reports or tests it requires. What a source can supply with the ash depends on the material, the source, the specification and the testing arrangement, so class, availability and the documents actually obtainable are put to the partner and reported back before a quotation is issued rather than promised in advance. A representative requirement is described under fly ash for an industrial facility, and the inquiry format is in how to request a materials quote.
State the intended use, the class and the specification cited, any project-specific limit such as maximum LOI, the quantity in metric tonnes, bulk or bagged delivery, the delivery location and dates, and the documents your specification requires.
FAQ
Can Class C be substituted for Class F on a project that specifies Class F?
Not without the engineer's approval. The specification usually calls for Class F because of heat, sulfate or ASR concerns, and Class C does not reliably solve those. Expect to submit test data.
Does fly ash reduce concrete strength?
It reduces early strength, especially Class F at higher doses. FHWA notes that fly ash mixes designed for equivalent early strength ultimately exceed straight-cement mixes; strength is a mix-design question, not a fly ash yes/no question.
What documents should I ask for with a fly ash delivery?
Buyers normally ask for a certificate of analysis reporting the ASTM C618 chemical and physical results for the lot, together with the source plant, the sampling date and the test methods. Whether a given source issues one, and what it covers, varies with the plant, the lot and the testing arrangement. State what your specification requires and it is put to the source and confirmed before an order is accepted.
What happens if fly ash gets wet in storage?
Class F cakes into lumps that will not disperse in the mixer; Class C can harden into a solid mass. Either way the affected material should not go into concrete.
Request a quote
Tell us the class, specification, quantity, delivery location and schedule your project needs through the quote form, by email to cjandulana@stonedepotph.com, or by call or Viber to +63 945 643 5233. Submitting an inquiry starts a sourcing conversation; it does not create a binding order.
Sources
- ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete — ASTM International (https://store.astm.org/c0618-19.html)
- Fly Ash Facts for Highway Engineers (FHWA-IF-03-019), Chapter 1: Fly Ash, An Engineering Material — Federal Highway Administration, U.S. Department of Transportation (https://www.fhwa.dot.gov/pavement/recycling/fach01.cfm)
- Fly Ash Facts for Highway Engineers (FHWA-IF-03-019), Chapter 3: Fly Ash in Portland Cement Concrete — Federal Highway Administration (https://www.fhwa.dot.gov/pavement/recycling/fach03.cfm)
- Fly Ash Facts for Highway Engineers (FHWA-IF-03-019), Chapter 4: Fly Ash in Stabilized Base Course — Federal Highway Administration (https://www.fhwa.dot.gov/pavement/recycling/fach04.cfm)
- Fly Ash Facts for Highway Engineers (FHWA-IF-03-019), Chapter 5: Fly Ash in Flowable Fill — Federal Highway Administration (https://www.fhwa.dot.gov/pavement/recycling/fach05.cfm)
- ASTM C1567, Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method) — ASTM International (https://store.astm.org/c1567-23.html)
- ACI 232.2R, Report on the Use of Fly Ash in Concrete — American Concrete Institute
Disclaimer
This article is general information. Final suitability of any material must be confirmed against the buyer's project specifications, the approved mix design, laboratory test results and the project engineer's requirements. Stone Depot Trading coordinates sourcing and supply; it does not certify materials or guarantee acceptance by any agency.