IS FLY ASH
IN CONCRETE
GOOD OR BAD?
Neither, and the framing is the problem. Fly ash is a supplementary cementitious material, which means it changes how the finished concrete resists the mechanisms that make concrete fail, so the question with an answer is not whether fly ash is present but which class it is, at what replacement level, and whether anybody wrote that down on the day.
Fly ash is a variable, not a verdict. Asking whether it is good or bad is like asking whether an ingredient is good or bad without naming the dish.
A supplementary cementitious material is not filler and it is not a cost saving substitution, although it has been sold as both. The relevant materials change how a slab resists alkali silica reaction, sulfate attack and chloride ingress. So the honest answer is that the right fly ash at the right replacement level, specified against the exposure your slab actually faces, is a durability decision. The wrong one, or the right one swapped for a different one without the mix being redesigned, is also a durability decision. Which of those happened on your job is a question about paperwork.
Two industries sell this word, and they sell it in opposite directions.
If you search this question you will find flat answers in both directions, delivered with equal certainty. Both are describing something real and neither is describing your slab.
The version that says it is cheap filler. This one is usually told by someone selling you the alternative, and the tell is the word filler. A supplementary cementitious material is not filler and is not a cost saving substitution, although it has been sold as both. It participates in the chemistry of the finished concrete, which is the whole reason it changes durability behaviour at all.
The version that says it is a durability upgrade. This one is closer to the literature and it is still not a property of the word. It is a property of a specific class, at a specific replacement level, designed against a specific exposure. Stated as a blanket good, it quietly promises something no material can promise on its own.
What both versions leave out. Neither one tells you what was in your truck. That is not a rhetorical point. It is the difference between a discussion about concrete and a fact about your driveway, and only one of those is retrievable.
This is the pattern PSIP keeps running into. A question about a material gets answered as though materials had moral qualities, when the answerable version is always a question about what was specified, what arrived and whether it was recorded.
Class F and Class C are two materials. One word covers both.
Fly ash is supplied as Class F or Class C. They differ in chemistry, they differ in alkali silica reaction mitigation behaviour, and they are not interchangeable at the same replacement percentage. Almost every confident answer about fly ash is silently about one of them.
The low calcium ash.
This is the one most of the alkali silica reaction mitigation literature is anchored to. When someone tells you fly ash helps with ASR, this is usually the material they are describing, whether or not they say so.
The elevated calcium ash.
It behaves partly like cement itself, contributing early strength while delivering a different long term durability profile. It is a legitimate material. It is not the material the ASR literature is anchored to.
That is the entire reason this page exists. The risk is not that somebody chose the wrong material on purpose. It is that the material moved underneath a specification that nobody reopened.
Fly ash is a coal byproduct. The coal fleet has been changing for years.
Fly ash does not come from a fly ash factory. It is a byproduct of burning coal for electricity, which means its regional availability tracks decisions made by utilities and regulators rather than by the concrete industry. When generation changes in a region, what a local plant can source changes with it.
PSIP maintains a separate evidence page on that supply picture, because it is a subject with its own primary sources and it deserves more than a paragraph here. The relevant consequence for a homeowner is narrow and it is this: the answer to what is in the regional mix is not a constant, so a mix specified some years ago and reordered by habit is not guaranteed to be the same concrete today.
Four questions with no answer, and the four underneath them that do.
Each question on the right is after the same information as the one on the left, in a form that has a factual answer somebody can be held to.
Is fly ash good or bad?
Both answers exist online with equal confidence, and neither is about your concrete.
Which supplementary cementitious material is in this mix, and at what replacement level?
A mix design either states it or it does not, and you can ask before the truck is ordered.
Should I ask them to leave the fly ash out?
Removing a durability input to solve a worry is a change to the concrete, made for a reason that is not about the concrete.
What exposure class is this mix specified to, and does the SCM strategy follow from it?
Exposure class is the input, and everything else follows from it. The governing codes are ACI CODE-318-25 for structural concrete and ACI CODE-332-20 for residential concrete.
Is the concrete my neighbour got the same as mine?
Two slabs poured the same month from the same plant can differ, and the finished surfaces will not show it.
What does the batch ticket say arrived on the truck?
The batch ticket is the producer's own record of what was delivered, and it already exists.
Has anything changed since we did this last time?
Asked of a market rather than a document, this gets you an impression.
Is the mix design being reissued for this job, or reused from a previous one?
Reuse is often correct. Reuse without anybody checking whether the inputs still describe the same material is the risk.
Five questions that work on any contractor, before or after.
These are not a test anyone should fail on a technicality. A contractor who works with mix designs will answer them from experience and will not be annoyed at being asked.
What a good answer sounds like: a named material. What is more informative than the answer itself: whether the question registers as a normal thing to be asked. Awareness that this is a variable at all is the signal.
Why the second half matters as much as the first: the two classes are not interchangeable at the same replacement percentage, so a class name without a level is half an answer. Listen for: a figure, or a straight statement that the plant sets it and the mix design records it.
Why it is a fair question and not an accusation: carrying a proven mix forward is ordinary good practice. The risk is carrying it forward across a period in which the regional supply moved. Listen for: when it was last reviewed.
Why it settles the whole page: everything above is about what was intended. The batch ticket is the third party record of what actually arrived, it already exists, and keeping it costs nobody anything. Hesitation here is the most informative moment in the conversation.
What you are listening for: a specific artifact. A closeout packet covering what was specified, what arrived, the conditions and the cure. A folder of photographs is not that, and neither is a verbal assurance about materials.
We score the record. We do not grade the mix.
This page would be worth less if it ended by telling you the answer is us. So here is exactly what PSIP is and what it is not, in the same terms as everything above.
- ×PSIP does not recommend a material. Not a class, not a replacement level, not a brand. Those are mix design decisions that belong to the people specifying concrete for your site and your exposure. What PSIP scores is whether the decision was recorded.
- ×PSIP does not say fly ash is good or bad. That is the framing this page exists to retire. A material is specified against an exposure, and the specification is the thing that can be right or wrong for a job.
- ×PSIP does not rank contractors. Records are sorted, never ranked. Sorting is an order you choose and can reverse. Ranking is a judgment made on your behalf, and a body that scores documentation has no basis for making it.
- ×A record is not a warranty. Certification is a documentation and scoring credential. It is not a warranty, guarantee, or insurance of the contractor's work; not engineering or design services; and not a government license or code approval. PSSG does not supervise, direct, inspect, or perform the contractor's concrete work, and is not a party to any contract between the contractor and the customer. The contractor is solely responsible for its own work, methods, safety, code compliance, licensing, and customer relationships. A closeout packet establishes what was specified and what was delivered, which is the factual ground a warranty or a claim would be argued from.
The useful takeaway is smaller than the debate and more durable than it. You do not have to resolve whether fly ash is good. You have to know which one is in your concrete, and that is a fact somebody either wrote down or did not.
Asked and answered.
Neither. Fly ash is a supplementary cementitious material, which means it changes how the finished concrete resists alkali silica reaction, sulfate attack and chloride ingress. It is not filler and it is not a cost saving substitution, although it has been sold as both. What decides whether it helps your slab is which class it is, at what replacement level, specified against the exposure your slab actually faces, and whether that was recorded at the time.
Class F and Class C are the two fly ash designations you will meet on a mix design. They differ in chemistry, they differ in alkali silica reaction mitigation behaviour, and they are not interchangeable at the same replacement percentage. Class F is the low calcium ash that most of the ASR mitigation literature is anchored to. Class C carries elevated calcium and behaves partly like cement itself, contributing early strength while delivering a different long term durability profile.
That request removes a durability input for a reason that is not about durability, so it is worth understanding what you would be changing. The more useful question is what exposure class the mix is specified to and whether the supplementary cementitious material strategy follows from it. ACI 318 and ACI 332 define the exposure categories, and the material choice is downstream of that decision rather than independent of it.
Strength and durability are different properties and the question usually conflates them. A supplementary cementitious material changes the durability behaviour of the finished concrete, and the two fly ash classes differ from each other in how they do that, with Class C contributing early strength while delivering a different long term durability profile from Class F. What a specific mix does is a property of that mix design rather than of the word fly ash, which is why the answerable question is what was specified and what arrived.
Fly ash is a byproduct of burning coal for electricity, so its regional availability tracks decisions made by utilities and regulators rather than by the concrete industry. When a regional supply shifts from one class to the other, the concrete changes and the paperwork often does not, because nothing on an order form has to change for the material in the truck to change. PSIP keeps the supply side evidence on a separate page at /coal-crisis.
Ask for the batch ticket, which is the ready mix producer's record of what was actually in the truck. It is the difference between what was ordered and what arrived, it already exists on an ordinary residential pour and is produced by the plant rather than by your contractor, and nothing in the ordinary process puts a copy in your hands. If your contractor cannot produce it, the plant that batched the load may hold its own copy, and what you need before calling either is the exact pour date and the delivery address.
Fly ash is one supplementary cementitious material among several, slag being another. The category term describes a material blended with cement that changes the durability behaviour of the finished concrete. Both the category and the two fly ash classes have standalone definitions in the PSIP dictionary.
Alkali silica reaction mitigation is one of the behaviours supplementary cementitious materials affect, and it is the reason the class distinction matters rather than being a detail. Most of the ASR mitigation literature is anchored to Class F, the low calcium ash, so a general statement that fly ash helps is usually a statement about that specific material. PSIP keeps alkali silica reaction on its own page at /concrete-cancer.
Every term on this page is defined once.
Supplementary cementitious material, Class F and Class C fly ash, exposure class, batch ticket, closeout packet and alkali silica reaction each have a single standalone definition in the PSIP dictionary, with the primary sources behind them.