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Neither rice husk ash (RHA) nor fly ash is a universal winner. Both are supplementary cementitious materials (SCMs) that can replace part of the cement, but their concrete performance changes with ash chemistry, fineness, processing, mixture proportions, curing and test conditions. Published reviews report different replacement levels as “optimum,” so those percentages are evidence from particular studies—not a specification for every project.
For a real job, compare characterized material lots in the same trial mix, then evaluate strength, fresh-concrete behavior, durability requirements, acceptance criteria and delivered cost. The available evidence does not establish a universal strength ranking, a universally cheaper ash or a guaranteed local supply.
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The direct answer
Choose between RHA and fly ash by qualifying the actual products available to your project, not by treating the material names as performance grades. A 2020 review reported 30% fly ash and 7.5% RHA as the optimum replacement levels in the set of studies it collected. Those studies used different ashes, mixtures and conditions; they were not a controlled head-to-head comparison. A 2026 review compiles RHA studies with reported optima from 5% to 25%, reinforcing that no single dosage transfers reliably from one project to another.
Cost is equally project-specific. No comparable current delivered prices, freight rates, minimum order quantities or supplier quotations were established for a particular location. A lower ex-works price can disappear after transport, processing, testing, storage and any mix adjustment. Availability also differs by region: a 2026 review describes fly ash suitable for SCM use as constrained by quality variation and says long-term supply is uncertain as coal-fired electricity generation changes. The same evidence does not prove that RHA is consistently available at a given jobsite.
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What fly ash and rice husk ash are—and why the names are not enough
In concrete, both materials are used as partial cement replacements. Their useful contribution depends on the characteristics of the ash and on the binder system into which it is introduced. The comparative review notes that ash compounds vary with how the original feedstock was processed. Two products carrying the same broad name can therefore behave differently in water demand, setting, early strength, later strength and durability-related performance.
Fly ash
Fly ash is the finely divided ash associated with coal combustion. The supply that is suitable for SCM use is not unlimited: the 2026 agricultural-ash review links usable quantity to quality variation and describes long-term availability as uncertain because coal-based electricity generation is not sustainable. Treat any statement about supply as regional and time-sensitive; a plant, distributor and quality grade must be verified for the project.
Rice husk ash
RHA is produced from rice-husk feedstock and can serve as an SCM when its properties meet the project’s requirements. Its performance is particularly sensitive to processing and characterization, which is why studies report a wide range of replacement levels. A review of agricultural waste ashes discusses RHA as a potential alternative in contexts where fly ash, slag and silica fume are difficult to obtain, including examples from sub-Saharan Africa. That is a regional observation, not proof of a dependable source or a construction-grade product in every market.
What the strength evidence actually shows
The most frequently repeated percentages come from review articles that aggregate separate investigations. They are useful for framing a trial program, but they should not be read as equivalent batches tested side by side.
| Evidence | Replacement levels represented | Reported result | How to use it |
|---|---|---|---|
| Civil Engineering Journal review (2020): fly ash studies | 0%, 10%, 20%, 30%, 40% and 50% cement replacement; observations at 7–28 days | 30% fly ash was identified as the optimum in the review’s collected data | Use as a historical reference point for trial percentages, not as a universal design value |
| Civil Engineering Journal review (2020): RHA studies | 0%, 5%, 7.5%, 10%, 12.5% and 15% replacement; observations at 7–28 days | 7.5% RHA was identified as the optimum in the review’s collected data | It is a synthesis of unlike investigations, not a direct comparison with the fly-ash percentage |
| Agricultural-waste-ash review (2026): RHA studies | Examples of reported optima span 5% to 25%; the review also discusses studies around 10–15% | One cited account reports approximately 11.4% improvement in compressive strength | Report the cited mix, ash characterization, curing age and application; the percentage and improvement are not guaranteed project outcomes |
| 2025 roller-compacted-concrete paper | Fly ash and RHA in pavement-oriented roller-compacted concrete | The surfaced information establishes the comparison’s scope but not sufficient numerical results to name a winner | Do not infer a head-to-head ranking from the title or abstract information alone |
These figures answer the question “what levels have been reported?” They do not answer “which ash will produce the highest strength in my concrete?” For that, the binder, aggregate grading, water content, admixture system, compaction, curing and ash lot must be held sufficiently consistent in a project trial.
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Why published optimum percentages conflict
Ash chemistry and physical properties
Reactive constituents, fineness and other physical properties influence how an ash participates in cement hydration and how much water the mixture needs. Processing of the original feedstock changes the resulting compounds. A percentage that worked with one finely processed, consistent lot may not work with a coarser or chemically different lot.
Replacement basis and mixture design
“10% replacement” is meaningful only when the basis is clear—usually a stated fraction of the cementitious binder, but projects and papers can define mixtures differently. Compare the complete binder formulation, not the number by itself. Changing the total binder, water-to-binder relationship, aggregate proportions or chemical admixture can move the apparent optimum even when the ash percentage is unchanged.
Age and curing
The 2020 review summarized results observed between 7 and 28 days. A material that develops strength more slowly can look weaker at an early test age and closer to the reference concrete later. Curing temperature, moisture retention and test procedure also affect the measured result. Always attach the test age and curing regime to a reported strength.
Application and performance target
Concrete for a structural element, a pavement, a mass pour or a self-compacting mixture may prioritize different properties. The existence of a roller-compacted-concrete study does not make its findings transferable to conventional cast concrete, and a result in one application should not be generalized to another without a comparable trial.
Strength is only one part of performance
Before selecting an ash, define the required fresh and hardened-concrete outcomes. The practical comparison should include:
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- Fresh-concrete behavior, including whether the candidate changes workability or requires a different admixture adjustment.
- Strength at the ages that govern the project’s decisions, with the same curing and test conditions for every candidate.
- Durability-related requirements and exposure conditions relevant to the structure.
- Batch-to-batch consistency, because a variable ash can produce variable concrete even when the nominal replacement percentage is constant.
- Acceptance requirements in the project specification and the jurisdiction where the concrete will be placed.
The reviews emphasize variation in material properties and concrete outcomes. That makes a characterized lot and a controlled trial more informative than importing an optimum from a different publication.
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There is no defensible universal answer to “which one costs less?” The available evidence does not provide comparable current prices, transport costs, minimum order quantities or suppliers for a particular location. A fair comparison must use equivalent quality and documentation and the quantity actually delivered to the batch plant or site.
Build a delivered-cost comparison
- Quote the same basis. Ask each supplier for a price per tonne or other unit, the quoted quantity, product grade, documentation and whether the price is recurring or promotional.
- Add freight and handling. Include transport distance, unloading, storage, moisture protection and any special equipment required for the delivery.
- Include processing. If the RHA or fly ash needs additional grinding, classification, blending or conditioning to meet the project requirement, price that work rather than assuming it is free.
- Price verification. Include sampling, characterization and concrete testing needed to qualify each lot. A nominally cheap material that requires extensive corrective testing may not be the low-cost option.
- Account for mix consequences. Compare the cost of the complete concrete mixture at the required performance, including any change in cement content, water demand, admixture dosage, production rate or curing provisions.
- Check continuity. A one-off low quote is not equivalent to a reliable supply for every pour. Ask how often the same grade can be delivered and how variability will be controlled.
Only after those items are priced can a project calculate a meaningful delivered cost per cubic metre. Without local quotations, any claim that RHA or fly ash is cheaper is speculation.
Availability and supply risk
Fly ash supply
The 2026 review states that the quantity of fly ash suitable for SCM use is limited by quality variation. It also describes long-term availability as uncertain because coal-based electricity generation is not sustainable. A nearby coal plant therefore does not automatically mean a dependable source: the usable grade, consistency and future operating pattern still need confirmation.
RHA supply
RHA may be attractive where rice processing creates a local feedstock and conventional SCMs are scarce, but the evidence does not establish a consistent supply for any particular jobsite. Confirm the processor, production schedule, ash handling, processing controls, storage conditions and lot-to-lot documentation before making it part of a construction program.
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Questions to ask a prospective supplier
- Where was the ash produced, and what process created it?
- What chemistry, fineness and other relevant physical-property data are available for the exact lot?
- How often are lots tested, and how are out-of-specification deliveries handled?
- What quantity can be delivered on the project schedule, and what is the minimum order?
- What is the delivered price to the batch plant, including freight and unloading?
- Can the supplier provide a retained sample for independent testing and trial batching?
Can rice husk ash replace fly ash?
RHA can replace part of the cement as an SCM, just as fly ash can, but it should not be treated as a one-for-one drop-in substitute. The two materials can differ in chemistry, fineness, water interaction and consistency. Start with a replacement basis that is explicitly defined, then test RHA at several levels around the range supported by the available literature rather than copying the fly-ash percentage.
The 2020 review’s 30% fly-ash and 7.5% RHA figures are not interchangeable dosage instructions. The 2026 review’s examples from 5% through 25% show why RHA trials may need more than one candidate level. The selected level must satisfy the project’s strength, durability, fresh-concrete and acceptance requirements at the tested ages.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A project-specific selection workflow
- Set the performance brief. Record required strength ages, durability exposure, placement method, curing conditions, production volume and acceptance criteria before comparing materials.
- Define the replacement basis. State exactly what fraction of the cementitious binder is being replaced and keep that basis consistent across candidates.
- Qualify each ash lot. Obtain verified chemistry, fineness and other relevant physical-property information. Check consistency across more than one delivery if the project is long or high consequence.
- Select trial levels. Use literature ranges only to choose sensible starting points. Include a reference mixture with no ash and several candidate replacement levels for each material.
- Batch under controlled conditions. Keep aggregate sources, water, admixtures, mixing sequence, placement and curing comparable so that differences can be attributed to the ash and the mixture design.
- Test at decision ages. The reviewed studies commonly report 7–28-day observations; test the ages that actually govern the project and record curing conditions with every result.
- Evaluate fresh concrete and hardened performance. Do not accept a strength result that fails workability, placing, finishing, durability or production requirements.
- Check repeatability. Repeat promising mixtures with another representative lot or additional batches. A single successful batch does not prove supply consistency.
- Compare delivered cost and supply. Use the qualified mixture’s actual dosage, testing burden, freight and available quantity—not an unverified catalogue price.
- Document acceptance. Keep the product data, trial results, supplier commitments and approval by the responsible engineer or specification authority together with the mix decision.
How to interpret the available comparison studies
The 2025 paper specifically examines fly ash and RHA in roller-compacted concrete for pavement applications. The available information confirms the application and comparison but does not expose enough quantitative results to support a winner. It is therefore appropriate to use the paper as a reason to investigate the pavement application, not as evidence that one ash is stronger or cheaper in your project. The paper is available at Taylor & Francis.
For self-compacting concrete and related mixtures, a review of RHA and calcined clay discusses the use of RHA as an SCM and the importance of mixture behavior. It can provide context for material selection, but it does not remove the need to qualify the actual product and meet the project’s own acceptance requirements: PMC review.
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Common mistakes and how to avoid them
Importing a published optimum
Mistake: specifying 30% fly ash or 7.5% RHA solely because a review reports that value.
Fix: treat those values as trial starting points and verify the complete mixture, ash lot and test age.
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Comparing nominal percentages without the binder basis
Mistake: calling two mixtures equivalent because both say “10% replacement.”
Fix: confirm the reference cementitious quantity, total binder and water-to-binder relationship before comparing results.
Using a retail or unverified ash product
Mistake: assuming any product labelled rice husk ash or fly ash is suitable for construction.
Fix: require composition, fineness, lot consistency, testing records and project acceptance before ordering production quantities.
Choosing on material price alone
Mistake: comparing a supplier’s ex-works quote with no freight, processing or testing allowance.
Fix: calculate delivered cost for the qualified concrete mixture and include supply continuity.
Assuming local availability from a broad regional statement
Mistake: treating a review’s discussion of RHA in regions with limited conventional SCMs as proof of supply at the project site.
Fix: obtain a current local quotation, delivery schedule, minimum quantity and representative sample.
Decision checklist
- Have both ashes been characterized for the exact lots proposed?
- Are the replacement percentages stated on the same binder basis?
- Were a no-ash reference and multiple replacement levels tested under the project’s curing conditions?
- Do fresh-concrete behavior, strength and durability results meet the project brief?
- Can the supplier provide consistent deliveries for the full program?
- Does the delivered-cost calculation include freight, processing, testing, storage and mix adjustments?
- Has the responsible specification authority accepted the material and trial evidence?
The defensible conclusion is conditional: select the ash that meets the verified performance and supply requirements at the lowest reliable delivered cost for this project. Current reviews support using both materials as candidate SCMs, but they do not support a universal strength, price or availability ranking.
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