Autoclaved Fly Ash Bricks: Production and Plant Setup

Autoclaved fly ash bricks are pressed from fly ash, lime, gypsum and a sand or slag aggregate, then hardened in an autoclave in saturated steam. On the lines we supply, curing runs at about 1.4 MPa and 195 °C for about 7 hours, during which the lime reacts with the fly ash to form calcium silicate hydrates. Strength is reached in the autoclave, not in weeks of water curing.

This guide is for investors and plant managers looking at a fly ash brick plant. It explains how the autoclaved route differs from the water-cured fly ash bricks made by many small plants, what to check in a fly ash source, each step of production, the standards the bricks are tested against, and what to plan for when you set up the plant.

Autoclaved vs Non-Autoclaved Fly Ash Bricks

“Fly ash brick” covers two different products. Both are pressed from fly ash with a lime or cement binder. What differs is how the binder hardens.

The non-autoclaved figures come from the fly ash brick guideline of the Fly Ash Resource Centre at the State Pollution Control Board, Odisha, which notes that the mix “may vary” with raw material quality and that about 50–80% fly ash may be used (SPCB Odisha guideline).

The two routes suit different businesses. Water-cured plants are small and need little equipment, which is why they dominate where fly ash bricks are made by many small producers. An autoclaved plant is an industrial project built around a large, steady fly ash supply: it gives consistent strength within hours of pressing and frees the yard for finished stock. If you plan to use cement rather than lime, and cure without steam, the brick making machinery line is the closer fit.

Fly Ash Quality: What to Check Before You Plan the Plant

Fly ash is the main raw material, and it is a by-product: nobody makes it to your specification. The US Federal Highway Administration notes that fly ash particles are generally spherical and typically 10–100 microns, that Class F ash, from bituminous and anthracite coals, usually has less than 10% CaO, and that Class C ash, from sub-bituminous coals, typically has more than 20% (FHWA, Fly Ash Facts for Highway Engineers). The classes are defined in ASTM C618.

What to test, and why it matters for bricks:

• Chemistry. Silica and alumina are what react with lime in the autoclave. High-calcium ash brings some binder of its own, which changes the lime dose.
• Loss on ignition. FHWA describes loss on ignition as a measure of unburned carbon in the ash. India’s fly ash-lime brick standard limits bottom ash used as a sand replacement to no more than 12% loss on ignition (IS 12894). High carbon weakens and darkens the brick.
• Fineness. Finer ash reacts more completely and packs better under the press.
• Consistency. FHWA notes that ash color is usually very consistent for each power plant and coal source, so the coal source is what to watch: ask the power plant how often it changes or blends its coal, and test again when it does.
• Moisture and condition. Dry ash from electrostatic precipitators is easier to dose than wet ash from ponds, which has to be dried or its water allowed for in the mix.

Send samples from the actual source, collected over several weeks if possible. The mix and the curing schedule are set from that test, before the line is configured.

Autoclaved Fly Ash Brick Mix: Lime, Gypsum and Aggregate

On the lines we supply, the typical mix is:

The lime is usually quicklime, which has to slake completely before pressing. The Odisha guideline also lists gypsum, with lime, as a binder. India’s standard specifies hydrated lime of Class C to IS 712 for fly ash-lime bricks, and recommends that clay and silt in the sand stay below 5% (IS 12894). Where slag is used instead of sand, check its grading and any free lime it contains.

Compared with a sand-lime brick, a fly ash brick uses far less natural sand. The sand-lime route uses lime at 8–12% with the rest sand; the fly ash route replaces most of that sand with ash. For the sand-lime version of the same plant, see the sand-lime brick manufacturing process.

Autoclaved Fly Ash Brick Production Process

The line runs in eight steps:

1. Storage. Fly ash, lime and gypsum in silos with dust-tight filling; aggregate in bins or a covered yard. Fly ash is a fine powder, so filters on every silo and transfer point matter.
2. Batching. Each material weighed into the batch. Weighing accuracy for lime is the most important, because the lime dose controls strength.
3. Mixing. Fly ash, lime, gypsum, aggregate and water mixed intensively.
4. Digestion. The mix rests in a digester (reactor) until the quicklime has slaked completely. Unslaked lime would expand later inside the pressed brick.
5. Re-mixing. Water is corrected to press-ready moisture, measured on line.
6. Pressing. On the lines we supply, double-sided hydraulic presses in the 500–800 t class press from both sides in two stages at 20–25 MPa, with 26 standard bricks per cycle in 18–22 seconds. Mold changes switch between the 240 × 115 × 53 mm standard brick and the 240 × 115 × 90 mm multi-hole brick.
7. Setting and autoclaving. Green bricks are stacked on autoclave cars with gaps for steam, then cured in the autoclave.
8. Unloading and packing. Bricks are inspected, packed and sent to the yard.

Pressing sets the density, and density sets most of the final properties. An autoclave cannot rescue a brick pressed too dry, too wet or under-filled, so press moisture and mold filling are the main shift checks before curing.

Autoclave Curing of Fly Ash Bricks

At room temperature, lime and fly ash react slowly; that is why water-cured bricks need days in the yard. In saturated steam, the reaction runs in hours. On the lines we supply:

Fly ash differs from sand in what it brings to the reaction. Its glassy particles contain alumina as well as silica, so the products include aluminum-bearing phases. In a laboratory study of fly ash and lime autoclaved at 160 °C for 8 hours, the reaction products included C-S-H, a hydrogarnet-type phase and a small amount of tobermorite, alongside unreacted quartz and mullite (Zhu et al., Frontiers in Physics, 2021). The phases that form, and so the strength, depend on the ash, the lime dose and the curing schedule, which is why the curing schedule is set by a raw material test.

Log both pressure and temperature on every cycle. In saturated steam each pressure has one temperature, so a gap between the two readings signals trapped air or condensate, and bricks that will not cure evenly. For autoclave sizing, door types and cycle steps, see autoclave for an AAC plant: the vessel technology is the same.

Standards for Autoclaved Fly Ash Bricks

Fly ash bricks are covered by national standards rather than one international one:

Two points for producers:

• IS 12894 does not prescribe a curing route. It says the bricks are “subjected to curing cycles at different temperatures and pressures”, so autoclaved and water-cured bricks are tested against the same limits. Those include average water absorption of no more than 20% up to class 12.5 and 15% above it, and average drying shrinkage of no more than 0.15%.
• Carbonation is a fly ash brick property. Lime-based binders react with CO₂ from the air over time. GB 26541 tests how much strength the brick keeps after carbonation, so ask for that result alongside compressive strength.

Check the current edition of each standard for its limits before you fix the product specification, and tell your supplier which one your market uses: the molds, lime dose and curing schedule are set for it.

Fly Ash Brick Plant Setup: What to Plan

An autoclaved fly ash brick plant is planned around its ash supply. The points that decide a project:

• Location. Close to the power plant or ash pond, because long haulage erodes the reason for using a by-product. Autoclaved brick plants are often sited next to power plants or in industrial parks that manage by-products.
• Supply contract. Quantity, quality limits, how ash is delivered (dry from silos or wet from ponds), and what happens when the power plant changes coal or runs less. Coal plant output can fall; a second ash source or a sand option protects the plant.
• Market policy. Some governments promote fly ash products. In India, power stations generated 142.07 million tonnes of fly ash in April–September 2022 and utilized 78.14%, with 14.46 million tonnes going into bricks and tiles, about 10% of generation by our arithmetic (Central Electricity Authority, 2023).
• Lime supply. Quicklime of steady reactivity, delivered and stored dry.
• Steam and pressure vessels. A boiler for the fuel you can secure, and autoclaves registered under your country’s pressure vessel rules. Check those rules before the contract.
• Dust and water. Filters on fly ash and lime handling, and water for mixing and the boiler.
• Commissioning. The team includes an experienced autoclave and steam specialist; travel and accommodation of our engineers are at the buyer’s cost.

If you are preparing a project report for a lender, the chapters are the same as for any brick plant, with the ash supply contract in place of the clay deposit. See brick making business plan. If you are still choosing between products, compare AAC block vs fly ash brick.

FAQ

What is the difference between autoclaved and ordinary fly ash bricks?

Both are pressed from fly ash with a lime or cement binder. Ordinary fly ash bricks harden slowly at ambient temperature; one Indian state guideline describes two days on pallets followed by 10–14 days of water curing. Autoclaved bricks harden in saturated steam at about 1.4 MPa and 195 °C in about 7 hours on our lines, which gives faster, more consistent strength.

What is the mix for autoclaved fly ash bricks?

On the lines we supply, fly ash 60–70%, lime 8–12%, gypsum 2–3% and sand or slag aggregate 20–30%. The exact proportions depend on the chemistry, fineness and carbon content of your fly ash, the reactivity of the lime and the strength class required, so they are fixed by a raw material test before the line is configured.

Can any fly ash be used for autoclaved bricks?

Not without testing. Ash varies with the coal and the power plant. Check chemistry, loss on ignition, fineness, moisture and consistency over time. High unburned carbon weakens the brick, and high-calcium ash changes the lime dose. Samples from your actual source, collected over several weeks, show what the plant will really receive.

Which standard applies to autoclaved fly ash bricks?

It depends on the market. In China, JC/T 239-2014 covers autoclaved fly ash bricks and GB 26541-2011 autoclaved fly ash perforated bricks. In India, IS 12894:2002 covers fly ash-lime bricks whatever the curing route. Elsewhere, check whether national standards for calcium silicate units, such as EN 771-2 or ASTM C73, accept fly ash units.

How does an autoclaved fly ash brick plant differ from an AAC plant?

Both use fly ash, lime, autoclaves and a steam boiler. An AAC plant casts a slurry that rises with aluminum, then wire-cuts the cake into lightweight blocks. A fly ash brick plant presses a damp mix into dense bricks in steel molds, so it needs presses and lime digestion instead of molds, a pouring mixer and a cutting line. The products serve different walls.

Planning an Autoclaved Fly Ash Brick Plant?

For the presses, autoclaves and steam boiler we configure around a fly ash supply, see our autoclaved brick production line. To see how the product compares with clay brick in the wall, read calcium silicate vs clay bricks.

Sources

• Fly Ash Resource Centre, State Pollution Control Board, Odisha, Guidelines for Fly Ash Bricks: https://ospcboard.odisha.gov.in/wp-content/uploads/2019/05/Guidelines-for-Fly-Ash-Bricks.pdf
• US Federal Highway Administration, Fly Ash Facts for Highway Engineers, Chapter 1: https://www.fhwa.dot.gov/pavement/recycling/fach01.cfm
• ASTM C618, Standard Specification for Coal Ash and Raw or Calcined Natural Pozzolan for Use in Concrete: https://store.astm.org/c0618-23e01.html
• Bureau of Indian Standards, IS 12894:2002 Pulverized Fuel Ash-Lime Bricks — Specification (with Amendment No. 1): https://law.resource.org/pub/in/bis/S03/is.12894.2002.pdf
• C. Zhu, Y. Lou, X. Shen, H. Xu and J. Yang, Influence of CaO Content on the Fly Ash–Lime System Hydrothermal Synthesis Reaction Under Autoclave Curing, Frontiers in Physics 9, 782309 (2021): https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2021.782309/pdf
• JC/T 239-2014 Autoclaved fly ash brick (replaces JC 239-2001), standard record: https://www.chinesestandard.net/Related.aspx/JCT239-2014
• GB 26541-2011 Autoclaved fly ash perforated brick, standard record: https://www.chinesestandard.net/Related.aspx/GB26541-2011
• Central Electricity Authority (India), Fly Ash Generation at Coal/Lignite Based Thermal Power Stations and its Utilization in the Country for the 1st Half of the Year 2022-23 (March 2023): https://cea.nic.in/wp-content/uploads/tcd/2023/05/Half_Yearly_Ash_Report_2022_23.pdf