Fly Ash Brick Manufacturing Process: Mix, Pressing and Curing

The fly ash brick manufacturing process mixes fly ash with a lime or cement binder, sand and water, presses the damp mix in a machine, then cures the bricks. One Indian state guideline uses 70% fly ash, 10% lime, 5% gypsum and 15% sand, with about two days on pallets followed by 10–14 days of water curing.

This guide covers the non-autoclaved route: pressing plus water curing or low-temperature steam, the way many small and mid-size plants work. It explains the raw material ratio, each process step, what curing does, the Indian standard’s test limits and why bricks effloresce. If you plan to cure at high pressure, read autoclaved fly ash bricks instead, because the plant and the mix logic are different.

Fly Ash Brick Types by Binder and Curing

The name covers several different products. Pick the binder and curing route first, because they set the mix, the equipment and the yard. The comparison below is our assessment.

The Odisha State Pollution Control Board guideline notes that cement can replace lime and gypsum as the binder. Lime supply and local cost usually decide the choice. For a comparison with lightweight walling, see AAC block vs fly ash brick.

Fly Ash Bricks Raw Material Ratio

The mix most often quoted is the Odisha guideline’s. It says fly ash may range from about 50% to 80% and that the ratio can change with raw material quality.

Source: IS 12894:2002. The Odisha guideline also asks for lime with at least 40% CaO.

Research suggests the published ratio is a starting point. In work at the Indian Institute of Science (Gourav, 2010), lime contents of 10–17% gave the highest strength under 28-day wet burlap curing, and the best gypsum content was 2%. A related paper reports the same 2% figure. That is well below the guideline’s 5%, and those results came from one ash under lab conditions. Run trial mixes at several lime and gypsum levels with your own ash. Excess gypsum adds sulfate, as the section on efflorescence explains.

What to check in the fly ash

Ash quality depends on the coal and the power plant, and the source can change without notice. The US Federal Highway Administration describes low-calcium Class F ash and high-calcium Class C ash, and treats loss on ignition as a measure of unburned carbon. For a press-and-cure plant, check fineness, loss on ignition, calcium, sulfate and moisture. Sample over several weeks to see how much they vary. Pond ash is wet and variable, so measure its water before batching.

Fly Ash Brick Manufacturing Process, Step by Step

1. Receive and store. Fly ash goes in silos or covered storage, kept dust-free. Lime and gypsum stay dry; sand is screened.
2. Batch. Weigh each material. Volume batching with shovels or boxes risks uneven bricks.
3. Mix. A pan mixer blends the dry materials, then water is added until the mix is uniform (Odisha guideline).
4. Convey. A belt conveyor feeds the machine. Keep the time between mixing and pressing short and consistent, especially with cement.
5. Press. A hydraulic or power press forms the bricks.
6. Hold. Bricks rest for one to two days before they can be moved. The guideline places them on wooden pallets.
7. Cure. Water curing in the open for 10–14 days in the guideline, or low-temperature steam.
8. Sort and test. Bricks are sorted and tested before dispatch.

Pressing: Moisture, Density and Machine Choice

Pressing is the step that sets most of the final quality. Gourav (2010) found that compressive strength rises with dry density, whatever the stabilizer content or curing type. A curing yard cannot repair a brick that was pressed too loose.

What to control:

• Mix moisture. Too dry and the brick is weak and crumbles at the edges. Too wet and it sticks to the mold and slumps when released. Find the moisture that gives the highest density in your press, then hold it.
• Mold filling. Uneven filling gives bricks with uneven density and height. Weigh green bricks regularly as a quick density check.
• Aggregate grading. In Waheed et al. (2022), a study of fly ash cement sand bricks, coarse sand with press forming performed best of the options tested in the first phase.
• Mold size. IS 12894 lists modular bricks of 190 × 90 × 90 mm and 190 × 90 × 40 mm, and non-modular sizes of 230 × 110 × 70 mm and 230 × 110 × 30 mm. Solid bricks may have a frog 10–20 mm deep.

Machines for this route range from small mechanical presses to pallet-based block machines. On the block making lines we supply, a machine combining vibration with hydraulic pressing under PLC control forms each batch in a 12–20-second cycle. Changing the mold switches between products such as 240 × 115 × 53 mm standard bricks, 390 × 190 × 190 mm hollow blocks, perforated bricks and pavers. For machine types and how to specify them, see hollow block machine and interlocking brick machine.

Those lines are built for cement-bound mixes that can take fly ash and slag, cured without firing or steam. If you plan a lime-based mix, or a mix very high in fly ash, run a pressing trial first, because a block machine’s cycle depends on how the mix compacts and holds its shape.

Fly Ash Brick Curing Process: Water vs Steam

Lime and fly ash react slowly at ambient temperature, which is why the curing stage is long and why it cannot be cut short.

Water curing. After the holding period, bricks are stacked in the open and kept wet. The Odisha guideline gives 10–14 days. Stacks must have gaps so water reaches every brick, and each stack should carry its pressing date so that nothing leaves early. In hot, dry climates such as the Gulf and much of Africa, stacks dry quickly, so cover them or wet them more often. In cold seasons the reaction slows, so a fixed day count becomes less reliable; test strength rather than relying on the calendar. Water use and recycled water also need planning where water is scarce.

Low-pressure steam. Heating speeds up the reaction. Gourav (2010) and Reddy and Gourav (2011) found that about 24 hours of steam curing at around 80 °C achieved nearly the maximum strength for fly ash-lime mixes. A study by Arıöz et al. of fly ash, FGD gypsum and lime bricks steam-cured for 6 or 10 hours and then moist-cured found that steam time mainly affected early strength, with longer steam also improving water absorption. Steam adds chambers, a heat source and temperature control. It makes sense when yard space is tight, the climate is cold, or sales depend on faster turnaround.

Curing chambers. Cement-bound bricks and blocks on our block making lines cure in chambers without steam, so the line needs no kiln, autoclave or boiler.

The comparison below is also our assessment.

Fly Ash Brick Quality Limits Under IS 12894

This guide uses India’s standard as the worked reference because its text is public. It is written for fly ash-lime bricks, lists cement as a permitted additive, and does not prescribe a curing route.

Sampling follows IS 5454. Always check the current edition before fixing your specification, along with any amendments. Bricks above about 30 N/mm² wet strength fall outside this standard’s scope. Outside India, ask which standard your buyers and approving engineers accept.

Test bricks wet. The standard’s strength is a wet strength, and fly ash-lime bricks lose a lot when saturated. Gourav (2010) recorded wet-to-dry strength ratios of 0.55–0.67, and 8–10 MPa wet strength at 28 days for fly ash-lime-gypsum bricks. A plant that tests dry can overstate its class by a wide margin. Compare the test methods for clay bricks if you need to explain the difference to a buyer.

Plant checks for a fly ash brick line

For site planning around the yard and stacks, see brick plant layout.

Fly Ash Brick Efflorescence: Causes and Control

Efflorescence is the white salt deposit that appears on a brick or wall. The Brick Industry Association (TN 23A) says it needs three things together: soluble salts, water in contact with them long enough to dissolve them, and a pore structure that lets the solution reach the surface. The most common salts are sulfates and carbonates of sodium, potassium, calcium, magnesium and aluminum. Remove any one of the three and it stops.

Likely sources in a fly ash brick plant (our assessment; test your own materials to confirm):

• Sulfate in the ash. It varies with coal and flue gas treatment.
• Excess gypsum. Gypsum is calcium sulfate, so a dose above what the reaction uses leaves soluble sulfate behind.
• Salts in sand and water. Common with saline groundwater and sea-sand sources, a real concern in coastal and arid regions.
• Under-curing. Poorly cured bricks are more porous and have more unreacted lime available to carbonate at the surface.
• Low density. Loose pressing opens the path for water and salts.
• The wall itself. BIA notes that salts often come from mortar, grout, sand and other materials rather than from the masonry unit.

How to test and control it:

1. Test every new ash, sand and water source for soluble salts and sulfate before running production.
2. Use trial mixes to find the lowest gypsum dose that gives the needed strength.
3. Press to a consistent density and finish curing in full.
4. Run the IS 3495 Part 3 test regularly. Bricks stand in 25 mm of distilled water until it evaporates, twice, and are then rated. IS 3495 defines “slight” as a thin deposit over no more than 10% of the exposed area and “moderate” as a heavier deposit over up to 50%, without powdering or flaking.
5. Protect stacked bricks from rain on site, and tell builders what to expect on new walls. BIA describes new-building bloom as occurring in walls less than a year old and usually dissipating as they dry and weather. IS 3495 excludes powdering or flaking from its “moderate” rating, so investigate either one.

FAQ

What is the raw material ratio for fly ash bricks?

A widely quoted mix from an Indian state guideline is 70% fly ash, 10% lime, 5% gypsum and 15% sand, with fly ash anywhere from about 50% to 80%. Cement can replace lime and gypsum. Laboratory work suggests less gypsum may be enough, so fix your ratio through trial mixes using your own ash, lime and sand.

How long does fly ash brick curing take?

The Odisha guideline describes one to two days of drying on pallets followed by 10–14 days of water curing. Laboratory studies found about 24 hours of steam at around 80 °C can reach nearly maximum strength for fly ash-lime mixes. Climate and mix change the result, so confirm the schedule with wet strength tests rather than a fixed count.

Why do fly ash bricks show efflorescence?

Efflorescence needs soluble salts, water and a path through the pores to the surface. In a fly ash plant, likely salt sources are sulfate in the ash, excess gypsum, and saline sand or water. Poor pressing and short curing open the pores. Test incoming materials, limit gypsum, and run the IS 3495 Part 3 test regularly.

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

Both are pressed from fly ash and a binder. Ordinary fly ash bricks gain strength through water curing, chamber curing or low-temperature steam. Autoclaved bricks harden in high-pressure saturated steam, which requires autoclaves, a boiler and pressure vessel compliance. India’s IS 12894 sets the same test limits for both, whichever curing route is used.

What strength can fly ash bricks reach?

IS 12894 sets classes from 3.5 to 30 N/mm² based on average wet compressive strength. Laboratory fly ash-lime-gypsum bricks in one Indian Institute of Science study reached 8–10 MPa wet at 28 days. The class you can reliably sell depends on density, binder and curing, so test production bricks wet and in the state your standard requires.

Planning a Fly Ash Brick Plant?

Our brick making machinery line presses cement-bound mixes, including fly ash and slag, into bricks, blocks and pavers with one machine and interchangeable molds. Send your ash analysis and target products so the mix can be checked first. If your project is built around a large ash supply and high-pressure curing, compare the autoclaved brick production line.

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
• 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
• Bureau of Indian Standards, IS 3495 (Parts 1 to 4): 1992 Methods of Tests of Burnt Clay Building Bricks: https://law.resource.org/pub/in/bis/S03/is.3495.1-4.1992.pdf
• K. Gourav, Studies on Compacted Stabilised Fly Ash Mixtures and Fly Ash Bricks for Masonry, Indian Institute of Science (2010): https://etd.iisc.ac.in/handle/2005/676
• B. V. Venkatarama Reddy and K. Gourav, Strength of lime-fly ash compacts using different curing techniques and gypsum additive, Materials and Structures 44(10), 1793–1808 (2011): https://eprints.iisc.ac.in/42272
• Ö. Arıöz et al., Properties of fly ash-FGD gypsum-lime based products, Journal of the Australasian Ceramic Society, Anadolu University repository record: https://earsiv.anadolu.edu.tr/xmlui/handle/11421/21959
• A. Waheed et al., Mechanical and durability properties of fly-ash cement sand composite bricks, Innovative Infrastructure Solutions 7, 24 (2022): https://link.springer.com/doi/10.1007/s41062-021-00630-w
• US Federal Highway Administration, Fly Ash Facts for Highway Engineers, Chapter 1: https://www.fhwa.dot.gov/pavement/recycling/fach01.cfm
• Brick Industry Association, Technical Note 23A: Efflorescence – Causes and Prevention: https://www.gobrick.com/media/file/23a-tn23a.pdf