Kumar IronWhat each brick is made of
Red bricks are made of clay, moulded and burnt in a kiln. They are covered by IS 1077, the standard for common burnt clay building bricks. Fly ash bricks are made mostly of fly ash (a by product of coal power plants) with lime or cement and sand or stone dust, moulded under pressure and cured rather than burnt. Fly ash lime bricks are covered by IS 12894.
Side by side
| Point | Fly ash brick | Red clay brick |
|---|---|---|
| Indian Standard | IS 12894 (fly ash lime bricks) | IS 1077 (burnt clay bricks) |
| Making | Pressed and cured, not fired | Moulded and burnt in a kiln |
| Typical size | Uniform, often 230 x 110 x 75 mm or modular | Varies by kiln, modular is 190 x 90 x 90 mm |
| Compressive strength | Class from 7.5 N/mm² up, graded by class | Class 3.5 N/mm² up to 45 N/mm² under IS 1077 |
| Water absorption | Limit set by the standard, 20% maximum | Limit 20% maximum in IS 1077 for common classes |
| Shape and finish | Even, sharp edges, plaster needs less mortar | Less uniform, thicker mortar bed needed |
| Weight of one brick | Roughly 2.5 to 3.5 kg depending on size | Roughly 3 to 3.5 kg for modular size |
Always ask for the class marking and a test report from the maker. A brick that does not carry a compressive strength class is not a graded brick.
Strength and how to choose a class
For a load bearing single storey or G+1 house, a brick of at least 7.5 N/mm² is common practice, with higher classes for taller walls. For infill walls in an RCC frame and for boundary walls, lower classes work. The structural engineer decides the class. Do not pick a lower class than the drawing says to save money.
Weight and wall dead load
Density of fired clay brickwork is about 1800 kg/m³, and fly ash brickwork is in a similar range, somewhat lighter in many products. The difference per brick is small, so dead load on the structure does not change much between the two. If lightness is the goal, AAC blocks are the lighter choice; see our comparison of AAC blocks and red bricks.
What changes the cost
- Distance from the kiln or plant to your site, since bricks are heavy and freight matters.
- Brick class and size: higher class costs more.
- Breakage and wastage on the truck and during unloading.
- Mortar use: uniform fly ash bricks usually need less mortar and less plaster to level the wall, but red bricks laid with care can give a similar result.
- Seasonal supply: kiln bricks are affected by the brick making season and rain.
- Labour: skilled masons are comfortable with both, but wall height and bond matter more than brick type.
Because these change from week to week, ask for the current rate and compare on cost per cubic metre of finished wall, not on cost per brick.
Practical tips
- Soak red bricks before laying. Fly ash bricks need less soaking, so follow the maker's advice.
- Cure fly ash brickwork for at least 7 days.
- Check a sample: drop a brick from about 1 m, and a good one will not break. A ringing sound when two are tapped together suggests good burning in a clay brick.
- Match the mortar to the brick. Use cement sand mortar of the grade the engineer specifies.
Frequently asked questions
- Are fly ash bricks stronger than red bricks?
- It depends on the class. Both come in graded strength classes under their own standards. Compare the stated N/mm² class rather than the type.
- Which IS code covers fly ash bricks?
- IS 12894 covers fly ash lime bricks. IS 1077 covers common burnt clay bricks.
- Are fly ash bricks lighter than red bricks?
- Slightly lighter in many products, but the difference per brick is small. AAC blocks are the much lighter option.
- Is fly ash brick good for plastering?
- Yes. The even shape lets you plaster with less thickness. Cure the wall and use the mortar mix your engineer specifies.