Fly Ash Bricks — Complete Guide
Fly Ash Bricks are eco-friendly masonry units manufactured from coal fly ash (a by-product of thermal power plants), cement or lime, sand, and gypsum. They are produced through a cold-binding hydraulic pressing process — no kiln firing required — which saves enormous amounts of energy and prevents air pollution.
Fly ash bricks are governed by IS 12894 — Pulverized Fuel Ash-Lime Bricks — Specification and are also called FaL-G bricks (Fly Ash – Lime – Gypsum bricks).
What are Fly Ash Bricks?
Fly Ash Bricks are pressed cement-based bricks made by mixing fly ash (60–75%), sand (5–10%), cement or hydrated lime (15–20%), and gypsum (5%), with water. The mixture is hydraulically pressed into moulds and water-cured (not kiln-fired) for about 21 days to develop strength.
Fly ash bricks are the modern, eco-friendly answer to traditional
red clay bricks — they consume industrial waste, do not require topsoil mining, and have a much lower carbon footprint.
Composition / Raw Materials
| Material | Percentage | Function |
|---|
| Fly Ash (Class F) | 60 – 75% | Main binder + filler |
| Sand / Stone Dust | 5 – 15% | Improves density |
| Hydrated Lime / Cement | 10 – 20% | Activates pozzolanic reaction |
| Gypsum (Calcium Sulphate) | 3 – 5% | Setting agent |
| Water | As required | Hydration |
Standard Sizes
| Type | Size (L x W x H) |
|---|
| Modular Fly Ash Brick | 230 x 110 x 70 mm |
| Standard Fly Ash Brick | 190 x 90 x 90 mm |
| Large Format | 230 x 110 x 100 mm |
| Fly Ash Block | 400 x 200 x 100 / 200 mm |
Compressive Strength Classes (IS 12894)
| Class | Min. Compressive Strength (MPa) | Typical Use |
|---|
| Class 7.5 | 7.5 | Non-load-bearing walls, partition |
| Class 10 | 10 | Residential load-bearing |
| Class 12.5 | 12.5 | Multi-storey load-bearing |
| Class 15 | 15 | Heavy duty applications |
| Class 17.5 | 17.5 | Specialty / industrial |
| Class 20 | 20 | Highest grade |
Physical Properties
| Property | Value |
|---|
| Compressive Strength | 7.5 – 20 MPa |
| Water Absorption (24 hr) | 10 – 15% |
| Density | 1500 – 1800 kg/m³ |
| Weight per Brick | 2.5 – 3.0 kg |
| Efflorescence | Nil |
| Thermal Conductivity | 0.7 – 0.9 W/m.K |
| Dimensional Tolerance | ± 2 mm |
| Drying Shrinkage | < 0.15% |
Manufacturing Process
- Raw Material Mixing: Fly ash, sand, lime/cement, gypsum, water in a pan mixer
- Hydraulic Pressing: Mixture pressed in steel moulds at 100–200 kg/cm²
- Demoulding: Green bricks released from mould
- Curing: Water curing for 21 days (or steam curing 24–48 hrs)
- Sorting & Stacking: Quality check, dispatch
No firing in kilns — saves coal/wood and prevents CO₂ emissions.
Tests for Fly Ash Bricks
- Compressive Strength Test — IS 3495 (Part 1) or IS 12894
- Water Absorption Test — IS 12894
- Efflorescence Test
- Drying Shrinkage Test — IS 12894
- Dimensional Tolerance Check
- Density Test
- Soundness Test (visual inspection)
- Visual Inspection for cracks, edge sharpness
Fly Ash vs Red Clay Bricks — Comparison
| Parameter | Fly Ash Brick | Red Clay Brick |
|---|
| Source | Industrial waste + cement | Topsoil clay |
| Manufacturing | Cold pressing + curing | Kiln firing at 900°C+ |
| CO₂ Emissions | Very Low | High |
| Compressive Strength | 7.5 – 20 MPa | 5 – 35 MPa |
| Water Absorption | 10 – 15% | 15 – 20% |
| Weight per Brick | 2.5 – 3 kg | 3 – 3.5 kg |
| Mortar Consumption | Less (smooth surface) | More (rough surface) |
| Surface Finish | Smooth, ready for plaster | Rough |
| Wastage | < 2% | 5 – 10% |
| Cost | Lower (Rs 5–8) | Higher (Rs 8–12) |
| Dimensional Accuracy | Excellent (± 2 mm) | Variable |
| Efflorescence | Nil | Possible |
| Environmental Impact | Eco-friendly (uses waste) | High (topsoil mining) |
Uses of Fly Ash Bricks
- Load-bearing walls (Class 10 and above)
- Partition walls in RCC framed structures
- Boundary walls and compound walls
- Internal and external walls in residential buildings
- Multi-storey building walls (up to G+4)
- Industrial buildings
- Sound-insulating walls
- Garden walls and landscape features
- Replacement for clay bricks in any masonry work
Advantages
- Eco-friendly — uses industrial waste (fly ash)
- No topsoil destruction (unlike clay bricks)
- Zero kiln emissions (no firing)
- Uniform size and shape (better dimensional accuracy)
- Less mortar consumption due to smooth surface
- Less plastering needed
- No efflorescence
- Light weight (10–15% lighter than clay bricks)
- Higher compressive strength than ordinary clay bricks
- Lower water absorption
- Faster construction
- Less wastage (1–2%)
- Cost-effective (Rs 5–8 per brick)
- Government incentives (CGST exemption)
- Fire resistant
Disadvantages
- Lower strength than first-class clay bricks (in some grades)
- Limited large sizes (mostly standard brick size)
- Edges can chip during transport
- Initial scepticism among traditional masons
- Variable quality between manufacturers
- Curing time of 21 days at factory
- Requires good quality fly ash (Class F preferred)
- Not suitable for very high compressive applications
Quantity Estimation
Same as clay bricks: approximately 500 fly ash bricks per cubic metre of wall (for 230 x 110 x 70 mm size with 10 mm mortar joint).
| Wall Type | Bricks per m² |
|---|
| 9-inch (230 mm) wall | ~105 |
| 4.5-inch (115 mm) wall | ~55 |
Recommended Mortar Mix
| Wall Type | Mortar Mix (C:S) |
|---|
| Load-bearing (External) | 1:5 |
| Load-bearing (Internal) | 1:6 |
| Partition (4.5 inch) | 1:6 / 1:8 |
| Boundary Walls | 1:6 |
Best Practices for Fly Ash Brick Masonry
- No soaking required before laying (unlike clay bricks)
- But sprinkle water before applying mortar (avoid moisture absorption from mortar)
- Use 1:5 or 1:6 cement-sand mortar
- Maintain 10 mm uniform joints
- Stagger vertical joints
- Cure walls for 7 days after laying
- Use only bricks aged 21+ days from manufacture
- Verify ISI mark / IS 12894 compliance
Cost (Approximate, Indian Market)
| Type | Price per piece (Rs) | Price per 1000 (Rs) |
|---|
| Class 7.5 Fly Ash Brick | 4 – 6 | 4,000 – 6,000 |
| Class 10 Fly Ash Brick | 5 – 8 | 5,000 – 8,000 |
| Class 12.5 & Above | 7 – 10 | 7,000 – 10,000 |
| Premium / Exposed | 10 – 15 | 10,000 – 15,000 |
Applicable Standards
| Standard | Description |
|---|
| IS 12894 | Pulverized Fuel Ash-Lime Bricks — Specification |
| IS 3812 (Part 1) | Fly ash — for use as pozzolan in cement |
| IS 3495 (Parts 1–4) | Methods of tests of burnt clay building bricks (also used for fly ash) |
| IS 1905 | Code of practice for structural use of unreinforced masonry |
| IS 2212 | Code of practice for brickwork |
| IS 12440 | Specification for precast concrete stone masonry blocks |
Conclusion
Fly ash bricks represent the future of sustainable Indian masonry. They consume industrial waste, save energy, prevent air pollution, and deliver consistent quality — while being cost-competitive with traditional red clay bricks.
For any new construction in India where fly ash bricks are available, they are an excellent choice for both load-bearing and non-load-bearing walls. Always procure Class 7.5 or higher bricks conforming to IS 12894, ensure 21-day curing, and check for proper ISI marking.