AAC Blocks (Autoclaved Aerated Concrete) — Complete Guide
AAC Blocks (Autoclaved Aerated Concrete Blocks) are precast lightweight building blocks made from cement, fly ash, lime, gypsum, and an aerating agent (aluminium powder). The aluminium reaction creates hydrogen gas bubbles, producing a cellular, porous structure with millions of tiny air pockets — making AAC blocks 3 to 4 times lighter than red clay bricks.
AAC blocks are governed by IS 2185 (Part 3) — Concrete Masonry Units — Specification for Autoclaved Cellular (Aerated) Concrete Blocks.
What are AAC Blocks?
AAC stands for Autoclaved Aerated Concrete — a precast, lightweight concrete material with a cellular structure. The blocks are cured under high pressure steam in an autoclave at about 190°C, which gives them excellent dimensional stability and strength.
AAC blocks were invented in Sweden in 1924. In India they are popularly known by trade names like Siporex, Aerocon, Magicrete, Biltech.
Composition / Raw Materials
| Material | Percentage | Function |
|---|
| Fly Ash / Silica Sand | 65 – 75% | Main filler |
| Cement (OPC 53) | 15 – 20% | Binder |
| Quick Lime (CaO) | 10 – 15% | Activator |
| Gypsum | 2 – 5% | Setting control |
| Aluminium Powder | 0.05 – 0.08% | Aerating agent |
| Water | As required | Hydration |
Manufacturing Process
- Slurry Mixing: Fly ash, cement, lime, gypsum, aluminium powder, water mixed
- Pouring into Moulds: Slurry poured into large rectangular moulds
- Rising / Pre-Curing: Aluminium reacts with lime, generating hydrogen bubbles. The cake rises like bread
- Cutting: The cake is cut into precise blocks using wire-cutters
- Autoclaving: Blocks steam-cured at 190°C and 12 bar pressure for 8–12 hours
- Packing & Dispatch: Blocks palletised and shipped
Standard Sizes
| Length (mm) | Height (mm) | Thickness (mm) | Application |
|---|
| 600 | 200 | 75 | Light partition |
| 600 | 200 | 100 | Internal partition |
| 600 | 200 | 125 | Partition / external (light) |
| 600 | 200 | 150 | External / partition |
| 600 | 200 | 200 | External load-bearing |
| 600 | 200 | 225 | External / load-bearing |
| 600 | 200 | 250 | External thick / sound walls |
| 600 | 300 | various | Faster construction (taller block) |
One AAC block (600 x 200 x 200 mm) replaces approximately 8 standard red clay bricks.
Physical Properties
| Property | Value |
|---|
| Density | 500 – 750 kg/m³ (lightest among masonry) |
| Compressive Strength | 3 – 6 MPa (Grade 1 to 3) |
| Water Absorption | 10 – 15% |
| Drying Shrinkage | < 0.04% |
| Thermal Conductivity (K) | 0.16 – 0.21 W/m.K (excellent insulation) |
| Sound Insulation | 40–45 dB (for 200 mm wall) |
| Fire Resistance Rating | 4 – 6 hours (depending on thickness) |
| Dimensional Tolerance | ± 1.5 mm (highly accurate) |
| Weight per Block (600x200x200) | 9 – 11 kg |
Grades (IS 2185 Part 3)
| Grade | Dry Density (kg/m³) | Compressive Strength (MPa) |
|---|
| Grade 1 (Class A) | 451 – 550 | ≥ 3.0 |
| Grade 1 (Class B) | 551 – 650 | ≥ 4.0 |
| Grade 2 (Class A) | 651 – 750 | ≥ 5.0 |
| Grade 2 (Class B) | 751 – 850 | ≥ 6.0 |
Uses of AAC Blocks
- Internal partition walls in RCC framed buildings
- External walls (with 200–250 mm thickness)
- High-rise buildings (significantly reduces dead load)
- Commercial complexes and IT parks
- Hotels and hospitals
- Educational institutions
- Industrial buildings
- Areas requiring thermal insulation
- Areas requiring sound insulation (theatres, recording studios)
- Fire-rated walls
- Lift shafts and stair walls
- Boundary walls (less common)
AAC Mortar / Adhesive
AAC blocks are typically laid using thin-bed mortar / adhesive — a polymer-modified cementitious mortar applied in 2–3 mm thick layer. Traditional cement-sand mortar (1:6) can also be used but with thicker 10 mm joints.
Popular AAC Mortars
- Dr. Fixit Roff AAC Block Adhesive
- Magicrete Block Joining Mortar
- Birla A1 Block Fix
- Fosroc Brickaid
- Sika MonoTop Block Joining Mortar
Advantages
- Lightweight (1/3 the weight of clay bricks)
- Reduces dead load — smaller foundation and steel needed
- Excellent thermal insulation (K = 0.16 – 0.21 W/m.K) — lowers AC bills
- Excellent sound insulation (40–45 dB)
- Fire resistant (4–6 hours)
- Faster construction (1 block = 8 bricks)
- Lower mortar consumption (5–7% vs 25% for bricks)
- Smooth uniform surface — less plaster needed
- Eco-friendly (uses fly ash, no kiln firing)
- Termite resistant
- Earthquake resistant (light weight reduces seismic load)
- Easy to cut, drill, chase for services
- Government / green building incentives (LEED, IGBC credits)
- Dimensionally accurate (± 1.5 mm)
- Lower total construction cost when all factors considered
Disadvantages
- Higher per-block cost than red clay bricks
- Lower compressive strength — not suitable for heavy load-bearing
- Brittle edges chip easily during transport
- Cannot bear point loads — needs special anchor fasteners
- Requires special wall plugs/anchors for fixings
- Initial absorption of water from mortar (needs proper wetting)
- Special skill needed for proper installation
- Plastering technique slightly different
- Limited availability in remote areas
- Wastage during cutting
AAC vs Red Clay Brick — Cost Analysis
| Parameter | AAC Blocks | Red Clay Bricks |
|---|
| Material Cost | Higher (Rs 60–100 / block) | Lower (Rs 8–12 / brick) |
| Mortar Consumption | 5–7% | 25–30% |
| Construction Speed | 3–4x faster | Slower |
| Plaster Required | Less (smooth surface) | More (rough) |
| Foundation Load | Lower | Higher |
| Steel Required | Less (reduced loads) | More |
| Energy Savings (AC bills) | 15–20% lifetime savings | Standard |
| Total Project Cost | Often equal or lower | Standard reference |
Quantity Estimation
| Block Size | Blocks per m² of wall |
|---|
| 600 x 200 x 100 mm | 8.33 |
| 600 x 200 x 150 mm | 8.33 |
| 600 x 200 x 200 mm | 8.33 |
| 600 x 300 x 200 mm | 5.55 |
For 1 cubic metre of 200 mm thick wall: approximately 41 blocks (600x200x200 mm).
Tests for AAC Blocks (IS 6441 / IS 2185)
- Compressive Strength Test — IS 6441 (Part 5)
- Dry Density Test — IS 6441 (Part 1)
- Drying Shrinkage Test — IS 6441 (Part 2)
- Moisture Movement Test
- Thermal Conductivity
- Dimensional Tolerance Check
- Water Absorption Test
- Visual Inspection (cracks, edges)
Best Practices for AAC Block Masonry
- Use thin-bed adhesive mortar (2–3 mm joints)
- Pre-wet the block contact surface (don't soak the block)
- Maintain staggered joints (overlap by 100 mm)
- Use special anchors / chemical anchors for heavy fixings
- Provide RCC bands at lintel and roof levels
- Use chicken mesh at junctions with RCC columns/beams
- Apply 8–10 mm plaster (sufficient due to smooth surface)
- Cure walls for 7 days
- Store blocks under cover to avoid moisture absorption
Cost (Approximate, Indian Market)
| Size (mm) | Price per block (Rs) | Price per m³ (Rs) |
|---|
| 600 x 200 x 75 | 30 – 45 | 4,000 – 5,500 |
| 600 x 200 x 100 | 40 – 60 | 4,200 – 5,800 |
| 600 x 200 x 150 | 55 – 80 | 4,300 – 6,000 |
| 600 x 200 x 200 | 75 – 100 | 4,500 – 6,200 |
| 600 x 200 x 230 | 85 – 120 | 4,600 – 6,500 |
Popular AAC Block Brands in India
- Siporex (Pioneer brand in India)
- Aerocon (HIL)
- Magicrete
- Biltech
- UltraTech Xtralite AAC
- Birla Aerocon
- Renacon AAC
- JK Smartblox
- Wonder AAC
- Eco Green AAC Blocks
Applicable Standards
| Standard | Description |
|---|
| IS 2185 (Part 3) | Concrete masonry units — AAC blocks |
| IS 6441 (Parts 1–14) | Methods of test for autoclaved cellular concrete products |
| IS 1905 | Code of practice for structural use of unreinforced masonry |
| IS 6042 | Code of practice for construction of light-weight concrete block walls |
| IS 4326 | Earthquake resistant design and construction of buildings |
Conclusion
AAC blocks have revolutionised modern Indian construction. With dramatically lower weight, superior thermal and sound insulation, fire resistance, eco-friendly manufacturing, and faster construction speed — they offer benefits that traditional red clay bricks simply cannot match.
For RCC framed buildings (all sizes), high-rise residential and commercial projects, hotels, hospitals, and any structure where reducing dead load matters — AAC blocks are the preferred modern choice. Always specify IS 2185 (Part 3) compliant blocks, use proper thin-bed adhesive mortar, and follow installation best practices for crack-free walls.