Autoclaved Aerated Cement Blocks (AAC Blocks)-Properties and Advantages

Fasi Ur Rahman

4 years ago

Reading time: 1 minute

Autoclaved Aerated Concrete is an eco-friendly and certified green building material which is lightweight, load-bearing, high-insulating, durable building blocks and 3 times lighter when compared to red bricks.

Fig 1: Autoclaved-Aerated-Cement Blocks Masonry.

AAC was developed in 1924 by a Swedish architect, who was looking for alternate building material with properties similar to that of wood – good thermal insulation, solid structure and easy to work with – but without the disadvantage of combustibility, decay and termite damage.

In this article, we understand the manufacturing process, technical specification, comparison, advantages and disadvantages of AAC Blocks.

Manufacturing Process of AAC Blocks

Materials Used

1. Cement

The suitable Cement for the manufacture of AAC blocks is OPC grade 53 that sets and hardens and can bind other materials together.

2. Fly Ash

Fly ash is a waste industrial product used for the reduction of construction cost. The density of fly ash ranges from 400-1800 kg/m³. It provides thermal insulation, fire resistance, and sound absorption. The type of fly ash used is Class C which contains 20% lime (CaO) and loss of ignition not be more than 6%.

3. Limestone

Limestone is obtained either by crushing to fine powder at the AAC factory or by directly purchasing it in powder form from a merchant.

4. Aluminum Powder

Aluminum is an expansion agent. When the raw material reacts with aluminum powder, the air bubble introduced due to the reaction between calcium hydroxide, aluminum and water, and hydrogen gas is released.

Fig 2: Flow Chart of Manufacturing Process of AAC Blocks.

Step 1: Raw Material Preparation

The preparation of raw material consists of mixing fly ash with water to form fly ash slurry so that the fly ash can be mixed with other raw materials such as cement, gypsum, and aluminum powder in the required proportion.

Step 2: Dosing and Mixing

This process is an important one as the quality of the final product is dependent. The ratio in which the raw materials are to be added is decided based upon the required final product.

The ratio of Mix for AAC blocks manufacture is-

Fly Ash/Sand : Lime : Cement : Gypsum = 69 : 20 : 8 : 3

Aluminum is about 0.08% of total dry materials in the mix and the water ratio is 0.6 - 0.65.

Fly ash is pumped into a container. Once the desired weight is poured in, pumping is stopped.
Similarly, lime powder, cement, and gypsum are poured into individual containers using conveyors.
Once the required amount of each ingredient is filled into their individual containers control system releases all ingredients into the mixing drum.
Once the mixture has been churned for a set time, it is ready to be poured into molds using the dosing unit.

Step 3: Casting, Rising and Curing

The molds could be of various sizes depending upon the required quantity of mix.
Before casting, molds are coated with a thin layer of oil in order to ensure that green-cake does not stick to molds.
Aluminum reacts with Calcium Hydroxide and water and releases hydrogen gas. This leads to the formation of tiny cells causing the slurry mix to expand.
Such expansion may be thrice its original volume. The bubble size is about 2- 5mm. Thus, this is the reason behind the lightweight and insulating properties of the AAC block.
When the rising process is over, green-cake is allowed to settle and cure.
Usually rising and the pre-curing process takes around 60-240 minutes.
Autoclave Aerated concrete is cured in an autoclave – a large pressure vessel.
An autoclave is normally a steel tube of 3m diameter and 45 meters long. Steam is fed into the autoclaved at high pressure, typically reaching a pressure of 800kPa to 1200 KPa and a temperature of 180°C.

Fig 3: High-Pressure tank for Steam Curing.

Step 4: Demoulding and Cutting

Once it had achieved cutting strength, it is de-molded and cut as per requirements.
Generally, market available sizes of AAC Blocks are
600 x 200 x 100, 600 x 200 x 150, 600 x 200 x 200.

Technical Specification of AAC Blocks and Clay Bricks

Property	Units	AAC Block	Clay Brick
Size	mm	600 x 200 x (75 to 300),	230 x 75 x 115
Size Tolerance	mm	± 1.5	± 05 to 15
Compressive Strength	N/mm 2	3 – 4.5 (IS 2185 part 3)	2.5 to 3.5
Normal Dry Density	Kg / m 3	550 – 650	1800
Sound Reduction Index	Db	45 for 200 mm Thick Wall	50 for 230 mm Thick Wall
Fire Resistance	Hrs.	2 to 6 (Depending on Thickness)	2
Thermal Conductivity “K”	W / m-k	0.16 – 0.18	0.81
Drying Shrinkage	%	0.04% (Size of block)	-

Comparison between AAC Blocks and Clay Brick

Parameter	AAC Block	Clay Bricks
Structural Cost	Steel Saving Upto 15%	No Saving
Cement Mortar for Plaster & Masonry	Requires less due to flat, even surfaces & less number of joints	Requires more due to irregular surface and more number of joints.
Breakage	Less than 5%	Average 10 to 12 %
Construction speed	Speedy construction due to its big size, light weight & ease to cut in any size or shape	Comparatively slow
Quality	Uniform & Consistent	Normally varies
Fitting & Chasing	All kind of fitting and chasing possible	All kind of fitting and chasing possible
Carpet Area	More due to less thickness of walling material	Comparatively low
Availability	Anytime	Shortage in monsoon
Energy Saving	Approx. 30% reduction in air-conditioned load	No such saving
Chemical Composition	Sand/Flyash used around 60 - 70 % which reacts with Lime & Cement to form AAC	Soil is used which contains many inorganic impurities like sulphates etc. resulting in efflorescence

Advantages of AAC Blocks

1. Eco-Friendly and Sustainable

The use of recycled industrial waste (fly ash), non-toxic ingredients, no emitting gases, and fewer energy consumptions makes the ACC Blocks eco-friendly and sustainable.

2. Lightweight

The AAC Blocks are 3 to 4 times lighter than bricks, 30% lighter than that of concrete which helps in reducing the dead load of the building, thereby allowing construction of taller buildings.

3. Thermally Insulated & Energy Efficient

Tiny air pores and thermal mass of blocks provide excellent thermal insulation, thus reducing heating and air conditioning costs of a building.

4. Fire Resistant

Non-combustible and fire-resistant up to 1600° C which can withstand up to 6 hours of direct exposure.

5. Acoustic Performance

As the AAC block is porous in nature, the sound absorption quality is superior. It offers sound attenuation of about 42 dB, blocking out all major sounds and disturbances which makes it ideal for schools, hospitals, hotels, offices, multi-family housing and other structures that require acoustic insulation.

6. Easy Workability and Design Flexibility

AAC blocks can be easily cut, drilled, nailed, milled and grooved to fit individual requirements.

7. Seismic Resistant

Lightweight blocks reduce the mass of a structure, thus decreasing the impact of an earthquake on a building. Non-combustible nature provides an advantage against fires, which commonly accompany earthquakes.

8. Faster Construction

Construction of AAC Blocks reduces the construction time by 20%. As different sizes of blocks help reduce the number of joints in wall masonry. The lighter weight of the blocks makes it easier and faster to transport, place and construct the masonry.

Disadvantages of AAC Blocks

Installation during rainy weather aircrete is known to crack after installation, which can be avoided by reducing the strength of the mortar and ensuring the blocks are dry during and after installation.
As the AAC Blocks are brittle nature, they need to be handled more carefully than clay bricks to avoid breakage.
The brittle nature of the blocks requires longer, thinner screws when fitting cabinets and wall hangings and wood-suitable drill bits or hammering in.
Insulation requirements in newer building codes of northern European countries would require very thick walls when using AAC alone. Thus many builders choose to use traditional building methods installing an extra layer of insulation around the entire building.