Fly ash bricks vs AAC blocks – Features, Preparation, and difference

Fly ash bricks and AAC blocks both are used to construct walls. They have their own properties. In this article, I have given Fly ash bricks vs aac blocks in detail. So, let us discuss one by one.

What is Fly ash brick?

Fly ash brick is masonry units of building construction, which is made up of fly ash, sand, cement, and gypsum (or sometimes directly Ordinary Portland cement) along with proper mix with water.

The fly ash is generally the remnants of the burnt coals along with different gases fumes like Sulphur, carbon, mercury, produced generally from thermal plants.

The fly ash dust contains harmful chemical compounds byproduct which if left in the environment without proper disposal then can affect the environment, atmosphere, and ecology of the surrounding.

Fly ash bricks vs aac blocks - Features, Preparation, and difference

And moreover, the fine dust disperses very fast in the air which can cause respiratory problems to the beings and even plants. Thus, fly ash brick is the proper utilization of such waste biohazardous byproducts as such dust is compacted in solid form along with the proper mix.

It must be noted that fly ash alone is biohazardous and cause-effect to the environment but, the fly ash brick are not hazardous to human use and environment.

How fly ash bricks are made?

Fly ash brick is made by proper mix formation of fly ash, sand, cement, and gypsum along with water sprinkles. The mix is generally prepared or batch in the mechanical mixture.

The strength of the fly ash bricks depends upon the different composition percentages of these materials. The semi-dry mix is subjected to highly vibratory mechanical press for compaction.

The mechanical press can apply pressure up to 28 MPa. Then the compacted form of brick is kept in sunlight for 24 hrs – 48 hrs.

After which the sun-dried blocks are kept in shade for air dry to 1 day- 2days. The bricks are finally left for curing 14days – 21 days after the steam bath at 66° C.

Features of fly ash brick

The fly ash brick is formed of the unused hazardous byproduct of the thermal plants so these bricks help to recycle such byproduct and prevent the environmental biohazard.

Moreover, these bricks are very good against sulfate and acid attacks as fly ash is formed with fumed gases of Sulphur and other gases with burnt coal remnants.

There is no efflorescence in such bricks compared to clay bricks as lime or carbonaceous material use in such brick is very low as well as this brick is well cured.

The bricks are almost made up of ash so, they are very good at resistant to fire. Thus can also used to form the chimney. This brick has a comparatively low water absorption rate, which is about 15 percent.

They are generally lightweight and of modular sizes so easy for a mason to handle n work with it. The compressive strength of such bricks ranges normally between 7MPa to 10 MPa

However, the brick is only useful for the subtropical area where the climate is warm and not useful for the cold environment as it doesn’t have well thermal conductivity.

And such bricks are advantageous only to the area nearby the thermal plants where there is an abundance of the fly ash otherwise such bricks might not be economically best.

What is Autoclaved aerated concrete block – AAC block

Autoclaved aerated concrete blocks are another type of modern masonry units formed of a mix of lime, sand(along with fly ash), cement, and admixture ( generally aluminum paste).

As the name itself explains, these blocks act kind of concrete blocks that are made with micro air-entrained inside it. Thus such blocks are very light weighted. The aluminum paste added to the mix made such bricks more lightweight, finely microporous, and high insulation.

Fly ash bricks vs AAC blocks - Features, Preparation, and difference

The quartz from sand and calcium hydroxide from cement and lime in mix form calcium silicate hydrates which are responsible for its strength.

The aluminum powder also reacts with calcium hydroxide to form hydrogen gas, which is mainly responsible to create microspores giving cellular structure to concrete.

The brick along with a comparison of its equivalent density of other masonry units seems to have high compressive strength. But the strength alone of brick is not suitable to use it as a structural construction material.

How Autoclaved aerated bricks are made?

The mix is made with the proper proportion of the lime, sand, cement, additional admixture (like an aluminum paste) along with the sprinkle of the water. And the wet mix is subjected to dosing units.

The dosing units provide the required quantity of mix to mould and then moulded into the shapes. Dosing and mixing should be done continuously in the cycle in order to prevent rapid hardening. After moldings, the moulded blocks are executed to autoclave under heat and steam pressure for 12hrs. 

The heating temperature in the autoclave is maintained around 190°C and steam pressure is maintained around 8- 12 bars.

Due to steam in Autoclave, the curing process also occur simultaneously. And the blocks taken freshly from autoclave are known as green cakes which are further subjected to the demoulding and cutting process.

This process depends upon the use of the mechanical equipment and desire of the owner to provide the required size. Then the blocks are distributed to users.

Features of AAC Blocks

AAC Blocks contains numerous micropores of size 1mm-5mm forming the cellular structure within concrete.

The air-entrained is about 60%— 85 %Thus due to this air-entrained inside the concrete mass the weight of block very low. About 80 percent of a dead load of similar concrete volume mass is decreased. So such blocks are very useful to decrease dead load due to the nonstructural components.

Moreover, due to such air-entrained, there is very good thermal and sound insulation, as we know air is a very good insulating medium. Thus such bricks are used in construction like auditorium, hall, acoustic buildings.

They are very good for thermal insulation as well as fire-resistant, so they maintain good room temperature. Due to their lightweight, they are very useful to use for masons to handle and transport.

Although there is the presence of the numerous pores they are moisture-resistant because of the even distribution of such pores. They are eco friendly too as it uses the naturally available materials and doesn’t produce any toxic byproduct.

Fly ash bricks vs AAC blocks

Fly ash bricks vs aac blocks - Features, Preparation, and difference
Major componentFly ash bricks use the fly ash as major raw material.In Acc blocks , aluminum acts as major component factor.
Percentage compositionFly ash – 50%-80% Sand – 20%-40% Cement – 5%-30% Gypsm – 2% -5%Lime – 10%-15% Sand (often with fly ash)– 65%-70% Cement –20%-25% Gypsm – 1%-2% Aluminium – 0.05%- 0.08%
Specific gravity and densityDensity – 550kg/m³- 650 kg/m³ Sp. Gravity – 0.6- .65Density – 1700kg/m³-1850 kg/m³ Sp. Gravity – 1.8- 2
Size (mm)190*90*90(modular) 230*110*70/30 (non modular) ) (market size nearly equal as suggested from IS 13757)  600*200*75/300 (market size nearly equal as suggested from IS 2185 part 3)  
Thermal conductivityNot good insulating units. Which is about 0.9 to 1.05 Watt/m² °CVery good insulation. Which is about 0.16 to 0.18 W/Mk
Dead load reduction (comparision to concrete)30% -35%Upto 80%
Compressive Strength7N/mm² -10 N/mm²4 N/mm²- 8 N/mm²
Water Absorption15%-20%Relatively high due to capillary suction
EfflorescenceComparatively less as it is cured more and contains less lime contentComparatively high as it contains more amount of lime
Sound resistance40 dB45dB – 200 dB (depends on thickness)
Fly ash bricks vs AAC blocks

I hope this article on “fly ash bricks vs aac blocks” remains helpful for you.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Rajan Shresth

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"Structural Engineer" with over 5 years of experience in estimation, structural design, and surveying. I am passionate about using his skills to create safe and sustainable structures. I am also a keen writer, and I enjoy sharing my knowledge and experiences with others.

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