Fly ash brick

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
Fly ash bricks

Fly ash brick (FAB) is a building material, specifically masonry units, containing class C or class F fly ash and water. Compressed at 28 MPa (272 atm) and cured for 24 hours in a 66 °C steam bath, then toughened with an air entrainment agent, the bricks last for more than 100 freeze-thaw cycles. Owing to the high concentration of calcium oxide in class C fly ash, the brick is described as "self-cementing". The manufacturing method saves energy, reduces mercury pollution, and costs 20% less than traditional clay brick manufacturing.

Ever since FaL-G (Fly ash-Lime-Gypsum) process is introduced in 1991, fly ash brick activity has been revolutionised in India. FaL-G technology ( developed and patented by Dr Bhanumathidas and Kalidas has simplified the process by adding gypsum to fly ash+lime/cement, converting the calcium aluminates into calcium alumino-sulphates resulting in to achieve high early strengths.Thus FaL-G brick does not need any pressure and gets cured at ambient temperature of 20-40 °C. By avoiding both press and heating chamber, FaL-G process has brought down the multi-million plant cost to a few hundred thousand, within the reach of micro units. This has facilitated proliferation of over 18000 units in the country as of 2016.


Coal dust has historically been collected as a waste product from homes and industry. During the nineteenth century coal ash was taken by 'scavengers' and delivered to local brick works, where the ash would be mixed with clay. The income from the sale of ash would normally pay for the collection of waste.[1]

During the formation of coal, clay is entrapped through natural process. During the combustion, carbon is burnt leaving the incombustible clay particles as ash. In age old grate boilers, through prolonged residential time these incombustible ash used to agglomerate as cinder. But, ever since pulverised coal technology is put to practice in order to improve energy-efficiency, the ground clay remains in hearth for a fraction of second and flies (escapes) along with flue gases, settling as ash in bag filters or electro static precipitators (ESPs). Since it is 'flied ash' it is named as 'fly ash'.[2]

The raw materials[edit]

Funton Creek. Presumably this was a navigable channel in the days when the brick works was serviced by barges bringing fly-ash from London and returning with loads of bricks.

The raw materials for Fly Ash Bricks are:

Material Mass
Fly ash 60%
Sand/Stone dust 30 %
Ordinary Portland Cement/(Lime+Gypsum) 10%
Total formula of material 100%

The strength of fly ash brick manufactured with the above compositions is normally of the order of 7.5 N/mm2 to 10 N/mm2. Fly ash bricks are lighter and stronger than clay bricks.

Main ingredients include fly ash, water, quicklime or lime sludge, cement, aluminum powder and gypsum. The block hardness is being achieved by cement strength, and instant curing mechanism by autoclaving. Gypsum acts as a long term strength gainer. The chemical reaction due to the aluminum paste provides AAC its distinct porous structure, lightness, and insulation properties, completely different compared to other lightweight concrete materials. The finished product is a lighter Block - less than 40% the weight of conventional Bricks, while providing the similar strengths. The specific gravity stays around 0.6 to 0.65. This is one single most USP of the AAC blocks, because by using these blocks in structural buildings, the builder saves around 30 to 35% of structural steel, and concrete, as these blocks reduce the dead load on the building significantly.

Notwithstanding above, FaL-G process is practiced in two ways: lime route and cement (OPC) route where the latter is availed as source of lime. In lime route the composition is fly ash (50%) slaked lime (30%) and anhydrite gypsum (20%) to which 3 to 4 times of stone dust, sand or any inert filler material can be added. In cement route the composition is fly ash(76%), OPC (20%) and anhydrite (4%) to which 3 to 4 times of filler material can be added.

There are three important ingredients of fly ash which affect the strength and look of fly ash brick.

  1. Loss on Ignition (LOI); fly ash loses weight when it burns at about 1000 °C due to presence of carbon and water. The weight loss happens due to carbon combustion and moisture evaporation is called "Loss on Ignition(LOI)". This is expressed as percentage. The lower the loss of Ignition, the better will be fly ash. As per BIS it should not be more than 5%.
  2. Fineness; the fine fly ash has more surface area available to react with lime, thus more will be the pozzolanic activity of fly ash. The greater pozzolanic activity contributes to the strength of fly ash brick. As per BIS it should not be more than 320 m2/kg.
  3. Calcium (CaO) content; the pozzolanic reactivity of fly ash is more in high calcium fly ash. The greater the pozzolanic activity leads to higher the strength of fly ash brick. As per ASTM C618 fly ash is classified into two; Class C contains more than 10% lime and Class F fly ash contains less than 10% lime.[3]

However, based on boiler operations, the inventors of FaL-G process, Dr Bhanumathidas and Kalidas have accorded additional classification as LT (low temperature) and HT (high temperature) fly ash. LT fly ash containing amorphous phases is generated where boiler temperature is not more than 800 °C, whereas HT fly ash containing glassy reactive phases is generated at more than 1000 °C in super thermal plants. LT fly ash reacts well with lime whereas HT fly ash reacts well with OPC (Ref. Fly ash for Sustainable Development authored by Dr N Bhanumathidas and N Kalidas; published by Ark Communication 2002).


  1. It reduces dead load on structures due to light weight (2.6 kg, dimension: 230 mm X 110 mm X 70 mm).
  2. Same number of bricks will cover more area than clay bricks
  3. High fire Insulation
  4. Due to high strength, practically no breakage during transport and use.
  5. Due to uniform size of bricks mortar required for joints and plaster reduces almost by 50%.
  6. Due to lower water penetration seepage of water through bricks is considerably reduced.
  7. Gypsum plaster can be directly applied on these bricks without a backing coat of lime plaster.
  8. These bricks do not require soaking in water for 24 hours. Sprinkling of water before use is enough.


  1. Mechanical strength is low, but this can be rectified by adding marble waste or mortar between blocks.
  2. Limitation of size. Only modular size can be produced. Large size will have more breakages.
  3. It is only good for the places like subtropical area or area where climate is warm because it doesn't absorb heat. But during cold it is not helpful.


  1. ^ Edwin Chadwick (1842). Report to Her Majesty's Principal Secretary of State for the Home Department, from the Poor Law Commissioners, on an Inquiry into the Sanitary Condition of the Labouring Population of Great Britain. Clowes for HMSO. p. 53.
  2. ^ Fly ash for Sustainable Development authored by Dr N Bhanumathidas and N Kalidas; Ark Communications 2002
  3. ^ Magadh, Brick. "Effect of quality of fly ash on Brick".