Fly ash brick are building materials, specifically masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

 units, containing Class C fly ash

Fly ash

Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...

 and water. Compressed at 4,000 psi and cured for 24 hours in a 150 °F (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

Calcium oxide

Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

 in class C fly ash, the brick can be described as "self-cementing". The manufacturing method is said to save energy, reduce mercury pollution, and costs 20% less than traditional clay brick manufacturing.

The raw materials

The raw material that is used for fly ash brick are: Fly ash, Sand/Stone dust, Lime, Gypsum and Cement. Flyash bricks are lighter than clay bricks.

• Fly ash
• Sand
  Sand
  Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
  
   or Stone dust
• Lime
  Agricultural lime
  Agricultural lime, also called aglime, agricultural limestone, garden lime or liming, is a soil additive made from pulverized limestone or chalk. The primary active component is calcium carbonate...
  
  
• Gypsum
  Gypsum
  Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
  
  
• Cement
  Cement
  In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
  
  

Advantages

1. Due to high strength, practically no breakage during transport
   Transport
   Transport or transportation is the movement of people, cattle, animals and goods from one location to another. Modes of transport include air, rail, road, water, cable, pipeline, and space. The field can be divided into infrastructure, vehicles, and operations...
   
    & use.
2. Due to uniform size of bricks mortar
   Mortar (masonry)
   Mortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...
   
    required for joints & plaster reduces almost by 50%.
3. Due to lower water penetration seepage of water through bricks is considerably reduced.
4. Plaster of Paris / Gypsum Plaster can be directly applied on these bricks without a backing coat of plaster
5. These bricks do not require soaking in water for 24 hours. Only sprinkling of water before use is enough.

Dis-advantages

1. Poor quality and outlook in colour without plastering
2. Mechanical bonding strength is weak. But this can be rectified by adding marble waste.
3. Limitation of size. Only modular size can be produces. Large size will have more breakages