Fly ash brick

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 can 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 in the environment, and often costs 20% less than traditional clay brick manufacturing.
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.
Clay is typically entrapped during the formation of coal. When coal is burnt, the incombustible clay particles are left behind as ash. In grate boilers, incombustible ash agglomerates as cinders through prolonged residential time. Nowadays, pulverised coal technology is preferred due to its improved energy-efficiency. In this case, the ground clay escapes along with flue gases, settling as ash in bag filters or electro static precipitators (ESPs). This gives rise to the name 'fly ash'.
A possible material mix for the production of fly ash brick is as follows:
The strength of fly ash brick manufactured with the above compositions ranges between 7.5 and 10 MPa (1,090 and 1,450 psi). 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. Autoclaving increases the hardness of the block by promoting quick curing of the cement. Gypsum acts as a long term strength gainer. The chemical reaction due to the aluminum paste provides AAC its distinct porous structure, lightness, and insulating properties. The aforementioned properties set it apart from 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. Using these blocks in buildings reduces the dead load, allowing one to save around 30 to 35% of structural steel, and concrete.
Commercial processes fall into two categories; the lime route, and the cement (OPC) route where the latter is used as a source of lime. In the lime route, the composition is fly ash (50%), slaked lime (30%), and anhydrous gypsum (20%), to which 3 to 4 times as much stone dust, sand or any inert filler material can be added. In the cement route, the composition is fly ash (76%), OPC (20%), and anhydrite (4%), to which 3 to 4 times as much filler material can be added.
The following properties of fly ash affect the strength and look of fly ash bricks.
Based on boiler operations, fly ash can be additionally classified as LT (low temperature) and HT (high temperature). LT fly ash containing amorphous phases is generated where boiler temperature is not more than 800 °C (1,470 °F), 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.