LEED notes - fly ash

a couple of notes on fly ash concrete - 

how is fly ash made?

from the combustion of coal.  fly ash is the residue gathered from the chimneys of coal-fired power plants.  it is one of two kinds of coal ash.  the other is bottom ash, which is, you guessed it, found on the bottom of the coal furnaces.

what is fly ash made of?

many, many different compounds.  silicon dioxide, calcium oxide, and other toxic constituents (including arsenic, beryllium, mercury, thallium, et cetera).

how is it stored?

at coal power plants and in landfills.  approximately 43% of the fly ash is recycled, which is utilized in the production of Portland cement.  it is also used in the synthesis of geopolymers and zeolites.

how is fly ash classified?

it is broken down into two distinct grades - class F fly ash and class C fly ash.  the difference between the classes are the trace amounts of calcium, silica, alumina, and iron in the coal ash.  these four elements all highly influence the way coal is burned during the process of fly ash formation.  

what is class F fly ash?

class F fly ash burns anthracite and bituminous coal.  it is pozzolanic (cementitious with calcium hydroxide) in nature, containing less than 10% lime.  it requires the presence of a cementing agent, typically portland cement with water, in order to cure.

what is class C fly ash?

class C fly ash is produced from the burning of lignite or sub-bituminous coal.  it is also pozzolanic, but also has some self-cementing properties.  it also contains 20% lime content, and unlike class F, does not require an activator for the hardening process.

how is fly ash recycled?

not easily.  with increasing landfill costs and with national efforts to push sustainable design, too many u.s. coal-fired power plants are reporting minimal tonnages of fly ash recycled.  this in turn causes unnecessary purchases of unwanted acreages in landfill space.  

what are the environmental benefits of fly ash?

the main benefit is the reduced demand for virgin materials that would need quarrying.  it can also easily substitute for similarly strong materials (such as portland cement).

what are the uses of recycled fly ash?

some include: portland cement and grout, embankments and other structural fill, waste stabilization, cement clinkers, soft soils, aggregate, and cellular units of concrete (geopolymers, roofing tiles, paints, etc.).

all in all, fly ash accounts for an innovation in design credit for the LEED AP exam, and it is fairly useful (not to mention enlightening) to know that even a sturdy unit of construction, which concrete is, can still be recycled economically and (somewhat) efficiently.