4.3.2 Fly Ash
Reduce the use and depletion of finite materials, e.g. limestone, through the reuse of waste products such as fly ash as a percentage of Portland cement in concrete production.
Fly ash is a fine, glass-like powder which is a pozzolan - a material that reacts with calcium hydroxide (lime) in the presence of water to form cement. Additional waste products that are cement substitutes include ground iron slag from blast furnaces, silica fume, and rice hull ash.
Ash that is captured as it flies from boiler chimneys at coal burning power generation facilities can replace a percentage of Portland cement used in concrete production. Typically, up to 30% of the mass of Portland cement in a concrete mix can be replaced by Class F fly ash, but High Volume Fly Ash concrete mixes can utilize 50% cement replacement.
Fly ash can be a cost effective partial substitute for Portland cement.
Using fly ash or other waste products in concrete diverts them from landfills and reduces demand for virgin material. Concrete containing fly ash can be more durable and easier to work with, as the glass-like powder acts as a lubricant making it easier to pump and fill form work with up to 10% less water. It also reduces the need for cement production, which requires large amounts of energy; reducing the Portland cement content of concrete reduces it’s embodied energy and carbon footprint. In addition, using silica fume as an admixture increases concrete’s strength and weathering performance. Concrete containing a high percentage of slag is well suited for installations that will be exposed to sea water or other sources of sulfates.
The project schedule must accommodate increased curing time due to cement substitute content. Slag properties are closest to Portland Cement and have the least schedule impact. The current high demand for cement substitutes has also increased procurement time and, in some case, price. Silica fume, for instance, has become an expensive admixture used primarily for structures such as bridges that require the added strength and weathering performance that it provides.
4.3.1 Recycled Content