Benefits of Fly Ash in Soil Modification, Soil Stabilization, and Full Depth Reclamation

What is Fly Ash?

Fly ash is a byproduct from burning coal which makes steam to generate electricity.  When burning coal, combustion particles rise out of the combustion chamber with flue gasses. They are captured in filters to prevent them from reaching the atmosphere and collected for disposal or beneficial reuse.  These particles are called fly ash.

There are two types of fly ash, Class C and Class F.  Class C has self-cementing properties and is used in the production of concrete as a substitute for Portland Cement, and as a chemical stabilizing & modifying agent to dry and/or strengthen poor soils.  Class F has very little self-cementing properties, but can be combined with additives such as quicklime, hydrated lime, or cement (portland or hydraulic) to create cementitious compounds for the same purposes.

Fly Ash in Modification, Stabilization and FDR

Soil Modification: Given its cementitious properties, fly ash can dry down wet soils and increase the strength of each fill layer. It works best in sandy/silty soils, but can be very effective in lean clays as well.

Lime Kiln Dust

Soil & Base Stabilization: Using fly ash to strengthen the top 8”-14” of subgrade can decrease the thickness of aggregate base and/or pavement (asphalt or concrete) needed to achieve the structural design strength of the overall pavement section.  Again, it works best in sandy, silty soils, but is also a great option to stabilize existing aggregate base when performing parking lot repairs or upgrades.

Lime Kild Dust

Full Depth Reclamation: Depending on the existing make-up of the current pavement & aggregate base, fly ash can be used by itself, or in combination with other additives, to rehabilitate entire pavement sections in place.

Fly ash yields less strength gain than Portland Cement.  Typically, you need to use twice as much product to achieve similar results when compared to cement.  However, if a fly ash source is within proximity of a project, utilizing it could be a better value and is worth comparing.


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