Today’s Fly Ash
Fly ash is formed from the non-combustible minerals found in coal. The powdered coal is conveyed by air to a furnace where the carbon is ignited in an atmosphere of 1900 to 2100 degrees F. The non-combustible minerals become molten as they are carried through the firing zone by the air stream. The molten minerals solidify in this moving air stream which gives approximately 60% of the fly ash particles a spherical shape. Similar to the fact that Portland cement is manufactured by firing raw materials at 2700 degrees F., the non-combustible minerals in the coal become reactive due to the formation of amorphous silica in the coal-fired furnace.
Chemical Interaction of Fly Ash with Concrete-Making Materials:
The elements of chemistry of both Portland cement and fly ash are similar. Of course, the compounds formed from this elemental chemistry will be somewhat different. As can be seen from the comparative chemistries of Portland cement and fly ash, the most glaring difference in chemistry is due to the calcium oxide content.
When Portland cement hydrates, calcium hydroxide is liberated as a byproduct of the hydration process. In each cubic yard of Portland cement concrete, approximately 20% of the weight of cement is converted to calcium hydroxide. This compound contributes nothing to the strength of concrete. Calcium hydroxide is soluble in water and may be removed from concrete by leaching action. The calcium hydroxide is also available to chemical attack.
When fly ash is employed with a Portland cement this hydrated lime combines with the fly ash forming a stable cementitious compound which contributes to strength. With the calcium hydroxide forming this stable cementitious compound, the calcium hydroxide is unavailable for leaching or for chemical attack.
Mechanical Interaction of Fly Ash with Concrete-Making Materials:
Fly ash also mechanically interacts with concrete-making materials. The fly ash particles’ spherical shape acts as “ball bearings” within the concrete mix which can significantly lower the water requirement for a given slump of concrete. Fine aggregate gradation and texture determine the amount of water reduction with the use of fly ash. Fly ash used in concrete with area sands will give a water reduction of approximately one to three gallons.