What are typical proportions of portland cement and fly ash for a concrete containing fly ash? And what are the typical cost savings when a fly ash mix is compared with a non-fly ash concrete having the same strength? First we have to differentiate between the two types of ash available.


Class F ash is produced from bituminous coals commonly available in the central and eastern parts of the country. Class F ash is generally low in lime and by itself has little or no cementitious value. It slowly combines with calcium hydroxide released during portland cement hydration to form new cementing compounds. Although ultimate strength is improved, use of Class F ash sometimes results in longer setting times and lower early strengths. Class C ash is produced from subbituminous or lignite coals from western sources and has a higher lime content than Class F ash. This gives the Class C ash some cementitious qualities of its own--it will chemically react with water, even if cement isn't present. Concrete setting times and early strengths are affected far less when Class C ash is used.


Because a Class C ash has cementitious qualities of its own and Class F ash does not, the amount of Class C ash that may be used is usually greater. Cold weather is also a consideration. Early strength gain is reduced with the use of either Class C or Class F ash.


Because cement, fly ash and admixture prices vary throughout the country, cost differences will also vary. Depending on the strength requirements of the concrete, the proximity of the ash source and comparative local cement and ash prices, the cost savings for cementitious materials can range from $1.50 to $4.00 per cubic yard.


The contractor or owner benefits from the use of fly ash concrete include improved concrete placeability and finishability, reduced bleeding, easier and more efficient pumping, reduced heat of hydration and a longer period of significant strength gain.