Not only can fly ash be used as a partial substitute for Portland cement when making concrete, but it can be an important ingredient to manufacture Portland cement itself.
To manufacture Portland cement, raw materials containing the elements Ca, Al and Si are mixed together and heated at high temperature in a kiln to make cement clinker. The clinker is then ground to a fine powder which we know as Portland cement. The amounts of the various raw ingredients are carefully controlled to produce different types of cement.
The raw materials can come from a variety of sources, depending on availability and chemical composition. Sources of calcium can be limestone, chalk or even oyster shells. Common alumina sources are shale and clay while and silica typically comes from sand and clay.
Since coal combustion ash contains both Al and Si, large quantities of bottom ash and fly ash are frequently used as a raw ingredient to make cement, reducing or even eliminating the need to mine clay or sand. In addition, FGD gypsum is sometimes added to the ground cement to change some of the setting properties.
By using coal combustion by-products as raw ingredients to make Portland cement, not only is by-product utilization increased, but the amount of minerals that need to be mined is also reduced.