Portland cement and fly ash can both be key ingredients in concrete mixes. Although concrete is sometimes use interchangeable with cement, concrete is actually a mixture of a base with larger aggregate materials. The aggregates are typically different types of stone, but the base is most often Portland cement, a mixture of clays, metallic elements and minerals like calcium and silica. While Portland cement can have fly ash in it, fly ash concrete is typically made with a base of full fly ash instead of any Portland cement.
Portland cement is mined and manufactured from a variety of sources. Portland cement is not a brand name, and many manufacturers can create it with their own mix of minerals. Many of these minerals are recovered from limestone mines or other excavation processes, where they are processed and mixed to create cement. Fly ash is a type of residue recovered from the exhaust of coal mills, where the ash is a contaminant in the smoke.
Cost and Use
Fly ash is easy to use and is recycled from a process already at work, making it less expensive than Portland cement. Since Portland cement can come in different grades and must be manufactured from a variety of compounds it typically has a higher price. Fly ash can also have different grades, and its low price may mean low-quality concrete if the fly ash mixture is too coarse.
Fly ash absorbs moisture more easily than Portland cement. If the concrete contains a high amount of fly ash, the water used to create it will be absorbed more readily and the concrete mixture will become thicker and more resistant to movement. This is a negative attribute if the concrete needs to be carefully molded or sculpted into specific patterns.
The same qualities that cause fly ash to absorb moisture more easily than cement also make fly ash stronger. Concrete created with fly ash tends to be more stable and exhibit a higher grade of overall strength than concrete made with traditional cement products.