The correct answer is Fly ash. Key Points - Fly ash is a byproduct of burning pulverized coal in electric power generating plants. - Fly ash utilization, especially in concrete, has significant environmental benefits including: - Increasing the life of concrete roads and structures by improving concrete durability - Net reduction in energy use and greenhouse gas and other adverse air emissions when fly ash is used to replace or displace manufactured cement - Reduction in the amount of coal combustion products that must be disposed of in landfills - Allotropy is the existence of a chemical element in two or more forms, which may differ in the arrangement of atoms in crystalline solids. - Allotropes of the carbon atom are diamond, graphite, graphene, and buckminsterfullerene.