Fly Ash

Fly ash is a fine powdery byproduct of burning pulverized coal in power plants, captured from flue gases, used in construction chiefly as a partial replacement for Portland cement in concrete. Its fine, glassy particles react with calcium hydroxide released during cement hydration in a process called the pozzolanic reaction, which can improve the finished concrete's workability, long-term strength and resistance to sulfate attack while reducing the volume of cement, and therefore carbon dioxide emissions, that a given batch of concrete requires. Its use in concrete became widespread in the second half of the twentieth century as power plants sought a use for a byproduct that would otherwise go to landfill, and it remains one of the most common supplementary cementitious materials in structural concrete today, alongside slag and silica fume.