definition of slag
Definition. Waste content of metal ores that floats on the surface during the process of extracting or refining metal. Containing mainly silicates, sulfates, and phosphates of calcium, slag is used principally in road building and concrete admixtures.Also called tailing.aste content of metal ores that floats on the surface during the process of extracting or refining metal. Containing mainly silicates, sulfates, and phosphates of calcium, slag is used principally in road building and concrete admixtures.