Blast Furnace Slag

Blast Furnace Slag  Blast furnace (BF) slag is a nonmetallic by product produced during the process of iron making in a blast furnace. It consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. The molten slag usually absorbs most of the sulfur from the blast furnace charge. Blast furnace slag is mildly alkaline and exhibits a pH in solution in the range of 8 to 10. Although blast furnace slag contains a small component of elemental sulfur (1 % to 2 %), the leachate tends to be slightly alkaline and does not present a corrosion risk to steels in pilings or to the reinforcement steels embedded in concrete structures made with blast furnace slag cement or aggregates. In certain situations, the leachate from blast furnace slag may be discolored (characteristic yellow/green color) and have a sulfurous odor. These properties appear to be associated with the presence of stagnant or slow moving water that has come in contact with the slag. The stagnant water generally exhibits high concentrations of calcium and sulfide, with a pH as high as 12.5. When this yellow leachate is exposed to oxygen, the sulfides present react with oxygen to precipitate white/yellow elemental sulfur and produce calcium thiosulfate, which is a clear solution. Aging of blast furnace slag can delay the formation of yellow leachate in poor drainage conditions but does not appear to be a preventative measure, since the discolored leachate can still form if stagnant water is left in contact with the slag for an extended period. BF slag is normally produced in two forms (Shown in Fig. 1). These two forms are described below.  Fig 1 Granulated and air cooled BF slag 1.   Granulated blast furnace slag – When liquid slag is cooled and solidified by rapid water quenching to...