What Is Ground Granulated Blast Furnace Slag?

A by-product of the manufacturing process when steel and iron are melted down, ground granulated blast furnace slag is most commonly used in cement products. Also known as GGBS or GGBFS, the dry powder is produced by grinding down the product that results from soaking molten-iron slag. Slag is the term applied to the waste produced during the iron smelting process.

Ground granulated blast furnace slag and ordinary portland cement (OPC) are usually combined to form two cement products. Of these two products, the most commonly used is Portland Blast Furnace Cement (PBFC), which contains approximately 30% of blast furnace slag. The alternative product is known as High Blast Furnace Cement (HSBFC), and it contains a higher concentration of blast furnace slag, usually up to 70%. Both products are used to make batches of ready-mixed concrete for structural applications.

There are several benefits to using ground granulated blast furnace slag in ready-mixed concrete applications. These advantages are in addition to the obvious environmental sustainability to be gained from utilizing a secondary waste product in lieu of a primary source material. The inclusion of the blast furnace slag helps to prevent chloride ingress to the concrete structure. Chloride ingress can result in the advanced corrosion of steel reinforcement members — a condition commonly known as concrete cancer.

Financially, using blast furnace slag as an admixture in concrete is generally less expensive than standard OPC mixtures. The product is also resistant to sulfate attack and alkali-silica reaction, both of which can significantly reduce the life of a concrete structure. The concrete can also be more aesthetically pleasing due to the white appearance of the cured product in comparison to the dull gray coloring provided by regular OPC mixes.

The inclusion of ground granulated blast furnace slag in ready-mixed concrete means that the product requires a longer curing period — the timescale needed for the concrete to achieve its strength rating — than OPC products. While this can be an advantage when considering the reduced risk of cold joints being required, it can become disadvantageous when a tight timescale of concrete pours is scheduled.

Cold joints occur when sections of a concrete wall or similar structure are left to fully cure before an abutting section can be poured. As a result of this, the new section will not be able to bond correctly to the existing one due to the smooth finish. To overcome this, it is necessary to break the existing surface back to a rough finish. This allows the new section to bond with the previously poured section. The longer curing period provided by the inclusion of ground granulated blast furnace slag means that the existing surface is more workable for an extended period of time.