Ground granulated blast furnace slag is a byproduct of steel and iron smelting used in concrete products. It prevents chloride ingress, is financially cheaper than OPC mixes, and is resistant to sulphate attack and alkali-silica reaction. It requires a longer curing period but reduces the risk of cold joints.
A byproduct of the manufacturing process when steel and iron are smelted, ground granulated blast furnace slag is most commonly used in concrete products. Also known as GGBS or GGBFS, dry powder is made by grinding the product resulting from immersion of 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 cementitious products. Of these two products, the most commonly used is Portland blast furnace cement (PBFC), which contains approximately 30% blast furnace slag. The alternative product is known as blast furnace cement (HSBFC) and contains a higher concentration of blast furnace slag, usually up to 70%. Both products are used to make ready-mix concrete batches for structural applications.
There are several advantages to using ground granulated blast furnace slag in ready-mix concrete applications. These benefits add to the obvious environmental sustainability that is achieved by using a secondary waste product instead of a primary source material. The inclusion of blast furnace slag helps prevent chlorides from entering the concrete structure. Chloride ingress can lead to advanced corrosion of steel reinforcing members, a condition commonly known as concrete canker.
Financially, using blast furnace slag as an additive in concrete is generally less expensive than standard OPC mixes. The product is also resistant to sulphate attack and alkali-silica reaction, which can significantly reduce the life of a concrete structure. Concrete can also be more aesthetically pleasing due to the white appearance of the cured product as opposed to the dull gray coloration provided by ordinary OPC mixes.
The inclusion of ground granulated blast furnace slag in the ready-mix concrete means that the product requires a longer curing period – the time required for the concrete to reach its strength grade – compared to OPC products. While this can be an advantage when considering the reduced risk of cold joints, it can become disadvantageous when a tight concrete pouring schedule is anticipated.
Cold joints occur when sections of a concrete wall or similar structure are allowed to fully cure before an adjacent section can be poured. As a result, the new section will not be able to adhere properly to the existing one due to the smooth finish. To overcome this, you need to revert the existing surface to a rough finish. This allows the new section to adhere to the previously poured section. The longer curing period provided by the inclusion of milled granulated blast furnace slag means that the existing surface is more workable over a longer period of time.
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