Granulated blast furnace slag

Iron blastfurnaces produce iron and slag at the furnace hearth at very high temperatures (around 1500°C). Molten slag taken from the bottom of the furnace is granulated. Granulation is the controlled quenching of the slag in cool water, thus not giving time for crystalline growth to take place. Large volumes of water are required - 10 parts water to 1 part slag being optimum. The resultant glassy disordered structure is truly hydraulic: indeed good granulation is key to producing an effective cementitious material.

Granulate is produced in modern, high cost, state of the art granulating plants which are computer controlled and environmentally friendly. Regular checks are carried out on the granulate to monitor the amount of glass produced as a control on the effectiveness of the quenching process.

Granulated slag is light in colour; around 3mm down to dust in size and has a relatively high moisture content. The material is allowed to drain by stocking in heaps. This process must be carefully managed: granulate will hydrate and if left too long in stock will form large lumps which are hard to break up and handle. On the other hand sufficient time needs to be allowed for excess moisture to drain to minimize further processing energy needs.

Granulated blastfurnace slag is produced by Tarmac Ltd. at three locations in the UK:

• Port Talbot, South Wales.
• Scunthorpe, North Lincolnshire.
• Teesport, Cleveland.

In total there are approximately two million tonnes of granulated blastfurnace slag produced in the UK per annum.

The vast majority of granulated slag is subjected to further processing to become Ground Granulated Blastfurnace Slag (ggbs) but small quantities have been used in road construction and also in the precast industry.

There is currently no British or European Standard for granulated blastfurnace slag however there is reference to it in BS EN 197-1: 2000, common cements.

For more information contact John Bullock

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