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
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
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
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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