Description
Ground granulated blastfurnace slag (ggbs) is a by-product from the
blast-furnaces used to make iron. These operate at a temperature of
about 1,**0 degrees centigrade and are fed with a carefully
controlled mixture of iron-ore, coke and limestone. The iron ore is
reduced to iron and the remaining materials form a slag that floats
on top of the iron. This slag is periodically tapped off as a
molten liquid and if it is to be used for the manufacture of ggbs
it has to be rapidly quenched in large volumes of water. The
quenching optimises the cementitious properties and produces
granules similar to a coarse sand. This 'granulated' slag is then
dried and ground to a fine powder.
The major use of GGBS is in ready-mixed concrete, and it is
utilised in a third of all UK ‘ready-mix’ deliveries. Specifiers
are well aware of the technical benefits, which GGBS imparts to
concrete, including:
better workability, making placing and compaction easier lower
early-age temperature rise, reducing the risk of thermal cracking
in large pours elimination of the risk of damaging internal
reactions such as ASR high resistance to chloride ingress, reducing
the risk of reinforcement corrosion high resistance to attack by
sulphate and other chemicals considerable sustainability
benefits
In the production of ready-mixed concrete, GGBS replaces a
substantial portion of the normal Portland cement content,
generally about *0%, but sometimes up to *0%. The higher the
proportion, the better is the durability. The disadvantage of the
higher replacement level is that early-age strength development is
somewhat slower.
GGBS is also used in other forms of concrete, including
site-batched and precast. Unfortunately, it is not available for
smaller-scale concrete production because it can only be
economically supplied in bulk. GGBS is not only used in concrete
and other applications include the in-situ stabilisation of
soil.
