Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 20

2.4 GROUND-GRANULATED
BLAST-FURNACE SLAG (GGBS)
2.4.1 Properties of GGBS
The properties of cementitious and pozzolanic materials depend on their
chemical composition, their physical state, and their fineness. This is par-
ticularly the case with blast-furnace slag. Since it is a by-product of the
production of iron, its composition may differ from different sources but is
likely to be reasonably consistent from a given source. Table 2.1 shows its
composition to be more similar to that of cement than to typical pozzolanic
materials. However, to develop satisfactory properties it is essential that
the molten slag be rapidly chilled (by quenching with water) as it leaves
the furnace. This causes the slag to granulate, that is, break up, into
sand-sized particles. More important it causes the slag to be in a glassy
or amorphous state in which it is much more reactive than if allowed to
develop a crystalline state by slow cooling. In the latter state it is suitable
as a concrete aggregate but not as a cementitious material. It is important
to note that the unground granulated material does not make a good fine
aggregate because often the grains are weak, fluffy conglomerates rather
than solid particles.
To use as a cementitious material, the granulated slag must be ground
as fine or finer than cement. The fineness of grind will (along with the
chemical composition and extent of glassiness) determine how rapidly the
slag will react in concrete.
Slag cannot be used alone to make concrete but can be used in much
larger proportion than pozzolanic materials. Portland cement clinker or
some other activator is required to initiate the hydration of the slag. The
latter may comprise 80% or more of the total cementitious material but
50% to 65% is more usual in most parts of the world. In North America,
GGBS is typically used at replacement levels of 20% to 30% similar to fly
ash. However, the authors do not fully understand why North America
foregoes the advantages of higher replacement levels. An alternative acti-
vator is calcium sulfate, producing a product known as “supersulfated
cement”. This cement is no longer produced. Although it offered the
valuable properties of chemical resistance and very low heat generation, it
required special care and understanding in use to offset its slow setting and
strength development, and needs very thorough extended curing.
In Portland blast-furnace cement, the slag may be interground with
the cement clinker or added as a separate material. The Portland cement
clinker is softer than the slag and therefore will be more finely ground when
the materials are interground. Even when sold as a composite “blended
cement” (the term is also applied to other blends) the GGBS cement may
have been either interground or postblended.