Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 21

2.4.2 Properties of GGBS concrete
Concrete using GGBS cement will tend to develop early strength more
slowly than pure Portland cement concrete except when very finely ground.
However, if thoroughly cured, it may have as good or better eventual strength.
It normally has a greater resistance to chemical attack and is particularly
suitable for marine works. Its normally greater fineness may confer resistance
to bleeding in the fresh state and lower permeability when hardened.
The glassy surface of the slag may give a slightly reduced water require-
ment even though it does not have the favorable particle shape of fly ash.
The water requirement may however be substantially dependent on the
fineness of grind.
It can be added as a separate ingredient at the mixer but is often sold
blended with Portland cement. There is a long history of extensive use in this
form as Portland blast-furnace cement, particularly in Europe and the for-
mer Soviet Union. The proportion of GGBS can exceed 80% of such cement.
To some extent this product is sometimes seen as a low-grade cement,
since it develops strength more slowly and sometimes has a lower strength
at 28 days. Obviously the properties of such a material will be dependent
upon the composition of the particular slag. Since GGBS is a by-product
material, there may be a wide variation in quality between materials from
different sources. For chloride and sulfate resistance the key consideration
is the aluminate component. For chloride resistance, the more the better
but too much can reduce sulfate resistance. The BRE Special Digest 1 states
that where the alumina content of the slag exceeds 14%, the tricalcium
aluminate content of the Portland cement fraction should not exceed 10%
to achieve additional sulfate resistance.
When used in lower proportion (less than 30%) in blended cement, it
may be marginally cheaper and may gain strength more slowly depending
on the fineness of the GGBS. It is by no means necessarily inferior.
2.4.3 Heat generation
There are three aspects to consider with heat generation. These are cold
weather concreting, hot weather concreting, and mass concrete. Because it can
be used in large proportion, GGBS can give rise to problems with slow setting,
slow strength gain, and lack of early resistance to frost in cold weather. These
same properties can be very advantageous in hot weather. The assumption
may be made that the slag cement will provide reduced peak temperatures
in mass concrete as does fly-ash concrete. In fact unless a high proportion
of GGBS (over 60%) or a very coarse grind is used, the GGBS cement can
give rise to even higher temperatures than with normal Portland cement.
Figure 2.2 shows the adiabatic temperature rise for blended cements contain-
ing 25% GGBS (GB), 65% GGBS (LH), and 60% GGBS and 7% silica fume