Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 22

60
50

40
30
20
10
0

0 20 40 60 80 100

Time (hrs)

GP LH MLH GB

Figure 2.2 Adiabatic temperature rise in GGBS mixes. (After Pettinau, C. B., The Effects
of the Type and Quantity of Binder on the Adiabatic Temperature Rise in
Mass Concrete, final year project, Curtin University of Technology, Kent,
Australia, 2003.)

(MLH). GGBS replacement levels at 25% (GB) reduced the rate of early tem-
perature rise slightly but increased the peak temperature. GGBS replacement
levels at 65% (LH) profoundly reduced the rate of temperature rise but did
not change the peak temperature. The ternary blend with 60% GGBS and
7% silica fume (MLH) also profoundly reduced the rate of temperature rise
and reduced the peak temperature. As silica fume is extremely fine and nor-
mally accelerates hydration, the reduced peak temperature is believed to be
due to the silica fume reducing availability of calcium hydroxide to the GGBS.
The effect of GGBS replacement levels on the temperature rise for dif-
ferent concrete thicknesses is shown in Figure 2.3. The benefit of using
GGBS reduces with increased section thickness. The authors have found
that ponding massive concrete elements with say 75 mm (3 inches) of water
is an excellent way of maximising the heat loss from massive elements as
well as preventing thermal shock and providing excellent curing.

2.4.4 Blue spotting
GGBS concrete is notorious for the early development of discoloured patches,
known as “blue spotting”. This is caused by the initial formation of iron sul-
fide, which oxidises to colourless ferric salts on drying but can be a problem in
continuously damp conditions or where a transparent sealer has been applied.

2.4.5 Ternary blends
Ternary (i.e., triple) blends of GGBS, fly ash, and cement are sometimes
used, and have a good reputation. The addition of different proportions of
fly ash during batching can give a flexibility of properties to a fixed blend
of GGBS and cement.