Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 16

2.2.3.3 Surface chemistry effects
It appears that fly ash can act as a catalyst or a nucleation site for crystal
growth in the cement paste. Such effects are beyond the scope of this topic,
but it should be realised that there is more to the story than has been told
here. This may provide some explanation for a smaller early age strength
reduction than chemical effects alone would predict when equal mass
substitutions are made.
Malcolm Dunstan in the United Kingdom and Mohan Malhotra in
Canada (Malhotra and Ramezanianpour, 1994) have done interesting
work on roller compacted and other concrete with 50% to 60% of fly-ash
substitution. A revealing point is that good results are obtained with high-
volume fly ash in dry roller compacted concrete or a low water to cementious
ratio concrete using a superplasticiser. However poor results are obtained
with high-volume fly ash at normal water contents (Odler, 1991). It could
be said that the w/c verses strength relationship is even more marked in
the case of fly ash than in the case of cement. Perhaps higher Type F fly
ash replacement may be possible in low w/c concrete where full hydration
will not occur. However, at higher w/c, the presence of fly ash beyond 30%
reduces the amount of calcium silica hydrate produced.
Figure 2.1 shows the effect of fly ash on adiabatic temperature rise of
concrete with a constant cementitious content of 400 kg/m 3 . At 30%

60

50

40

30

20

10

0

0 20 40 60 80 100

Time (hrs)

30% Collie fly ash
30% Collie fly ash

40% Collie fly ash
30% Gladstone fly ash

60% Collie fly ash
GP

Figure 2.1 Adiabatic temperature rise curves for fly ash concretes. (From 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.)