African Design Magazine ADM #40 May 2018 | Page 53

TECHNOLOGY
72 hours at 25°C. The water to binder
ratios kept 0.4 and the total mass of
binders was 6 g in each experiment.
The cumulative hydration heat and
exothermic rate of all the samples are
exhibited in Figure 4.
Figure 4: Hydration heat curves of
composite cementitious materials.
in binders. In comparison with pure
cement, the second exothermic peak
of binder with grade I fly ash addition
appeared later, while with ground fly ash
addition, the second exothermic peak
appeared earlier. In addition, the total
hydration heat and exothermic rate of
Sample GFA-25% were both higher than
Sample FA-25%.
In the process of hydration, the
precipitation of C–S–H and Ca(OH)2
required Ca2+ concentration in the
solution reaches the saturation. When
replacing part of cement with grade I
fly ash, some of Ca2+ was adsorbed on
the surfaces of fly ash particles and the
decrease of the cement content led to
the decrease of Ca2+concentration in
the liquid phase, thus delaying the time
of Ca2+ to reach saturation. Therefore,
adding grade I fly ash has an effect
of delaying the early-age hydration
and further delay the time of the
appearance of the second exothermic
peak. However, the particle size of
ground fly ash is significantly smaller
than that of grade I fly ash. In the early-
age hydration of Sample GFA-25%, the
fly ash particles act as nucleation of the
cement’s hydration products, which to
some extent can promote the hydration
and thus slightly advance the second
exothermic peak.
Figure 4 shows that the additions of
grade I fly ash and ground fly ash both
lowered the hydration heat compared to
that of pure cement, because replacing
part of cement with fly ash resulted
in the reducing content of cement
3.2. Thermal analysis
The content of Ca(OH)2 in
hardened pastes was measured by
thermogravimetry (TG). In order to
prevent carbonisation of hardened
pastes during the heating process,
N2 was used as the protective
atmosphere. The heating rate was 10°C/
AFRICAN DESIGN MAGAZINE © | MAY 2018
53