Page 359 - Handbook of Thermal Analysis of Construction Materials

336 Chapter 8 - Supplementary Cementing Materials

A detailed study on the influence of binary and ternary blends of
portland cement mixed with silica fume, fly ash, and slag has been presented
[4]
by Uchikawa and Okamura. Applying conduction calorimetry, it was
found that in blends with fly ash and slag, the hydration of the interstitial
phase is accelerated, and that of alite at stages I and II is slightly delayed.
In blends with slag and silica fume, the hydration of the interstitial phase
and that of alite was accelerated with the reduction in the induction period.
In fly ash-silica fume blends, the hydration of the interstitial phase de-
creased, and the induction period for alite hydration was lengthened.
Fly ash is available in large quantities and is more economical to
use than silica fume. The addition of fly ash to cement causes an increase
in the setting time and a reduction in strength. Improvement in hydration
rate could be obtained by the addition of silica fume. By blending 5–10%
silica fume with 15–30% fly ash, the rate of hydration as well as the strength
is improved, as confirmed by conduction calorimetry and DTA investi-
gations. [82] The heat of hydration of mortars containing a mixture of slag
(30–90%) and silica fume (10–30%) indicates that the values decreased
with the silica fume content. The results would not be the same if they were
based on the cement content. [83]
The relative activities of pozzolans can be followed by estimating
the lime content at different times in the cement-pozzolan-water systems.
The DTG/DTA/TG techniques are well suited to estimate the amount of
lime in these systems. Chatterjee, et al., [84] compared the amounts of lime
in the systems containing fly ash, silica fume, and three natural pozzolans
(Fig. 33). The pozzolan:cement ratio was kept at 1:4 by weight. In the
control and that containing Segni and fly ash, the amount of calcium
hydroxide increases with time. The other mixes indicate a dip in CH content
at 7 days. Segni and fly ash have lower lime-fixing ability. The dip in the
curve may indicate the pozzolan has been completely consumed by this
time. Any further increase in CH is due to continued hydration of the
cement, or the rate of reaction between the pozzolan and CH has been
reduced without a corresponding drop in the hydration rate of portland
cement. Only low amounts of lime are recorded in mixtures with silica
fume, diatomaceous earth, and Sacrafano indicating the consumption of
lime by these pozzolans.
DTA has been used successfully to identify the products formed in
mixtures of calcined gypsum, slag or fly ash, and cement (with or without
hydrated lime) that were cured at 55, 70, and 85°C for different periods. [85]
The compounds C-S-H, calcium trisulfate aluminate hydrate, calcium