Page 356 - Handbook of Thermal Analysis of Construction Materials

Section 8.0 - Relative Effects of Pozzolans and Their Mixtures 333

vary significantly. Attempts have, therefore, been made to study the
properties of cement paste and concrete into which are blended two or more
additives in order to derive the optimum properties and benefits. These have
implications in terms of availability of the materials and economy. This
section describes the effect of supplementary cementing-material mixtures,
and also the comparative effects of different supplementary cementing
materials on cement hydration. Kasai, et al., [78] applied DTA/TG tech-
niques to characterize zeolites and other supplementary materials. In
zeolites, peaks at 80–400°C represented the loss of zeolitic water. An
exothermic peak at 600°C in fly ash signified combustion of carbon, and
that at 800–900°C corresponded to the phase transition of the glassy phase.
Silica fume exhibited a small peak at 500°C for the crystallization of a
cristobalite from the amorphous phase.
Not much data is available on the relative effects of pozzolans on
the individual cement phases. The DTA curves of β-C S treated with 5% fly
2
ash, silica fume, or slag hydrated for 1 to 90 days are compared with respect
to the untreated sample in Fig. 31. [79] The reference C S exhibits endother-
2
mal effects at 100–150, 160–300, 350–450, 500–600, 700–800, and 800–
900°C. The endothermal effects in the range 100–150°C are attributed to
the loss of free water and the dehydration of interlayer water from the C-S-
H phase. The decomposition of calcium carbonate occurs with the peak at
700°C. The sample, hydrated for one day, shows the peaks for the presence
of C-S-H, Ca(OH) , and CaCO which increase in intensity with the curing
3
2
period. It is also evident that Ca(OH) is converted to CaCO at 90 days. In
2
3
the presence of slag, C S shows several endothermal effects including an
2
exothermal peak that can be ascribed to the oxidation of sulfide in the slag.
The presence of smaller amounts of Ca(OH) and larger amounts of C-S-H
2
indicate that Ca(OH) is consumed by the slag. In samples containing silica
2
fume, an exothermal peak at 870°C is indicative of the formation of
wollastonite from C-S-H (I). This peak appears even at 1 day of hydration.
The rapid consumption of calcium hydroxide is followed by a correspond-
ing enhancement of the exothermal effect. The fly ash-cement mix also
indicates the formation of the C-S-H (I) phase and a peak at about 230°C due
to C AH or gehelinite hydrate. In general, the amount of hydration
4
13
products in the β-C S treated with pozzolans is higher than that in the
2
untreated sample. Conduction calorimetry shows that the total heat evolved
in cement blends with 30% sand, fly ash, or silica fume is higher than that
produced by the reference cement paste (Fig. 32). [22] These results indicate
that fly ash, silica fume, and ground sand accelerate the hydration of
cement, silica fume being the most effective admixture.