Page 352 - Handbook of Thermal Analysis of Construction Materials

Section 7.0 - Natural Pozzolans 329

etc., exceed 600°C. The TG/DTA/TMA curves of various pozzolans such
as kaolin, tuff, basalt, diatomite, and activated kaolin have been pub-
lished. [35]
Conduction calorimetry and DTA have been applied to the study of
the effect of natural pozzolans on the hydration of cement phases. [1][20][72]
Ogawa, et al., [72] examined the role of five pozzolans on the hydration of the
tricalcium silicate phase using TG/conduction calorimetric techniques
(Fig. 28). The samples V (white clay), F (volcanic ash), and R (volcanic ash)
are natural pozzolans whereas T is a fly ash and S is a slag. All were hydrated
at a w/c ratio of 0.5 and C S:pozzolan ratio of 6:4. The amount of Ca(OH) 2
3
formed was estimated by DSC/TG. All the pozzolans modify the calorimet-
ric curves to a great extent. The natural pozzolans have an accelerating
effect on hydration as evidenced by the intense peaks which appear
somewhat later with respect to the reference. The induction period does not
vary. The accelerating effect is explained by the surface of the pozzolan
+2
acting as a precipitation site and also by its ability to adsorb Ca ions.
Lowering of the calcium ion in the solution phase leads to the acceleration
effect. The amount of CH determined by thermal methods does not follow
the same trend as the heat evolution data. This can be explained by the
reaction and adsorption of CH by the pozzolans, especially those having
high surface areas and reactivity. Conduction calorimetry of C S containing
3
montmorillonite and magnesium silicate clays indicates that the peaks in
the clay-containing samples are of larger intensity than that of C S and also
3
appear earlier. The induction period is reduced from 6 hours to 2 hours in
pozzolan-containing samples. This confirms that these clays accelerate the
hydration of C S.
3
The products of the lime-pozzolan reaction include calcium silicate
hydrate, calcium aluminate hydrate of the form C AH , in which H varies
4 x
from 9 to 13, hydrated gehelinite (C ASH ), calcium carboaluminate
8
2
(C A•CaCO •H ), high sulfoaluminate ettringite (C A•3CaSO •32H O),
3 3 12 3 4 2
and low sulfoaluminate (C A•CaSO •12H O). All the above compounds
3
4
2
yield characteristic thermal effects in DSC/DTA/DTG. In general, all the
products will not be present at any one time. For example, calcium
aluminate mono-sulfate hydrate exists at early stages of hydration and is
converted to ettringite subsequently. Only pozzolans with high alumina
contents form gehelinite, and they also reduce the amount of C-S-H.
Jambor, [35][38][73] studied extensively by thermal methods, the system lime-
natural pozzolan-water containing various types of pozzolanic materials
(Fig. 29). The first ten mixtures (A–J) contained natural pozzolans, the 11 th
th
th
sample (K) was activated kaolin, the 12 to 15 referred to fly ashes (L–O),