Civil Engineering Reference
In-Depth Information
7.0
NATURAL POZZOLANS
A
pozzolan
may either be a material of natural origin or an artificial
preparation. The natural pozzolans are generally of volcanic origin or are
derived from sedimentary rocks. The well-known natural pozzolans are
volcanic glass, tuffs, and siliceous materials such as diatomite and rocks.
Natural pozzolans are used in combination with lime or portland cement,
and the resultant product possesses good sulfate resistance and produces
low heats. Some of them are known to reduce the alkali-aggregate expan-
sion reaction. These pozzolans may serve as partial replacements of
cement. The reactivity of the pozzolan is determined by its chemical and
mineralogical composition, morphology, the amount of glassy phase, and
fineness. Most natural pozzolans have a high SiO
2
+ Al
2
O
3
content and a
glassy or amorphous structure, with the exception of the zeolite. The
products formed in the pozzolan-cement paste mixtures are similar to
those formed in pure cement pastes. The products that are produced by
the reaction of a natural pozzolan with lime include C-S-H (calcium
silicate hydrate), C
3
A•CaCO
3
•12H
2
O (calcium carboaluminate),
C
3
A•3CaSO
4
•32H
2
O (ettringite), C
4
AH
x
(
hexagonal calcium aluminate
hydrate), C
2
ASH
8
(hydrated gehelinite), and C
3
A•CaSO
4
•12H
2
O. The
types and the amounts of products depend on the chemical constituents in
the pozzolan and the curing conditions. In general, the addition of pozzolan
promotes the hydration of cement. Also, iron and aluminum can be
incorporated in the C-S-H structure. Adequate strengths, improved durabil-
ity, and low permeability can only be obtained in pozzolan concrete after
a longer curing period, compared to that required for normal portland
cement concrete.
Studies have been conducted on the thermal behavior of natural
pozzolans. For example, the TG analysis indicates that the weight loss
between 60°
and 400°C in zeolites amounts to 80-90% of the total amount.
This is due mainly to the loss of capillary, inner layer, and zeolitic water.
A very small weight loss occurs between 600° and 700°C. The loss at higher
temperatures is due to the dehydration of the zeolitic water. Zeolites may
be classified by the decomposition reactions that they undergo when
subjected to thermal analysis. The stability range of harmotone, phillipsite,
pualingite, etc., is between ambient temperature and 250°C. Gismondine,
yugawaralite, stilbite, etc., have the stability range between 250° and
400°C. Laumonite, faujasite, natrolite, and others have the stability range
between 400° and 600°C, and the values for analcime, erionite, offretite,
