A pozzolan is a siliceous or siliceous
and aluminous material which, in itself,
possesses little or no cementitious
value but which will, in finely divided
form and in the presence of water, react
chemically with calcium hydroxide at
ordinary temperature to form compounds
possessing cementitious properties. The
broad definition of a pozzolan imparts
no bearing on the origin of the
material, only on its capability of
reacting with calcium hydroxide and
water. A quantification of this
capability is comprised in the term
pozzolanic activity. Pozzolana are
naturally-occurring pozzolans of
volcanic origin.
History
Mixtures of calcined lime and finely
ground reactivesilicate materials were
pioneered and developed as inorganic
binders in the Antique world.
Architectural remains of the Minoan
civilization on Crete have shown
evidence of the combined use of slaked
lime and additions of finely ground
potsherds for waterproof renderings in
baths, cisterns and aqueducts. Evidence
of the deliberate use of volcanic
materials such as volcanic ashes or
tuffs by the ancient Greeks dates back
to at least 500–400 BC, as uncovered at
the ancient city of Kameiros, Rhodes. In
subsequent centuries the practice spread
to the mainland and was eventually
adopted and further developed by the
Romans. The Romans used volcanic pumices
and tuffs found in neighbouring
territories, the most famous ones found
in Pozzuoli, hence the name pozzolan,
and in Segni. Preference was given to
natural pozzolan sources such as German
trass, but crushed ceramic waste was
frequently used when natural deposits
were not locally available. The
exceptional lifetime and preservation
conditions of some of the most famous
Roman buildings such as the Pantheon or
the Pont du Gard constructed using
pozzolan-lime mortars and concrete
testify to both the excellent
workmanship reached by Roman engineers
and to the durable properties of the
utilized binders.
Much of the practical skills and
knowledge regarding the use of pozzolans
was lost at the decline of the Roman
empire. The rediscovery of Roman
architectural practices as described by
Vitruvius in De architectura, also led
to the reintroduction of lime-pozzolan
binders. Particularly the strength,
durability and hydraulic capability of
hardening underwater made them popular
construction materials during the
16th–18th century. The invention of
other hydraulic lime cements and
eventually Portland cement in the 18th
and 19th century resulted in a gradual
decline of the use of pozzolan-lime
binders, which develop strength less
rapidly.
Over the course of the 20th century the
use of pozzolans as additions to
Portland cement concrete mixtures has
become common practice. Combinations of
economical and technical aspects and,
increasingly, environmental concerns
have made so-called blended cements,
i.e. cements that contain considerable
amounts of supplementary cementitious
materials the most widely produced and
used cement type by the beginning of the
21st century.
Pozzolanic materials
The general definition of a pozzolan
embraces a large number of materials
which vary widely in terms of origin,
composition and properties. Both natural
and artificial materials show pozzolanic
activity and are used as supplementary
cementitious materials. Artificial
pozzolans can be produced deliberately,
for instance by thermal activation of
kaolin-clays to obtain metakaolin, or
can be obtained as waste or by-products
from high-temperature process such as
fly ashes from coal-fired electricity
production. The most commonly used
pozzolans today are industrial
by-products such as fly ash, silica fume
from silicon smelting, highly reactive
metakaolin, and burned organic matter
residues rich in silica such as rice
husk ash. Their use has been firmly
established and regulated in many
countries. However, the supply of
high-quality pozzolanic by-products is
limited and many local sources are
already fully exploited. Alternatives to
the established pozzolanic by-products
are to be found on the one hand in an
expansion of the range of industrial
by-products or societal waste considered
and on the other hand in an increased
usage of naturally occurring pozzolans.
Natural pozzolanas are abundant in
certain locations and are extensively
used as an addition to Portland cement
in countries such as Italy, Germany,
Greece and China. Volcanic ashes and
pumices largely composed of volcanic
glass are commonly used, as are deposits
in which the volcanic glass has been
altered to zeolites by interaction with
alkaline waters. Deposits of sedimentary
origin are less common. Diatomaceous
earths, formed by the accumulation of
siliceous diatom microskeletons, are a
prominent source material here.
Use
The benefits of pozzolan use in cement
and concrete are threefold. First is the
economic gain obtained by replacing a
substantial part of the Portland cement
by cheaper, pollution free, natural
pozzolans or industrial by-products.
Second is the lowering of the blended
cement environmental cost associated
with the greenhouse gases emitted during
Portland cement production. A third
advantage is the increased durability of
the end product. Additionally, the
increased blending of pozzolans with
Portland cement is of limited
interference in the conventional
production process and offers the
opportunity to create value by
converting large amounts of industrial
and societal waste into durable
construction materials.
Current practice may permit up to a 40
percent reduction of Portland cement
used in the concrete mix when replaced
with a carefully designed combination of
pozzolanic materials. Pozzolans can be
used to control setting, increase
durability, reduce cost and reduce
pollution without significantly reducing
the final compressive strength or other
performance characteristics.
The properties of hardened blended
cements are strongly related to the
development of the binder
microstructure, i.e., to the
distribution, type, shape and dimensions
of both reaction products and pores. The
beneficial effects of pozzolan addition
in terms of higher compressive strength
performance and greater durability are
mostly attributed to the pozzolanic
reaction in which calcium hydroxide is
consumed to produce additional C-S-H and
C-A-H reaction products. These
pozzolanic reaction products fill in
pores and result in a refining of the
pore size distribution or pore
structure. This results in a lowered
permeability of the binder.
The contribution of the pozzolanic
reaction to cement strength is usually
developed at later curing stages,
depending on the pozzolanic activity. In
the large majority of blended cements
initial lower strengths can be observed
compared to the parent Portland cement.
However, especially in the case of
pozzolans finer than the Portland
cement, the decrease in early strength
is usually less than what can be
expected based on the dilution factor.
This can be explained by the filler
effect, in which small SCM grains fill
in the space between the cement
particles, resulting in a much denser
binder. The acceleration of the Portland
cement hydration reactions can also
partially accommodate the loss of early
strength.
The increased chemical resistance to the
ingress and harmful action of aggressive
solutions constitutes one of the main
advantages of pozzolan blended cements.
The improved durability of the
pozzolan-blended binders enables to
lengthen the service life of structures
and reduces the costly and inconvenient
need to replace damaged constructions.
One of the principal reasons for
increased durability is the lowered
calcium hydroxide content available to
take part in deleterious expansive
reactions induced by e.g. sulfate
attack. Furthermore, the reduced binder
permeability slows down the ingress of
harmful ions such as chlorine or
carbonate. The pozzolanic reaction can
also reduce the risk of expansive
alkali-silica reactions between the
cement and aggregates by changing the
binder pore solution. Lowering the
solution alkalinity and increasing
alumina concentrations strongly
decreases or inhibits the dissolution of
the aggregate aluminosilicates.
See also
Alkali-aggregate reaction
Calcium silicate hydrate
Energetically modified cement
Cement chemist notation
References
^ Mehta, P.K.. "Natural pozzolans:
Supplementary cementing materials in
concrete". CANMET Special Publication
86: 1–33. 
^ Snellings, R.; Mertens G.; Elsen J..
"Supplementary cementitious materials".
Reviews in Mineralogy and Geochemistry
74: 211–278. doi:10.2138/rmg.2012.74.6. 
^ Spence, R.J.S.; Cook, D.J.. "Building
Materials in Developing Countries".
Wiley and Sons, London. 
^ Idorn, M.G.. Concrete Progress from
the Antiquity to the Third Millennium.
London: Telford. 
^ Schneider, M.; Romer M., Tschudin M.
Bolio C.. "Sustainable cement production
- present and future". Cement and
Concrete Research 41: 642–650.
doi:10.1016/j.cemconres.2011.03.019.  
^ Chappex, T.; Scrivener K.. "Alkali
fixation of C-S-H in blended cement
pastes and its relation to alkali silica
reaction". Cement and Concrete Research
42: 1049–1054.
doi:10.1016/j.cemconres.2012.03.010. 
Cook D.J. Natural pozzolanas. In: Swamy
R.N., Editor Cement Replacement
Materials, Surrey University Press, p.
200.
McCann A.M. "The Roman Port of Cosa",
Scientific American, Ancient Cities, pp.
92–99, by Anna Marguerite McCann.
Covers, hydraulic concrete, of
"Pozzolana mortar" and the 5 piers, of
the Cosa harbor, the Lighthouse on pier
5, diagrams, and photographs. Height of
Port city: 100 BC.
