To
understand what a pozzolan is and how the addition of one can benefit concrete,
we need to define the term. The American
Concrete Institute defines a pozzolan as, “a siliceous or silico-aluminous
material that will, in finely divided form and in the presence of moisture,
chemically react with calcium hydroxide at ordinary temperatures to form
compounds having cementitious properties” Pozzolan are found in natural, artificial and
silica fume substances. To have
pozzolanic properties, the material must be amorphous (lacking a crystalline
structure, without solid form) to create the required chemical reaction.
The
word “pozzolan” comes from the name of a town in Pozzuoli, Italy that is
located near the site of Mount Vesuvius and is where the Romans mined ash from the
volcanic activity to use in the construction of many of their buildings. Natural pozzolans are found in materials such
as volcanic ash, pumice and tuffs as well as inorganic material such as
calcined clay and shale and metakaolin.
Artificial
pozzolans are found in materials that include Class C fly ash, Class F fly ash,
ground slag, and silica fume and rice hull ash. All of these materials are waste or by-products
of industry. Silica fume is a waste
product of the silicon metal industry while fly ash results from the burning of
coal in electric power plants.
The
addition of pozzolans to concrete has many benefits. One of the foremost benefits is they offer an
economic savings as they replace a substantial portion of the more costly
portland cement.
Pozzolans can also create a higher strength concrete and increase
durability by being more resistant to the environment. Furthermore since they decrease the water
content that is needed in the concrete, they help to reduce drying
shrinkage.
Sources:
concrete.org/topicsinconcrete/topicdetail/pozzolans
en.wikipedia.org/wiki/Pozzolan
ferrocement.net/ferro/files/pozzolans-uwe.html
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