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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