Pozzolans are siliceous or aluminous materials blended with Portland cement. They interact with the calcium hydroxide produced during the hydration of Portland cement and contribute to improved strength and durability of concrete. The pozzolanic activity, a measure of a pozzolan's effectiveness, is typically assessed using the strength activity index, as defined in ASTM C 618-93, which calculates the ratio of the compressive strength of cement mixtures with and without pozzolan.
Fly ash is a common artificial pozzolan originating from the electrostatic or mechanical collection of flue gases from coal combustion. Other notable pozzolans include rice husks, metakaolin, and silica fume. Rice husks yield an amorphous material when burned between 500 and 700°C. Metakaolin results from the calcination of kaolinitic clay, enhancing concrete's mechanical properties and durability. Silica fume, a by-product of silicon and ferrosilicon production, consists of fine, highly reactive glassy particles that fill voids in concrete, enhancing its strength and durability.
Portland-pozzolan cements, specifically, develop strength more gradually than ordinary Portland cements, necessitating a longer curing period. However, they achieve high long-term strength, depending on the pozzolan proportion used in the cement mix. Pozzolans like fly ash are beneficial in roller-compacted concrete, low-heat concrete, and sulfate-resistant applications.
Portland cement is sometimes interground or blended with pozzolans, which are siliceous and aluminous materials.
The finely divided pozzolans, in the presence of moisture, react with the calcium hydroxide released during cement hydration to produce cementitious compounds.
Pozzolans are naturally available, such as volcanic ash and pumicite, or they are synthesized, such as fly ash, which is obtained from the flue gases of coal-fired power plants.
Pozzolans are added with cement used in roller-compacted concrete, mass concreting, and chemically resistant concrete.
When cement is replaced with pozzolan on a weight basis, the volume of pozzolan equivalent to the cement it replaces will be higher, as the specific gravity of pozzolan is less compared to cement.
The hydration of pozzolans is slower compared to that of Portland cement, resulting in a low rate of heat development, longer setting time and longer curing periods.
Lastly, the pozzolanic activity with cement can be assessed using the strength activity index, which is the ratio of the compressive strength of cement partially replaced with pozzolans to that without any replacement.
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