pozzolan

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pozzolan

[′pät·sə·lən]
(geology)
A finely ground burnt clay or shale resembling volcanic dust, found near Pozzuoli, Italy; used in cement because it hardens underwater.
(materials)
Cement made by mixing and grinding together slaked lime and pozzolan without burning; sometimes used for concrete not exposed to the air.

pozzolan, pozzolona, pozzuolana

A siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.