cement, Agent that binds concrete and mortar. Cements are finely ground powders that, when mixed with water, set to a hard mass. The cement of 2,000 years ago was a mixture of ash and lime. Volcanic ash mined near the city of Puteoli (now Pozzuoli), near Naples, was particularly rich in essential aluminosilicate minerals, giving rise to the pozzolana cement of the Roman era. See also portland cement.