According to the researchers, the Roman builders took great care in selecting a mortar to hold walls together. They knew the diverse material properties of the rocks, and developed concretes that employed granular volcanic ash and chunks of various volcanic rocks. To find the ash they turned to the material erupted from the Alban Hills volcano some 456,000 years ago. The area features three pozzolanic (cement-like) deposits, known as Pozzolane Rosse, Pozzolane Nere and Pozzolanelle. "Of these, the type that makes a harsh, grating, rasping noise when rubbed vigorously in the hand is best," the ancient Roman writer and architect Vitruvius wrote in his De Architectura, a 10-volume treatise on architecture. Indeed, when rubbed between the hands, Pozzolane Rosse, the volcanic ash identified in the Trajan Market sample, crackles. The recipe for a robust concrete worked very well. "The mortar was very well compacted; the ratio of lime, pozzolanic volcanic ash and water was carefully controlled," Jackson said. Related Links: |
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