Mid-Pleistocene Pozzolanic Volcanic Ash in Ancient Roman Concretes

摘要

The extraordinarily durable concretes of Imperial Age (27 B.C. through 3rd century A.D.) monument construction in Rome contain scoriaceous, highly potassic, altered volcanic ash from the Pozzolane Rosse ignimbrite, erupted at 456 3 ka from the Alban Hills volcano as pozzolanic mortar aggregate. Stratigraphic, micromorphological, and chemical investigations demonstrate that during the relatively warm, humid period preceding marine isotope stage 11, intensive hydrolytic pedogenesis produced dense illuvial clay coatings in the upper horizon of the ignimbrite. Moderately altered ash of the Pozzolane Rosse transitional Bt to Bw soil horizon has opal coatings overlain by limpid, illuvial halloysite coatings. Less weathered ash from a lower C horizon apparently was altered in ground water. Where the ignimbrite illed certain paleovalleys, zeolitic alteration produced phillipsite and chabazite textures. Builders selected ash from these intermediate and least altered horizons for the highest-quality mortars of the Imperial Age, as for the Forum and Markets of Trajan (A.D. 107 to 113). The alumina- and alkali-rich compositions of cementitious complexes give some preliminary insights as to why the reaction of hydrated lime with the altered, alkali-rich Pozzolane Rosse ash produced pozzolanic cements that have remained resistant to decay for nearly 2000 years. The results of the geological analyses fully confirm empirical observations made by Esther B. Van Deman in 1912 regarding the durability of the ancient mortars and the technical choices of Roman builders.