ALTERED VOLCANIC ASH AS FINE AGGREGATE (HARENAE FOSSICIAE) IN THE POZZOLANIC MORTARS OF ANCIENT CONCRETES, ROME, ITALY

Altered, granular volcanic ash forms the sand-sized aggregate of ancient Roman pozzolanic mortars, which presage modern pozzolanic cements. Petrographic and XRD analyses show that early, friable mortars (2C BCE) contain reworked volcanic sands from the Plio-Pleistocene San Paolo and Via Aurelia Formations on the Capitoline Hill. The architect Vitruvius, writing for Octavian (later, Emperor Augustus) 2000 years ago, considered these earthy (terrosa) sands unsuitable for mortar (De Architectura 2.4.1). His ideal (optima) excavated sands (harenae fossiciae) correspond to Pozzolane Rosse (PR) (457± 4 KA) aggregate in Augustan (late 1C CE-early 1C CE) and later Imperial age (1C-3C CE) mortars. The PR ignimbrite has low-silica, high-potassium compositions; all scoria are weathered or altered to clay, mainly poorly crystalline 7Å and 10Å halloysite. Stratigraphic, petrographic, and geochemical studies define three main alteration facies, first identified by R. L. Hay. An upper, most altered facies has yellow-brown dispersive scoria, leucite replaced by analcime (or dissolved), illuvial clay coatings, and sparse kaolinite; an intermediate facies has reddened non-dispersive scoria, fine leucite in scoria replaced by analcime (or dissolved), and translucent clay (1.53≤η≥1.54) and opal (η<1.46) surface coatings; and a deeper, least altered facies has gray-brown non-dispersive scoria, fresh leucite, and opal and halloysite surface coatings (w/ K-clinoptilolite?). Pedogenic weathering produced the upper leaching and lower enrichment zones. Certain Augustan mortars contain the greatest and intermediate facies, quarried from the upper ignimbrite at least 2.5 km S of the Roman Forum. Imperial mortars “of flint-like hardness” (Van Deman 1912a, b) have the least altered and intermediate facies, quarried from a lower silication zone. Their siliceous (Si:Al ~2.5-3:1) cementitious matrix apparently formed by reaction of portlandite (Ca(OH)₂) with authigenic components of Pozzolane Rosse.