REACTIVE COMPONENTS OF ROMAN POZZOLANE IN THE POZZOLANIC CEMENTS OF ANCIENT ROME

Ancient Roman concrete consists of coarse clasts of leucitic tuff or lava laid in a durable mortar made of lime (CaO) mixed with water and the granular volcanic ash, or pozzolana, of Rome that is commonly reported to have glassy and/or zeolitic compositions. The pozzolana combines with portlandite (Ca(OH)₂) to form enduring Ca silicate and aluminate compounds that have maintained long-term strength in the concrete (unlike portlandlite, which altered to calcite). The Roman pozzolane, Pozzolane Rosse (PR), Pozzolane Nere, and Pozzolanelle, are Middle Pleistocene (~500–400 ka) unwelded fine-grained ignimbrite deposits. Numerous ancient excavations for PR exist south of Rome, and PR is still used in modern mortar. Nearly all the vitric ash and leucite of the pozzolane have been diagenetically altered, chiefly to non-dispersive siliceous clay in which the principal clay mineral is smectite. Clay coats volcanic clasts, forms a matrix and/or cement, and occurs as claystone pellets. Much of the authigenic zeolite (principally phillipsite and chabazite) has been replaced by smectitic clay and opaline silica, which commonly retain zeolite morphology. Opaline silica (probably Opal-A) forms cements, fills vesicles, replaces zeolites, and is the constituent that makes clays non-dispersive. X-ray diffraction patterns of three pozzolane indicate 20–50 wt% clay mineral and ≤4% non-analcimic zeolite in two of the three samples. However, d-spacings of clay minerals and zeolites other than analcime are lacking in the diffraction patterns of four samples of cementitious matrix in late-republican and mid-imperial age Roman mortar. Hence reactants with portlandite to yield the binding cement were clay mineral, opal, and possibly zeolite. Like fly ash and microsilica added to modern cements, the opaline silica, siliceous clay, high porosity and reactive surface area of Roman pozzolana enhanced its capacity to form hydraulic mortars that have remained robust for two millenia.