BUILDING FOR ETERNITY

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described here have led the authors of this book to regard this as a reasonable hypothesis, our analytical results and those of previous studies are seldom perfectly conclusive. As a result, the association of the pyroclastic materials in the ancient concretes – mainly pumice and tuff – with the Gulf of Naples volcanic deposits is often expressed in a tentative manner. This approach may surprise readers accustomed to the confident attributions seen in many archaeological publications (see below pp. 2–5), but the reasons for this caution are explained in Chapter 7. We refer to the material that is the focus of this book as “Roman maritime concrete” or “Roman marine concrete,” rather than “Roman hydraulic concrete.” The latter is a general, generic term that refers to concretes that harden by reacting with water and form a water-resistant product. Romans did not use kiln-fired cements as we know them. Instead they relied on the reaction of hydrated lime with volcanic ash to produce stable binding cementitious hydrates. Most ancient Roman concretes used on land, as well as that in the sea, remain intact when saturated in water, and even develop new cementitious phases. The earliest synthetic lime mortars, simple mixes of slaked
lime and quartz sand, appear in the archaeological record in the Near East as early as 12,000 BC, and these were applied to architectural uses by 10,000 BC (Gourdin and Kingery 1975; Kingery et al. 1988). Probably not by accident, and possibly in connection with early ceramic production or metallurgy, it was discovered that heating limestone to 800–900˚ C produced a caustic alkaline powder, calcium oxide (CaO). The principal component of most limestone is calcite, or crystalline calcium carbonate (CaCO3). During calcination in kilns, calcium carbonate releases CO2 gas and decomposes to calcium oxide
(CaO), called lime (or quicklime). When quicklime is mixed with water, or “slaked,” an exothermic hydration reaction takes place that produces hydrated lime (Ca(OH)2), or portlandite. Vitruvius described this reaction as it occurred during the slaking of lime with fresh water to form putty for the volcanic ash mortars of architectural concrete structures (De arch 2.6.3; pp. 17–19, Passage 7). When slaked lime putty is mixed with quartz sand, the portlandite carbonates in the presence of atmospheric CO2 to form a calcite cement binder. The resulting mortar develops some compressive strength and resistance to shrinking and
cracking. This type of mortar is non-hydraulic, and it may deteriorate during long term saturation in water after having set. Nevertheless, simple lime mortars provided adequate strength and water resistance to serve as plaster on floors, walls, and roofs, and for the lining of cisterns throughout the Mediterranean
area for many centuries. Plasters were widely used during the Bronze Age (Shaw 2009). Pozzolanic materials add durability and long-term strength to
modern cementitious materials, even in maritime environments. In antiquity, the most common pozzolans were pyroclastic rocks – mainly poorly consolidated volcanic ash or glassy tuffs. Vegetable ash pozzolans were also sometimes used on a large scale (Lancaster 2012: 146). The altered volcanic tuffs, or trass, of the Rhine region were finely ground and used as pozzolan in the mortars of Roman concrete structures at Cologne during the first and second centuries (Lamprecht 1996: 61, 75, 87; Elsen 2006). Ground-up potsherds and brick also produce pozzolanic reactions with lime, and these were frequently used by the Romans, producing opus signinum for loors and cistern linings (Italian, cocciopesto; Blake 1947: 322–23; Lancaster 2005a: 58–59). In Latin, Vitruvius’ term for the pumiceous, poorlyconsolidated volcanic ash that crops out “in the vicinity of Baiae and the territory of the municipalities around Mount Vesuvius” in the northwest sector of the Gulf of Naples was pulvis, “powder” or “dust” (De arch 2.6.1). This term refers to its finer grain size distribution, as compared with the granular scoriaceous ash or excavated sands (harenae fossiciae) of the region around Rome.

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