Module: Building Bricks and
                 Monumental Glue

Module: Building Bricks and Monumental Glue

Theme:

Monuments (from the Latin monere, to remind) reflect that which a society intends to be culturally lasting. The fabric of such constructions-- the bricks and mortar-- provides information about the choices made in manipulation of the materials of building in the service of aesthetic and religious goals. This module explores the materials science and engineering of mortars, the monumental "glue" used literally to hold together civilizations.

Module program:

Five case studies will be examined: Egyptian gypsum mortar in the third dynasty of the Old Kingdom (c. 2600 BCE), Greek lime mortar of the fifth century BCE, Babylonian fired-clay brick of the eighth century BCE, Roman pozzolanic concrete of the first century AD Roman empire, and Victorian Portland cement. In lecture, the cultural and social contexts will first be explored, followed by consideration of the physical and social circumstances governing materials choices. Finally we will consider elaboration of the materials science and engineering underlying successful employment of the materials chosen. Coordinated hands-on laboratory sessions will introduce participants to the "feel" of the materials, so necessary to acquire a real appreciation of the materials choices made and their elaboration.

Material culture component:

Five corresponding monuments will be studied, in the cultural context of the societies erecting them: the Great Pyramid of Khufu at Giza, the ziggurat of Birs-Nimrod in Babylon (the "Tower of Babel"), the Parthenon in Athens, the Pantheon in Rome, and Kresge Auditorium at MIT. An important objective of this module is to explore comparatively the reasons underlying the respective materials choices in the construction of each monument. In some cases, availability of resources is seen to govern the mortar choice made: Egyptians brought gypsum in large quantitites from the Nile Delta, 100 miles distant, to the limestone-rich Giza plateau, because the low-temperature pyrotechnology demands of gypsum mortars and plasters were more compatible with the local absence of wood fuels than would have been the high-temperature lime-burning technologies necessary for the later development of Hellenistic lime mortars and plasters . In some cases, the choice is dictated by materials property needs: the expanding Roman maritime empire had great need of hydraulic mortars (which set underwater) and, after the fire in Rome, for inexpensive fireproof construction on a vast scale. In other cases, aesthetics play a role: a driving force for popularization (and the name) of Portland cement was its fortuitous resemblance to the quality building stone of the day. As just one example of the societal implications of these technologies, the rise of an immense building trade, based on brick-faced concrete, forged a new middle class in Roman society, initially originating in cadres of freed slaves overseeing brick and mortar production on the estates of the landed class.

Because all of the building materials studied are still in use today, participants will be asked to consider the cultural implications of their present-day employment in our own western society. To encourage the making of such connections, field trips will be made to Eero Saarinan's Kresge Auditorium (1953) on the MIT campus, the first monumental use of a concrete dome since Roman times; and to the Stiles & Hart brickworks in Bridgewater, MA, the principal source of traditional restoration brick (used in the Faneuil Hall renovation, for paving the freedom trail in Boston, and at many other historic sites throughout the East Coast), whose facilities and methods (box-molding of bricks, batch beehive kilns, coal firing) have changed little since its 19th-century establishment.

Materials science/engineering and laboratory component:

Study of building materials has traditionally not been a major part of historically metals-dominated materials science and engineering educational curricula, in part because the building industry is necessarily conservative in its materials choices. Yet these seemingly prosaic media encompass fascinating lessons in atomic structure, environmental and solid state reactions, materials processing, and mechanical properties. In particular, they serve as a far richer introduction to ceramic science and technology than does the more traditional vehicle of fired pottery that is featured in most archaeological curricula. In each unit, the materials science behind the mortar preparation and setting reactions will be explained: the crystal structures and microstructures of the antecedents, the chemistries and morphologies of the reaction products, and the (physical, chemical, mechanical) bases of the setting process. Likewise mechanical properties will be investigated through this unique vehicle. The materials limitation of Greek trabeated (post-and-lintel) construction, set by fracture toughness of the lintel material, and the materials-strength advantage of compressive loading in the Roman arch will both be derived and demonstrated.

The laboratory: The hands-on laboratory sessions, in which mortars will be produced by participants from raw materials (gypsum and limestone rock, volcanic ash, clay), provide an appreciation of the labor-intensity of the production methods, the importance of developments in associated pyrotechnologies, and the concepts of compressive and tensile mechanical strength and fracture mechanics with which builders both ancient and modern have had to develop an empirical appreciation. Participants will proceed through the five units in sequence with the lecture material.

Unit 1: Egyptian gypsum mortar. Participants will crush gypsum rock by hand, using stone and copper hammers, then dehydrate the crushed material at 160 °C, grind by hand, and mix with sand and water.

Unit 2: Greek lime mortar. Limestone rock will similarly be crushed by hand, calcined at 1050 °C to form lime, and mixed with sand and hydrated to form slaked lime mortar.

Unit 3: Babylonian brick. Clay extracted from a pit deposit in western Massachusetts will be hand-thrown into wet wooden box molds, dried to the green state, and fired at 1050 °C. Drying and sintering shrinkages, impact strength and permeability will be measured.

Unit 4: Roman concrete. Volcanic ash from Santorini (the ancient Greek island of Thera, whose volcanic explosion likely contributed to the demise of the Minoan civilization on nearby Crete) will be ground and mixed with lime and sand to produce the pozzolanic mortar that forms the basis of Roman concrete. Types F and C fly-ash, which constitute modern pozzolanic equivalents, will also be formulated into analogous mortars for comparison.

Unit 5: Portland cement. Dried aluminosilicate clay will be crushed and co-calcined with limestone at 1100 °C to form cement clinker, which will be ground by hand and mixed with sand and water.

Cylinders of each mortar will be respectively tested for mechanical strength in an Instron machine and the failure modes noted. The microstructural origins of the setting reactions will be investigated using environmental scanning electron microscopy of the set mortar pastes. The relationship between microstructure and strength is particularly evident in these comparisons, especially the role of reaction product morphology.