Ancient Building Materials Giuseppe Zappia CONTENTS Introduction ...309 Ancient and Modern Building Materials: Historical Excursus...310 Structural Materials...310 Bricks...310 Stones ..
Trang 1Ancient Building Materials
Giuseppe Zappia
CONTENTS
Introduction 309
Ancient and Modern Building Materials: Historical Excursus 310
Structural Materials 310
Bricks 310
Stones 311
Binder Materials 312
Air-Setting Binders 312
Hydraulic Binders 312
Polymers 313
Conglomerates 314
Mortars 314
Concretes 315
Environment-Related Deterioration of Building Materials: State of the Art 315
Damage on Historic Buildings and Monuments 316
Laboratory Exposure Tests 317
Field Exposure Tests 322
Concluding Remarks 325
References 325
INTRODUCTION
The deposition of atmospheric pollutants (gas and aerosol) on the surfaces of monuments and buildings of historical interest exposed to today’s urban environment constitutes one of the main
building materials due to environmental factors is of fundamental importance for both the conser-vation of modern buildings and to guarantee correct methods of restoration on works of historic
or artistic interest
oxidation process
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Numerous authors have dealt with the effects of carbonaceous particulate and heavy metals on
differences in the experimental set-ups involved and in the extrapolation of laboratory studies, based
on differing model carbons, to atmospheric conditions; thus, to date, the problem remains open
So far, there has been a proliferation of studies on the environment-related deterioration of the
Far less numerous are studies on the effects of atmospheric pollutants on other construction materials, both traditional (air-setting mortars, bricks) and modern (hydraulic mortars, concretes)
ANCIENT AND MODERN BUILDING MATERIALS:
HISTORICAL EXCURSUS
Although the use of building materials over the centuries has often been limited to local materials, due to the high cost of transportation and subordinate to the know-how and quality controls prevalent within different geographical areas, it is nonetheless possible to trace a parallel, often interconnect-ing development of technologies and types of materials in correspondence with the various phases human history
Thus, we find in prehistoric eras rudimentary shelters built by mixing a wide and varied range
of natural materials (soil, mud, straw, etc.) with water The most ancient civilizations of the Mediterranean area and Near East already used rough bricks or stones laid one above the other with no binding materials and there is no shortage of remarkable examples of this type of con-struction work; for example, the fortifications of Cape Soprano (Gela, Sicily), built in still well-preserved rough bricks, and the dome-shaped rooms of Mycenae, where small wedges placed between the large stone slabs ensured the stability of the joints However, it was not until the acquisition of the process of firing common raw materials (clay and stone) that the first great revolution in construction technology came about, enabling the fabrication of bricks, tiles, gypsum, and lime In particular, the discovery of binders opened the way to the development of mortars, which have since become a fundamental component of all forms of construction work
The technology of mortar-making reached its peak with the Romans who, with their careful preparation techniques and introduction of pozzolan sands, obtained hydraulic mortars with excel-lent mechanical properties and waterproof, allowing the construction of impressive, long-lasting works that have been preserved up to the present day The Medieval decline that followed the fall
of the Roman Empire saw the abandonment of the refined methods developed by the Romans and
a generalized fall in the quality of building work that lasted up to the 12th century; only in the Renaissance did it return to a level comparable to that obtained in Roman times
The milestones marking the subsequent phases of development up to the modern era were the discovery of hydraulic lime mortars, attributed to J Smeaton (1756), and the invention of Portland cement during the mid-19th century, commonly attributed to J Aspdin, although the previous works
of Vicat (1812) and the subsequent ones of I.C Johnson were more crucial in this regard Before moving on to discuss the environmental degradation of building materials, it is
of ancient and modern building materials
STRUCTURAL MATERIALS
B RICKS
The drying and firing of clay in order to obtain construction materials has taken place in many areas of the world since time immemorial Clay is a sedimentary rock mainly composed of aluminium silicate hydrates; its basic property is that of forming a plastic mass when mixed with water
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The origin of bricks is thought to lie in Asia, in the regions west of the Euphrates, where their use underwent a long period of expansion on account of the scarcity of natural stones and timber for construction Bricks were initially utilized in their raw form (i.e., by simply sun-drying the clay after modeling) It was soon realized, however, that their mechanical resistance and durability could
be much improved upon by firing in kilns Due to the wide availability and accessibility of the raw materials, as well as the ease with which artifacts of any shape could be obtained, the clay-firing technique spread rapidly and is still used today, with little modification over the centuries Bricks are now produced by firing clay at 950 to 1000°C The clay minerals first lose their
or alumina in excess after the constitution of mullite are found in an amorphous state within the brick, alongside other impurities of the clay
Stones have always been the main raw material for the construction of permanent works: houses, palaces, monuments, etc.; their basic property is therefore durability, understood as the capacity of
a material to maintain its properties in time Stones have been tied in with the entire history of building, as they are employed, either directly or after processing, in all construction components:
as a structural or ornamental component in masonry, in the production of binders (lime, cement),
TABLE 15.1 Classification of the Main Ancient and Modern Building Materials
Structural materials
Bricks
Stones
Marbles Limestones Sandstones
Binders
Air-setting
Gypsum Lime
Hydraulic
Pozzolan Hydraulic lime Cement Polymers
Conglomerates
Mortars
Jointing mortars Rendering mortars
Concretes
Ancient concretes Cement-based
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and as aggregate in the production of mortars and concretes Because of the variety of uses to which they can be put, virtually all types of stones are utilized; however, three types prevail over others: marbles, limestones, and sandstones
they have saccharoidal granules and may be white, veined, or polychromatic Known and admired since antiquity, marble has always been used for works of great prestige: the facades of palaces and churches, columns, capitals, friezes, and sculptures The most greatly admired marbles are those of Greece and Italy in which the finest works of classic Greek art and Imperial Rome were realized The finest Greek marbles are Parian and Pentelic (used in the building of the Parthenon), while Carrara marble is the most prized among the Italian ones
minerals, including clay minerals The principal formation process is based on the action of
which gives rise to calcareous deposits One of the finest limestones is Travertine, which the Romans extracted from the Tivoli quarries for the construction of many important buildings and monuments, including the Colosseum
The term “sandstone” refers to a very numerous group of rocks composed of silicate granules
of approximately one millimeter in diameter bound by a cement made up of CaCO3 The granules are mainly formed of quartz, feldspars, plagioclases, and other minerals
BINDER MATERIALS
A IR - SETTING BINDERS
These inorganic substances, when mixed with water, form a plastic mass that has the property of setting and hardening in the air The most important air-settig binders are gypsum and lime, little used today although widespread in the past
(selenite, sericolite, alabaster, etc.) that on firing at 120 to 150°C transform into the hemihydrate form Today, gypsum is used almost exclusively for plasters, stuccoes, and ornamental work, while
in the past it was also employed as a jointing mortar Because it is easy to produce, it was the first binder ever used in history; the Egyptians, for example, used it as a jointing mortar in the con-struction of the Cheops Pyramid (2500 B.C.)
The discovery of lime came about much later on account of its far more complex production process Although the Egyptian already prepared rudimentary forms of lime, it was not until the Greeks and Romans that lime of high quality was achieved and used on a regular basis
Lime is obtained by firing calcareous rocks with a clay content not exceeding 5% at 900 to
Unlike air-setting binding materials, hydraulic binders do not require the presence of air in order
to set, but can harden even in water The term “hydraulic” is commonly taken to refer not only to this property but also to all the other excellent properties of these materials: low porosity, water resistance, high mechanical strength, etc The most important hydraulic binders are lime-pozzolan, hydraulic lime, and Portland cement
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vitreous state and high specific surface area, react with lime and water to form calcium silicates and aluminate hydrates Pozzolans can be either natural or artificial Natural pozzolans are obtained
by crushing volcanic tuff or can be found already in the form of sand or fossil flour Italy has an abundance of natural pozzolan in the regions of Campania and Latium; other countries that produce pozzolan are Greece, Germany, and the United States Artificial pozzolans, obtained in the past using crushed bricks and tiles or finely ground ceramics (cocciopesto), are today composed of fly-ash or silica fume
Hydraulic lime is produced by firing a marly limestone with a clay content of about 15% at
1000 to 1100°C Alongside CaO, this process also gives rise to bicalcium silicate and monocalcium aluminate, due to the presence of silica and alumina in the clay Firing is followed by hydration
of the CaO, using only the stoichiometrically necessary amount of water to avoid hydration of the silicate and aluminate that must take place when the binder is in use Artificial hydraulic limes can
be obtained by mixing, prior to firing, more or less pure limestones with the required quantity of clay, or by mixing Portland cement with fillers
The hydraulic capacity of limes depends on the amount of clay present in the limestone and clay can be evaluated using the hydarulicity index (I) given by the relation
(15.1)
magnesium oxides, respectively
Portland cement owes its name to the resemblance of the hardened cement to Portland stone and is the most important and widely used hydraulic binder It is made by firing at 1450°C marly limestones with a clay content of 25% or by mixing limestone and clay so as to reach the said composition The product obtained (clinker), composed of a mixture of bi- and tricalcium silicate, tricalcium aluminate, and tetracalcium ferrite aluminate, is then cooled and ground, with the addition
of approximately 3% of gypsum in order to regulate setting; in this state, the cement is ready for sale Subsequent phases of setting and hardening are characterized by the hydration reactions of the cement components
Natural polymers have been used since ancient times and, over recent decades, synthetic types have been increasingly utilized for both the construction of new buildings and the restoration of old ones However, since the study of polymeric materials does not fall within the province of this work, they receive only brief mention here
Some natural organic substances are the oldest examples of polymers used for construction: wood as a structural and ornamental element, waxes, and animal and vegetable fats as protective substances Today, widespread use is made of synthetic resins, particularly as consolidants and protective coatings
Organic consolidants consist of polymers that, when dissolved in suitable solvents and after evaporation of the solvent, form a continuous film that covers the walls of the pores of a material, welding together their crystalline grains and impeding water adsorption; consolidants have a good capacity of penetration and are flexible as well as waterproof The problems that may arise during use are: a different dilatation coefficient to that of the material, a reduction of permeability to vapor, and a diminished durability Among the consolidants most commonly used are epoxy resins, which also constitute the most suitable materials for use as adhesives Since epoxy resins become fragile
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and yellow with exposure to atmospheric agents, in particular to ultraviolet rays, use must be limited
to the deepest areas of cracks, while acrylic resins or fiber-glass-reinforced polyester resins should
be used on surface areas
The protection of materials is obtained by covering their external surfaces with the finest and most uniform film possible of a polymeric material that is waterproof and cannot be altered by the substances present in the environment or in the treated material In general, the requirements for
a protective coating are similar to those prescribed for consolidants, although waterproofing, transparency, chromatic invariability, and permeability to water vapor assume even greater impor-tance The last of these requisites is particularly essential: a polymeric film impermeable to water vapor will prevent the treated material from drying out naturally, should water accidentally penetrate inside it The most commonly used coatings are acrylic and silicon resins
Polymer deterioration can be brought about both by physical agents (e.g., heat, light, high-energy radiations, and mechanical stress), and by chemical agents (e.g., oxygen, ozone, acids, bases, water, etc.) Decay reactions affecting the polymeric chain are highly complex depolymerization reactions that inevitably lead to a break in the chain The most frequent reactions are: radicalic depolymerization, thermo-oxidative, photoxidative, and chemical-mechanical degradation and bio-degradation
CONGLOMERATES
Conglomerates are generally marketed in the form of powders and, to assume the plastic properties necessary for use, must be mixed with water In order to minimize shrinkage and for greater economy, they undergo the addition of materials that do not participate in the hardening of the mixture, called aggregates (sand, gravel, crushed stones, etc.), of an appropriate granulometry If the aggregate granules are of a diameter not exceeding 5 mm, the conglomerate is called mortar; otherwise, it is referred to as concrete
Mortar is a conglomerate obtained by mixing a binder and sand in water; it is mainly used for the fixing of structural components (jointing mortar) and for plastering (rendering mortar) A mortar
is defined according to the type of binder adopted for its composition, whose characteristics it assumes; thus, we have air-setting mortars when the binder is lime or gypsum and hydraulic mortars when the binder is lime and pozzolan, hydraulic lime, or cement
In the past, mortars were used only after the acquisition of the technological know-how required for the firing of natural raw materials The most remote examples were among the Egyptians, who
as early as 2500 B.C made use of gypsum mortar in which lime impurities have been found The
while the Greeks and Romans also knew and extensively employed hydraulic mortars
With regard to the latter, since ancient times, different materials have been added to lime in order to obtain hydraulic mortars It is known that as early as the 10th century B.C., the Phoenicians and Israelites were familiar with the techniques of producing hydraulic mortars for the protection
of all their hydraulic works (aqueducts, ports, water tanks, etc.), where washing used to cause the rapid decay of ordinary mortars The drinking-water reservoirs that King Solomon commissioned
in Jerusalem were protected by hydraulic mortar obtained by mixing lime and crushed ceramics The Greeks employed pozzolanic sand obtained by adding volcanic ash from the island of Thera, today’s Santorini However, it was the Romans who were the first to fully understand the importance
of pozzolan and utilized it regularly in the preparation of hydraulic mortars They discovered that the use of sand of volcanic origin (of the type present near Pozzuoli) to substitute ordinary sand
in lime mortar, caused it to become hydraulic Thus, the term “pozzolanic” is used to refer to a type of sand able to transform lime mortar into a hydraulic mortar, although the binder used is L829/frame/ch15 Page 314 Wednesday, February 2, 2000 9:59 AM
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itself air-setting In more recent times, the Dutch were renowned for their hydraulic works for which a mixture of lime and trass were used Trass is a volcanic tuff with properties similar to pozzolan, imported from Andernach, on the Rhine border, near Koblenz in Germany
The next milestone in the development of mortars was the invention of hydraulic lime, a special type of lime that, independent of the presence of pozzolan, has the ability to harden under water This did not take place until the 16th century and is attributed to the Italian architect, Andrea
Smeaton was to reach a fuller understanding of hydraulic reactions in 1756, while attempting
to make a water-resistant lime From the chemical analysis of the limestone used for the production of natural hydraulic lime, he found that the presence of clay in limestones is the decisive factor of hydraulicity Hydraulic lime represents the link between lime and Portland cement discovered in the mid-19th century The use of cement to prepare hydraulic mortars spread rapidly toward the end of the 19th century to assume the position of absolute predominance that it still occupies today
The technique of building masonry by mixing crushed stones and bricks with lime, sand, and water
works still preserved today that were built with this technique; for example the Appian Aqueduct and the dome of the Pantheon in Rome In the Medieval period, concrete was used almost solely
as a filling between external hancings in bricks and stones, which functioned as permanent form-works However, it was the advent of cement that gave rise to the widescale expansion of this building technique, lasting up to the present day where most cement is produced for the manufacture
of concretes
Modern concrete is a conglomerate made up of water, cement as binder, and sand and gravel
as aggregates To improve its mechanical properties, concrete is reinforced with steel bars, a combination exempt from any problems of a physical or chemical nature; in fact, steel adheres well to concrete, the thermal dilatation coefficients of the two materials are more or less the same, ensuring their adherence even with temperature variations, and, finally, the base environment set
up in the concrete after the hydration reactions of the cement protects the steel from corrosion
ENVIRONMENT-RELATED DETERIORATION OF BUILDING
MATERIALS: STATE OF THE ART
The main damage product resulting from the interaction between today’s atmosphere and building
in which it occurs: on the one hand, due to its greater solubility compared to the original compounds
of the materials, once gypsum forms on a surface, it is easily washed off from artifacts that are exposed to rainfall On the other hand, the reaction of gypsum formation leads to the growth of black surface patinas on materials with low porosity (marbles and limestones) or occurs up to a depth of approximately 1 cm in those with high porosity (sandstones and mortars)
The black patinas can be considered as the areas where the products of material deterioration and the deposition of atmospheric gas and aerosol accumulate The color is ascribed to the presence
of carbonaceous atmospheric particles, mainly soots, that are embedded within the crust during its
including automobile exhaust fumes, and their carbonaceous matrix is composed of elemental and
calcium carbonate
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Over recent decades, as part of the effort to ensure a more efficient protection of the architectural
other building materials are entirely lacking
The occurrence of gypsum formation on masonry is particularly dangerous in the case of cement
mortars, concretes, and hydraulic binders in general, because two seriously damaging expansive
Ettringite
Thaumasite Ettringite is produced during the early hours of the hydration process and the reaction generally
involves all the sulfate present in the cement; in this case, the process causes no damage as the
mortar is in a plastic state during setting Subsequently, however, if new sulfate interacts with the
calcium aluminate hydrates in the binder paste, the formation of new ettringite, referred to as
the pores of the cement structure, with spalls and cracks that can lead to the total destruction of
the material
To date, the role of sulfate-rich and sea waters in the formation of secondary ettringite in
cement-based mortars is known However, secondary ettringite formation due to environmental
governing this process In the case of blended cements (with natural or artificial pozzolan addition)
and traditional pozzolanic binders (lime-pozzolan mortars), the relationship between the formation
unknown
Even less information is available on the mechanisms and kinetics of thaumasite formation
Although thaumasite was first observed as early as 1965 on damaged concretes and in repair mortars
used for the conservation of the architectural heritage, so far no correct explanation has been
provided for its formation process However, it has been shown that thaumasite formation can be
produced at low temperatures (2 to 5°C) when gypsum and calcium carbonate interact with CaO
requires a detailed knowledge of the thermodynamic parameters controlling the formation and
stability of such compounds
DAMAGE ON HISTORIC BUILDINGS AND MONUMENTS
With the aim of studying the environment-related damage on historic monuments and buildings,
samples of black alteration patinas were collected from the most common building materials: stones
(marbles, limestones, and sandstones), bricks, and mortars Only black patina samples were selected,
excluding other types of deterioration, as the crusts constitute the areas of maximum accumulation
of alteration products and environmental deposition
The samples were collected in three large cities (Rome, Milan, and Bologna) and in four
maritime sites of central northern Italy (Venice, Ravenna, La Spezia, and Ancona) In the laboratory,
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which they underwent the following analytical procedures X-ray diffractometry (XRD; Philips PW 1730) and infrared spectroscopy (FTIR; Nicolet 20 SX) were used to identify the main chemical species The gypsum and carbonate contents of the samples were quantified by differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) (Netzsch Simultane Thermoanalyze STM
429 apparatus) Carbon and sulfur were measured by combustion and IR techniques (Carbon-Sulfur
using a Dionex 4500I ion chromatograph
Figure 15.1 shows the X-ray diffractogram of a black patina removed from an ancient lime
averaged for the single materials and site typologies The data confirm that, as widely reported in the literature for marbles and limestones, the main damage mechanism affecting all components
percentage of gypsum in the brick patinas is similar to that of marbles and limestones, while the
degree of sulfation is more greatly influenced by the microstructure of the material than its calcium carbonate content Finally, the degree of sulfation in marbles and limestones from the large urban centers turned out to be 18% greater than that found for the maritime sites
The analysis of cations showed, in order of abundance after calcium, Fe, K, Al, Na, Mg, and small quantities of Sr, Mn, and Ba The most abundant anions, after sulfates, were chlorides, nitrates, oxalates, fluorides, phosphates, and traces of bromides In all the samples analyzed, appreciable
amounts of Cnc were found, ranging between 0.6% and 1.8% From these results, it would appear
black patinas on marbles and limestones can be extended also to the patinas found on other building materials
LABORATORY EXPOSURE TESTS
A series of laboratory exposure tests in controlled atmosphere was performed on building stones (marbles and limestones), mortars, and a brick The mortars were prepared in order to reproduce the composition adopted both in antiquity and in modern mortars, the latter being used not only
in contemporary building projects but also in the conservation of historic buildings
Three types of mortars were prepared: (1) a traditional air-setting mortar, composed of lime and sand, 1:3 ratio; (2) a traditional hydraulic mortar composed of lime, volcanic pozzolan, and sand in the ratio 1:1:6; and (3) a modern hydraulic mortar composed of cement and sand in a 1:3 ratio (all ratios are expressed in weight) The constituents used in mortar preparation were: powder
of hydrated lime, natural pozzolan (from Segni, Italy), high-strength Portland cement, and siliceous sand The fresh mortars were poured onto a glass plate and molded by hand into a 5-mm thickness During setting, once a suitable consistency was reached, the fresh mortar was cut into sections
days The samples then underwent chemical-physical characterization in order to determine poros-ity, measured by a mercury porosimeter (Carlo Erba); specific surface, measured by nitrogen
In order to determine their heavy metal content, elemental analyses of the lime, pozzolan, and cement were carried out by inductively coupled plasma spectroscopy (ICPS; Perkin Elmer 5500)
content turned out to be 0.5% for lime, 6% for pozzolan, and 2% for cement
A part of the mortar and stone samples was used blank, while another part was utilized to
oxide, activated carbon, or carbonaceous particles (soots) In order to ensure that the particles were
Trang 10FIGURE 15.1 X-ray diffractogram of a black patina removed from an ancient lime mortar.
© 2000 by CRC Press LLC