E. Sebastián

Carbonate and silicate phase reactions during ceramic firing

Mineralogical, textural and chemical analyses of clay-rich materials following firing, evidence that initial mineralogical differences between two raw materials (one with carbonates and the other without) influence the tex- tural and mineralogical evolution of the ceramics as T increases from 700 to 1100° C. Mineralogical and textural changes are interpreted considering local marked disequilibria in a system that resembles a small-scale high- T meta- morphic process ( e.g., contact aureoles in pyrometamorphism). In such conditions, rapid heating induces significant overstepping in mineral reaction, preventing stable phase formation and favoring metastable ones. High- T transfor- mations in non-carbonate materials include microcline structure collapse and/or partial transformation into sanidine; and mullite plus sanidine formation at the expenses of muscovite and/or illite at T ‡ 800° C. Mullite forms by mus- covite-out topotactic replacement, following the orientation of mica crystals: i.e., former (001) muscovite are ^ to (001)mullite. This reaction is favored by minimization of free energy during phase transition. Partial melting followed by fingered structure development at the carbonate-silicate reaction interface enhanced high- T Ca (and Mg) silicates formation in carbonate-rich materials. Gehlenite, wollastonite, diopside, and anorthite form at carbonate-silicate interfaces by combined mass transport (viscous flow) and reaction-diffusion processes. These results may add to a better understanding of the complex high- T transformations of silicate phases in both natural ( e.g., pyrometamor- phism) and artificial ( e.g., ceramic processing) systems. This information is important to elucidate technological achievements and raw material sources of ancient civilizations and, it can also be used to select appropriate clay com- position and firing temperatures for new bricks used in cultural heritage conservation interventions.

How does sodium sulfate crystallize? Implications for the decay and testing of building materials

The fundamental behavior of sodium sulfate crystallization and induced decay in concrete and other building materials is still poorly understood, resulting in some misinterpretation and controversy. We experimentally show that under real world conditions, both thenardite (Na2SO4) and mirabilite (Na2SO4·10H2O) precipitate directly from a saturated sodium sulfate solution at room temperature (20°C). With decreasing relative humidity (RH) and increasing evaporation rate, the relative proportion of thenardite increases, with thenardite being the most abundant phase when precipitation occurs at low RH in a porous material. However, thenardite is not expected to crystallize from a solution at T<32.4°C under equilibrium conditions. Non-equilibrium crystallization of thenardite at temperatures below 32.4°C occurs due to heterogeneous nucleation on a defect-rich support (i.e., most porous materials). Anhydrous sodium sulfate precipitation is promoted in micropores due to water activity reduction. Fast evaporation (due to low RH conditions) and the high degree of solution supersaturation reached in micropores before thenardite precipitation result in high crystallization pressure generation and greater damage to porous materials than mirabilite, which crystallizes at lower supersaturation ratios and generally as efflorescence. Data from the environmental scanning electron microscope (ESEM) show no hydration phenomena following wetting of thenardite; instead, thenardite dissolution occurs, followed by thenardite plus mirabilite crystallization upon drying. These results offer new insight into how damage is caused by sodium sulfate in natural geological, archaeological, construction and engineering contexts. They also help explain some of the controversial results of various commonly used sodium sulfate crystallization tests.

Influence of mineralogy and firing temperature on the porosity of bricks

The changes in brick porosity upon firing (700 up to 1100 � C) and its relation to the mineralogical composition are examined. Two types of raw clay with a composition representative of that used in brick-making industry were selected to manufacture the bricks: one contains notable amounts of carbonates, with a grain size of under 1 mm, and the other is predominantly quartzitic and lacking in carbonates. We demonstrate that the presence or absence of carbonates strongly influences the porosity development and, therefore, the brick texture and physical-mechanical properties. The carbonates in the raw clay promote the formation of fissures and of pores under 1 mm in size when the bricks are fired between 800 and 1000 � C. The absence of carbonates results in a continuous reduction in porosity and a significant increase in the pore fraction with a radius (r) > 1 mm as the firing temperature rises and smaller pores coalesce. Porosity and pore size distribution results obtained from the combined use of hydric tests (HT), mercury intrusion porosimetry (MIP) and digital image analysis (DIA) of scanning electron microscopy photomicrographs are compared. A clear correlation between the water absorption and drying behaviour of the bricks and the porosity plus pore size distribution is observed. DIA discloses the evolution of size, shape and connectivity of macropores (r > 1 mm) and evidences that MIP results underestimate the macropore content. Conversely, MIP gives a good estimate of the open porosity and of the distribution of pores with r <1 mm. It is concluded that the combined use of these complementary techniques helps to fully characterise the pore system of bricks. These results as well as the study of the evolution of the speed of ultrasound waves vs. time yield useful information to evaluate the bricks physical–mechanical behaviour and durability. The relevance of these findings in the conservation of historic buildings is discussed. # 2003 Elsevier Ltd. All rights reserved.

Role of particulate matter from vehicle exhaust on porous building stones (limestone) sulfation

Abstract This work, for the first time, experimentally demonstrates the relationship between motor vehicle emissions and the decay of ornamental calcareous stone, by means of sulfation processes (the well-known phenomenon of Black-crust formation). The critical catalytic effects of carbon (soot) and metal-rich particles from vehicle exhaust result in the acceleration of the rate of fixation of atmospheric SOZ to form gypsum on the limestones (calcarenites) used to build Granada Cathedral (Spain). The analysis of particulate matter deposited on the building (carbonaceous and metal-rich particles), as well as of emissions from both leaded-gasoline and diesel motor vehicles confirms that the origin of the particulate matter found in the surface of decayed building stones from Granada Cathedral is consistent with having been contributed by motor vehicle exhaust. Experimental data indicate the role played by this particulate matter in the fixation of atmospheric SOZ as sulfates (gypsum) on calcareous materials in the presence of humidity. We have also experimentally demonstrated that there is a close relationship between the composition of the particulate matter and the fixation rates of the SO 2 in the form of sulfate: (a) diesel engine exhaust, which is primarily composed of soot and metallic particles bearing Fe and Fe-S as major elements and of Cr, Ni, Cu, and Mn as trace elements, plays the largest part in the catalytic oxidation rates of SO 2 ; (b) the emissions from gasoline engines, composed of minor quantities of soot and high concentrations of Pb- and Br-bearing particles, cause a lower rate of SO 2 fixation as gypsum on limestones. From these experimental findings, a new hypothesis is proposed concerning the sulfation of the limestones.

TEM study of mullite growth after muscovite breakdown

Abstract Mullite (Mul) formation after high-T muscovite (Ms) breakdown has been studied in phyllosilicaterich bricks. At T ≥ 900 °C Ms dehydroxylation is followed by partial melting that triggers the nucleation and growth of Mul acicular crystals. An analytical electron microscopy study reveals that the Mul is a 3:2-type with a [6](Al1.686Ti0.031Fe0.159Mg0.134)[4](Al2.360Si1.649)O9.82 formula and an O atom vacancy of x = 0.18. This is consistent with X-ray diffraction results [i.e., unit-cell parameters: a = 7.553(7), b = 7.694(7), and c = 2.881(1) Å, V = 167.45 Å3]. The initial stage of the process resulting in Mul growth followed the balanced reaction Ms → 0.275Mul + 0.667Melt + 0.244K2O + 0.01Na2O + 0.125H2O, yielding an alkali-poor peraluminous melt. H2O with K (and Na), which are lost along the (001) planes of dehydroxylated Ms, play a significant role as melting agents. The c-axes of the Mul crystals are oriented parallel to [010]ms or to the symmetrically equivalent <310>ms zone axis, while the (120)mul or (210)mul planes are subparallel to (001)ms (TEM results). These systematic orientations point to epitaxial Mul nucleation and growth on the remaining Ms substrate, which acts as a template for Mul heterogeneous nucleation. Randomly oriented Mul growth is also observed during the late stages of the process (i.e., melt cooling). The epitaxial nature of Mul growth after dehydroxylated Ms melting minimizes the energy requirement for nucleation. In addition, the water released after Ms breakdown and the multicomponent nature of the melt enable this high-T aluminum silicate to grow at T ~ 900 °C, almost 100 °C below the SiO2-Al2O3-K2O ternary system eutectic (after a melt with an end-member Ms composition).

Origins of honeycomb weathering: The role of salts and wind

Honeycomb weathering is a common surface phenomenon affecting a variety of rocks in a range of environments. It develops on building stones and it shapes ocean cliffs, rocks in hot deserts, and Arctic landscapes. Honeycomb weathering may also help alter rocks on other planets, such as Mars. Although first noted in the nineteenth century, its origins are still not well understood, and a dearth of laboratory experiments testing the many theories proposed for its development has added to the ambiguity. Incipient honeycomb weathering in a homogeneous limestone has been experimentally reproduced by wind exposure and salt crystallization. Our experiments show that heterogeneous wind flow over a stone surface is important in the development of this weathering pattern. Wind promotes evaporative salt growth between grains on a stone surface, resulting in the development of small, randomly distributed cavities. A reduction in air pressure within the cavities results in increased wind speed and rapid evaporation. A high evaporation rate and evaporative cooling of the saline solution in the cavity leads to more rapid and greater granular disintegration than in the surrounding areas. It seems that this local supersaturation and subsequent buildup of salt crystallization pressure ultimately result in the formation of honeycomb features. For the first time, these experimental results demonstrate the close relationship between salts, wind, and honeycomb weathering. They also offer new ways to understand the genesis of this striking and sometimes harmful weathering pattern.

Liesegang pattern development in carbonating traditional lime mortars

Liesegang patterns, generally rings, bands, spheres or spirals, form in far–from–equilibrium systems in nature and in the laboratory by self–organized periodic precipitation of sparingly soluble phases following a nonlinear reaction–diffusion process. Although Liesegang patterns have been known for more than hundred years, there is still disagreement as to the mechanisms underlying this phenomenon. Most studies have focused on Liesegang pattern formation in gels, quantitative studies of quasiperiodic patterns in non–conventional porous media (e.g. construction materials) being rare. Here, we report the development of ‘revert’ three–dimensional Liesegang patterns (i.e. concentric ellipsoids) in traditional lime mortars undergoing carbonation. Portlandite (Ca(OH)2) in a quartz (SiO2) sand aggregate, transforms into calcite (Ca(CO)3) in contact with atmospheric CO2, resulting in banded cementation of the lime mortar. Surprisingly, well–developed Liesegang patterns only occur in mortars prepared using ‘aged’ lime putty, kept under excess water for years, following an ancient Roman recipe to improve slaked lime quality; the carbonation of these mortars being faster than in pattern–less ones. The smaller Ca(OH)2 particle size in the long–term–aged putty enhances dissolution and increases the ion–concentration product, while creating a higher volume of pores with r < 0.1 &mgr;m. These small pores can sustain very high supersaturation ratios with respect to CaCO3, resulting in higher nucleation rates, a crucial fact for pattern development previously neglected. These results may have strong implications for the understanding of Liesegang patterns, as well as for the conservation of architectural heritage.

Behavior of Brick Samples in Aggressive Environments

The weathering of different brick samples ina range of aggressive environments has been studied.Brick samples were prepared using two clay types (fromGranada, Spain), different additives, and a range offiring temperatures (850–1100 °C). The brickscompositional and textural characteristics wereevaluated using XRD, SEM, hydric tests and mercuryintrusion porosimetry (MIP). The samples weresubjected to accelerate aging, including wet-dry,freeze-thaw and salt crystallization cycles. The decayof the bricks in polluted atmospheres was simulated ina static chamber containing sulfur dioxide (SO2)at 25 °C and 50% relative humidity. Samplesfired at 1000 °C proved to be the most durable,with better hydric behavior (fast drying and slowwater absorption) and fewer micropores. However, theywere not suitable for salt-rich environments (badperformance in the salt decay test). Samples fired at850 °C turned out to be more resistant to saltdecay, but they showed a poor hydric behavior (slowdrying and rapid water absorption) and littleresistance to freeze-thaw and wet-dry cycles. Samplesfired at 1100 °C had good hydric behavior, theyperformed well in the salt decay test, but they didnot perform as well as the samples fired at1000 °C in most accelerated aging tests. Gypsumformed on all the brick samples submitted to SO2atmosphere regardless exposure time (e.g. gypsumappears following just 24 h of exposure),composition, or firing temperature. Samples withdeposited particulate matter collected from vehicleexhausts (diesel, as well as leaded and non-leadedgasoline motor cars) resulted in the fastest gypsumdevelopment and greater abundance. On the other hand,the blank samples, and the samples withpollution-derived dust collected from historicalbuildings showed little gypsum development. Theimplications of these results in historicalbrick-building preservation in a range of aggressiveenvironments, and in polluted atmospheres inparticular, are discussed.

The role of sepiolite-palygorskite in the decay of ancient Egyptian limestone sculptures

An ancient Egyptian limestone sculpture was found to be undergoing major structural decay when stored in a museum environment. Mineralogical and petrographic analysis of the limestone showed a high proportion of clay (≥- 10% by weight) that was concentrated along bedding planes. The clay fraction consisted mostly of sepiolite (>90%) and palygorskite (<10%). Minor quantities (≤l%) of soluble salts (NaCl and NaNO3) were also found. Wetting/drying with distilled water and relative humidity cycling resulted in the same delamination cracking damage as that observed in the museum environment. Thermomechanical analyses (TMA) confirmed that the damage was due to expansion (>4.5%) parallel to bedding planes when the limestone was immersed in water. The expansion due to swelling of the clays was directly observed at high magnification in an environmental scanning electron microscope (ESEM) when wetting/drying cycles were performed. X-ray diffraction (XRD) analysis showed that crystalline swelling of sepiolite occurred. This was determined by a shift of (110) reflection (from 12.07 to 12.20 Å) and a decrease of (060) reflection (4.47 Å, to 4.44 and 4.41 Å), when in contact with ethylene glycol (EG) and dimethyl sulfoxide (DMSO), respectively. Swelling also occurred due to hydration of the clay surfaces and to electrostatic forces between clay particles, which, it was assumed, was promoted by the presence of Na counterions in water solution. Possible treatments for the conservation of these artistic objects are proposed and discussed.

THE ROLE OF CLAYS IN THE DECAY OF ANCIENT EGYPTIAN LIMESTONE SCULPTURES

One type of Egyptian limestone from Naga el-Deir (Abydos/Thebes region) exhibits an ongoing problem of deterioration typified in the form of continued delamination of the surface in a stela from Naga el-Deir acquired from archaeological investigations carried out in the early 20th century. Previous testing of this limestone type indicated the presence of sodium chloride and sodium nitrate. The sculptures have been treated and desalinated either by immersion in water or by aqueous poulticing, but the decay process was not halted, and major loss of surface stone was still noticeable after storage for a period of years. Mineralogical and petrographic data (x-ray diffrac- tion (XRD), scanning-electron microscopy (SEM), and optical microscopy) indicate that this stone has a high proportion of clays (up to 10% by weight). Lab- oratory tests suggest that the clays, concentrated along bedding planes, are largely responsible for the type of deterioration noted. The role of clay minerals in the decay of this type of limestone was demonstrated by performing a series of experiments, including wet- ting/drying cycles and relative humidity changes, thermomechanical analysis, and accelerated decay tests using water and ethylene glycol. One conclusion of this study is that in some instances, desalination procedures can induce more deterioration than can rigid environmental control. Another conclusion is that attribution of deterioration to the presence of salts may be insufficient, and further petrographic analysis should be initiated prior to desalination of clay-rich limestones. Unconventional methods for possible stabilization of the clay structure (by means of