Giuseppe Cultrone

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.

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.

The combined influence of mineralogical, hygric and thermal properties on the durability of porous building stones

It is a common practice to test building stones against an isolated decay factor when assessing the durability of building stones. Accordingly, the interpretation of results is often driven by the considerations of a limited number of properties thought as relevant to the action of that specific decay factor. In this way, the role of the synergies of decay processes and agents is often neglected. Contrarily, when different decay factors are included in the same experiment and the stone properties are examined conjunctly, the difficulties of isolating the effects of a specific decay factor or of isolating the contribution of “relevant” stone properties become apparent. The aim of this paper is to assess the combined contribution of mineralogical, hygric and thermal properties to the durability of different stones after exposure to laboratory-simulated cyclic decay combining wetness and temperature variations. For this purpose, 10 stone types were selected to emphasize the impact of the mineralogical heterogeneity on stone decay, in particular the coexistence of calcareous and siliceous grains and the occurrence of clays. The results emphasize the importance of considering the relations between decay factors and highlight how stone properties must be considered as a whole in order to assess and understand the durability of building stones.

Durability of bricks used in the conservation of historic buildings — influence of composition and microstructure

Differences in mineralogical and textural evolution during firing of calcareous and non-calcareous bricks are studied and correlated with their behaviour in hygric and weathering tests. Results reveal significant differences in the evolution of vitrification degree, porosity and pore size distribution. Such evolution depends mostly on raw clay composition and firing temperatures. A higher degree of vitrification and of compressive strength is displayed by calcareous rather than non-calcareous bricks at lower firing temperatures of between 700 and 900 °C. However, their resistance to salt crystallisation and freezing is not notably improved because of unfavourable pore size distribution and crack development. The latter are caused by the transformation of calcite into calcium oxide at around 800 °C, which reacts readily with moisture to form calcium hydroxide, thus leading to a volume increase (lime blowing). This problem can be avoided by closely controlling grain size and content of carbonates in the raw clays. High firing temperatures of 1100 °C in the case of calcareous clay and 1000 °C in the case of non-calcareous clay are required to produce durable bricks that remain unaltered upon weathering. The improved durability appears to be due to a more favourable pore size distribution and a reduction in porosity. Results from textural and hygric studies of the brick samples indicate that these parameters can to a significant extent be controlled by varying raw clay composition and firing temperature, thus making it possible to fabricate replacement bricks for particular conservation purposes. This paper addresses limitations regarding the interpretation of test results, as well as the lack of a systematic application of existing standards for evaluating the state of conservation of historic bricks and for establishing specifications for replacement bricks.

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).

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.

Innovative Analytical Methodology Combining Micro-X-Ray Diffraction, Scanning Electron Microscopy-Based Mineral Maps, and Diffuse Reflectance Infrared Fourier Transform Spectroscopy to Characterize Archeological Artifacts

Excavations at the 14th century Moorish rampart (Granada, Spain) unearthed a brick oven alongside black ash and bone stratigraphic layers. In situ evidence suggests the oven served to fabricate a wall coating including powdered burnt bones. Original ad hoc analyses improved on conventional methods were used to confirm this hypothesis. These methods enable (i) nondestructive micro-X-ray diffraction (mu-XRD) for fast mineralogical data acquisition (approximately 10 s) and moderately high spatial (approximately 500 microm) resolution and (ii) identification and imaging of crystalline components in sample cross-sections via mineral maps, yielding outstanding visualization of grain distribution and morphology in composite samples based on scanning electron microscopy-energy dispersion X-ray spectrometry (SEM-EDX) elemental maps. Benefits are shown for applying diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) vs transmittance-FT-IR (T-FT-IR) to analyze organic and inorganic components in single samples. Complementary techniques to fully characterize artifacts were gas chromatography/mass spectroscopy (GC/MS), optical microscopy (OM), conventional powder XRD, and (14)C dating. Bone-hydroxyapatite was detected in the coating. Mineralogical transformations in the bricks indicate oven temperatures well above 1000 degrees C, supporting the hypothesis.

Supramolecular structure of 5-aminosalycilic acid/ halloysite composites

This paper assesses the supramolecular structure of nanocomposites prepared by including the anti-inflammatory drug 5-aminosalycilic acid in halloysite nanotubes. Halloysite tubes have sub-micron individual lengths with outer diameters ∼0.1 µm, as observed by FESEM. The mercury intrusion plots showed bimodal profiles with pore dimensions ∼10 and 0.06 µm. X-ray diffraction and thermogravimetric results revealed changes in the hydration form of the clay after the interaction. The groups associated to the interaction were studied by FTIR. The location of the drug in the composites was determined after uranium staining of its amino groups by X-EDS microanalysis coupled with HREM. The drug was located both inside and on the surface of the halloysite nanotubes. These results confirm the occurrence of two concomitant interaction mechanisms: rapid adsorption of 5-ASA at the external halloysite surface followed by slow adsorption of the drug inside the tubes.

Three-way ANOVA interaction analysis and ultrasonic testing to evaluate air lime mortars used in cultural heritage conservation projects

Lime mortar has been used throughout history despite current substitution by hydraulic mortars (cements). The chemical composition of the lime used in its manufacture, however, is determined by local geology. In addition, the type of slaking, which depends on the amount of water used, gives rise to different types of lime. The result is that the behavior of lime mortar can vary depending on the composition and type of lime used. A three-way ANOVA analysis was carried out to determine the composition, type, and temporal evolution and the interactions of these three variables for evaluation of the characteristics of air lime mortars for their use in cultural heritage conservation projects.

The influence of aggregate texture, morphology and grading on the carbonation of non-hydraulic (aerial) lime-based mortars

This paper reports on investigations of the influence of the texture, morphology and grading of fine aggregates on the microstructure and macroscopic properties of aerial lime-based mortars. To understand the role of the aggregate in the outcome of the carbonation process, mortars set with two aggregates were cured for 2 years under standard conditions and studied at different time intervals by means of textural and mineralogical analyses and hygric and physical–mechanical tests. Both the initial and further development of the mortar properties are strongly influenced by compositional and textural differences induced by the type of aggregate used. Results show that a calcareous aggregate with continuous grading, consisting of sub-angular grains with a rough surface, induced better textural and physical–mechanical properties than a siliceous aggregate, with polished surface grains and less continuous grading. The use of a calcareous aggregate also provided compositional continuity between the binder and aggregate, apparently promoting further improvement in carbonation and physical–mechanical properties. Further work to quantify this influence is recommended.