J. L. Pérez-Rodríguez

Talc from Puebla de Lillo, Spain. II. Effect of dry grinding on particle size and shape

Abstract The effect of dry grinding on the structure, particle size and shape of talc from Puebla de Lillo (Leon, Spain), was studied by means of nitrogen adsorption, XRD, DTA-TG and by SEM-EDS methods. Grinding of talc produces an increase of the starting BET surface area value progressively up to a maximum of 110 M 2 /g at 30 min. A subsequent decrease of surface area with increasing grinding time was observed. The ground, activated particles with high surface energy start a reaggregation process recovering the original surface area value at 210 min. The values of coherent scattering thickness (crystallite size) indicated a reduction of thickness due to crystalline degradation along [00 l ], with initial delamination accompanied by a lesser degree of crystal structure breakdown and amorphization of the structure. The latter increases with increasing grinding time. A profound structural alteration, with progressive smearing of 00 l basal reflections, have been observed. Structural changes have been correlated with particles size, shape and coefficient of texture. The tendency of particles to preferred orientate decreases with grinding. A breakdown of the original particles due to mechanochemical effect produced more rounded particles, aggregates and agglomerates, as proved by SEM. With prolonged grinding, the rate of size reduction decreases with time, but reduction continues up to about 30 min. At this stage, particle size was reduced to about 0.02 μm in diameter and 0.009 μm in thickness, with aspect ratio 2.6, producing an increased degree of amorphism and a decrease in the surface area by reaggregation of the ground powder. After 30 min, the mechanical reduction of the original particles appears to have a limit. The thermal behavior is strongly dependent on the structural state of the material. Grinding produces a slight shift in the main endothermal DTA peak associated to talc dehydroxylation, as indicated also by the TG and DTG curves. An increase of weight loss on heating at lower temperatures than those observed in unground talc have also been detected. The exothermic DTA effect appears on grinding of talc. It has been detected from 10 min grinding time. This effect increases markedly in intensity during grinding, especially when structural breakdown is very important and the amorphous reactive solid is produced.

Sonication as a tool for preparing nanometric vermiculite particles

We have studied the effect of ultrasound on two different natural macroscopic vermiculite samples from Ojen and Santa Olalla (Spain). We describe the conditions for the preparation of nanometric particles by sonochemistry. Sonication produces not only a delamination effect in the [00l] direction, but also a breaking of layers in the other directions, while the crystalline character is retained. Some differences have been observed in the behaviour of both vermiculites. Thus, the Santa Olalla sample suffers an important decrease in crystallite size with up to 30 h treatment time; longer treatment times do not produce any effect on crystallite size. On the other hand, the Ojen sample suffers a decrease in its crystallite size with up to 80 h treatment time, remaining unchanged for longer treatments. The crystallite sizes in the [00l] direction are much smaller for the Ojen sample than for the Santa Olalla sample. In addition, the Santa Olalla sample maintains the typical laminar shape after sonication, while the Ojen sample suffers a morphological change. The different behaviours seem to be related to the different compositions and, therefore, the different layer charges of both vermiculite samples.

Particle size distribution in sonicated high- and low-charge vermiculites

Abstract High- and low-charge vermiculites from Spain (Santa Olalla and Ojen) were subjected to treatment in ultrasonic liquid processor in an attempt to reveal changes in particle size, morphology, and structure ordering. Sonication caused a rise of the S BET from the initial value below 1 m 2 /g for the raw samples to 36 and 54 m 2 /g at the final stage of processing of Santa Olalla and Ojen samples, respectively. These values, converted into equivalent spherical diameter (e.s.d.), prove that vermiculite from Ojen has finer particle size (0.042 μm) than the one from Santa Olalla (0.063 μm ). The low-charge vermiculite (Ojen) suffered an important delaminating effect. Some scrolling at the borders of particles was observed after first cycle of sonication. After eight cycles, the scrolling increased, yielding triangular-shaped particles that finally converted into tubes. The high-charge vermiculite (Santa Olalla) also showed delamination, but no scrolling was observed. The effect of sonication on the crystal structure of the high-charge vermiculite is negligible. The low-charge vermiculite shows broadening and smearing of the diffraction lines. It also shows wider spectrum of thickness of domains of coherent scattering, ranging from the initial value of 0.053 μm to the final one of 0.019 μm, as compared to 0.041 and 0.026 μm for the high-charge vermiculite. It was proved that, contrary to grinding, sonication produces particle division without important structural damages, and that this process is dependent on a layer charge of given vermiculite.

Calcite, vaterite and aragonite forming on cement hydration from liquid and gaseous phase

Cement hydration products were studied as influenced by the hydration conditions (hydration time in liquid phase; relative humidity, RH, in gaseous phase). The formation of calcium hydroxide (portlandite, P) and its transformation to calcium carbonates is mainly discussed here. More hydration products, including P, were formed in liquid phase (paste) than in water vapor (powder), due to the higher availability of water molecules. Full hydration was observed only in the paste hydrated for 6 month, otherwise the P content, estimated from its water escape, DM(400-800°C), increased after storage in water vapor of the prehydrated paste. All the three polymorphs of CaCO3 (calcite, vaterite and aragonite) were found on prolonged contact with air of the hydrated powder (XRD, HRTEM). Their content was dependent on sequence of RH conditions on hydration: higher after water retention, WR, on lowering RH=1.0→0.95→0.5, than after water sorption, WS, on increasing RH in the inverse order. It increased also on wetting and drying, both of hydrated powder and paste. Ca was found to accumulate on the micro-surfaces of WR samples (SEM, TEM), whereas more Al was observed on WS samples and the crystallinity of hydration products was here higher (ED). Dissolution-diffusion-recrystallization was possible: small Al-ions concentrated at one end and the bigger Ca ions - at the other end of some needles (TEM). At 400-500°C the P in cement transforms in air into CaCO3, which decomposes at 600-700°C. Thus the sensitivity to carbonation was estimated from ΔM(600-800°C). This value was similar in pastes hydrated for 1 month and in powder (WR). It was lower in powder WS and much lower in the paste (6 months). It increased pronouncedly when the prehydrated paste was stored in water vapor in WS. The nanocrystals of portlandite, vaterite and aragonite, embedded in the amorphous matrix, were observed by HRTEM in the hydrated powder. They may contribute to the cement strength.

Expansibility of Some Vermiculites

Abstract The thermal expansion of four samples of “vermiculite”, one sample containing both vermiculite and mica and one sample containing vermiculite and mica/vermiculite interstratification from various geological origins have been studied. All samples yield good pyro-expanded products. However, minerals containing mica or mica/vermiculite interstratification have the highest expansibilities. It is suggested that the sudden release of interlayer water is not the only factor influencing the thermal expansion of these samples, e.g. the presence of relicts of altered mica, loss of OH groups or chemical composition.

Characterization of illuminated manuscripts by laboratory-made portable XRD and micro-XRD systems

Illuminated Arabic manuscripts have been studied, employing two laboratory-made X-ray diffraction (XRD) systems developed recently in the C2RMF laboratory. The validity of the µ-XRD and XRD portable systems for the study of this type of artworks has been demonstrated. A common observation in all the analyses is the presence of calcite and rutile; also, hematite, goethite, cinnabar, brass, anatase and barite were detected in the various colours. Differences between the results obtained by both techniques due to acquisition mode are discussed. In addition, other techniques such as X-ray fluorescence (XRF) and micro-Raman were used for the complete characterization of the manuscripts.

Effect of dry grinding and leaching on polytypic structure of pyrophyllite

Abstract The polytypic structure of a purified pyrophyllite sample from Zalamea (Badajoz, Spain) and the structural alteration induced after dry grinding and leaching were studied by means of infrared spectroscopy, nitrogen adsorption, thermal analysis and by X-ray diffraction methods (reflection and transmission). Simulated X-ray powder diffraction patterns, using preparations with random and preferred orientations with coefficients of orientation g=1 and g=3, were compared with the experimental patterns. It was found that the pyrophyllite is in the polytypic modification 1Tc (1AA-II, 1). As compared to those in simulated diffractograms, the diminished intensity of 11l and 02l reflections is due to random translations along the b axis. A short grinding time caused further degradation of the 11l and 02l diffractions. This leads to disordered forms and not to the formation of a monoclinic polytype as has been claimed previously. DTA showed a temperature shift in the main endothermic peak after grinding. This shift was fastest during the first few minutes of grinding. In addition, DTA peak areas decreased with increasing grinding time and with increasing surface area of the ground silicate. These changes are related to a progressive delamination and structural breakdown during grinding. Prolonged grinding, up to 30 min, created a turbostratic-type structure and a drastic reduction in particle size to about 0.04 μm in diameter and about 0.02 μm in thickness, this size reduction being related to the thermal features. After 30 minutes of grinding the mechanochemical reduction of the original particles appears to have reached a limit. At longer times there is an increasing degree of amorphization and a decrease in the surface area by reaggregation of the ground powder. Thus, the thermal behaviour is strongly dependent on the structural state of the material. The polytypism of pyrophyllite is discussed. The X-ray results are compared with those for other pyrophyllite samples (Honami, Japan, the former Yugoslavia, Venezuela and the Ukraine). It is concluded that the formation of a one-layer triclinic polytype of pyrophyllite or disordered forms is more likely than the formation of the two-layer monoclinic variety.

XRF, μ-XRD and μ-spectroscopic techniques for revealing the composition and structure of paint layers on polychrome sculptures after multiple restorations.

This paper presents the novel application of recently developed analytical techniques to the study of paint layers on sculptures that have been restored/repainted several times across centuries. Analyses were performed using portable XRF, μ-XRD and μ-Raman instruments. Other techniques, such as optical microscopy, SEM-EDX and μ-FTIR, were also used. Pigments and other materials including vermilion, minium, red lac, ivory black, lead white, barium white, zinc white (zincite), titanium white (rutile and anatase), lithopone, gold and brass were detected. Pigments from both ancient and modern times were found due to the different restorations/repaintings carried out. μ-Raman was very useful to characterise some pigments that were difficult to determine by μ-XRD. In some cases, pigments identification was only possible by combining results from the different analytical techniques used in this work. This work is the first article devoted to the study of sculpture cross-section samples using laboratory-made μ-XRD systems.

Effect of pollution on polychromed ceramic statues

Abstract Pigments of different colours used to polychrome ceramic statues have been characterized using different experimental techniques. The pigments detected in the painting layers belong to the group of natural minerals widely used in antique paintings. The polluted air and the composition of dust and crust on the ceramic surface have been analysed. Environmental pollution has altered the surface of the ceramic sculptures, destroying the polychrome, so that only small parts covered by crust or dust remain. The environment is responsible for transformation of the chemical components of the pigments. The lead compounds minium and hydrocerussite are altered to anglesite, cerussite, hydrocerussite and lead sulphide. Azurite, forming some blue colour, is hydrated to malachite which is green.Atacamite is formed by the transformation of azurite in the presence of the chloride ion from environmental pollution. The environment supplies a variety of organic compounds—mainly alkanes (produced by petrol combustion)—which are responsible for the black coating on the polychrome., which serves as nutrition for microbiological growth.

Determination of impurity in sepiolite by thermal analysis

The aim of this work was to demonstrate, by means of thermoanalytical methods (DTA, TG and DTG), that sepiolite from Vallecas (Spain) is mixed with another mineral which is difficult to determine by X-ray diffraction and chemical analysis in routine mineralogical studies.The low temperature endothermic effect and weight loss shown by this material can be separated into two effects: one from sepiolite and the other from an accompanying mineral-identified as a magnesium smectile. The concentration of this latter mineral in the mixture attained up to 15%.ZusammenfassungMit vorliegender Untersuchung sollte mittels DTA, TG und DTG gezeigt werden, daß Sepiolith aus Vallecas (Spanien) mit einem anderen, in mineralogischen Routineuntersuchungen durch Röntgendiffraktion und chemische Analyse schwer bestimmbaren Mineral vermischt ist.Der durch diese Substanz bei niedrigerer Temperatur gezeigte endotherme Effekt und Gewichtsverlust kann in zwei Teileffekte untergliedert werden: der eine von Sepiolith herrührend, der andere von dem Begleitmineral, was als Magnesium-Smektit identifiziert wurde. Die Konzentration letzteren Mineralstoffes erreicht im Gemisch bis zu 15%.