Alberto Viani

The nature of disorder in montmorillonite by simulation of X-ray powder patterns

Abstract The planar disorder of Ca-montmorillonite (Fuller’s earth) has been investigated using structural simulations of X-ray powder patterns. A standard sample was fully characterized using chemical, microscopic, and diffraction methods. Earlier models of disorder taken from the literature and newly formulated combined models were used to generate simulated powder patterns to be compared with the experimental spectrum. A new model of disorder with random shifts of -a/3 and ±b/3, with a total density of defects of 75%, gives the best fit to the observed data. Thus, the sample cannot be classified as a turbostratic structure (fully disordered) and consequently turbostratic disorder does not invariably apply to all smectite samples. These findings open a debate on the nature and application of turbostratic disorder: is it possible for smectite samples to have intermediate degrees of disorder between a fully disordered stacking (turbostratic) and a highly faulted but well-defined stacking or is the result obtained for the Ca-montmorillonite just an exception? This model of disorder is useful for the quantitative phase analysis by X-ray powder diffraction based on the Rietveld method, which can now benefit from a more reliable initial structure model for Ca-montmorillonite and which will improve the accuracy of the weight-fraction estimates

Crystal structure-crystal chemistry relationships in the zeolites erionite and offretite

Abstract The new mineral tschortnerite, ideally Ca4(K,Ca,Sr,Ba)3Cu3(0H)8[Si12Al1204S]·ϰH20, ϰ > 20, occurs as well-formed cubes up to a maximum size of 0.15 mm in a Ca-rich xenolith at the Bellberg volcano near Mayen, Eifel, Germany. The light blue, transparent crystals are optically isotropic, n = 1.504(2). Microprobe analysis (in weight percent) gave CaO 13.10, CuO 9.64, SrO 4.49, BaO 1.93, K2O 1.37, Fe2O3 0.30, Al2O3 25.21, SiO2 30.25, H2O (calc, by difference) 13.71. The empirical formula based on 48 O atoms within the tetrahedral net is Ca5.60Sr1.04K0.70Ba0.30Cu2.90Fe0.09Al11.85Si12.06O48(OH)8.44·14.01H2O. Tschörtnerite is cubic, space group Fm3̅m [a = 31.62(1) Å, V = 31614 Å3, Z = 16]. The density is -Dmeas = 2.1 g/cm3, Dcalc = 2.10 g/cm3. Single-crystal X-ray investigations showed that tschortnerite is a zeolite; the structure contains interconnection of double six-rings, double eight-rings, sodalite cages, truncated cubo-octahedra, and previously unknown 96-membered cages (tschörtnerite cage). A new structural unit is the [Cu12(OH)2JCa8O24(H2O)8 cluster centered within the truncated cubo-octahedron. The cluster is formed by a rhomb- dodecahedron-like arrangement of comer connected CuO4 squares, the eight CaO7 poly- hedra are branched. The sodalite cage houses Ca4(OH)4O12 clusters of edge-sharing CaO6 octahedra. Half-occupied (K,Ca,Sr,Ba) positions were located in the basal and top face of the double eight-rings, i.e., the border to the tschortnerite cage. Within the large tschortnerite cage only H2O molecules were localized.

Crystal chemistry of the high temperature product of transformation of cement-asbestos.

In this work, the high-temperature inertization product of a representative batch of samples of cement-asbestos (CA) from different localities in Italy have been characterized with a multidisciplinary approach. All the raw CA samples were heated at 1200°C for 15 min. After firing, they underwent a series of solid state reactions leading to global structural changes of the matrix. Effects of annealing time and temperature on the crystallization kinetics were thoroughly investigated. Both factors acted in favour of equilibrium. Three classes of CA were identified with the aid of phase diagrams and of specific plots relating chemical and mineralogical parameters. This result was considered of importance in view of the potential use of transformed cement-asbestos as a secondary raw material. In principle, the content of CA packages removed from the environment and their corresponding heat-treated products can be classified simply using XRF. This method allows for the selection of appropriate fractions in function of the most suitable recycling solution adopted. Samples belonging to the class called larnite-rich, turned out to be of great interest as possible candidate for substituting a fraction of cement in many building materials and innovative green cement productions.

Recycling the product of thermal transformation of cement-asbestos for the preparation of calcium sulfoaluminate clinker.

According to recent resolutions of the European Parliament (2012/2065(INI)), the need for environmentally friendly alternative solutions to landfill disposal of hazardous wastes, such as asbestos-containing materials, prompts their recycling as secondary raw materials (end of waste concept). In this respect, for the first time, we report the recycling of the high temperature product of cement-asbestos, in the formulation of calcium sulfoaluminate cement clinkers (novel cementitious binders designed to reduce CO₂ emissions), as a continuation of a previous work on their systematic characterization. Up to 29 wt% of the secondary raw material was successfully introduced into the raw mix. Different clinker samples were obtained at 1250 °C and 1300 °C, reproducing the phase composition of industrial analogues. As an alternative source of Ca and Si, this secondary raw material allows for a reduction of the CO₂ emissions in cement production, mitigating the ecological impact of cement manufacturing, and reducing the need for natural resources.

Crystal chemistry of cement-asbestos

Abstract A study of a representative number of cement-asbestos (CA) samples removed from different localities in Italy has been accomplished with a combination of analytical techniques, including XRF, XRPD, SEM/EDS, micro-Raman, and electron backscattered diffraction (EBSD), to elucidate the mineralogical and chemical variability of this class of building materials on a large scale. We describe a complex mineralogy including phases of cement hydration, residual non-hydrated components, and a relevant fraction attributed to various processes of deterioration. With the aid of the CaO-MgOSiO2 compositional diagram, three groups of CAs have been identified on the basis of their chemical parameters. This result is important for environmental and waste management issues

X-ray powder diffraction quantitative analysis performed in situ at high temperature: application to the determination of NiO in ceramic pigments

Although nickel(II) oxide (NiO) is a potential carcinogenic agent, it is still used in the synthesis of ceramic pigments because during their preparation at high temperature, NiO is thought to combine with other compounds, crystallizing as new phases with spinel-like structures. Unfortunately, there are no widely accepted methods for the determination of free NiO in ceramics, the main reason being experimental difficulties. In fact, quantitative phase analysis (QPA) by X-ray powder diffraction (XRPD) may fail because diffraction peaks of NiO with space group Fm3¯m and a ≃ 4.18 A overlap with those of the spinel with space group Fd3¯m and a ≃ 8.4 A. To overcome this problem, in this work QPA has been performed in situ at high temperature to resolve the peak overlap of NiO and spinel by taking advantage of the different thermal expansion of each phase. It is believed that this is the first report of the application of this technique.

The concept of ‘end of waste’ and recycling of hazardous materials: in depth characterization of the product of thermal transformation of cement-asbestos

Abstract Selected samples of asbestos-containing material (ACM) with different Ca/Si ratios have been treated thermally at 1200ºC for 15 min to obtain an ‘end of waste geo-inspired material’. Before and after treatment, micro-Raman spectroscopy allowed the investigation of both powdered and massive samples by directing the laser beam onto crystals with elongated morphology, thin fibres and the matrix. In the raw samples, chrysotile and/or crocidolite were detected. After the thermal treatment, no asbestos phases were identified in the Raman spectra collected on fibrous or fibre-like morphologies. The scanning electron microscopy/energy dispersive spectroscopy investigations confirmed the onset of a pseudomorphic process during annealing, leading to the complete transformation of asbestos minerals into non-hazardous magnesium or calcium magnesium silicates such as forsterite, monticellite, åkermanite and merwinite. The identification of such mineral assemblages was inspired by the close inspection of a natural counterpart, the high-temperature contact metamorphic imprint due to the intrusion of a sill into carbonate rocks. The process turned out to occur largely at the solid state and involved substantial mobilization of Ca and Mg to form a spinel phase (namely MgFe2O4) which was recognized in the matrix and within, or close to elongated morphologies.

In vitro biodurability of the product of thermal transformation of cement–asbestos

To safely recycle the product of the thermal transformation of cement-asbestos as secondary raw material, its toxicity potential should be assessed by in vitro biodurability tests. In this work, the acellular in vitro biodurability of the products of transformation of cement-asbestos at 1200 °C (named KRY·AS) was tested using both inorganic and organic simulated lung fluids at pH 4.5. The dissolution kinetics were followed using chemical, mineralogical and microstructural analyses. The total dissolution time estimated from the experiments with inorganic HCl diluted solution is one order of magnitude higher than that determined from the experiments with buffered Gamble solution (253 days vs. 20 days). The key parameter determining the difference in dissolution rate turns out to be the solidus/liquidus ratio which prompts a fast saturation of the solution with monosilicic acid. The calculated dissolution rate constants showed that the biodurability in vitro of KRY·AS is much lower with respect to that of standard chrysotile asbestos (total estimated dissolution time of 20 days vs. 298 days, respectively). This proves a low potential toxicity of this secondary raw material.

Application of combined multivariate techniques for the description of time-resolved powder X-ray diffraction data

In this work, multivariate statistical techniques are employed to determine patterns and conversion curves from time-resolved X-ray powder diffraction data. For these purposes, time-window statistical total correlation spectroscopy is introduced for the pattern matching of the crystalline phase and is shown to be effective even in the case of overlapping peaks. When combined with evolving factor analysis and multivariate curve resolution–alternating least squares, this technique allows a definite estimation of patterns and conversion curves. The procedure is applied to in situ synchrotron powder diffraction patterns to monitor the setting reaction of magnesium potassium phosphate ceramic (MKP) from magnesia (MgO) and potassium dihydrogen phosphate. It is shown that the phases involved in the reaction are clearly distinguished and their evolution is correctly described. The conversion curves estimated with the proposed procedure are compared with the ones determined with the peak integration method, leading to an excellent agreement (Pearsons correlation coefficient equal to 0.9995 and 0.9998 for MgO and MKP, respectively). The approach also allows for the detection and description of the evolution of amorphous phases that cannot be described through conventional analysis of powder diffraction data.

Microstructural characterization of dental zinc phosphate cements using combined small angle neutron scattering and microfocus X-ray computed tomography

OBJECTIVE To characterize the microstructure of two zinc phosphate cement formulations in order to investigate the role of liquid/solid ratio and composition of powder component, on the developed porosity and, consequently, on compressive strength. METHODS X-ray powder diffraction with the Rietveld method was used to study the phase composition of zinc oxide powder and cements. Powder component and cement microstructure were investigated with scanning electron microscopy. Small angle neutron scattering (SANS) and microfocus X-ray computed tomography (XmCT) were together employed to characterize porosity and microstructure of dental cements. Compressive strength tests were performed to evaluate their mechanical performance. RESULTS The beneficial effects obtained by the addition of Al, Mg and B to modulate powder reactivity were mitigated by the crystallization of a Zn aluminate phase not involved in the cement setting reaction. Both cements showed spherical pores with a bimodal distribution at the micro/nano-scale. Pores, containing a low density gel-like phase, developed through segregation of liquid during setting. Increasing liquid/solid ratio from 0.378 to 0.571, increased both SANS and XmCT-derived specific surface area (by 56% and 22%, respectively), porosity (XmCT-derived porosity increased from 3.8% to 5.2%), the relative fraction of large pores ≥50μm, decreased compressive strength from 50±3MPa to 39±3MPa, and favored microstructural and compositional inhomogeneities. SIGNIFICANCE Explain aspects of powder design affecting the setting reaction and, in turn, cement performance, to help in optimizing cement formulation. The mechanism behind development of porosity and specific surface area explains mechanical performance, and processes such as erosion and fluoride release/uptake.OBJECTIVE To characterize the microstructure of two zinc phosphate cement formulations in order to investigate the role of liquid/solid ratio and composition of powder component, on the developed porosity and, consequently, on compressive strength. METHODS X-ray powder diffraction with the Rietveld method was used to study the phase composition of zinc oxide powder and cements. Powder component and cement microstructure were investigated with scanning electron microscopy. Small angle neutron scattering (SANS) and microfocus X-ray computed tomography (XmCT) were together employed to characterize porosity and microstructure of dental cements. Compressive strength tests were performed to evaluate their mechanical performance. RESULTS The beneficial effects obtained by the addition of Al, Mg and B to modulate powder reactivity were mitigated by the crystallization of a Zn aluminate phase not involved in the cement setting reaction. Both cements showed spherical pores with a bimodal distribution at the micro/nano-scale. Pores, containing a low density gel-like phase, developed through segregation of liquid during setting. Increasing liquid/solid ratio from 0.378 to 0.571, increased both SANS and XmCT-derived specific surface area (by 56% and 22%, respectively), porosity (XmCT-derived porosity increased from 3.8% to 5.2%), the relative fraction of large pores ≥50μm, decreased compressive strength from 50±3MPa to 39±3MPa, and favored microstructural and compositional inhomogeneities. SIGNIFICANCE Explain aspects of powder design affecting the setting reaction and, in turn, cement performance, to help in optimizing cement formulation. The mechanism behind development of porosity and specific surface area explains mechanical performance, and processes such as erosion and fluoride release/uptake.