1,721,187 research outputs found
Investigation of setting reaction in magnesium potassium phosphate ceramics with time resolved infrared spectroscopy
No full textCrucial information on reaction products in magnesium potassium phosphate ceramics, essential for material design, is lacking. Setting reaction has been followed with time resolved infrared spectroscopy, supported by 2D correlation spectroscopy. We found evidence of a first amorphous orthophosphate precursor, possibly MgKPO4·H2O, forming early, and of a second intermediate amorphous phase, more structurally similar to MgKPO4·6H2O, forming later. Crystallization of MgKPO4·6H2O occurs from this last phase. Presence of amorphous Mg(OH)2 and magnesium phosphates identified as precipitates from diluted suspensions, was excluded. This complex set of parallel reactions, bears analogies with reaction of zinc phosphate cements and is consistent with recent NMR results
Polyamorphism and frustrated crystallization in the acid-base reaction of magnesium potassium phosphate cements
No full textMagnesium potassium phosphate cements are a class of acid-base cements for bioengineering and civil engineering applications. The kinetics of their chemical reaction was investigated in situ with isothermal conduction calorimetry and powder X-ray diffraction, quantifying amorphous and crystalline products. The first reaction step, dissolution of MgO, with an apparent activation energy of 71 kJ mol-1, dictates the time evolution of two amorphous intermediate precursors and the crystalline product. The early crystallization of the latter has been described with an Avrami equation with an apparent activation energy of 81 kJ mol-1, pointing to a mechanism of deceleratory nucleation and growth in one direction, compatible with the acicular crystal habit observed with electron microscopy. The observed polyamorph transformation is controlled by a complex interplay between kinetic and thermodynamic factors, in which the changes in the chemical environment (increase in pH) driven by the MgO dissolution, play a crucial role. It is proposed that the onset of the amorphous-amorphous transformation hinders crystallization by decreasing ion mobility, raising the energy barriers to structural reorganization. The rate of MgO dissolution depends on the reactivity of the powder and the parameters of the mix (such as the amount of liquid) and influences the reaction pathways, impacting on material performance
Amorphous development during the setting reaction of magnesium potassium phosphate ceramics studied by in situ X-ray diffraction
No full textPhysical and nanomechanical properties of the synthetic anhydrous crystalline CaCO3 polymorphs: vaterite, aragonite and calcite
No full textThe synthetic anhydrous crystalline CaCO3 polymorphs—vaterite, aragonite and calcite—were tested using dilatometry and nanoindentation. Microstructural changes in the samples before and after measurements were observed under scanning electron microscope and their phase composition quantified with X-ray powder diffraction with the Rietveld method. The thermal expansion coefficients of vaterite and the hardness and elastic modulus of synthetic aragonite are reported for the first time. The physical and nanomechanical properties were measured under similar conditions for each CaCO3 polymorph. Aragonite, calcite and vaterite showed volumetric thermal expansion coefficient at 303 K of 49.2(8), 48.6(2) and 44.1(3) 10−6 K−1, respectively. The elastic modulus increased from 5(4), 16(7) to 31(8) GPa for aragonite, calcite and vaterite, respectively. Average hardness was found lower than values from the literature, ranging from 0.3 to 1.3 GPa. The results are considered of interest for the design of CaCO3-based materials for applications
Time-evolution of microstructure during hardening of magnesium phosphate cements from synchrotron x-ray micro-computed tomography
No full textRole of Glucose as Retarding Agent of Magnesium Phosphate Cement
No full textMagnesium phosphate cements (MPCs) find application as alternative inorganic binders in construction, for crack repair and recycling of hazardous wastes. For the most common formulation, setting occurs through the reaction in water between magnesium oxide and potassium dihydrogen phosphate. The products include MgKPO4·6H2O (MKP) and an amorphous phase. Their use is somehow limited by the short working time and excessive release of heat. In this work, glucose has been introduced in the formulation of MPC to extend the setting time and modulate the rate of heat evolution. This can be considered an inexpensive and sustainable solution. The mechanism of action of the additive has been studied by investigating the reaction with isothermal conduction calorimetry, whereas the microstructure and phase composition of the obtained cements have been studied with scanning electron microscopy and X-ray powder diffraction, respectively. Results indicated that the additive influenced the reaction path thanks to the interaction at the molecular level with the dissolution process of magnesium oxide, as well as with the nucleation and growth of MKP. This has been confirmed by the changes induced in the size and shape of MKP crystals observed after the experiments conducted on diluted systems
THE CONCEPT OF END OF WASTE IN VIEW OF DEVELOPING SUSTAINABLE SECONDARY RAW MATERIALS
No full textThe concept of end of waste, adopted by the European Commission on December 2005, regards under which conditions a waste could cease to be waste and could be regarded as a non-waste material. This is a revolutionary way to think of wastes not just as refuse to be dismissed but as secondary raw material to be exploited, with an enormous social and economic impact. In this scenario, the creativity of materialsscientists is highly stimulated and the key to success is the development of innovative and sustainable means to transform a waste into a secondary raw material of potential economic value. To this aim, stimulus comes from the so called geo-inspiring materials. This contribution presents the results of a long-term project aimed to recycle asbestos containing materials (ACMs), into secondary raw materials for various industrial applications. The high temperature product of transformation of ACM can be successfully recycled in clay bricks, rock-wool, glass–ceramics, ceramic pigments[1], geopolymers [2], concrete [3], and recently also for innovative formulations of calcium sulfoaluminate cement clinkers [4 ] and magnesium phosphate cements [5]
Use of organic additives as binders in raw mixtures for ceramic tiles
No full textThe aim of this work is the study of the role played by organic additives used as binders in raw mixtures for ceramic tiles. Such binders are extensively used for the production of advanced ceramic materials. In traditional ceramics, they can, in principle, allow for a drastic education of the clay fraction and consequently a very positive effect on the colour of the ceramic body without affecting its peculiar physical properties. Polyvinyl alcohol (PVA) was added in proportions of less than 0.5 wt% to different typical raw ceramic mixtures in which the weight fraction of clay minerals was reduced of about 50 wt%. In each case, it was possible to obtain very good mechanical properties of the green bodies. Clay minerals are the component of the mixture with the highest price and the highest content of iron and titanium (known to have negative effects on the whiteness of the ceramic body). Decreasing the amount of clay minerals makes possible to produce ceramic bodies with the same colour, using less quality (i.e. less expensive) raw materials. This can also lead to reconsider the use of several raw materials present on the national territory. Furthermore, superwhite ceramic bodies can easily be obtained even without the introduction of very expensive components such as zircon and alumina. Possible drawbacks appear to be not severe as the milling times are increased of less than 3% (in laboratory), and additional costs due to the organic additive are overwhelmed by the savings due to the choice of less expensive raw materials and the reduction of the clay fraction (up to 20%)
USE OF SMALL ANGLE NEUTRON SCATTERING AND X-RAY POWDER DIFFRACTION WITH THE RIETVELD METHOD IN THE ASSESSMENT OF FIRING CONDITIONS IN HISTORICAL FIRED-CLAY BRICKS
No full textIt is well-known that the description of the complexity of fired-clay bricks can be tackled only by integrating information from different analytical techniques [1]. At the same time, a nondestructive or low impact approach, aimed at the conservation of cultural heritage objects, is prompting for the search of new analytical methods for their characterisation. As a widely accepted practice, in restoration works, the best compatible materials should be employed. To this aim, the characterization of the original bricks is essential in order to choose the most suitable replacement material and prevent further damages. Mechanical properties and performance of fired-clay bricks are directly related to their microstructure and mineralogical composition, which in turn depend upon the employed raw clay and firing temperature. In this work, the potential of small angle neutron scattering (SANS) and X-ray powder diffraction (XRPD) with the Rietveld method (including the quantification of the amorphous fraction), is evaluated in the investigation of historical fired-clay bricks. SANS is a powerful tool for the characterization of the microstructure of porous solids. In bricks, XRPD allows to identify and quantify the minerals and the amorphous fraction, in order to describe the phase transformations taking place during firing [1,2]. Bricks produced from 2 raw clays have been fired at 5 temperatures (from 800 to 1100 in laboratory. Results have been used to derive information on the firing process experienced by a number of historical fired-clay bricks produced in Czech Republic. SANS allowed to retrieve the pore size distribution, characterise the fractal surface of pores as well as suggest an empirical relationship between surface area per unit volume of pores and firing temperature. The fractal quality of the surface area of pores is also suggested as a method to distinguish between bricks produced from different raw clays. Unit-cell parameter of spinel and amount of hematite (phases detected in the fired bricks) from XRPD are proposed as indicators of the firing conditions
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