Characterisation and development of fine porosity in magnesium potassium phosphate ceramics

Characterisation of porosity and microstructure in chemically-bonded ceramics was accomplished by small-angle neutron scattering. Microstructural parameters were also followed during the setting reaction and obtained kinetic curves were compared with results of XRD quantitative phase analysis up to 1 month. Results indicate that less reactive MgO yields a more compact microstructure, the same happens during the progress of the reaction. The reaction product forms from an amorphous precursor and its development was found to scale with the microstructural parameters. A change in reaction mechanism after 40 min is suggested

In situ synchrotron powder diffraction study of the setting reaction kinetics of magnesium-potassium phosphate cements

This work reports a kinetic study of the formation of magnesium-potassium phosphate cements accomplished using in-situ synchrotron powder diffraction. The reaction: MgO + KH 2 PO 4 + 5H 2 O → MgKPO 4 · 6H 2 O was followed in situ in the attempt of contributing to explain the overall mechanism and assess the influence of periclase (MgO) grain size and calcination temperature (1400-1600 °C) on the reaction kinetics. Numerical kinetic parameters for the setting reaction have been provided for the first time. The best fit to the kinetic data was obtained using a weighted nonlinear model fitting method with two kinetic equations, representing two consecutive, partially overlapping processes. MgO decomposition could be described by a first order (F1) model followed by a Jander diffusion (D3) controlled model. Crystallization of the product of reaction was modelled using an Avrami model (A n ) followed by a first order (F1) chemical reaction. A reaction mechanism accounting for such results has been proposed

Characterization of by-products from the combustion of solid fuels with SEM/EDS and micro-Raman spectroscopy

Coal is still one of the main sources of energy for producing electricity. The environmental impact of the solid products resulting from coal combustion (fly ashes) is mitigated by employing them as secondary raw materials. Their chemical and physical properties are strongly dependent upon the type of coal and the burning plant technology, making their characterization an essential prerequisite for recycling. In recent years, there has been also a concomitant increase in the amount of ashes produced from biomass combustion. Structurally and chemically different, they pose different problems in terms of ecological impact: one of the most relevant is the concentration of heavy metals. In this work, two samples from the combustion of coal and lignite, and two samples from the combustion of biomass, namely straw and hay, have been investigated by means of scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and micro-Raman spectroscopy (with laser wavelength 532 nm). Raman spectroscopy, with the aid of the optical microscope, allowed for addressing the laser beam on specific crystals for phase identification at the micrometric scale [1]. X-ray diffraction (XRD) was employed for bulk qualitative analysis of the crystalline fraction. Due to the operating temperatures above 1400 °C, fly ashes from coal combustion showed the presence of partly glassy spherical bodies (around 10 μm in diameter) with alumino-silicate composition. Crystallization of mullite (ideal Al6Si2O13) from the mass was documented. Euhedral to pseudoeuhedral iron oxide crystals were found as a ’coating’ on some these particles, suggesting ’condensation’ at the grain surface. Their presence in the mass is also an indication of their crystallization from the glass during cooling. These findings are in agreement with XRD results, showing mainly mullite, hematite (Fe2O3) and quartz. Ashes from the combustion of biomass consist mainly of unburned fuel residues, that represent up to 25% in weight. Silica is about 40% in weight and is mainly concentrated in spherical glassy-like particles from nanometric to micrometric in size. Both types of ashes from biomass are high in potassium and phosphates. Typical phases detected by XRD are Arcanite (K2SO4) and Monetite Ca(HPO4). Implications for the use of these by-product as secondary raw materials will be discussed

XANES and micro-Raman spectroscopy study of the barium titanosilicates BaTiSi2O7 and BaTiSi4O11

The coordination environment around Ti4+ in the photoluminescent compound BaTiSi2O7 and in BaTiSi4O11 was investigated with X-ray absorption near-edge structure spectroscopy and micro-Raman spectroscopy. The presence of VTi in TiO5 pyramidal units with one short Ti–O bond involving the apical oxygen was detected in both compounds. Interpretation of the vibrational signal from the silicate framework suggested that BaTiSi4O11 is a metasilicate containing building units of SiO4 tetrahedra, which are larger than in other barium titanosilicates. These results confirmed the same structural environment of Ti4+ as recently disclosed by structure refinement of BaTiSi2O7 and provided new insights into the unknown structure of BaTiSi4O11 in the light of the study of its physical properties as potential functional material