A. D. Chervonnyi

Thermal transformations of the SCAS monolith

Thermal conversions in SCAS monolith were studied in the temperature range 25–1200°C. As a result of temperature exposure, the monolith is found to undergo dehydration, dehydroxylation, and decarbonization. These processes are noticed to alter not only the amorphous component of the monolith, but also some of its crystalline phases (calcium carbonate polymorphs, melilite, and gypsum) to yield, in the ultimate product, β-dicalcium silicate, which is capable of forming a binder gel under secondary hydration reactions.

SCAS-monolith: II. Microstructure and mineralogical composition

We present a microscopic investigation using scanning electron microscopy and energy-dispersive X-ray microanalysis (SEM/EDX) of SCAS monolith manufactured from fly ash (a large-tonnage waste of combustion of coal dust at power plants), lime, and sand in a three-step process comprising filtration combustion under superadiabatic heating, fine milling, and pressing. The microstructure and mineral composition of the monolith are described after seven years of exposure in natural (laboratory) settings. The major crystalline phases are melilite minerals (including gehlenite) and dicalcium silicate. Inclusions are represented by dioctahedral calcium mica and xonotlite, as well as chromferide Fe3Cr0.4, magnesioferrite MgFe23+O4, and greigite-violarite minerals (Ni, Fe2+, Cu)Fe23+S4. Three types of glasses have been identified: cristobalite, calcium aluminosilicoferrite, and calcium aluminosilicate. The binder is a calcium hydrosilicate gel of variable composition (1.20–1.31)CaO · SiO2 · (0.68–2.04) · H2O. Sulfur is found in small amounts in the sulfate form. No direct evidence of formation of portlandite (Ca(OH)2) or carbonization products in the form of CaCO3 polymorphs is obtained.

Synthetic calcium aluminosilicate monolith: V. Change in the pore structure in hydration and carbonization

The physicochemical properties of synthetic calcium aluminosilicate (SCAS) monoliths produced from fly ash, limestone, and sand in a three-stage process (filtration combustion with superadiabatic heating, fine grinding, and pressing) were studied. It was found that hydration and carbonization in a SCAS monolith during long hardening under natural (laboratory) conditions lead to perfection of the structure of pores, which improves its physicochemical properties. The presence of unreacted β-Ca2SiO4 in the SCAS monolith throughout the hardening period ensures its high immobilizing properties under the action of the hydrosphere on the matrix containing hazardous (including radioactive) wastes because of calcium hydrosilicate gel formation, which decreases the pore space volume. Examples were given for determining the dependence of the total rate of leaching of SCAS monoliths by deionized water at 90°C on the treatment time (MCC-1 test). The rate of leaching of a SCAS-MRW monolith (where MRW is model radioactive waste of closed nuclear fuel cycle) was found to be 6.7 × 10−7, 7.2 × 10−7, and 8.3 × 10−7 g cm−2 day−1 at MRW contents of 10, 20, and 30 wt %, respectively. The possibility of integrated solutions of some environmental problems using energy- and resource-saving technologies was considered.

Synthetic calcium aluminosilicate monolith: IV. Mechanical properties

Synthetic calcium aluminosilicate (SCAS) containing dicalcium silicate Ca2SiO4 polymorphs was prepared by filtration combustion under superadiabatic heating of a batch comprising lime, sand, and fly ash (a large-tonnage waste of combustion of coal dust at power plants). Mechanical properties of SCAS monoliths manufactured by cold pressing (500 MPa, 2 min) were studied. During a long-term exposure (up to 7 years) under natural (laboratory) settings, Ca2SiO4 polymorphs react with atmospheric moisture and carbon dioxide to produce a material having a certain combination of two opposite properties, namely, high density on account of a stiff filler and a considerable fracture toughness thanks to the plastic properties of thin matrix layers. In static compression tests (under loads from 0 to 351 MPa), SCAS monoliths retain elastic strain while the sample length is reduced by 10.01%; its compression strength is 355 MPa, microhardness is 2.85 GPa, and crack resistance is 2.83 MPa m1/2.

Synthetic calcium aluminosilicate monolith: I. Specific features of the synthesis

A three-step method is suggested for monolith synthesis from fly ash (large-scale waste from the combustion of pulverized coal at heat and power plants), limestone, and sand, which includes filtration com-bustion with superadiabatic heating, fine grinding, and pressing. X-ray diffraction data for the monoliths at different hardening times are presented. The stock combustion conditions are chosen so as to stabilize the reactive α- and β-modifications of Ca2SiO4 and to prevent the undesired formation of γ-Ca2SiO4. Monolith hardening by pressing followed by hydration with atmospheric moisture affords a binder with a low water content and a layered turbostratic structure. No analogue of this binder is known now.

Thermodynamic properties of lanthanum and lanthanide halides: IV. Enthalpies of atomization of LnCl, LnCl+, LnF, LnF+, and LnF2

The results of measurement of equilibrium constants of 30 reactions involving lanthanum and lanthanide fluorides (LnF, LnF2, and LnF3) and 14 reactions involving lanthanum and lanthanide monochlorides (Ln = La-Lu) have been summarized. These constants have been used for calculating the enthalpies of reactions by the second and third laws, from which the enthalpies of atomization ΔatH00 of LnCl, LnF, and LnF2 have been determined. Comparison of the calculation results shows that the thermodynamic functions of LnCl and LnF (Ln = Ce-Yb) in which the electronic excitation contribution has been calculated from the excitation energies of Ln+ ions allow one to adequately determined the ΔatH00 values from experimental data. Using the trends in the change in ΔatH00 as a function of the atomic number of a lanthanide, the enthalpies of atomization of compounds for which experimental data are lacking have been estimated. The ΔatH00 values for LnCl+ ions have been calculated. The reliability of the ΔatH00 values for LnF+ ions have been assessed.

Thermodynamic properties of some lanthanum and lanthanide halides: II. Thermodynamic functions of gaseous molecules LnX (Ln = Ce−Lu, X = F, Cl)

The molecular constants of LnF (Ln = Ce−Lu) for the ground state have been estimated based on the available spectroscopic characteristics of lanthanum and lanthanide monofluorides. These constants have been used for calculating the reduced Gibbs energy (−[G0(T) − H0(0)]/T) of the compounds in the ideal gas state in the range 298.15–3000 K at the standard pressure p0 = 0.1 MPa. The rigid rotator-harmonic oscillator approximation with additional inclusion of the anharmonicity of oscillations (by the method of Mayer and Goeppert-Mayer) and correction for centrifugal stretching have been used. The contribution made by electronic excitation has been calculated by two methods: based on the energies of the excited states of molecules and based on the electronic excitation energy of the free Ln+ ion. Analogous data are reported for lanthanide monochlorides.

Kinetics of reversible polymorphic transitions in high-energy compounds. Phase transformations in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

The kinetics of reversible phase transitions (PTs) in various polymorphs (α, β, γ, δ, and ɛ) of polycrystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is investigated by means of differential isothermal and scanning calorimetry. The rate of the β → δ PT is limited by the nucleation process occurring during the induction period. In a general case, the distribution density for the induction times is a superposition of continuous and discrete functions. The reverse δ → β PT obeys the first-order kinetic law. The effects of mechanical exposure on the kinetics and the PT products of the different polymorphs of HMX is investigated by FTIR spectroscopy.

Composition of the gas phase over Al2O3 and the enthalpies of atomization of AlO, Al2O, and Al2O2

The A1, O, AlO, A12O, Al2O2, WO2, and WO3, partial pressures in the vapor over Al2O3 in a tungsten Knudsen effusion cell between 2300 and 2600 K were derived from A1+, O+, AlO+, A12O+, Al2O2+, WO2+, and WO3+, ion intensities. The mass spectrometer was calibrated against the equilibrium constant of the WO3(g) = WO2(g) + O(g) reaction. Refined values of the ionization cross sections of AlO and A12O2 were used in the partial pressure calculations. The enthalpies of atomization of aluminum suboxides were determined to be ΔatHo(AlO, g, 0) = 510.7 ± 3.3 kJ mol−1, ΔatHo(Al2O, g, 0) = 1067.2 ± 6.9 kJ mol−1, and ΔatHo(Al2O2, g, 0) = 1556.7 ± 9.9 kJ mol−1.

Thermodynamic Properties of Some Lanthanum and Lanthanide Halides: I. Thermodynamic Functions of LaF(gas) and LaCl(gas)

The experimental and theoretical data on the structures and spectra of gaseous LaF and LaCl have been surveyed with the aim of selecting molecular constants for the states with excitation energies in the range 0–10 000 cm−1. With the use of these data, the thermodynamic functions in the range 298.15–3000 K have been calculated from statistical sums in which the intramolecular component was found by direct summation over the energy levels and in the rigid rotator-harmonic oscillator approximation with additional inclusion of the anharmonicity of oscillations (by the method of Mayer and Goeppert-Mayer) and correction for centrifugal stretching. In the latter case, the contribution made by electronic excitation has been calculated by two methods: based on the energies of the excited states of molecules and based on the electronic excitation energy of the free La+ ion. The adequacy of the description of the thermodynamic functions is demonstrated by the calculations of the enthalpies of atomization of LaF and LaCl from experimental data obtained when studying high-temperature reactions with the participation of these halides.