N. A. Chervonnaya

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: III. The nature of binder and high-temperature behavior

We have studied the physicochemical properites of the SCAS monolith that was manufactured from fly ash (a large-tonnage waste of coal dust combustion at power plants), lime, and sand in a three-stage process comprising the stages of filtration combustion with superadiabatic heating, fine milling, and pressing, and then hardened in natural (laboratory) settings for 7 years. From the inspection of IR spectra together with the results of energy dispersive X-ray microanalysis, we discovered that calcium serpentine (Ca,Fe,Mg)3[(Si,Al)2O5](OH,H2O)4 was formed in the SCAS monolith during 7-year natural hydration/carbonization, this serpentine acting as a binder in the dispersion-hardened polymineral body. This inference matches the results obtained by X-ray powder diffraction. Differential scanning calorimetry/thermogravimetry (DSC/TGA) with simultaneous recording of the mass spectra of the products has been used to study dehydration, dehydroxylation, decarbonization, and recrystallization occurring in the SCAS monolith between 35 and 1000°C. We have demonstrated that the calcium carbonate polymorphs contained in the monolith have a reduced thermal destruction temperature. The tetrahedral calcium serpentine layer has not been destroyed as a result of heating to 1000°C.

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.

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.

Thermodynamic properties of dimeric molecules of lanthanum and lanthanide trichlorides Ln2Cl6(gas)

The partial pressures of dimeric molecules Ln2Cl6 in the saturated vapor over lanthanum and lanthanide trichlorides LnCl3 (Ln = La, ..., Nd, Sm, Gd, ..., Lu) have been determined by high-temperature mass spectrometry. From these data, the enthalpies of the gas-phase reaction Ln2Cl6 = 2LnCl3 and the enthalpies of sublimation of the compounds under consideration in the form of Ln2Cl6 dimers have been calculated by the third law. Analogous characteristics have also been calculated by the second and third laws from the available literature data on the partial pressures of Ln2Cl6 in the course of sublimation (evaporation) of LnCl3. Taking into account typical tendencies in the standard thermodynamic characteristics of lanthanum and lanthanide compounds, a set of recommended D2980 (LnCl3-LnCl3) values has been determined. This set has been used for calculating the enthalpies of atomization ΔatH2980 (Ln2Cl6), where Ln = La, ..., Lu.

Effect of CaCO3 polymorphs on the strength of a calcium aluminosilicate composite

The material prepared by the superadiabatic combustion of thermal plant fly ash mixed with a carbonate-containing component was hydrated and then carbonated for a long time to obtain a composite which had a compressive strength of ∼180 MPa and Youngs modulus of ∼100 GPa owing to the formation of a mixture of aragonite and calcite in the pores of the composite. Heat treatment of the composite in the range 20–750°С leads to structural changes associated with a number of consecutive processes: dehydration, the aragonite → calcite polymorphic transformation, dehydroxylation, and decarbonation. Each of these processes influences the mechanical properties of the resulting material. The temperature of the aragonite → calcite transformation in the composite is shown to be higher than that in pure aragonite. The transformation causes no dramatic decrease in the mechanical strength of the composite because of the polycondensation of the silicate and aluminosilicate constituents.