Enthalpy of Formation of Zircon

Using high-temperature solution calorimetry in molten 2PbO. B₂O₃, the enthalpy of reaction of the formation of zircon, ZrSiO₄, from its constituent oxides has been determined: Δ_r H ₉₇₇(ZrSiO₄) =−27.9 (± 1.9) kJ/mol. With previously reported data for the heat contents of ZrO₂, SiO₂, and ZrSiO₄ and standard-state enthalpies of formation of ZrO₂ and SiO₂, we obtain Δ_f H °₂₉₈· (ZrSiO₄) =−2034.2 (±3.1) kJ/mol and Δ_f G °₂₉₈ (ZrSiO₄) =−1919.8 kJ/ mol. The free energy value is in excellent agreement with a range previously estimated from solid-state reaction equilibria. At higher temperature also the data are in close agreement with existing data, though the data sets diverge somewhat with increasing T . The limitations of the data for predicting the breakdown temperature of zircon into its constituent oxides are discussed.     

Si-C-O-N High-Pressure Equilibria and ΔGfo for Si2ON2

Phase relations in the Si-C-O-N system were studied by subjecting mixtures of carbon and silica (C/SiO₂= 2 to 3) to high temperatures (up to 2000 K) and elevated nitrogen pressures (0.5 to 2.0 MPa) in a gas autoclave. The condensed-phase assemblages obtained at different temperatures, pressures, and N/Si ratios are reported and compared with thermodynamic calculations. Values of Δ G _f ^o(Si₂ON₂) consistent with the experimental data are -397 kJ/mol at 1780 K and -369 kJ/mol at 1890 K. The influence on the synthesis of seeding the starting material is discussed.     

Highly Reactive β-Dicalcium Silicate: IV, Ball-Milling and Static Hydration at Room Temperature

The ball-milling hydration of highly reactive β-Ca₂SiO₄ synthesized from hillebrandite, Ca₂(SiO₃)(OH)₂, was investigated and compared with its static hydration under a water/solid ratio of 10. In static hydration, the hydration is completed in 8 d to yield C-S-H with a Ca/Si ratio of 1.81 and Ca(OH)₂. There is no major change after this period. In the case of ball milling, the hydration is completed in 2 d to yield C-S-H with Ca/Si = 1.81 and Ca(OH)₂. After this, the two products react to form a C-S-H(II)-like monophase hydrate having a Ca/Si ratio of 1.98. The morphology and structure of this hydrate are different from those of the earlier hydrate, and afwillite is not formed. ²⁹Si MAS NMR measurements indicated the C-S-H to be a mixture of dimers and single-chain structures. Dimeric species are the main species up to completion of the reaction, but polymerization progresses very rapidly after the completion.     

Melting Phenomena in the System CaO–SiO2– H2O: I, The Join Ca2SiO,-Ca(OH)2

The liquidus-solidus relations along the join Ca₂SiO₄-Ca(OH), in the system CaO-SiO₂-H₂O have been determined at 1000 atm up to 1110°C. This join is binary and contains the calcium silicate hydrate, calciochondrodite, Ca5-(SiO₄(OH)₂. Calciochondrodite melts incongruently to Ca₂SiO₂+ liquid (composition 23 wt% Ca₂Si0₄) at 955°C. The eutectic between calcium hydroxide and calciochondrodite lies at 13% Ca₂Si0₄ and 822°C. Preliminary experiments, also at 1000 atm, in the ternary system CaO-Ca₂Si0₄-Ca(OH), indicate that the eutectic at which the fields of primary Ca(OH)₂, CaO, and Ca₂(Si0₄)₂(OH)₂ meet is close to the CaO-Ca. (OH), side of the triangle at approximately 805° C. The ternary reaction point Ca₂SiO_l+ liquid ⇌Ca₅(SiO₄)₂(OH)₂+ CaO + liquid is believed to lie in the low-CaO (<5%) high-Ca(OH)₂ (>70%) part of the system.     

Hydration of Beta Dicalcium Silicate Alone and in the Presence of CaCl2 or C2H5OH

Beta C₂S was hydrated at room temperature with and without added CaCl₂ or C₂H₅OH by methods previously studied for the hydration of C₃S, i.e. paste, bottle, and ball-mill hydration. The amount of reacted β-C₂S, the Ca(OH)₂ concentration in the liquid phase, the CaO/SiO₂ molar ratio, and the specific surface area of the hydrate were investigated. A topochemical reaction occurs between water and β-C₂S, resulting in the appearance of solid Ca(OH)₂ and a hydrated silicate with a CaO/SiO₂ molar ratio of ≃1. As the liquid phase becomes richer in Ca(OH)₂, the first hydrate transforms to one with a higher CaO/SiO₂ ratio. Addition of CaCl₂ increases the reaction rate and the surface area of the hydrate but to a much lesser extent than in the hydration of C₃S, whereas C₂H₆OH strongly depresses the hydration rate of β-C₂S, as observed for C₃S hydration.     

Assessment of the Thermodynamic Values for PuO1.5 and High-Temperature Determination of the Values for PuC1.5

Thermodynamic values for PUO_1.5 were assessed using an improved method for estimating fef ° 1.5 and new data for S°₂₉₈ 1.5. Based on the assessment, a value of ΔH°₂₉₈, 1.5=–828 kJ/mol is recommended. Measurements of (CO) pressure over the nominal equilibrium 1.5+ 1.5+ C were performed between 1348 and 1923 K, yielding pressures between 0.644 and 11600 Pa. Second- and third-law analyses were used to obtain a value for ΔH°₂₉₈ 1.5=–93.3°3.3 kJ/mol.     

Interpretation of the Parabolic and Nonparabolic Oxidation Behavior of Silicon Oxynitride

The oxidation process of Si₂N₂O, prepared by a hot isostatic pressing technique, has been studied by the thermogravimetric method. The oxidation has been performed in oxygen for 20 h in the temperature range 1300° to 1600°C, producing oxide scales of amorphous SiO₂ and α-cristobalite. The weight gain for T 1350°C does not begin to follow a parabolic rate law, until a certain time, t ₀. The A ₀ parameter in the parabolic rate law, (Δ w / A ₀)²= K _p t + B , represents the cross section area, A , through which the oxygen diffuses; in the derivation of this law A is assumed to be constant during the experiment. If crystallization occurs during the oxidation process, A will decrease with time. A function, A ( t ), describing the time dependence, has been developed and incorporated into the parabolic rate law, yielding a new rate law, which reads Δ W/A ₀= a arctan √ bt + c √ t . This new rate law is valid in the time interval t < t ₀, whereas, for t > t ₀, the oxidation process follows the equation (Δ w/A ₀)²= K °_p t + B ₀. The relation of the latter equation to the common parabolic rate law is described. All of the oxidation curves are described by these equations. The activation energy of the oxygen diffusion (and of the oxidation ( K _p)) is found to be 245 ± 25 kJ/mol, which is consistent with literature values reported for oxygen diffusion.     

Phase Composition of Hydrated DSP Cement Pastes

Cement pastes densified with small particles (DSP) containing up to 48% silica fume by weight of cement, and hydrated to up to 180 d at room temperature, have been analyzed using TMS-GPC, TGA, and ²⁹Si NMR to quantitatively estimate the amount of unreacted cement, Ca(OH)₂, and residual silica fume, respectively. Using a mass balance approach, the CaO/SiO₂ and H₂O/SiO₂ molar ratios of the C-S-H in the samples were calculated. For samples containing silica fume, the values of CaO/SiO₂ lie between 0.9 and 1.3, depending on the degree of hydration and silica fume content, whereas for samples without silica fume they were 1.6. Silicate polymerization analysis using TMS-GPC suggests that the molecular structure of the C-S-H is similar to that formed in conventional hydration. No cross-linking species were found, but the fraction of higher polymers (above octamer) increases as the CaO/SiO₂ ratio decreases.     

Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End-Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solutions

Solubility in the fully hydrated CaO–SiO₂–H₂O system can be best described using two ideal C-S-H-(I) and C-S-H-(II) binary solid solution phases. The most recent structural ideas about the C-S-H gel permit one to write stoichiometries of polymerized C-S-H-(II) end-members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)₂·6SiO₂·5H₂O and 10Ca(OH)₂·6SiO₂·6H₂O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients n _Si and n _Ca. Thermodynamic solid-solution aqueous-solution equilibrium modeling by applying the Gibbs energy minimization (GEM) approach shows the best generic fits to the available experimental solubility data at solid 0.8 < Ca/Si < 2.0 if both stoichiometry and thermodynamic constants of the end-members are normalized to n _Si= 1.0 ± 0.3. Recommended stoichiometries and thermodynamic data for the C-S-H end-members provide a reliable basis for the subsequent multicomponent extension of the ideal C-S-H solid solution model by incorporation of end-members for the (radio)toxic elements or trace metals.     

Standard Enthalpy of Formation of Lanthanum Zirconate

The enthalpy of formation of lanthanum zirconate, La₂Zr₂O₇, which is shown to have a distorted pyrochlore structure, was determined. High-temperature solution calorimetry in molten lead borate was used to determine the heat of formation from the constituent oxides as −135.8 ± 6.4 kJ/mol at 974 K. Using additional data on the enthalpies of formation of the oxides and heat contents, this value was converted to a standard enthalpy of formation from the elements: Δ_fH°_298.15=−4130.4 ± 6.8 kJ/mol.     

Characterization of C-S-H from Highly Reactive β-Dicalcium Silicate Prepared from Hillebrandite

β-dicalcium silicate synthesized by thermal dissociation of hydrothermally prepared hillebrandite (Ca₂(SiO₃)(OH)₂) exhibits extremely high hydration activity. Characterization of the hydrates obtained and investigation of the hydration mechanism was carried out with the aid of trimethylsilylation analysis, ²⁹Si magic angle spinning nuclear magnetic resonance, transmission electron microscopy selected area electron diffraction, and XRD. The silicate anion structure of C-S-H consisted mainly of a dimer and a single-chain polymer. Polymerization advances with increasing curing temperature and curing time. The C-S-H has an oriented fibrous structure and exhibits a 0.73-nm dreierketten in the longitudinal direction. On heating, the C-S-H dissociates to form β-C₂S. The temperature at which βC₂S begins to form decreases with increasing chain length of the C-S-H or as the Ca/Si ratio becomes higher. The high activity of β-C₂S is due to its large specific surface area and the fact that the hydration is chemical-reaction-rate-controlled until its completion. As a result, the hydration progresses in situ and C-S-H with a high Ca/Si ratio is formed.     

Calorimetric Study of Nickel Molybdate: Heat Capacity, Enthalpy, and Gibbs Energy of Formation

The thermodynamic properties of the α and β polymorphs of NiMoO₄ were directly investigated by calorimetry. The standard entropies of formation, Δ_f S ° _T , of α and β were determined from measuring the molar heat capacity, C _p,m, from near absolute zero (2 K) to high temperature (1380 K) by a relaxation method and differential scanning calorimetry. The standard enthalpies of formation, Δ_f H ° _T , of α and β were determined by combining C _p,m with the standard enthalpy of formation, Δ_f H °₂₉₈, at 298 K obtained from drop solution calorimetry in molten sodium molybdate at 973 K. The standard Gibbs energies of formation, Δ_f G ° _T , of α and β were determined from their Δ _f S ° _T and Δ _f H ° _T values. The Δ _f G ° _T values indicate that the polymorphic transformation from α to β occurs at 1000 K, consistent with the observed phase transformation at 1000 K.     

Vaporization of LaCrO3: Partial and Integral Thermodynamic Properties

The vaporization of LaCrO₃(s) and samples of the composition LaCrO₃+ La₂O₃ was investigated in the temperature range of 1887-2333 K by Knudsen effusion mass spectrometry using Knudsen cells made of tungsten lined completely with iridium. The species Cr(g), CrO(g ), CrO₂(g), and LaO(g) were identified in the vapor. Their partial pressures were determined by calibration with pure platinum solid. The thermodynamic activity of Cr₂O₃, a _cr2o₃ in LaCrO₃ for the Cr₂0₃-poor phase boundary of this phase was In a_Cr2o₃⁼ -(17953/T) - 0.485 (temperature T given in K) for the temperature range of the measurements with a probable overall error of ± 13%. The following values and temperature dependence of ΔG°_f,T resulted for the formation of LaCrO₃(s) according to the reaction 0.5Cr₂O₃(s) + 0.5La₂O₃(s) → LaCrO₃(s): ΔG°_f,2100= -78.9 ± 1.1 kj/mol, Δ H°_f,298= -76.8 ± 5.2 kj/mol, and ΔG°_r(kJ/mol) = -74.7 - 0.00202 T . Computations for the vaporization of LaCrO₃ were conducted to show the volatility of this material in different atmospheres at high temperatures.     

Highly Reactive β-Dicalcium Silicate: III, Hydration Behavior at 40–80°C

The effect of curing temperature (40°, 60°, 80°C) on the hydration behavior of β-dicalcium silicate (β-C₂S) was investigated. The β-C₂S was obtained by decomposition of hillebrandite, Ca₂(SiO₃)(OH)₂, at 600°C, has a specific surface area of about 7 m²/g, and is in the form of fibrous crystals. The dependence of the hydration reaction on temperature continues until the reaction is completed. The hydration is completed in 1 day at 80°C and in 14 days at 14°C. The hydration mechanism is different above and below 60°C, but at a given temperature, the reaction mechanism and the silicate anion structures of C-S-H do not change significantly from the initial to the late stages of the reaction. High curing temperature and long curing times after completion of reaction promote silicate polymerization. The Ca/Si ratio of C-S-H shows high values, being almost 2.0 above 60°C and 1.95 below 40°C.     

Precise Determination of the 3CaO·SiO2 Cells and Interpretation of Their X-Ray Diffraction Patterns

The cell dimensions of pure triclinic 3CaO·SiO₂ and monoclinic 3CaO·SiO₂ solid solution (54CaO·16SiO₂·Al₂O₃·MgO) were determined and the powder diffraction patterns were indexed by the method of precise measurement of the spacings. The lattice constants are expressed in terms of triclinic or monoclinic cells corresponding to pseudo-orthorhombic cells derived from Jeffery's trigonal cell. The apparent lattice constants for pure 3CaO·SiO₂ are a = 12.195 a.u., b = 7.104 au., c = 25.096 a.u., α= 90°, β= 89°44'γ= 89°44'; for 54CaO·16SiO₂.-Al₂O₃MgO, a = 12.246 a.u., b = 7.045 a.u., c = 24.985 a.u., β= 90°04'. Precise lattice constants of Jeffery's monoclinic lattice for 54CaO.-16SiO₂-Al₂O₃·MgO are derived as a = 33.091 a.u., b = 7.045 a.u., c = 18.546 a.u., β= 94°08'. High-temperature X-ray patterns showed that pure triclinic 3CaO·SiO₂ transformed to a monoclinic form at about 920°C. and then to a trigonal form at about 970°C. Monoclinic 54CaO.16SiO₂·Al₂O₃–MgO transformed to trigonal at about 830°C. These transitions were reversible and reproducible and were accompanied by only slight deformation of the structure forms.     

Cell Constants for Dicalcium Silicate-Alkali Fluoride Complexes

X-ray diffraction powder patterns were used to determine the cell constants for the complexes 2CaO·SiO_2·LiF and 2CaO·SiO₂·KF by Lipson's method of indexing, assuming orthorhombic structures. The resulting cell constants are α₀= 18.2, b₀=20.6, and c₀=7.2 Å for the Li complex and α₀=18.7, b₀=20.5, and c₀6.9 Å for the K complex.     

Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, Δ _SiO2, of SiO₂ in SiO₂–M₂O (M = Li, Na, K and Cs) binary and SiO₂–CaO–Al₂O₃ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. Δ _SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Δ _SiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q ⁴ and more Q ³ and Q ² species, but positive due to structural strain when the melt is composed of mostly Q ⁴ species. The Δ _SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.     

Densification of Alumina and Silica in the Presence of a Liquid Phase

Alumina and silica powders were sintered at 1250° to 1460°C in the presence of liquid phases containing CaO, A1₂O₃, and SiO₂. The data were analyzed using the equation D = K log t +C and the Arrhenius equation. The activation energy for sintering decreases with increasing amounts of liquid phase.     

Reaction of Beta-Dicalcium Silicate and Tricalcium Silicate with Carbon Dioxide and Water Vapor

The carbonation-reaction kinetics of beta-dicalcium silicate (2CaO·SiO₂ or β-C₂S) and tricalcium silicate (3CaO. SiO₂ or C₃S) powders were determined as a function of material parameters and reaction conditions and an equation was developed which predicted the degree of reaction. The effect of relative humidity, partial pressure of CO₂, surface area, reaction temperature, and reaction time on the degree of reaction was determined. Carbonation followed a decreasing-volume, diffusion-controlled kinetic model. The activation energies for carbonation of β-C₂S and C₃S were 16.9 and 9.8 kcal/mol, respectively. Aragonite was the principal carbonate formed during the reaction and the rate of carbonate formation was coincident with depletion of the calcium silicates; C-S-H gel formation was minimal.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).