29Si and 27Al MAS-NMR of NaOH-Activated Blast-Furnace Slag

b²⁹Si and ²⁷Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad ²⁹Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q ¹], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new ²⁹Si peaks with widths of ca. 2 ppm are formed at -78.5 Q ¹, –81.4 [ Q ²(1Al)J, and -84.3 [ Q ²]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, ²⁷Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)₄AH₁₃. ²⁹Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days.     

Dissolution of a Sphene Glass-Ceramic, and of Its Component Sphene and Glass Phases, in Ca-Na-Cl Brines

Leaching experiments for up to 569 d have been performed to examine the dissolution in Ca-Na-Cl groundwater of a sphene (CaTiSiO₅)-based glass-ceramic, a candidate material for immobilizing nuclear fuel recycle wastes. The experiments involved leaching of samples of a simulated-waste-loaded glass-ceramic, doped with ²²Na or ⁴⁵Ca, in a synthetic groundwater at 25° and 100°C. The results are compared with those from separate leaching experiments with ²²Na- or ⁴⁵Ca-doped samples of aluminosilicate glass and ceramic sphene, representing the component phases of the glass-ceramic. The comparison supports a model in which the glass-ceramic dissolution rate may be approximately derived from the weighted average of the separate dissolution rates of its component glass and ceramic phases. No synergistic effects in the leaching of the glass and ceramic phases in the glass-ceramic were found.     

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.     

Highly Reactive β-Dicalcium Silicate: II, Hydration Behavior at 25°C Followed by 29Si Nuclear Magnetic Resonance

The hydration behavior at 25°C of highly reactive β-dicalcium silicate synthesized from hillebrandite (Ca₂(SiO₃)(OH)₂) was studied over a period of 7 to 224 d using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C-S-H, contains Q² and Q¹ silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q¹ and Q² formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q²/Q¹ increases as the reaction progresses and as the curing time becomes longer. From the values of Q²/Q¹, it appears that the hydrate is a mixture of dimers and short single-chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q²/Q¹ ratio. It was also found that the NMR peak intensities allow quantitative assessment similar to XRD.     

Structure of Calcium Silicate Hydrates Formed in Alkaline-Activated Slag: Influence of the Type of Alkaline Activator

The influence of the alkaline activator (NaOH, waterglass, or Na₂CO₃) on the structure of the hydrated calcium silicate formed in alkali-activated slag (AAS) cement pastes has been investigated by FTIR, ²⁹Si and ²⁷Al magic-angle scattering nuclear magnetic resonance, and TEM/EDX techniques. In all cases, the main product formed after 7 d of activation, with activators giving an Na₂O concentration of 4%, is a semicrystalline calcium silicate hydrate with a dreierkette-type anion. In these structures, linear finite chains of silicate tetrahedra ( Q ² units) are linked to central Ca-O layers, and tetrahedral aluminum occupies bridging positions in the chains. The main chain length and the amount of aluminum incorporated in the tetrahedral chains depend on the activator used. The detection of Q ³ silicon entities in alkaline-activated slags is discussed in relation to the possible formation of cross-linked structures that may be responsible for increased flexural and compressive strengths in AAS mortars.     

MAS NMR Studies of Partially Carbonated Portland Cement and Tricalcium Silicate Pastes

²⁹Si, ²⁷Al, and ¹H MAS NMR studies of partially carbonated mature ordinary Portland cement (OPC) and tricalcium silicate (C₃S) pastes have been carried out. The water-to-solid ratios ( W/S ) have been varied between 0 and 1 at hydration temperatures of 23^o and 90^oC. Various Q ⁿi units with n =0, 1,2,3, and 4, and a Q³ (1Al) group have been identified using ²⁹Si NMR. Cross-polarization experiments, in addition, have made it possible to assign the OH groups. Two types of fourfold- and one type sixfold-coordinated aluminum have been distinguished using ²⁷Al NMR. In C₃S pastes for w/s >0.7, progressive carbonation leads to a nearly perfect three-dimensional network consisting of Q³ and Q⁴only. In contrast, in OPC pasted only about 40% of the highly polymerized silicate units are formed, partially copolymerized with AlO₄ tetrahedra.     

Third-Harmonic Generation from Some Chalcogenide Glasses

Third-order optical nonlinear susceptibilities (χ⁽³⁾) of some high-refractive-index chalcogenide glasses were evaluated from third-harmonic generation. Compared with oxide glasses whose χ⁽³⁾ has been known, χ⁽³⁾ of the present glasses was higher by an order of magnitude. The addition of selenium drastically increased χ⁽³⁾. The highest χ⁽³⁾ was 1.4 × 10^–11 esu, being comparable with those of high-χ⁽³⁾-organic compounds. Further, χ⁽³⁾ generally increased with increasing density in the present glasses.     

Phase Relations and Structures in the System PbO–Fe2O3

A detailed study of the system PbO-Fe₂O₃, largely by X-ray diffraction analysis, has been made. The results indicate the existence of three intermediate phases: (1) a phase (beta) extending from PbO·5Fe₂O₃ to PbO·6Fe₂O₃, (2) a phase (gamma) extending from PbO - 2Fe₂O₃ to PbO -2^1/2Fe₂O₃, and (3) a phase (delta) 2PbO·– Fe₂O₃. Structures are proposed for the beta and gamma phases that explain their solubility limits, magnetic properties, and very great similarity in structure and in lattice dimensions.     

X-ray Diffraction and 151Eu Mössbauer Spectroscopic Study of the Hf1−xEuxO2−x/2(0 ≤x≤ 1.0) System

Powder X-ray diffractometry (XRD) and ¹⁵¹Eu Mössbauer spectroscopy were performed for samples prepared in the temperature range 1500–1500°C for the hafnia–europia (HfO₂–Eu₂O₃) system Eu _x Hf_{1− x} O_{2− x /2}(0 ≤ x ≤ 1.0). The XRD results showed that two types of solid solution phases formed in the composition range 0.25 ≤ x ≤ 0.725: an ordered pyrochlore-type phase in the middle-composition range (0.45 < x < 0.575) and a disordered fluorite-type phase, enveloping the pyrochlore-type phase on both sides, in the composition ranges 0.25 ≤ x ≤ 0.40 and 0.60 ≤ x ≤ 0.725. ¹⁵¹Eu Mössbauer spectroscopy sensitively probes local environmental changes around trivalent europium (Eu³⁺) caused by the formation of these solid solution phases. In addition to the broad, single Mössbauer spectra observed in this study for the Eu³⁺, the values of isomer shift (IS) exhibited a pronounced minimum in the neighborhood of the stoichiometric pyrochlore phase ( x ≈ 0.5). Such IS behavior of Eu³⁺ was interpreted based on the XRD crystallographic information that the ordered pyrochlore phase has a longer (in fact, the longest) average Eu–O bond length than those of the disordered fluorite phases on both sides or the monoclinic (and C-type) Eu₂O₃at x = 1.0.     

Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Mechanism of the Blue Up-Conversion in Tm3+/Nd3+-doped Calcium Aluminate Glasses

Blue up-conversion fluorescence from the Tm³⁺:¹ G ₄→³ H ₆ (480 nm) transition has been observed from calcium aluminate glass codoped with Tm³⁺/Nd³⁺. The mechanism for the up-conversion process consists of a two-photon process. An excitation beam with a wavelength of 791 nm first excites Tm³⁺ to the ³ H ₄ level, where Tm³⁺ again absorbs the 1060 nm emission from Nd³⁺:⁴ F _3/2→⁴ I _11/2 to attain the Tm³⁺:¹G₄ level. Lifetime and intensity variations with compositions suggest the presence of an efficient energy transfer from Nd³⁺ to Tm³⁺. The highest 480 nm emission intensity has been obtained from the glass with 0.1 mol% of Nd₂O₃ and 0.2 mol% of Tm₂O₃.     

Structures and Phase Relations of Aluminum-Substituted Calcium Silicate Hydrate

The structures of aluminum-substituted calcium silicate hydrate (C-S-H) forming in a series of aqueous suspensions formulated with colloidal silica, reactive alumina, and lime and aged for 1 year have been studied using ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (with and without cross polarization (CP)), solution pH, electron microscopy, and X–ray diffraction. As in earlier work dealing with the nature of C-S-H in the system CaO-SiO₂H₂O₂ two aluminum-substituted C-S-H phases, having distinctly different anionic structures on the atomic level (Q₂ versus Q₁Q₂), were found to extend into the system CaO-Al₂O₃-SiO₂-H₂O₂ X-ray diffraction patterns of the two aluminum-containing C-S-H phases are nearly identical, suggesting that their intermediate-range order is very similar, but MASNMR spectra show that these two phases have markedly different silicate structures on the atomic-level scale. Both C-S-H structures can accommodate approximately 5 mol% of Al₂O₃ in tetrahedral and possibly octahedral coordination as well.     

Two-Dimensional Mapping of Er3+ Photoluminescence in CaF2 Crystal Lines Patterned by Lasers in Oxyfluoride Glass

The laser-induced crystallization method is applied to an oxyfluoride glass with the composition of 41.5SiO₂–21.3Al₂O₃–4.8CaO–12.6NaF–16.4CaF₂–2.9NiO–0.5ErF₃ (mol%), and the lines consisting of CaF₂ nanocrystals (diameter: ∼20 nm) are patterned on the glass surface. It is found from micro-photoluminescence (PL) spectra of Er³⁺ ions that Er³⁺ ions are incorporated into CaF₂ nanocrystals formed by laser (continuous-wave Yb:YVO₄ fiber laser with a wavelength of 1080 nm) irradiations. Two-dimensional mappings of the PL intensity for the ⁴S_3/2→⁴I_15/2 transition of Er³⁺ ions are measured for the surface and cross section of the patterned lines. It is found that two phases giving different PL intensities are formed in the laser-irradiated region, suggesting that the center part of the laser-irradiated region consists of Er³⁺-doped CaF₂ nanocrystals and the surrounding of the center part gives the fluoride ion rich coordination state for Er³⁺ ions. The formation mechanism of Er³⁺-doped CaF₂ nanocrystals is related to the temperature distribution of the laser-irradiated region.     

29Si Magic-Angle-Spinning Nuclear Magnetic Resonance Study of Hydrated Cement Paste and Mortar

This paper presents ²⁹Si magic-angle-spinning nuclear magnetic resonance measurements that trace the cement hydration process in cement paste and mortar specimens made from ordinary portland cement, type I. These specimens were moist-cured for 3, 7, 14, and 28/31 d at temperatures ranging from 21° to 80°C. Compressive strength for all tested specimens was also determined. The results show that the degree of hydration ( Q ¹+ Q ²) and the compressive strength increase with curing times and temperatures. However, at 80°C, the compressive strength decreases while the degree of hydration increases.     

Oxygen Tracer Diffusion in Lanthanum-Doped Single-Crystal Strontium Titanate

Strontium titanate (SrTiO₃) is known as a good high-temperature resistive oxygen sensor material; its response time depends on oxygen bulk diffusion and surface exchange processes. In the present work, ¹⁸O diffusion has been investigated in lanthanum-doped SrTiO₃, single crystals in the temperature range 700° to 900°C by secondary ion mass spectrometry (SIMS). Oxygen tracer diffusivities between 2 × 10⁻¹⁵ and 1 × 10⁻¹³ cm²/s have been calculated from the SIMS results. Low surface enrichment of ¹⁸O compared to the ¹⁸O concentration in the gas atmosphere gives clear evidence for a surface exchange reaction.     

Determination of the Oxygen Self-Diffusion Coefficients in Y2O3-Containing Tetragonal Zirconia Polycrystals by Raman Spectrometric Monitoring of the 16O-18O Exchange Reaction

A new method of determining the oxygen self-diffusion coefficients (D) in oxides has been developed. The method is based on Raman spectroscopy combined with the ¹⁶O–¹⁸O exchange technique. By using the time dependent ¹⁸O2 concentration measured in a quasi in situ manner by Raman spectroscopy, the oxygen self-diffusion coefficients are calculated. The calculation takes into account the influence of the surface exchange reaction and the limited gas volume. The result obtained in 2.8 mo1% Y₂O₃-containing tetragonal zirconia polycrystals is D = 1.55 (^+0.07_-0.07) x 10^-2 exp[(-120.0 0.4) (kJ/mol) / RT ] cm²/sec. It was demonstrated that Raman spectroscopy is useful tool for determining the oxygen self-diffusion coefficients in oxides.     

Limits of the Thermodynamic Stability of Cobalt—Iron—Manganese Mixed Oxides at 1200°C

The thermodynamic stability of spinel and rock-salt structure phases has been investigated for the quasi-ternary mixed oxide system Co-Fe-Mn-O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 10⁵ Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock-salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log₁₀ a _o2= 0 to log₁₀ a _o2= -12, by using CO/CO₂ and N₂/O₂ gas mixtures and monitored by an electrochemical, stabilized zirconia-based EMF cell. The maximum cobalt mole fraction used in the investigation was about 0.33.     

Stability Diagram for the System Yba2Cu3O7–x

The dependence of the degree of nonstoichiometry of YBa₂Cu₃0_7–x (123) on temperature and oxygen pressure has been determined by thermogravimetric analysis (TGA) in the temperature range 400° to 950°C and the oxygen pressure range 10^–6 to 1 atm (1 atm = 10⁵ Pa). The nature of the decomposition of 123 in the temperature range 750° to 950°C and the oxygen pressure range 10^–6 to 10^–2 atm has been determined by TGA and X-ray diffractometry (XRD). As the oxygen pressure decreases, the decomposition of 123 follows the sequence 123→ Y₂BaCuo₅ (211) + BaCuO₂° Cu₂O→ 211 ° BaCuO₂° BaCu₂O₂→ 211 ° YBa₃Cu₂O_x (132) ° BaCu₂O₂→ 211 ° BaCu₂O₂°BaO. The incongruent melting temperatures have been determined in the oxygen pressure range 10^–6 to 1 atm by differential thermal analysis, and the phases formed on solidification have been identified by XRD. The stability diagram for the composition 123 has been constructed.     

Thermochemistry of Framework Titanosilicate A2TiSi6O15 (A=K, Rb, Cs)

A new family of framework titanosilicates, A₂TiSi₆O₁₅ (A=K, Rb, Cs) (space group Cc ), has recently been synthesized using the hydrothermal method. This group of phases can potentially be utilized for storage of radioactive elements, particularly ¹³⁷Cs, due to its high stability under electron radiation and chemical leaching. Here, we report the syntheses and structures of two intermediate members in the series: KRbTiSi₆O₁₅ and RbCsTiSi₆O₁₅. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that they adopt the same framework topology as the end-members, with no apparent Rb/K or Rb/Cs ordering. To study energetics of the solid solution series, high-temperature drop-solution calorimetry using molten 2PbO·B₂O₃ as the solvent at 975 K has been performed for the end-members and intermediate phases. As the size of the alkali cation increases, the measured enthalpies of formation from the constituent oxides (     ) and from the elements (     ) become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs⁺, Rb⁺, and K⁺. This trend is consistent with the higher melting temperatures of A₂TiSi₆O₁₅ phases with increase in the alkali cation size.     

Structural Investigations of Prehydrolyzed Precursors Used in the Sol-Gel Processing of Lead Titanate

A variety of titanate gels were prepared from lead acetate and titanium isopropoxide in methoxyethanol. In the present study, ¹H and ¹³C Fourier transform-nuclear magnetic resonance and mass spectroscopic techniques were used to identify the structure of the lead and titanium precursors formed from lead acetate and titanium isopropoxide, respectively, when refluxed in methoxyethanol. The lead and titanium precursors were determined to be Pb(OOCCH₃) (OC₂H₄OCH₃). x H₂O, where x < 0.5, and Ti₂(OC₂H₄OCH₃)₈, respectively. Similar spectroscopic procedures were applied to identify the lead titanium complex formed from the reaction between lead and titanium precursors.