Effect of Gypsum on the Hydration and Strength of C3S Pastes

The paste hydration of C₃S containing different amounts of gypsum was studied. Reaction rates and the composition of hydrated products were evaluated. The gypsum influenced the quantity and quality of the hydrated products by accelerating the hydration process and, at the same time, lowering the intrinsic strength of the gel. Variations in intrinsic strength correlated linearly with the C/S ratio of the CSH gel. The effect of gypsum on strength was interpreted in terms of its influence on the extent of hydration and on the chemical composition of the gel.     

Coarsening Behavior of Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) Grains Dispersed in a Clinker Melt

Microstructural evolution during the heat treatment of cement clinker was investigated. Two model specimens, which consisted of faceted tricalcium silicate (C₃S) and spherical dicalcium silicate (C₂S) grains dispersed in a liquid matrix, were prepared with 5 wt% of large seed particles. The seed particles of faceted C₃S grains grew extensively, whereas those of the spherical C₂S grains grew rather slowly, relative to the matrix grains. As a consequence, C₃S grains exhibited a bimodal size distribution that was typical of exaggerated grain growth, whereas C₂S grains retained a uniform and normal size distribution. These results suggest that the growth of faceted C₃S grains was controlled by the interface atomic attachment, such as two-dimensional nucleation, and that of spherical C₂S grains was controlled by diffusion through the liquid matrix. The dependence of growth mechanisms on grain morphology has been explained in terms of the atomistic structure of the solid/liquid interface.     

NMR Relaxation Study of Adsorbed Water in Cement and C3S Pastes

The deuteron and proton spin-lattice and spin-spin relaxation times T ₁ and T ₂ of adsorbed water in commercial portland cement and tricalcium silicate pastes were studied as functions of the hardening time at room temperature. The time dependence of the water self-diffusion coefficient of tricalcium silicate pastes was also followed. The proton and the deuteron T ₁ and T ₂ decrease markedly as hydration increases and the pastes harden due to the increase in the active surface and the number of adsorptive sites, thus providing convenient tools for studying the nature of the hydration process.     

Changes in Zeta Potential During Hydration of C3S With and Without CaCl2 Additions

C₃S and C₃S+2% CaCl₂ were hydrated for varied times; the degree of hydration and zeta potential were determined. In the absence of CaCl₂, the duration of the induction period was 5 h, whereas when CaCl₂ was added, an induction period of 1 h was observed. The zeta potential was positive, maximum, and constant during induction .     

Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation

The structures of partially carbonated hardened C₃S cement pastes have been investigated by a combination of ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO₂ for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO₂ but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.     

Thermal Diffusivity/Conductivity of Compacts of C60 Buckminsterfullerene and a C60–C70 Mixture

The thermal diffusivity/conductivity at room temperature of a compact of C₆₀ buckminsterfullerene with a density ∼94.5% of theoretical was found to be about one-half of the literature value for a C₆₀ single crystal. With increasing temperature to about 500°C, the thermal diffusivity was found to decrease with ∼T⁻¹ dependence, indicative of phonon-phonon scattering. A compact of a C₆₀–C₇₀ mixture with density comparable to that of the C₆₀ compact exhibited a thermal diffusivity/conductivity about one-half that of the C₆₀ compact, this decrease possibly attributable to enhanced phonon scattering because of the mass differences of the C₆₀ and the C₇₀ molecules. The specific heat values of the C₆₀ and the C₆₀–C₇₀ compacts were about the same as that of graphite, with a similar dependence on temperature. Although both the C₆₀ and the C₆₀–C₇₀ compacts appeared to disintegrate spontaneously at elevated temperatures, this tendency appeared to be less pronounced for the C₆₀ than for the C₆₀–C₇₀ compacts.     

Electrical Conductivity of MgAl2O4 and Y3A15O12

The electrical conductivities of single crystal and polycrystalline MgAl₂O₄ and Y₃Al₅O₁₂ were measured to 1260 K using a three-contact, guard-ring technique. The electrical conduction mechanisms change with temperature, with anomalous oxygen pressure and time-dependent inflections in log σ versus T⁻¹ curves between 900 to 1000 K. The conduction processes of Y₃Al₅O₁₂ and MgAl₂O₄ appear to be similar and possibly related to A1³⁺ ion diffusion.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Effects of Oxide Content on the Glass-Forming Ability of the Ga2S3-Na2S System

Gallium–sodium–sulfide glasses have been shown to be the most promising host for the 1.3 μm fiber optic amplifier so far. However, the current study has revealed that the formation of stable glasses in this system relies on the existence of a certain amount of oxide in the composition. A glass-forming region was identified when 3 mol% Ga₂O₃ was introduced into the melts. Both bulk crystallization and surface crystallization can take place in this system. The thermal behavior of the glasses obtained was studied by DTA and characteristic infrared and UV/visible transmission spectra are presented. The current results are discussed and compared with those previously reported.     

Polymorphism of Ba3Aℓ2O6 and solid solution between Ba3Aℓ2O6, Sr3Aℓ2O6 and Ca3Aℓ2O6

Ba₃A?₂O₆ has a rapid, reversible phase transition at 450°C. The systems of Ca₃A?₂O₆ with Sr₃A?₂O₆ and Sr₃A?₂O₆ with Ba₃A?₂O₆ each contain two intermediate phases. The incongruent melting of M₃A?₂O₆ solid solutions gives one crystalline product, a pseudocubic phase of unknown composition. Powder X-ray data are given for the newly-discovered phases.     

C60 Nanowhiskers in a Mixture of Lead Zirconate Titanate Sol–C60 Toluene Solution

Fine fibrous structures of C₆₀ with a diameter on the order of nanometers were discovered in a lead zirconate titanate sol ultrasonically mixed with a toluene solution of C₆₀. By transmission electron microscopy observations, they were identified as single-crystalline fibers of C₆₀ with submicrometer diameters, i.e., nanowhiskers of C₆₀. The C₆₀ nanowhiskers showed thin slablike TEM images, and the growth axis of the nanowhiskers was parallel to the 〈110〉 close-packed direction of a fcc crystal system of C₆₀.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

C70 Nanowhiskers Fabricated by Forming Liquid/Liquid Interfaces in the Systems of Toluene Solution of C70 and Isopropyl Alcohol

C₇₀ whiskers with submicrometer diameters (C₇₀ nanowhiskers, or C₇₀NWs) have been successfully fabricated by forming liquid/liquid interfaces in systems that involve a toluene solution of C₇₀ and isopropyl alcohol. Transmission electron microscopy observations show that the C₇₀NWs have a 〈110〉 growth axis and that the intercluster distance of C₇₀ molecules in the C₇₀NWs are shortened by ∼3%, compared with that of face-centered cubic C₇₀ crystals in the 〈110〉 close-packed growth direction, which indicates the formation of strong chemical bonds between the C₇₀ molecules. It is suggested that the C₇₀ molecules are polymerized, with their short axis parallel to the growth axis of the C₇₀NWs, from observation via high-resolution transmission electron microscopy.     

Oxidation and Strength Retention of Monolithic Si3N4 and Nanocomposite Si3N4-SiC with Yb2O3 as a Sintering Aid

The oxidation behaviors of monolithic Si₃N₄ and nanocomposite Si₃N₄-SiC with Yb₂O₃ as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb₂Si₂O₇ and SiO₂ (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si₃N₄-SiC compared to the monolithic Si₃N₄. The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb³⁺ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.     

Microstructural Characterization of C60-Doped Zirconia

Zirconia powders doped with C₆₀ molecules were prepared from an aqueous solution of zirconium oxynitrate dihydrate, C₆₀ and C₁₆TMA, and sintered at 600°C under 5.5 GPa for 2 h. C₆₀ was found to be retained in the sintered specimens by HRTEM, and carbon was observed to be uniformly dispersed by the SEM-EDX analysis. HRTEM observations of the sintered specimens exhibited the formation of ZrO₂ crystal grains covered with thin graphitic or amorphous carbon films.     

Phases in the Binary Systems PbO—La2O3, Gd2O3, and Sm2O3

In the binary system PbO–La_zO₃ only one compound, 4PbO.La₂O₃, exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La₂O₃, corresponding to half the a parameter of the C form of La₂O₃. The solid solutions existing between the compositions La₂O₃–2Pb0 and pure La₂O₃ have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm₂O₃ and Gd₂O₈ are quite similar to that with La₂O₃. The compound Sm₂O₃.4Pb0 decomposes at 1000°C with evaporation of PbO; Sm₂O₃ remains in the B modification.     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.     

Glass–Ceramic Composites with Strontium Hexaferrite Ultrafine Particles in the SrO–Fe2O3–B2O3–SiO2 System

Glass samples with nominal compositions SrFe₁₂O₁₉+(12− n )SrB₂O₄+nSrSiO₃, n =3, 6, 9 were prepared by rapid quenching of the melt. Processes of glass devitrification were studied. The samples were annealed at temperatures of 600–900°C, and the resulting glass–ceramics was characterized by XRD, SEM, EDX, and magnetic measurements. SrFe₁₂O₁₉ crystallizes above 700°C and forms nano- and submicron platelet particles with the aspect ratio depending on the thermal treatment conditions. The glass–ceramic samples annealed at 900°C show coercive force values in the range of 422–455 kA/m.