Structural Features of C–S–H(I) and Its Carbonation in Air—A Raman Spectroscopic Study. Part II: Carbonated Phases

The effects of carbonation of mechanochemically prepared C–S–H samples under ambient conditions for upto 6 months have been investigated by Raman spectroscopy and X-ray diffraction. The type and extent of carbonation are strongly dependent on the initial CaO/SiO₂ (C/S) ratio of the samples. Amorphous calcium carbonate hydrate is formed within minutes upon exposure to air. It crystallizes, over time, to give primarily vaterite at C/S≥0.67 and aragonite at C/S≤0.50. Calcite was not observed as a primary carbonation product within the time frame investigated. Decalcification upon storage also initiates silicate polymerization. The dimeric silicate units seen in the calcium-rich phases polymerize rapidly to yield Q² silicate moieties. After 6 months, broad bands are seen in most spectra, ascribed to poorly ordered silica. C–S–H phases with C/S ratios of 0.75 and 0.67 are the most resistant to carbonation, and even after 6 months of storage, Q² silicate units still dominate their structures. The ability of Raman spectroscopy to probe the short-range order of poorly crystalline materials is ideal for investigations of C–S–H structure. Additionally, the technique's sensitivity toward the various calcium carbonate polymorphs illuminates the sequence of carbonation and decalcification processes during aging of C–S–H. Of particular importance is the identification of amorphous calcium carbonate as the first carbonation product. Additionally, the formation of aragonite as a carbonation product is related to the presence of SiO₂ gel in the aged samples.     

Analytical Electron Microscopic Studies of Doped Dicalcium Silicates

Dicalcium silicates having CaO/SiO₂ molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K₂O or Al₂O₃) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca_1.8SiO_3.8 or Ca_2.2SiO_4.2. Amorphous grain-boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO₂ ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO₂ ratio close to that of wollastonite. High levels of Al₂O₃ were observed to enter the β-Ca₂SiO₄ grains under lime-rich conditions (CaO/SiO₂= 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Quaternary System Al2O3–CaO–MgO–SiO2: II Study of the Crystallization Volume of MgAl2O4

Solid-state compatibility and melting relations of MgAl₂O₄ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching selected samples located in the 65 wt% MgAl₂O₄, plane followed by microstructural and energy dispersive X-ray analysis. A projection of the liquidus surface of the primary crystallization volume of MgAl₂O₄ was constructed from CaO, SiO₂ and exceeding Al₂O₃, not involved in stoichiometric MgAl₂O₄ formation; those three amounts were recalculated to 100 wt%. The temperature and character of six invariant points, where four solids co-exist with a liquid phase, were defined. One maximum point was localized and the positions of the isotherms were tentatively established. The effect of CaO, SiO₂, and Al₂O₃ impurities on the high temperature behavior of spinel materials was also discussed.     

Crystal Chemistry of Hydrous Calcium Silicates: I, Substitution of Aluminum in Lattice of Tobermorite

Mixtures of 0.8 moles of CaO per mole of SiO₂ plus Al₂O₃ were prepared from lime, kaolin, and tripoli (microcrystalline quartz); the amounts of SiO₂ to Al₂O₃ were varied to give from 0.2 to 20.7% Al₂O₃ by weight of dry solids. After hydrothermal treatment (170° to 175°C.), the products were examined by differential thermal analysis and by X-ray diffraction. A homogeneous solid identified as the mineral tobermorite (4CaO.5SiO₂.5H₂O) and containing up to 4 or 5% Al₂O₃ was obtained. Increasing the amount of Al₂O₃ in the raw mixture above about 5% resulted in the formation of the hydrogarnet 3CaO.Al₂O₃.SiO₂.4H₂O as a second phase. Allowing for the Al₂O₃ combined in this solid, it was indicated that slightly more Al₂O₃ was substituted in the tobermorite as the amount was increased in the raw mixture. It is suggested that the Al³⁺ ions probably assume tetrahedral coordination when substituting for the Si⁴⁺ ions.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Reaction Characteristics of Magnesia–Spinel Refractories with Cement Clinker

The adherence ability of cement clinker on magnesia–spinel refractories is investigated, using a sandwich test, at 1550°C for 30 min under a load of 5.3 kPa. Fractional factorial experiments determine that the silica ratio (SR)—SiO₂/(Al₂O₃+Fe₂O₃) and particle size of raw meal, as well as heating rate, have a significant effect on adherence ability. Microstructural analyses indicate that the adherence ability depends upon reactions between clinker and refractories at high temperature. Only spinel reacts with CaO and 3CaO·SiO₂ from clinker to form n -calcium aluminate (such as 3CaO·Al₂O₃, 12CaO·7Al₂O₃, CaO·Al₂O₃), but there is no reaction between MgO and the clinker. Fine crystalline spinel, evenly distributed in magnesia-based brick, is prone to reacting with lime-containing phases from clinker to form low melting phases and a belite-enriched zone at the clinker/brick interface. This reaction positively contributes to the high adherence on a magnesia−spinel brick. The high content of liquid in clinker with low SR accelerates reactions between spinel and clinker, while a limited reaction occurs at the brick/clinker interface with high silica.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

Effect of Secondary Crystalline Phases on Dielectric Losses in High-Alumina Bodies

An effort was made to determine the cause of high dielectric losses at 10⁶ cps and 25°C in dense, essentially alkali-free, high-alumina bodies. Emphasis was placed on secondary crystalline phases in the fired bodies. The flux systems BaO-SiO₂, CaO-MgO-SiO₂, and SrO-SiO₂ were investigated. All high-alumina bodies having high dielectric losses contained one of the synthetically formed feldspars, BaO.Al₂O₃.2SiO₂, CaO.Al₂O₃.2SiO₂, or SrO.Al₂O₃.2SiO₂. The mechanism by which the feldspars caused the high dielectric losses was tentatively attributed to thermal expansion mismatch. The expansion curves of the feldspars were similar, but they were lower than the expansion curve of alumina.     

Stability of the Aluminum Silicates

From the ternary phase diagrams of Al₂O₃–SiO₂ with CaO, MgO, or FeO, it can be concluded that the free energies of formation of kyanite, andalusite, and sillimanite, according to the reaction Al₂O₃+ SiO₂= Al₂O₃.SiO₂, are of the order of magnitude of 0 to –10 kcal. rather than the previously accepted value of –40 to –45 kcal. However, this result may be expected from the general variation of free energies of formation with the ionic potential of the silicate-forming cation; this conclusion is supported by a plot for some silicates, carbonates, and sulfates.     

Glass Formation and Recrystallization in the Lithium Metasilicate Region of the System Li2O–Al2O3–SiO2

The stability of the vitreous state in the lithium metasilicate region of the system Li₂O–Al₂O₃–SiO₂ was found to be a function of the concentration of lithia. The higher the lithia content, the less stable was the glass. The devitrification of glasses in this system was studied. In addition to the phases present at or near the liquidus, it was found that the β -eucryptite– β -quartz solid solution phase was metastable over most of the region. The Li₂O–SiO₂, β -Li₂O–Al₂O₃–4SiO₂ solid solution, β -Li₂O–Al₂O₃–2SiO₂ solid solution triple point was estimated to be near 62.5% SiO₂, 17% Al₂O₃, and 20.5% Li₂O (by weight). The thermal expansions of bodies in this region were measured and the values obtained are explained in terms of the phases present.     

Acicular Mullite Crystals in Vitrified Kaolin

The microstructure of vitrified kaolin ceramic tapes has been studied via scanning and transmission electron microscopy (SEM and TEM). The sintered samples contained crystalline phase of predominantly stoichiometric mullite (3Al₂O₃·2SiO₂), which consisted of high aspect ratio, acicular crystals that are often referred to as secondary mullite. These crystals were interlocked and embedded in an aluminosilicate glass matrix of inhomogeneous composition. The glass matrix contained an average of ∼3.63 wt% K as determined by energy-dispersive X-ray analysis (EDS), whose composition could be approximated to 5Al₂O₃·16SiO₂·0.1MgO·0.3K₂O·0.15TiO₂·0.12Fe₂O₃. The acicular crystals have approximately the stoichiometric composition of Al₂O₃:SiO₂= 3:2. They have grown along a specific crystallographic orientation along the [001] axis. The crystal growth front exhibited facetting on the {110) planes with microfacetting on both the {100) and {010) planes.     

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.     

Dissolution in Ceramic Systems: III, Boundary Layer Concentration Gradients

By use of electron microbeam probe analysis on quenched samples, the concentration distribution of CaO, A1₂O₃, and SiO₂ was determined across the boundary layer between molten calcium aluminum silicates and dissolving or growing sapphire and fused silica. A definite shift in the concentration ratios of the solvent components was found near the interface. Analysis of diffusion flux equations for a ternary system successfully related the shift in concentration ratio to the intrinsic diffusion coefficient for each component. For alumina dissolution in a melt rich in CaO, evidence of incongruent dissolution was observed with the formation of new phases, CaO· 6Al₂O₃ and CaO· 2A1₂O₃.     

Faceting Behavior of Alumina in the Presence of a Glass

The faceting of alumina interfaces in the presence of a glass affects both grain growth and grain-boundary mobility during liquid-phase sintering. The geometry and movement of facets that form during this sintering process are expected to play an essential role in the development of the final microstructure, in particular, by their influence on the topology of the grain boundaries which ultimately control the properties of Al₂O₃ compacts. A new method for studying the interaction between Al₂O₃ and a glass has been developed. A thin sample of Al₂O₃ suitable for examination in a transmission electron microscope is prepared and examined and then reacted with SiO₂ and CaO via the vapor phase. This experimental approach allows the faceting behavior of glass/Al₂O₃ interfaces to be studied systematically without introducing unnecessary complications during subsequent sample preparation. Faceting occurs almost exclusively on the (0001) and {1 1 02} planes. The interaction between glass and certain structured grain boundaries in alumina has been studied using polycrystalline thin films.     

Electrical Resistance of Some Refractory Oxides and Their Mixtures in the Temperature Range 600° to 1500°C.

Measurements of electrical resistance in the composition systems Al₂O₃–SiO₂, SiO₂–TiO₂, Al₂O₃–Cr₂O₃, and MgO–NiO were made using, in general, dry-pressed disks about 3 cm. in diameter and 0.4 cm. thick and fired to 1500°C. In the Al₂O₃–SiO₂ series minimum resistance was shown by the samples containing 50% SiO₂, 50% Al₂O₃. The resistance of Al₂O₃ was increased by the addition of small amounts of Cr₂O₃. The same effect was observed in the higher temperature range with small additions of NiO to MgO. In other instances the addition of the relatively inert SiO₂, Al₂O₃, and MgO to the semiconductors TiO₂, Cr₂O₃, and NiO resulted in a dilution effect. The resistance of Cr₂O₃ was decreased by the addition of a slight amount of MgO.     

Isothermal Section of a Cu2O–Al2O3–SiO2 Pseudo-Ternary System at 1150°C in Air

In this study, the isothermal section of a Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was analyzed by means of a scanning electronic microscope and powder X-ray diffraction of the quenched samples qualitatively, and the compositions of the tie-points of the tie-planes as well as their regions were determined by in situ high-temperature quantitative X-ray diffraction analysis and energy-dispersive X-ray spectroscopy. Then, the isothermal section of the Cu₂O–Al₂O₃–SiO₂ pseudo-ternary phase diagram at 1150°C was constructed; it was found that the isothermal section is composed of two single liquid-phase regions, five two-phase regions, and six three-phase regions.     

Potassium Vapor Attack in Refractories of the Alumina–Silica System

A graphite chamber was used for the reaction between samples of 45 or 55 wt% alumina and a mixture of metallurgical coke and potassium carbonate. Thermal treatments were conducted at 1000°C. The results suggest that the potassium attack in silica-alumina bricks is controlled by the following reactions: K₂O + SiO₂→ K₂O → SiO₂ in the glassy matrix; 3(K₂O · 2SiO₂) + 3Al₂O₃→ 2SiO₂· 3(K₂O · Al₂O₃· 2SiO₂) + 2SiO₂ for short times; and K₂O → Al₂O₃· 2SiO₂+ 2SiO₂· K₂O · Al₂O₃· 4SiO₂ for long times. In 55 wt% alumina bricks containing corundum and tridymite, potassium also attacks those phases forming a glassy phase. The formation of kaliophilite at the matrix/mullite grain interface causes a volumetric expansion of 55.5%, resulting in cracks in the matrix. Because the kaliophilite phase is not in equilibrion with mullite, the former will react with free silica to form leucite that is more thermodynamically stable.     

Calcium Hexaluminate and Its Stability Relations in the System CaO–Al2O3–SiO2

By a combination of solid-state sintering and quenching experiments the validity of calcium hexaluminate as a stable phase and the extent of its primary field in the system CaO–Al₂O₃–SiO₂ have been established. The size of the primary field is considerably reduced from that suggested by earlier work. The anorthite-corundum-calcium hexaluminate invariant point has been relocated at 28.0% CaO, 39.7% Al₂O₂, and 32.3% SiO₂ and at 1405°± 5°C.     

Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.