Activation of Pozzolanic Reaction of Hydrated Portland Cement Fly Ash Pastes in Sulfate Solution

The activation of the pozzolanic reaction of fly ash in portland cement paste immersed in sulfate solution has been studied. Mixtures of two Spanish fly ashes (ASTM class F) with 0%, 15%, and 35% replacement of portland cement by fly ash were immersed in Na₂SO₄ solution, of 2880 ppm SO₄²⁻ concentration, for a period of 90 days. The resistance of the different mixtures to the sulfate attack was evaluated using the Koch-Steinegger test. The results showed that all of the mixtures were sulfate resistant, despite the high Al₂O₃ content of the fly ash. The diffusion of SO₄²⁻ and Na⁺ ions through the pore solution activated the pozzolanic reactivity of the fly ashes, causing microstructural changes, which were characterized by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). As a result, the flexural strength of the mixtures increased, principally for the fly ash of a lower particle size and 35% of addition.     

A Practical Model for the Interactions Between Hydrating Portland Cements and Poly-β-Naphthalene Sulfonate Condensate Superplasticizers

The performance of poly-β-naphthalene sulfonate condensate superplasticizer (BNS) as a dispersant for cement in concrete is affected severely by slight differences in the characteristics of the cement. In order to be able to predict these effects, a model for estimating the fluidity of cement paste containing BNS is proposed. This model is based on an assumption that the fluidity of cement paste is proportional to the BNS adsorption amount per surface area of hydrated cement (Ad/Hy). BNS is known to show two types of sorption on hydrated cement: one is the bulk absorption into initial hydrates and the other one is the superficial adsorption onto hydrates. Only the superficially adsorbed BNS is expected to work as a dispersant. By assuming a competitive Langmuir-type adsorption on hydrates between BNS and SO₄²⁻, a simple method to estimate Ad/Hy is developed, with the concentrations of BNS and SO₄²⁻ as the only two independent parameters. The resulting estimates of Ad/Hy show a good correlation with paste flow and its change with elapsed time for a broad range of cements. The SO₄²⁻ concentration in the aqueous phase of the cement paste just after the beginning of the mixing is known to affect the performance of BNS as a dispersant. By using the proposed model to discriminate between the superficial adsorption and bulk absorption of BNS, this phenomenon is explained quantitatively.     

Oxygen Ion Activity and the Solubility of Sulfur Trioxide in Sodium Silicate Melts

The solubility of sulfur trioxide in sodium silicate melts was determined from 1150° to 1250°C by equilibrating melts in gas mixtures of known contents of sulfur dioxide and oxygen. Sulfate forms according to the reactions: O^2-+ SO₂+ 1/2O₂= O^2-+ SO₃= SO₄^2-. The data obtained at 1200°C were interpreted by the linear equation: log(SO₄^2-) = log( P so₂½ P o₂^1/2) + log Y in which Y is a function of the soda/silica ratio. A series of parallel lines was obtained. Relative free oxygen ion activities calculated for 1200°C were in good agreement with theoretical values calculated from the thermodynamic model of Toop and Samis.     

Hydroxyapatites Produced by Wet-Chemical Methods

Hydroxyapatite samples were produced by two different wet-chemical methods, and characterized by X-ray diffraction, infrared (IR), thermal gravimetric analysis (TGA), scanning electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectrometry, and compression strength measurements. The IR spectra showed the presence of CO₃²⁻ ions in all samples. After the sintering, the CO₃²⁻ bands almost disappeared, indicating a large release of CO₃²⁻ ions by the samples, which were also confirmed by TGA. By mixing samples produced by both methods, a bioceramic was prepared and, after sintering at 900°C for 1 h, compressive strengths of 26–30 MPa were obtained.     

Electrokinetic Properties of Nanosized SiC Particles in Highly Concentrated Electrolyte Solutions

In this research, the electrokinetic behavior and stability of nanosized SiC particles suspended in various electroplating solutions were studied. Analyses were performed using electrophoretic mobility photometry and streaming current (SC) techniques. The electrolytes included NiCl₂, Ni(SO₃NH₂)₂, and Na₃Co(NO₂)₆, which are currently used in composite plating solutions with concentrations as high as 0.5 M . The results showed that the adsorption of dissolved Ni²⁺ ions onto the surface of the SiC in the pH range 4–8 changed the sign and magnitude of the surface potential. Moreover, trivalent complex species Co(NO₂)₆³⁻ replaced nickel species on the SiC surface and decreased the surface charge of SiC to between pH 3 and pH 5. Even in a highly concentrated electrolyte solution, the SiC particles still maintained a positive charge in a Ni(SO₃NH₂)₂ suspension with nickel coplating on the cathode. The difference between the SC reading and the zeta potential, as well as the surface adsorption of various species onto the SiC, are discussed here.     

Hydrothermal Formation of Hydroxyapatite Layers on the Surface of Type-A Zeolite

Type-A zeolite evenly covered with hydroxyapatite thin layers was prepared using hydrothermal treatment at 120°C for 8 h under autogenous pressure. The hydroxyapatite needlelike nanocrystals, 100–200 nm in diameter and 30 nm in thickness, were grown under the reaction between discharged Ca²⁺ ions from type-A zeolite and PO₄³⁻ ions in (NH₄)₃PO₄ solution. The preferential orientations of the c -axis of hydroxyapatite crystals perpendicular to a zeolite surface were observed using transmission electron microscopy. The crystal structure of type-A zeolite was not destroyed under the reaction, but the surface morphology was changed only with complete covering of scaly hydroxyapatite particles.     

Thermodynamic Assessment of the Lithium-Borate System

The Li₂O–B₂O₃ quasi-binary system is assessed. A two-sublattice ionic solution model, (Li¹⁺) _P (O²⁻, BO₃³⁻, B₄O₇²⁻, B₃O_4.5) _Q , is adopted to describe the liquid phase. All solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on phase diagram and thermodynamic properties is obtained by using the CALPHAD technique.     

Sulfate Decomposition and Sodium Oxide Activity in Soda–Lime–Silica Glass Melts

Reaction equilibrium constants for the sulfate decomposition process, which releases oxygen and sulfur oxide gas in soda–lime–silica glass melts, have been determined. The chemical solubility of SO₂, probably in the form of sulfite ions in soda–lime–silica melts, has also been determined. The chemical solubility value of SO₂, dissolving as sulfite, ranges between 0.02 and 0.06 wt% SO ₃ ²⁻ at 1 bar SO₂ pressure in the temperature range of 1600–1800 K. Results of square-wave-voltammetry studies and measurements of the temperature-dependent sulfur retention after the fining process of commercial float glass melts and a model soda–lime–silica melt, with 74 wt% SiO₂, 16 wt% Na₂O, and 10 wt% CaO, are presented. The measured sulfur retention data and the results of the square-wave-voltammetry studies are used to determine the equilibrium constant of the sulfate decomposition reaction in the temperature range of 1600–1800 K. The thermodynamic relations and properties found for sulfate decomposition are used to derive activities of sodium oxide in soda–lime–silica melts. Literature values for sodium oxide activities in these glass melts are rare. In this study, these activities have been determined by a method, based on the measurement of sulfate decomposition equilibrium constants and the residual sulfate concentrations in glass melts, equilibrated with almost pure sodium sulfate galls.     

Apatite Formation on Calcium Phosphate Invert Glasses in Simulated Body Fluid

Calcium phosphate invert glasses, which contain P₂O₇²⁻ and PO₄³⁻ ions, have been prepared via the addition of a small amount of TiO₂. The formation of bonelike calcium phosphate apatite on the surface of the phosphate invert glasses was examined in simulated body fluid (SBF) at a temperature of 37°C. Soaking for 20 d resulted in the deposition of leaflike apatite particles on 6CaO·3P₂O₅·TiO₂ invert glass (based on molar ratio). The glass had much-greater chemical durability against SBF, in comparison with a metaphosphate glass; P ions were not dissolved excessively from the 6CaO·3P₂O₅·TiO₂ glass, so the apatite formation was not suppressed.     

Crystal Structure of Zirconia Prepared with Alumina by Coprecipitation

Zirconia was prepared by firing the coprecipitate from ZrOCl₂and AlCl₃mixed aqueous solution with ammonia. When fired above 600°C, the products were fine crystalline tetragonal zirconia of crystallite size <10 nm. In previous studies, the tetragonal phase had been assumed to be a (Zr_{1− x} ⁴⁺Al _x ³⁺)O_{2− x /2}solid solution, where x ≤ 0.25. However, X-ray diffraction pattern simulation and Al K -edge XANES spectroscopy confirmed the present product to be a mixture of t -ZrO₂fine powder with a small amount of δ-Al₂O₃of very low crystallinity, even below the expected compositional range of x ≤ 0.25 in the (Zr_{1− x} ⁴⁺Al _x ³⁺)O_{2− x /2}solid solution.     

Formation of BaCrO4 NanoCrystallites within Thermally Evaporated Sodium Bis-2-Ethylhexyl-Sulfosuccinate and Stearic Acid Thin Films

This paper describes the growth of barium chromate (BaCrO₄) nanocrystallites within thermally evaporated thin films of stearic acid (StA) and sodium bis-2-ethylhexyl-sulfosuccinate by a process of Ba²⁺ ion entrapment followed by in situ reaction with CrO₄²⁻ ions. Dense spherical assemblies of BaCrO₄ nanocrystallites of very uniform size (∼50 nm) were obtained within the two different host matrices. The spherical assemblies were composed of smaller (ca. 5–10 nm size) BaCrO₄ crystals indicating that efficient size control over crystal size may be exercised by the matrix. Contact angle measurements of the BaCrO₄–StA and BaCrO₄–sodium bis-2-ethylhexyl-sulfosuccinate films indicated that they were hydrophobic, thus pointing to the possible role of hydrophobic interaction between the StA and sodium bis-2-ethylhexyl-sulfosuccinate monolayer-covered BaCrO₄ crystals in the assembly process.     

Microstructural Changes during the Reduction/Reoxidation Process in Donor-Doped BaTiO3 Ceramics

Microstructural changes occurring during oxidation of the reduced form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X} ⁴⁺Ti _X ³⁺O₃) and during reduction of the oxidized form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X /4}⁴⁺( V _Ti^) _{X /4}O₃) were studied using TEM. Samples of both types of solid solutions, containing different La concentrations (from 2 to 20 mol% La), were prepared by sintering under reducing conditions and in air, respectively. The reduced form of donor-doped BaTiO₃ was oxidized by annealing at high temperatures (1150° and 1350°C) in air, while the oxidized form was reduced by annealing under reducing conditions. Because of oxidation of the reduced phase of donor-doped BaTiO₃, the Ti-rich phases Ba₆Ti₁₇O₄₀ and BaLa₂Ti₄O₁₂ were precipitated. Reduction of the oxidized form caused precipitation of the Ba-rich phase Ba₂TiO₄ preferentially inside the matrix grains. All precipitates had well-defined orientational relationships with the perovskite matrix.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

Magnetic Resonance In Situ Study of Tricalcium Aluminate Hydration in the Presence of Gypsum

The hydration of tricalcium aluminate, in the presence of gypsum, is investigated using in situ "real-time"¹H NMR spin–spin relaxation, X-ray diffraction, and scanning electron microscope experiments. Aside from rapid ettringite formation within the first 45 min, it is shown that a re-distribution of water within the first 2 h contributes significantly to the retardation of the rate of hydration. From the ¹H NMR component with T ₂ of approximately 20 μs, the continual disappearance of ettringite and the production of the layered monosulfate structure is monitored. In addition, the technique makes possible the monitoring of the change in the quantity of interlayer water in monosulfate, as well as the time scale associated with the decrease in the effective interlayer spacing, resulting from the ionic substitution of SO₄²⁻ for OH⁻. The ionic substitution within monosulfate starts after approximately 9 h. Although it has been believed that the time scale for this reaction is fast, we have shown that it can take days to reach completion.     

Electrochemical Cells and Electrical Conduction of Pure and Doped Al2O3

Data concerning the types of charge carrier responsible for electrical conduction in single-crystal aluminum oxide are reviewed, and explanations are offered for discrepancies in terms of the experimental conditions. Measurements on undoped and Co²⁺ and Mg²⁺ doped crystals, made using a volume guard to avoid surface and gas conduction, are used to describe Al₂O₃ as an ionic conductor with Al interstitial ions as the principal charge carriers at high oxygen activity. A defect model is proposed and the mobility of Al _i ³⁺, the concentration of Al _i ³⁺ in undoped crystals, and the equilibrium constant for defect formation at high temperatures are estimated.     

Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide

The relationship between electronic polarizability of oxide(—II), α_O²⁻, and the electron donor power, expressed as the optical basicity, ∧, indicates that ∧(TiO₂) is much greater than ∧(SiO₂) and is approximately the same as ∧(CaO). Such a high basicity is supported by the trend in the Racah B parameter for the Ni²⁺ ion in crystalline hosts and is also indicated from α_O²⁻ values of glasses containing TiO₂. Electronic polarizability and other data for zirconium(IV) media indicate that ZrO₂ also has a high basicity, but that ∧(ZrO₂) is somewhat less than ∧(TiO₂).     

Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are discussed.     

Acid-Base Equilibria in the System PbO–SiO2

The acid-base equilibria in the liquid silicates in the system PbO–SiO₂ are discussed, Data reported by Richardson and Webb, wherein the PbO activity is determined over a composition range of 0 to 60 mole % SiO₂, are used for comparison with activities computed from structural models with consideration of the acid-base equilibria. The results suggest that the liquid silicates in the system PbO–SiO₂, for the composition and temperature ranges studied, are constituted of a relatively low number of anionic species and that these anions are of a relatively small size (i.e., O_2–, SiO_4–, (SiO₃)₃⁶⁻. and (SiO_2.5)₆⁶⁻).     

Hydrothermal Synthesis of Tetragonal Barium Titanate from Barium Chloride and Titanium Tetrachloride under Moderate Conditions

Tetragonal BaTiO₃powders were prepared hydrothermally at 240°C, in only 12 h, using BaCl₂·2H₂O and TiCl₄, which are rather easy to manipulate. Characterization via X-ray diffractometry, scanning electron microscopy, Brunauer–Emmett–Teller analysis, and differential scanning calorimetry confirmed that increasing the NaOH excess concentration (from 0.5 M to 2.0 M ) and decreasing the initial TiCl₄concentration (from 0.625 M to 0.15 M ) promotes the formation of tetragonal BaTiO₃powders. After reaction, the powders were proved to be phase-pure BaTiO₃, with no impurities, such as Cl⁻ and CO₃²⁻.