Effect of Simulated Radioactive Liquid Waste on the Microstructure of Cementitious Materials: Portlandite Orientation and Saturation Factors in the Pore Solution

The microstructural implications of the preferred orientation of portlandite crystals and portlandite saturation factors in the pore solution of a fly-ash–pozzolanic-cement mortar that is used in Spain for the disposal of low- and medium-level radioactive waste have been discussed. Changes in the saturation of portlandite, which were promoted by the interaction of two types of simulated radioactive liquid waste (SRLW) with the mortar, were evaluated during a period of 365 d at a temperature of 40°C. The mortar was immersed in two SRLW materials, whose main ions were SO₄²⁻ (0.68 M ), PO₄³⁻ (0.89 M ), and Cl⁻ (0.51 M ) in one case and SO₄²⁻ (2.05 M ) and Na⁺ (4.1 M ) in the other case. The diffusion of those ions through the porous mortar microstructure and related dissolution–precipitation reactions influenced both the quantity and the degree of orientation of portlandite crystals at the paste/sand and paste/fly-ash interfaces. All these effects will lead to a pore refinement of the mortar that improves the strength and durability of a mixture of fly ash and cement mortar.     

Effect of Fly Ashes with High Total Alkali Content on the Alkalinity of the Pore Solution of Hydrated Portland Cement Paste

The influence of two Spanish fly ashes (ASTM class F) with high total alkali content (equivalent to 2.0% and 2.6% Na₂O) on the alkalinity levels of the pore solutions expressed from hydrated portland cement pastes was studied during a period of 90 days from mixing. Mixtures with 0%, 15%, and 35% replacement of cement by fly ash were prepared with a water/mixture ratio of 0. 4. The effect of the fly ash on the pore solution depended mainly on the age and fineness of the fly ash.     

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.     

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.     

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.     

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.     

Analytical Study of Pure and Extended Portland Cement Pastes: II, Fly Ash- and Slag-Cement Pastes

A range of pastes of portland cement interground with low-calcium fly ash or granulated blast furnace slag was studied by X-ray diffraction, analytical electron microscopy, thermo-gravimetry, and determinations of CO₂ and of unreacted fly ash or slag. Partial replacement of clinker by fly ash results in increased reaction of the alite from at least as early as 3 d. The amount of Ca(OH)₂ formed from a given weight of clinker increases at 3 d due to the enhanced reaction, but from 28 d onward, it decreases due to the pozzolanic reaction. The mean Ca/SI ratio of the C-S-H decreases with time or fly ash content, toward a lower limit of ∼1.4. Partial replacement of clinker by slag also decreases both the amount of Ca(OH)₂ formed from a given weight of clinker and the mean Ca/Si ratio of the C-S-H, but to a lesser extent. Using the methods described in Part I of this paper, the results of the experimental methods were tested for mutual consistency, and volume percents of phases, porosities, and related quantities were calculated.     

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.     

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.     

Hydroxyapatite Coating on Thermally Oxidized Titanium Substrates

Titanium substrates were oxidized in oxygen or air at temperatures of 600°–800°C, then immersed in solutions of 2.0m M – 20.7m M CaCl₂ and 1.2m M –12.4m M KH₂PO₄ for aging periods of 0.5–10 d. The titanium surface was successfully coated with hydroxyapatite (HAP) when the substrates were oxidized in oxygen gas at 610°C for 1 h and then aged in a solution of 2.00m M Ca²⁺ and 1.20m M PO₄³⁻. The Ca/P ratio of the surface coating increased toward its stoichiometric HAP value (return 10/6) as the aging time increased; the Ca/P ratio attained a value of 1.66 after 10 d.     

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⁻).     

Chemical Speciation of Trace Zinc in Ordinary Portland Cement Using X-ray Absorption Fine Structure Analysis

Chemical change of trace zinc in ordinary portland cement (205.1 ppm) was investigated in hydration process using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Intensities of the peaks appearing at the same energy of ZnO in XANES spectra were decreased with cement hydration. The interatomic distances and the coordination numbers of the first and the second shells calculated from EXAFS spectra indicated that ZnO hydrolyzed to zincate ion [Zn(OH)₄]²⁻ with cement hydration keeping their fundamental structure of ZnO₄ tetrahedra.     

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.     

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).     

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.     

Solid-State SO2 Sensor Using a Sodium-Ionic Conductor and a Metal–Sulfide Electrode

All solid-state sulfur oxides (SO _x ) sensor devices combined with a sodium ionic conductor (Na₅DySi₄O₁₂) disk and metal sulfide-sensing electrodes synthesized via solution routes have been systematically investigated for the detection of SO₂ in the range of 20–200 ppm at 150–400°C. Among the various sulfide-sensing electrodes tested, the metal monosulfide-based electrodes gave good SO₂ sensitivity at 400°C. The Pb_{1− x} Cd _x S ( x =0.1, 0.2)-based solid electrolyte sensor element showed the best sensing characteristics, i.e., the EMF response was almost linear to the logarithm of SO₂ concentration in the range between 40 and 400 ppm, with a 90% response time to 100 ppm SO₂ of about 3–15 min, and also showed high selectivity to SO₂ at 400°C.     

Hydration of fly ash-portland cements

Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.     

Synthesis of Anatase-Type TiO2 Nanocrystallites Via a Redox Route

Anatase nanocrystallites showing high surface area (∼62 m²/g) and good photocatalytic property have been obtained by pyrolyzing at 600°C for 4 h an ammonium titanyl double sulfate precursor (α-(NH₄)₂TiO(SO₄)₂) synthesized via a redox approach, that is, by oxidizing an aqueous solution of titanium trichloride (TiCl₃) with ammonium peroxodisulfate ((NH₄)₂S₂O₈), followed by reacting with ammonium sulfate ((NH₄)₂SO₄).