Calcium Aluminate System Research Articles

Hydration kinetics of C3A: effect of lithium, copper and sulfur-based mineralizers

AbstractCalcium aluminate phases have a particular effect on the early heat release during setting initiation and have a substantial influence on the further workability of ordinary Portland cement. The nature of the calcium aluminate hydration products and its kinetics strongly depends on sulfate content and humidity. The effect of mineralisers on melt formation and viscosity is well described for calcium silicate systems, but information is still lacking for calcium aluminates. Therefore, the synergistic effect on the crystal structure and hydration mechanism of the tricalcium aluminate phase of the addition of mineralizers, i.e. Li2O, CuO, SO3 to the raw meal is here investigated. Co-doped calcium aluminate structures were formed during high-temperature treatment. Thermal analysis (TG–DTA and heating microscopy) was used to describe the ongoing high-temperature reaction. Resulting phase composition was dependent on the concentration of the mineralizer. While phase pure system was prepared with low mineralizer concentrations, with increasing mineralizer content the secondary phases were formed. Raman spectroscopy and XPS analysis were used to investigate the cation substitution and to help describe the cations bonding in co-doped calcium aluminate system. Prepared powders have been hydrated in a controlled manner at different temperatures (288, 298, 308 K). The resulting calorimetric data have been used to investigate the hydration kinetics and determine the rate constant of hydration reaction. First-order reaction (FOR) model was here applied for the activation energy and frequency factor calculations. The metastable and stable calcium aluminate hydrates were formed according to initial phase composition. In phase pure systems with low S content, the formation of stable and metastable hydrates was depended on the reaction temperature. Conversely, in systems with secondary phases and higher S content, the hydration mechanism resembled that which appears in calcium sulfoaluminates.

Behavior and mechanism of in-situ synthesis of auxiliary electrode for electrochemical sulfur sensor by calcium aluminate system

The behavior and mechanism of in-situ synthesis of the auxiliary electrode for sulfur sensor were investigated in this work, aiming for better application of calcium aluminate system in synthesizing the auxiliary electrode used for sulfur sensor. The in-situ reaction experiment was developed. In addition, the thermodynamic and kinetic calculations were adopted to further study the in-situ reaction possibility and the reaction rate. The results indicated that the value of lg(a[S]/a[O]) should be greater than a particular value to ensure the occurrence of the in-situ reaction. After immersion into the molten iron, the CaS phase was synthesized in the calcium aluminate system. The relationship between the reaction rate and reaction time was exponential, and the initial reaction rate was affected by the CaO content incorporated in the calcium aluminate system and the sulfur content in the molten iron. The initial in-situ reaction rate greatly increased with the increase of the CaO content and sulfur content. For example, the initial reaction rate was as high as 14.87 s-1 when the calcium aluminate system containing 60 wt% of CaO and for a sulfur content of 0.077 wt% in the molten iron. Moreover, the reason that the sulfur sensor fabricated by the ZrO2(MgO) tube with the calcium aluminate coating with different components had the same response time when measuring the different sulfur contents in the molten iron was further explained.

Solubility of Sulfur in the Solid Oxide of the Calcium-Aluminate System

Solubility of Sulfur in the Solid Oxide of the Calcium-Aluminate System

Configurational constraints on glass formation in the liquid calcium aluminate system

We report new time-resolved synchrotron x-ray diffraction (SXRD) measurements to track structural transformations in calcium-aluminate (CaO)x(Al2O3)1−x liquids during glass formation, and review recent progress in neutron diffraction with isotope substitution (NDIS) experiments, combined with aspherical ion model molecular dynamics (AIM-MD) simulations, to identify the atomic-scale configurational constraints on glass-forming ability. The time-resolved measurements reveal substantial changes in ordering on short- and intermediate-range occurring during supercooling. In the equimolar composition x = 0.5 (CA), the liquid undergoes a remarkable structural re-organisation on vitrification as over coordinated AlO5 polyhedra and oxygen triclusters breakdown to form a network of predominantly corner-shared AlO4 tetrahedra. This is accompanied by the formation of branched chains of edge-and face-sharing Ca-centred CaOy polyhedra contributing to cationic ordering on intermediate length-scales. The Ca-rich end-member of the glass-forming system x = 0.75 (C3A) is largely composed of AlO4 tetrahedra, but % unconnected AlO4 monomers and Al2O7 dimers are present, representing a threshold after which the glass can no longer support the formation of an infinitely connected network. Overall, the AIM-MD simulations are in excellent agreement with the SXRD and NDIS experiments suggesting an accurate potential model. However, small discrepancies between the simulated glass structures and experimental measurements are apparent, indicating a small degree of liquid-like ordering persists in the simulated glass trajectories. This may be due to the short simulation time-scales which are unrepresentative of the viscous kinetic processes involved in supercooling and glass formation. One approach to improve future models could be the integration of rare event sampling techniques into MD simulation codes to massively extend equilibration time-scales and more accurately model vitrification and structural configurations in real glass systems.

In-Situ Kinetic Investigation of Calcium Aluminate Formation

Rapid in-situ non-ambient X-ray diffraction represents a powerful tool for characterizing the evolution of crystalline materials in real time. The calcium aluminate system and formation of Ca12Al14O33 (C12A7) is particularly sensitive to processing conditions. This report characterizes the kinetic pathways to thermodynamic equilibrium as a function of atmosphere (ambient, dry, and vacuum) and reactant heterogeneity (as-received, milled, and sol-gel reactants). When reactants are heterogenous (as-received and milled), intermediary phases of Ca3Al2O6 (C3A) and CaAl2O4 (CA) are observed as the route to C12A7 formation and Ca5Al6O14 (C5A3) is only observed as a decomposition product of C12A7. When reactants are heterogenous, C12A7 is only thermodynamically favorable under ambient conditions due to the stability provided by hydration. When reactants are homogenous (sol-gel), direct crystallization of C12A7 from an amorphous precursor is observed at low temperature regardless of atmosphere defining C12A7 as the kinetic equilibrium. These findings accurately define the heterogenous formation pathways and report for the first time the formation of C12A7 under a carbon-free vacuum environment.

Properties investigation of CaO–Al2O3–SiO2–Li2O–B2O3–Ce2O3 mould flux with different w(CaO)/w(Al2O3) for heat-resistant steel continuous casting

ABSTRACTThe new CaO–Al2O3–SiO2–Li2O–B2O3–Ce2O3 mould flux was devised to realise smooth continuous casting of Ce-bearing heat-resistant steel. The new devised mould flux was based on calcium-aluminate system, so the w(CaO)/w(Al2O3) has great influence on the properties of the slag, which is similar to the basicity in the silicate system. The melting temperature, viscous properties, slag structure and crystalline phases with different w(CaO)/w(Al2O3) were investigated. The melting temperature of the mould flux could remain relatively steady with w(CaO)/w(Al2O3) in the range of 1.0–1.82. The main network former in the new slag was [AlO4]-tetrahedron. The network formed by [AlO4]-tetrahedron was destroyed by increasing w(CaO)/w(Al2O3), the viscosity decreased consequently. The mould flux show weaker crystallisation tendency with increasing w(CaO)/w(Al2O3). When the temperature decreased to 1100°C, the change of the fully crystallised phases with increasing w(CaO)/w(Al2O3) was as follows: Li2O·Al2O3 + 2CaO·Al2O3·SiO2 → Li2O·Al2O3 → Li2O·Al2O3 + 3CaO·Al2O3 + CaCeAlO4.

Evaluation of surface tension of Ce-bearing slag based on calcium aluminate system

The surface tension of the Ce-bearing calcium aluminate slag was studied by the modified Butler’s equation. By taking the slag structure into account, the parameter related to AlO4-tetrahedron has been modified, imparting the model with good reproducibility. Ce2O3 and Li2O were surfactants. Ce2O3 can decrease the surface tension because of the decrease in the total ion moment of the slag and the weaker interactions between Ce3+ and O2−. Li+ had the weakest ionic potential, and increasing Li2O content decreased the total ion moment, which led to the decrease in the surface tension. The dissociated Ca2+ ions showed stronger absorption of the anions in the surface, so the surface tension increased with increasing the content of CaO. Increasing the value of w(CaO)/w(Al2O3) decreased the surface tension, because of the decrease in the total polymerised anions in the slag and the increase in the total ion moment.

Crystallization Behavior of the CaO-Al2O3-MgO System Studied with a Confocal Laser Scanning Microscope

The crystallization behavior of a calcium-aluminate system with various MgO content from 2.5 to 7.5 wt pct and CaO/Al2O3 ratios between 0.8 and 1.2 has been examined using a confocal laser scanning microscope (CLSM). CCT (continuous cooling transformation) and time temperature transformation (TTT) diagrams were constructed to identify the primary crystal phase of slag at different compositions and at cooling rates between 25 and 800 K/minutes. In the slag at a CaO/Al2O3 ratio of 1.0, crystallization temperature increased during isothermal and continuous cooling with higher MgO content, and the shortest incubation time was observed at 5 wt pct MgO. When MgO content was fixed to be 5 wt pct, crystallization temperature increased with lower CaO/Al2O3 ratio. According to the slag composition, cooling rates and temperature, the primary phase could be CA, or C5A3, or C3A, or C3MA2, or MgO, and the crystal morphology changes from dendrites to faceted crystals to columnar crystals in this composition range.

Optimization of boric acid content in developing efficient blue emitting, long persistent phosphor

The active role of boric acid (H3BO3) in enhancing the Eu2+ photoluminescence; varying persistence times and formation of several crystalline phases of calcium aluminate system is presented. The luminescence characteristics of CaAl2O4 : Eu2+, Nd3+ (CA) phosphor took an immense leap forward with the addition of very small quantities of H3BO3 (as low as 1–2 mol%) to the initial oxide mixture. The maximum PL augmentation is observed for an H3BO3 content of ∼10 mol%, and for higher concentrations PL decreased. The persistence of the Eu2+ afterglow is in the range of 10–12 h, which is relatively high compared with the value reported so far. Based on the results, we identified two major roles for H3BO3 in tailoring efficient blue phosphorescence of CA phosphors suitable for many practical display applications. In the lower (<10 mol%) concentration range, H3BO3 acted as a better fluxing agent in promoting formation of the required crystalline phase for producing efficient luminescent characteristics, whereas in the higher (≥20 mol%) range it behaved as one of the precursor materials for formation of aluminoborate complexes, which in turn cloaked the luminescence. Finally, a correlation has been drawn between the amount of H3BO3 and the energy conversion efficiency of Eu2+ in tailoring an efficient blue emitting, long persistent phosphor.

Chemical and biological integration of a mouldable bioactive ceramic material capable of forming apatite in vivo in teeth

Chemically bonded ceramics have several advantages compared with conventional ceramics to be used as biomaterials. Especially the possibilities to harden the material at room temperature and to control the rheology are very beneficial. This paper investigates the interface formed in vivo between a calcium aluminate based dental filling material and teeth. Class 1 occlusal fillings were made in wisdom teeth and extracted after up to four weeks. Polished cross-sections of the teeth were studied with scanning electron microscopy (SEM), focused ion beam microscopy (FIB) and transmission electron microscopy (TEM). In order to analyse the distribution of elements at the interface elemental mapping was performed using STEM and EDX. The results showed that a tight bond forms between the filling material and tooth and no gap could be found even at high magnification. A 100–200 nm wide zone with an increase in oxygen was detected in the enamel next to the filling. The zone was denser than the rest of the enamel. Elemental mapping indicated an increase of silicon and a decrease of Ca at the interface. Dark field imaging and EDX mapping showed that the calcium aluminate system formed apatite in situ during hardening through precipitation.

A thermochemical study of glasses and crystals along the joins silica-calcium aluminate and silica-sodium aluminate

Enthalpies of solution in molten 2PbO · B 2O 3 at 985 K are reported for series of glasses xCa 0.5 AlO 2-(1− x) SiO 2 ( O ≤ x ≤ 0.99) and xNaAlO 2-(1− x) SiO 2 (0 ≤ x ≤ 0.56). The data are compared to values for the corresponding crystalline aluminosilicates and to preliminary data for systems containing KAlO 2 and Mg 0.5AlO 2. The enthalpies of mixing of glasses become more exothermic with increasing basicity of the mono- or divalent oxide. The tendency toward immiscibility on the silica-rich side, indicated by the shape of the heat of mixing curve between x = 0 and x = 0.4, is pronounced in the calcium aluminate system, but not in the sodium aluminate system. The shape of the heat of mixing curve, which is roughly symmetrical about x = 0.5, can be rationalized in terms of glass structure by considering essentially random substitution of Si and Al on a continuous three dimensional tetrahedral framework, with stabilization arising from electrostatic interactions between aluminum and the nonframework cation balancing the destabilizing effects arising from perturbation of the aluminosilicate framework by the nonframework cation. These trends are consistent with the variation of physical properties of aluminosilicate melts.