CaAl2O4 Research Articles

Key-role of the recipe formulation in the solution combustion synthesis of monocalcium aluminate, CaAl2O4

The preparation of monocalcium aluminate, CaAl2O4, via solution combustion synthesis is discussed in a side-by-side evaluation of the single-fuel and fuel-mixture approach. Single-fuel recipes based on urea or glycine, and corresponding metal nitrates, generated low combustion temperatures (480 °C and 748 °C), which did not enable the formation of CaAl2O4. Under identical experimental conditions, urea and glycine fuel mixture generated a much higher combustion temperature (1558 °C), which promoted the formation of single-phase CaAl2O4 directly from the combustion reaction. The as-prepared CaAl2O4 had a crystallite size of 60 nm and a BET surface area of 0.8 m2/g. Laser granulometry measurements evidenced that 90% of the CaAl2O4 particles were smaller than 28 µm. Combustion synthesis of CaAl2O4 powders requires a careful recipe formulation, as single-fuel approach and fuel-mixture approach may lead to different results in terms of combustion reaction evolution and phase composition of the resulted powder.

Impact of initial CA dissolution on the hydration mechanism of CAC

Usually, for basic investigations on the hydration behaviour of CA (monocalcium aluminate, CaAl2O4) preferably pure samples without the presence of other hydraulic phases are used. In the presented study we apply an alternative approach investigating mixes that contain tiny to low amounts of CA (0.5–20 wt%) besides CA2 (CaAl4O7) or C6AF2 (Ca6Al2Fe4O15). The hydration of the mixes is investigated by isothermal heat flow calorimetry, in-situ XRD and pore solution analysis at 23 °C. A modified thermodynamic database enables the calculation of saturation indices for relevant hydrate phases in the investigated systems. CA similarly influences the hydration of both CA2 and C6AF2 although the exact impact strongly depends on the initial CA/water ratio in the mixes. Up to a certain amount CA is completely consumed during the first minutes after mixing. Beyond this amount the presence of undissolved CA causes an induction period and retards the hydration of CA2 or C6AF2.

Effect of iron on mineral composition of quaternary phase clinker and its hydration properties

The present task is to investigate the influence of iron on the mineral composition and microstructure of the phase Q (C20A13M3S3, in the text below C = CaO, A = Al2O3, M = MgO, S = SiO2, F = Fe2O3) clinker and its hydraulic property by using XRD, Rietveld refinement, BSE, isothermal calorimeter, DSC, non-evaporable water content and compressive strength measurements. Results reveal that incorporating 4% of iron leads to the formation of the accompanying minerals which are spinel (MA), gehlenite (C2AS) and tetracalcium aluminoferrite (C4AF), and 6% of iron results in the formation of 15.3% of monocalcium aluminate (CA). Additionally, the lattice parameters of phase Q change with the doped-iron content which could be due to Ca2+ and Si4+ being substituted by Fe3+. Although the formation of C4AF coating layer around the clinker grains inhibits the hydraulic activity of clinker within the first 1 day, it has no adverse impacts on the later-age (28 days) hydration and is favorable to improve the 28-day compressive strength. The formation of CA accelerates the hydration of clinker, shortens the time span of the acceleration and deceleration periods, and may influence the hydration reaction of phase Q. Considering compressive strength, the threshold limit of iron incorporated into phase Q raw meal is found to be 4.0 mass%, beyond which an obvious decrease in 28-day compressive strength is detected.

Simultaneous removal of calcium and sulfate ions from flotation water of complex sulfides

This paper presents and discusses the results of an experimental study designed to investigate the feasibility of removing calcium and sulfate from saturated solutions by using calcium aluminate-based compounds. The results obtained show that the treatment of saturated solutions (0.016 mol/L CaSO4) alkalinized at pH above 12 by adding 0.75 g/L CaO and treated with a calcium aluminate-based compound (C70: 70% Al2O3, 30% CaO), promotes the precipitation of calcium sulfate as ettringite, which is a hydrated calcium sulfoaluminate, in equilibrium with residual calcium and sulfate concentrations below 200 mg/L. The active species of compound C70 is the monocalcium aluminate (CaO·Al2O3), which is the responsible of promoting ettringite precipitation. The use of a large excess of monocalcium aluminate significantly increases the removal kinetics but has the disadvantage that the excess is eventually hydrolyzed releasing calcium and aluminate ions to the solution. It was also observed that an excessively long contact time (e.g., 50 h), results in the acidification of the suspension due to absorption of atmospheric CO2, causing the dissolution of the ettringite, which is stable at pH above 11.5. The best experimental conditions observed are the use of a slight excess of reactants (with respect to the stoichiometrically required): 0.75 g/L CaO and 2 g/L of C70 and a reaction time of 6 h.

A high-pressure X-ray diffraction study of the crystalline phases in calcium aluminate cement paste

Calcium aluminate cement (CAC) has wide application in civil engineering and castable refractory materials. The main binding phases of hardened CAC paste, CaO·Al2O3·10H2O (CAH10), inevitably converts to 2CaO·Al2O3·8H2O (C2AH8), 3CaO·Al2O3·6H2O (C3AH6) and Al(OH)3 (AH3), leading to a significant change in the mechanical properties of the CAC matrix. This work investigates the mechanical properties of the main crystalline components in hydrated and/or converted CAC systems, using synchrotron-radiation-based high-pressure X-ray diffraction. The anisotropic deformations of CaAl2O4 (CA), CAH10 and C3AH6 along each crystallographic direction are investigated, along with their bulk moduli. The density-driven stiffening hypothesis is validated for the studied phases and other cement-based minerals. An atom-scale topological analysis is proposed to explain the unusually high stiffness of CAH10. The results provide fundamental information to understand the mechanical properties of single cement-based phases at molecular level, and enable predicting the changing mechanical properties of converted CAC matrix using homogenization models.

Strength properties of Bayer red mud stabilized by lime-fly ash using orthogonal experiments

Bayer red mud (RM) was produced from aluminum ore according to Bayer process. In order to study the feasibility of stabilized Bayer RM for use in road construction, strength properties of RM stabilized by lime-fly ash using orthogonal experimental method were evaluated. In test design, the normalized orthogonal table of L9 (34) was adopted. The influence of ratio of lime-ash, the dosage of lime and fly ash, and the type of Bayer RM (Two kinds of Bayer RM from different producers) on the unconfined compressive strengths (UCS) of 7-day and 28-day curing age were studied. The experimental results showed that these factors significantly influenced the UCS. The principal factors influencing 7-day UCS was ratio of lime-ash, followed by chemical compositions of Bayer RM and the dosage of lime and fly ash in last. The principal influencing factor of 28-day UCS was chemical compositions of RM, followed by ratio of lime-ash and the dosage of lime and fly ash in last. Then the optimum proportion for the UCS was determined through the orthogonal analysis. The X-ray fluorescence (XRF) showed that chemical compositions and contents of two types of RM were significantly different in iron, silicon and calcium. The microscopic analysis of the raw material and stabilized material employed X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) showed that C3A(Ca3Al2O6), C2S, and calcium carbonate increased under the optimal conditions of lime-fly ash stabilization, and hydrated silica aluminate and CSH were gradually generated. The stabilization mechanism of Bayer RM by lime-fly ash is related to hydration like aluminate cement. There is no essential difference in reaction mechanism for two kinds of Bayer RM and the difference in the strength properties is essentially originates from the active material content.

Solid-phase combustion synthesis of calcium aluminate with CaAl2O4 nanofiber structures

Calcium aluminate with CaAl2O4 (CA) nanofiber structures was fabricated through a facile solid-phase combustion synthesis method with raw materials of CaO2, CaCO3, Al, and Al2O3 in Ar atomosphere at a pressure of 0.1MPa. The results indicated that the relative content of CA decreased, but that of Ca12Al14O33 (C12A7) increased with the increase of r-value from 0 to 0.5 (r is molar substitution coefficient of CaCO3 for CaO2). Interestingly, CA nanofibers with tens of micrometers in length and about 200nm in diameter were observed in the combustion products with r = 0.2, 0.3, 0.4, respectively. The more nanofibers can be found in the products, as the content of CaCO3 in raw material ratios increased. And the yield of nanofibers in the combustion product with r = 0.4 is the highest. The typical round droplet on head of nanofiber indicated that the growth process of nanofibers was governed by vapor-liquid-solid (VLS) mechanism with base growth mode. It is proposed that both higher reaction temperature and reducing atmosphere are requirements for the growth of CA nanofiber during the process of combustion synthesis. The nanofiber cannot be generated in the samples r0, and r5, because the gaseous Ca is absent due to oxidizing atmosphere and lower reaction temperature, respectively.

Intrinsic mechanical properties of calcium aluminate crystals via the linear comparison composite method coupled with nano-indentation

Monocalcium aluminate and monocalcium dialuminate are minerals with a monoclinic crystal structure, that result from the fusion of limestone and bauxite. They are involved in many applications including high alumina cement, advanced biomaterials, or cement-polymer composites such as macro-defect-free cement. Despite several theoretical and experimental studies, predicting the nonlinear behavior of cement-polymer composites remains a challenge. Our research goal is to connect the effective strength behavior to the micro- and nano-constituents for cement-polymer composites, using nonlinear micromechanics, computational mechanics and experimental nano-mechanics. To this end, we extend the linear comparison composite method to non-porous granular materials. The nonlinear strength upscaling scheme is integrated into a finite element model so as to represent the mechanical behavior of calcium aluminate-polyvinyl alcohol nanocomposites—or macro-defect-free cements— across multiple length scales, while taking into account the chemistry, internal friction and grain size. At the sub-micron level, grid nano-indentation tests are carried out. An inverse scheme is implemented to predict the behavior of the basic unit, mono-calcium aluminate and monocalcium di-aluminate crystals, at the molecular level. Meanwhile, a forward approach is selected to predict the uniaxial tensile and compressive strength at the macroscopic level. The theoretical predictions agree well with independent experiments reported in the scientific literature. Macro-defect-free cements owe their extraordinary mechanical properties to a high packing density of calcium aluminate crystals, a granular morphology and the lack of porosity. The inter-disciplinary framework built can be applied to yield fundamental insights into the behavior of a broad class of engineered nano-composites.

Correlation of Structure, Tunable Colors, and Lifetimes of (Sr, Ca, Ba)Al₂O₄:Eu2+, Dy3+ Phosphors.

(Sr, Ca, Ba)Al2O4:Eu2+, Dy3+ phosphors were prepared via a high temperature solid-state reaction method. The correlation of phase structure, optical properties and lifetimes of the phosphors are investigated in this work. For the (Sr, Ca)Al2O4:Eu2+, Dy3+ phosphors, the different phase formation from monoclinic SrAl2O4 phase to hexagonal SrAl2O4 phase to monoclinic CaAl2O4 phase was observed when the Ca content increased. The emission color of SrAl2O4:Eu2+, Dy3+ phosphors varied from green to blue. For the (Sr, Ba)Al2O4:Eu2+, Dy3+ phosphors, different phase formation from the monoclinic SrAl2O4 phase to the hexagonal BaAl2O4 phase was observed, along with a shift of emission wavelength from 520 nm to 500 nm. More interestingly, the decay time of SrAl2O4:Eu2+, Dy3+ changed due to the different phase formations. Lifetime can be dramatically shortened by the substitution of Sr2+ with Ba2+ cations, resulting in improving the performance of the alternating current light emitting diode (AC-LED). Finally, intense LEDs are successfully obtained by combining these phosphors with Ga(In)N near UV chips.

Synchrotron Radiation Pair Distribution Function Analysis of Gels in Cements

The analysis of atomic ordering in a nanocrystalline phase with small particle sizes, below 5 nm, is intrinsically complicated because of the lack of long-range order. Furthermore, the presence of additional crystalline phase(s) may exacerbate the problem, as is the case in cement pastes. Here, we use the synchrotron pair distribution function (PDF) chiefly to characterize the local atomic order of the nanocrystalline phases, gels, in cement pastes. We have used a multi r-range analysis approach, where the ~4–7 nm r-range allows determining the crystalline phase contents; the ~1–2.5 nm r-range is used to characterize the atomic ordering in the nanocrystalline component; and the ~0.2–1.0 nm r-range gives insights about additional amorphous components. Specifically, we have prepared four alite pastes with variable water contents, and the analyses showed that a defective tobermorite, Ca11Si9O28(OH)2.8.5H2O, gave the best fit. Furthermore, the PDF analyses suggest that the calcium silicate hydrate gel is composed of this tobermorite and amorphous calcium hydroxide. Finally, this approach has been used to study alternative cements. The hydration of monocalcium aluminate and ye’elimite pastes yield aluminum hydroxide gels. PDF analyses show that these gels are constituted of nanocrystalline gibbsite, and the particle size can be as small as 2.5 nm.

Matrix microstructure optimization of alumina-spinel castables and its effect on high temperature properties

In this paper, submicron spinel-containing calcium aluminate (SP-CA) cements were prepared at temperatures of 1200°C (RA12), 1300°C (RA13), and 1450°C (RA14) using reactive alumina, magnesium hydroxide, and calcium carbonate as raw materials. The microstructural characteristics and high temperature properties of refractory castables bonded by SP-CA cement were investigated. The results indicated that the mineralogical compositions of SP-CA were spinel (MgAl2O4, MA) and CaAl2O4 (CA). The spinel phases presented no definite borders or shapes, with average size ranging from 0.1μm to 1.2µm at different sintering temperatures. After heating alumina-spinel castables, the submicron spinel particles bonded with corundum and hibonite (CA6) grains could strengthen the castable body. However, the large sized (< 88µm) spinel did not form links between spinel and alumina in the castable with added preformed spinel (castable AS). Submicron spinel can decrease the pore sizes of refractory castables from 2µm to 0.5µm. Extended finite element method (XFEM) calculations showed that smaller pores prevent stress concentration and result in increased deformation with limited crack propagation. Hence, large numbers of 0.5µm pores can act as crack-stoppers to improve the thermal shock resistance of castables with added SP-CA cements. Thermal shock resistance tests showed that R'''' of castable RA12 was 39.7m, which is significantly higher than that of castable AS (19.8m). The corrosion rate of castable RA12 was 5mm2/min, which is significantly lower than that of castable AS (7.4mm2/min). Small pore size and submicron spinel played relevant roles in corrosion behavior.

Simple and Efficient Fabrication of Mayenite Electrides from a Solution-Derived Precursor.

Mayenite (12CaO·7Al2O3, C12A7) electride with an anti-zeolite nanoporous structure has attracted intense attention due to its versatile promising application potentials. However, the synthesis difficulty because of extremely harsh conditions (e.g., reduction in sealed calcium or titanium vapor) significantly obstructs its realistic applications. In this work, we employed a simple, efficient, and cost-effective route for synthesizing mayenite electrides (C12A7:e-) in both powder and dense ceramic. C12A7:e- powders with efficient electron doping (3.5 × 1020 cm-3) were obtained via simple graphite reduction of a novel mixture precursor of CaAl2O4 (CA) and Ca3Al2O6 (C3A) derived from a modified Pechini method. The structural evolution during the electride formation was investigated, and it was found that reduction below 1300 °C induced the formation of Ca5Al6O14 (C5A3), while reduction above 1400 °C helped retain the mayenite structure. Fully dense C12A7:e- ceramics were also fabricated via graphite reduction of presintered pellets with a relative density of 97.9% starting from the CA+C3A mixture. Careful studies improved the mechanism cognition of graphite treatment that the electrons injection was probably initiated by surface reduction with involatile C species (e.g., C22-) rather than previously proposed CO, during which the mixed conduction of oxygen ions and electrons played an important role. Furthermore, the stability of C12A7:e- in water as well as in the presence of moisture was discussed. These results not only suggest a novel precursor for fabricating high-quality mayenite electrides but also provide in-depth insights into the stability of the mayenite structure toward practical applications.

Chlordetect: Commercial Calcium Aluminate Based Conductimetric Sensor for Chloride Presence Detection.

Chloride presence affects different environments (soil, water, concrete) decreasing their qualities. In order to assess chloride concentration this paper proposes a novel sensor for detecting and measuring it. This sensor is based on electric changes of commercial monocalcium aluminate (CA) when it interacts with chloride aqueous solutions. CA is used as a dielectric material between two coplanar capacitors. The geometry proposed for this sensor allows to assess the chloride content profile, or to make four times the same measurement. Besides, the experimental design gives us the possibility of study not just the chloride effect, but also the time and some geometric effects due to the sensor design. As a result, this sensor shows a limit of detection, sensitivity, and response time: 0.01 wt % Cl− and 0.06 wt % Cl−, and 2 min, respectively, comparable with other non invasive techniques as optical fibre sensors.

Effect of fuel molecules on properties and activity of KOH/calcium aluminate nanocatalyst for biodiesel production

New technologies such as microwaves have gained a large deal of attention from scientists and industries who sought increased rate of production processes. In this study, microwave irradiation was utilized to produce a novel KOH/Ca12Al14O33 nanocatalyst used for biodiesel production. As support, calcium aluminate was prepared by microwave combustion method using different fuels including urea, glycine, sorbitol, and citric acid. The samples were then impregnated by KOH to improve their catalytic activities for microwave-enhanced transesterification of canola oil for biodiesel production. Results of XRD, BET, FTIR, TG, EDX, and FE-SEM analyses showed differences in physicochemical properties of the samples when using different fuels with different flame characteristics and combustion temperatures. Only the urea-fueled sample showed the crystalline structure of monocalcium aluminate (CaAl2O4), with the other samples exhibiting amorphous structure of CaO–Al2O3. However, all samples, except for that prepared by citric acid, transformed to crystalline structure of Ca12Al14O33 by calcination during KOH impregnation. Among the samples, the KOH/Ca12Al14O33 nanocatalyst prepared by sorbitol showed the highest activity in microwave-enhanced biodiesel production because of its large surface area, pore size, and basicity, converting 93.4% of canola oil to biodiesel at a methanol-to-oil molar ratio of 18, catalyst concentration of 4 wt%, and microwave output power of 450 W in 60 min of reaction time. Moreover, the sample showed well-distributed particle sizes without any agglomeration, so that it could easily maintain its level of activity for several rounds of use.

Stable conversion of metastable hydrates in calcium aluminate cement by early carbonation curing

The hydration of monocalcium aluminate in calcium aluminate cement forms metastable products such as CAH10 and C2AH8, which undergo phase transformations into stable products, inducing significant volumetric instability. The present study adopted carbonation curing, in which a calcium aluminate cement paste is exposed to a CO2 concentration of 10%. X-ray diffractometry, thermogravimetry and mercury intrusion porosimetry were conducted on the carbonation-cured and on reference samples at 28days. The results showed that the metastable phases in the carbonation-cured sample were converted into stable phases and calcium carbonate polymorphs without inducing any significant change in the pore structure. This study may provide new insight into innovative means of avoiding the unstable conversion of hydrates in calcium aluminate cement by utilizing CO2 collected from industrial sources.

Effect of the phosphorous content and synthesis method on the in vitro bioactivity and mechanical properties of calcium aluminate cements

Abstract The effect of the synthesis method and the phosphorous content on the in vitro bioactivity, compressive strength and setting time of phosphorous-containing calcium monoaluminate cements (CACs) was studied. In order to obtain pure monoclinic calcium aluminate (CA), two methods were used: Pechini and solid state reaction, incorporating the phosphorous to CA during synthesis. After 7 days of immersion in a simulated body fluid (SBF), a Ca-P rich compound was formed on the surface of all the CACs. The amount of this compound was increased as phosphorous content and immersion time were increased and its morphology was finer for the CACs obtained from CAs synthetized by Pechini method. Phosphorous-containing CACs showed a higher compressive strength than those with no P after 21 days of immersion in SBF. Final compressive strength of CACs elaborated with CA synthetized by solid state reaction was higher than that of CACs elaborated with CA synthetized by Pechini method due to the smaller particles obtained by this advanced method which promote a rapid hydration. Setting times decreased as phosphorous content was increased, being shorter in cements elaborated with CA synthetized by Pechini method due to the smaller particle size which has a strong effect on the hydration kinetics.

Synthesis and comparative study of Ce3+ ion in calcium aluminates

Phase pure monoclinic CaAl2O4 (CA2), CaAl4O7 (CA4) and hexagonal CaAl12O19 (CA12) doped with 1 mol% Ce3+ phosphor were prepared by combustion method at 500 °C in a few minutes. Synthesized phosphor has been well characterized by X-ray diffraction. In the present paper, we discussed the comparative luminescence properties of the phosphors with respect to their crystal structures and the various sites of Ca2+. This paper provides evidence for different coordinated sites of Ca2+ ion in the host used. Comparison between calculated and experimental value of position of energy in the lower d-band edge for Ce3+ ion is discussed. Ce3+ ions were doped in order to study the emission characteristics in the prepared sample with respect to their crystal symmetry. Different emission spectrum is observed despite of their same crystal structure. The photoluminescence emission spectra of the CaAl2O4:Ce, CaAl4O7:Ce, and CaAl12O19:Ce phosphors show strong Ce3+ emission at around 370, 330, and 320 nm for the excitation at 300, 273, and 264 nm wavelengths respectively. The emission characteristics are credited to 5d→4f (2F5/2 and 2F7/2) type transitions in Ce3+.

Quantitative phase analysis of the calcium silicate slag as the residue of extracting alumina from high-alumina fly ash

Calcium silicate slag is the residue of process of pre-desilication alkali lime sintering applied in the high-alumina fly ash to extract the alumina. The quantitative phase analysis (QPA) of the calcium silicate slag has been performed by the Rietveld method based on the powder X-ray diffraction (XRD) with the aid of noncommercial software GSAS-EXPGUI. A known weight of crystalline internal standard (10% CaF2) was added to the calcium silicate slag to calculate the fraction of amorphous phase and other crystalline phases on an absolute basis. Besides, the calcium silicate slag was characterized by X-ray fluorescence (XRF) and thermo gravimetric (TG) differential scanning calorimetry (DSC) to test the QPA results and investigate its other characters. Finally, the results show that the amorphous fraction is 17.5% (hereinafter, the percentages refer to the mass fraction), and the major crystalline phases detected in the calcium silicate slag consist of 23.5% Beta-Ca2SiO4, 10.0% bredigite, 10.3% Ca3Al2O6 (C3A) and 21.6% CaCO3.

Optimization of the activity of KOH/calcium aluminate nanocatalyst for biodiesel production using response surface methodology

In this study, alumina and calcium aluminate were prepared through microwave combustion method (MCM) and then were modified by potassium hydroxide as solid base catalysts. The catalytic activities were evaluated in the transesterification reaction of canola oil. The characteristic properties of the samples determined by XRD, FTIR, TG, BET surface area, basicity by Hammett indicator, SEM and EDX showed that the alpha phase of alumina and monocalcium aluminate (CaAl2O4) were successfully synthesized by MCM. However, the samples showed less basicity and activity whereas these properties were meaningfully increased by KOH loading. Moreover, the surface area of monocalcium aluminate was increased from 38.9 to 48.1m2/g by loading of potassium components. To obtain a catalyst with highest activity and basicity, the calcium oxide and potassium group precursor dosages on aluminum oxide were optimized using the response surface methodology (RSM). The optimal parameters obtained were calcium oxide/alumina molar ratio of 1.48:1 and 23wt.% potassium hydroxide to CaO-Al2O3. The yield in the optimal condition was 96.7% (the predicted yield was 98.3%) where the transesterification reaction was performed in conditions of 65°C, 3.5wt.% catalyst, 12:1 molar ratio of methanol-to-oil and 4h reaction time. The catalyst maintained its activity for at least three times.

Calcium silicate/calcium aluminate composite biocement for bone restorative application: synthesis, characterisation and in vitro biocompatibility

Pure β-dicalcium silicate and monocalcium aluminate powder were prepared by Pechini method. A series of calcium silicate/calcium aluminate cements (CSC/CAC) were prepared. The setting time, crystalline phases, microstructures, compressive strength, cells attachment and silicon release of the cements were investigated. The results indicate that the setting time of CSC/CAC was shorter than that of either CSC or CAC. The hydration products in CSC/CAC composite are gehlenite (Ca2Al2SiO7·8H2O), calcium aluminate hydrate (Ca3Al2O6 × H2O), and katoite (Ca2Al2O6·6H2O). Platelike crystals were found in the microstructure. The liquid to powder ratio has a significant effect on the porosity and the strength of CSC/CAC. The MC3T3 cells attached well to the surfaces of CSC/CAC. However, the cells proliferation on the surface of 7S3A was better than that of 3S7A due to its higher silicon release. In general, CSC/CAC exhibits good biocompatibility and relative high strength, and may be suitable for some non-load bearing bone restorative applications.