Synthesis and characterization of calcium aluminate compounds from gehlenite by high-temperature solid-state reaction

The mineral transition mechanism and self-pulverization property of the sintered products in the Ca₂Al₂SiO₇-CaO system were systematically studied using pre-synthesized gehlenite determined by XRD, SEM, FTIR and particle size analyses. The minerals of Ca₁₂Al₁₄O₃₃, CaAl₂O₄, Ca₃SiO₅ and Ca₂SiO₄ are formed by the direct reactions of Ca₂Al₂SiO₇ with CaO. CaAl₂O₄ reacts with CaO to form Ca₁₂Al₁₄O₃₃ or Ca₃Al₂O₆, while Ca₃SiO₅ reacts with Ca₂Al₂SiO₇ to form Ca₂SiO₄ and calcium aluminate compounds. The sintered products mainly contain CaAl₂O₄, Ca₁₂Al₁₄O₃₃ and Ca₂SiO₄ at 1350?°C or above 1500?°C when the molar ratio of CaO to Al₂O₃ is 1.0. Increasing the sintering duration or the CaO consumption promotes the transition of Ca₂Al₂SiO₇ to Ca₂SiO₄ and calcium aluminate compounds when sintered at 1350?°C, which accordingly improves the self-pulverization property of the sintered products. The formed minerals of Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₂SiO₄ transform into Ca₂Al₂SiO₇ again when the sintering temperature is between 1400?°C and 1450?°C, and the corresponding self-pulverization property of the sintered products deteriorates sharply.     

Grain boundary diffusion effect on mineral transition of calcium sulpho-aluminate with sodium dopant

The mineral formation-transition mechanism, microstructure morphology evolution, pulverization property and chemical reactivity of calcium sulpho-aluminate with sodium dopant are systematically studied with the molar ratio of CaO to Al₂O₃ is 0.8, the molar ratio of CaO to SiO₂ is 1.5, the mass ratio of Al₂O₃ to SiO₂ is 2.0, and the SO₃ accounts for 4%. The results show that sodium dopant could promote 3CaO·3Al₂O₃·CaSO₄, CaO·2Al₂O₃ and 2CaO·Al₂O₃·SiO₂ transform into 2CaO·SiO₂, CaO·Al₂O₃, 12CaO·7Al₂O₃ and 2Na₂O·3CaO·5Al₂O₃, and it also can decrease the decomposition temperature of reactive materials and the initial phases formation temperature significantly. Sodium diffusion can destroy the clear grain boundary of calcium aluminate and calcium silicate phases, change the macro-microstructure morphology, improve the crystalline degree of the samples, and then improve the pulverization and alumina leaching property. However, excessive sodium dopant can decrease the melting temperature, inhibit the transformation process of 2CaO·SiO₂ from β to γ, and then deteriorate the pulverization property significantly again.     

The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

In this work the impact of Al₂O₃ amount on the synthesis (200?°C; 4–8?h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al₂O₃ plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al₂O₃ only calcium silicate hydrates (CSH) intercalated with Al³⁺ ions were formed. While in the mixtures with a higher amount of Al₂O₃ (5.3–15.4%), calcium aluminate hydrate – C₃AH₆, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4?h of hydrothermal treatment, in the mixtures with 15.4% of Al₂O₃. It was proved that synthesized C₃AH₆ remain stable up to 300?°C and at higher temperature (945?°C) recrystallized to mayenite (Ca₁₂Al₁₄O₃₃), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca₂Al₂SiO₇). Moreover, the synthesized C₃AH₆ addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5?h, while with addition – only 2–2.5?h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis.     

Influence of different parameters on calcium hexaluminate reaction sintering by Spark Plasma

Calcium hexaluminate (CaAl₁₂O₁₉) is the most alumina-rich intermediate compound of the CaO–Al₂O₃ system. The formation of this aluminate is produced by the reaction between calcium oxide and alumina. Intermediate compounds with lower alumina content (CaAl₂O₄, CaAl₄O₇,…) are formed during the synthesis of calcium hexaluminate with increasing temperature. With the aim to obtain dense and pure calcium hexaluminate by reaction sintering method using Spark Plasma Sintering (SPS) the variation of sintering parameters was studied. Final densities close to the theoretical and calcium hexaluminate formation rates over 93% were achieved by this method. Once the sintering parameters were optimized, a study of the flexural strength and the hardness of the samples were performed.     

Corrosion mechanism of polycrystalline corundum and calcium hexaluminate by calcium silicate slags

The chemical reactions involved in the corrosion of polycrystalline alumina (Al₂O₃) and calcium hexaluminate–hibonite (CaAl₁₂O₁₉) ceramics by two dicalcium silicate slags with additions of fluorspar (CaF₂) were studied using a hot-stage microscopy (HSM) up to 1600 °C. The corrosion mechanism was investigated on post-mortem corroded samples and the phases formed at different stages of the dissolution process were characterised by reflected optical light microscopy (RLOM) and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS) microanalysis system.The attack of the fused slags on dense alumina substrates takes place through an interdiffusion mechanism producing successive layers of calcium aluminates. In porous hibonite samples chemical interactions were observed although only a layer of calcium dialuminate was formed. A sintering process in presence of liquid phase was also detected behind the reaction interphase.Thermodynamic calculations, based on the Al₂O₃–CaO–SiO₂, Al₂O₃–CaO–SiO₂–MgO, and Al₂O₃–CaO–SiO₂–CaF₂ phase equilibrium were used to further knowledge of the corrosion mechanism.     

High-temperature stability of Mg(Al,Cr)2O4 spinel co-existing with calcium aluminates in air

Cr₂O₃ is a well-known corrosion resistant oxide used in refractory applications. However, it can oxidize into toxic and water-soluble Cr(VI) compounds upon reaction with calcium aluminate cement phases in the presence of oxygen, which subsequently causes disposal problems after use. This study describes the extent to which chromium in the spinel Mg(Al,Cr)₂O₄ phase can be oxidized to Cr(VI) when it reacts with the calcium aluminate cement phases C₁₂A₇, CA, CA₂ and free CaO at 1300 °C in air, using XRD, XPS and leaching tests (TRGS 613 standard) as analytical tools. On reaction with CaO, the Mg(Al,Cr^III)₂O₄ spinel mainly transformed into hauyne (Ca₄Al₆Cr^VIO₁₆) and Ca₅Cr₃O₁₂ which contains both Cr(IV) and Cr(VI). The reaction of C₁₂A₇ and CA with the spinel phase also resulted in the formation of Ca₄Al₆CrO₁₆. Conversely, the reaction of Mg(Al,Cr^III)₂O₄ spinel with CA₂ resulted in the formation of only a trace amount of Cr(VI). Water-soluble Cr(VI) leached in large quantities (>100 mg/L) from samples where the Mg(Al,Cr^III)₂O₄ reacted with either C₁₂A₇ or CA. Almost no Cr(VI) leached from the sample when Mg(Al,Cr^III)₂O₄ reacted with CaO, using the standard TRGS 613 leach test, but a significant amount of Cr(VI) was released into solution when leached with a HCl solution for 12 h. Both Cr(IV) and Cr(VI) present in the Ca₅Cr₃O₁₂ dissolved into acidic solution. Only a small amount of Cr(VI) leached from the sample that resulted when spinel was reacted with CA₂, even after a prolonged HCl leach. Cr(III) in spinel Mg(Al,Cr)₂O₄ is very stable and does not leach in either distilled water or acidic solution.     

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 (Ca₃Al₂O_6?×?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.     

CaO–SiO2–Al2O3–Y2O3 glasses as model grain boundary phases for Si3N4 ceramics

The glasses with compositions derived from the eutectic composition [37.78 (Y₃Al₅O₁₂)·62.22 (SiO₂)] of the quasi-binary glass system (Y₃Al₅O₁₂)-(SiO₂) with addition of up to 20 mol.% CaO were investigated as model grain boundary phases for Si₃N₄ ceramics. The influence of CaO as model impurity on the physical properties of the glass (density, thermal expansion) and on the crystallisation behaviour was studied. Although the initial composition of the basic glass was that of yttrium-aluminium garnet (Y₃Al₅O₁₂–YAG), no crystalline YAG was detected. Apart from yttrium disilicate (Y₂Si₂O₇), anorthite (CaAl₂Si₂O₈), tricalcium aluminate (Ca₃Al₂O₆), and calcium yttrium oxide silicate (Ca₄Y₆O(SiO₄)₆), a new phase was detected, not found in the powder diffraction file (PDF) database. Cavities were formed within the devitrified glass due to the volume contraction after crystallisation. Possible implications for the mechanical properties of Si₃N₄ ceramics sintered with addition of Y₂O₃–Al₂O₃ are discussed in terms of the observed compositional dependences of the physical properties of CaO–Y₂O₃–Al₂O₃–SiO₂ glasses.     

Selected Phase Equilibria Studies in the Al2O3–CaO–SiO2 System

The Al₂O₃–CaO–SiO₂ system provides the basis for describing many important chemical processes. Although the system has previously been extensively studied, recent advances in experimental technique have provided the opportunity to obtain accurate liquidus measurements in the low‐silica region at fixed temperatures. The experimental procedures involve equilibration of high‐purity oxide powder mixtures at selected temperatures, rapid quenching, and accurate measurement of phase compositions using electron probe X‐ray microanalysis. The liquidus isotherms have been determined at selected temperatures between 1503 and 1873 K in the anorthite, gehlenite, pseudowollastonite, corundum, CaAl₁₂O₁₉, CaAl₂O₆, lime, Ca₃SiO₃, and Ca₂SiO₄ primary phase fields. The results are compared with currently available thermodynamic model predictions of the phase chemistry.     

Formation characteristics of sodium calcium silicate compounds based on the solid-state reaction

The formation thermodynamics, phase transition and stability of sodium calcium silicate compounds under different calcination parameters in the Na₂O–CaO–SiO₂ system were studied using XRD, FTIR and SEM-EDS methods. As the Na₂O/SiO₂ ratio increases from 0.3 to 0.7 when the CaO/SiO₂ ratio is 1.0, the formation sequence of sodium calcium silicate compounds is Na₂Ca₃Si₂O₈→Na₆Ca₃Si₆O₁₈→Na₂Ca₂Si₂O₇→Na₂CaSiO₄; as the CaO/SiO₂ ratio increases from 0.3 to 1.2 when the Na₂O/SiO₂ ratio is 0.5, the formation sequence is Na₆Ca₃Si₆O₁₈→Na₂Ca₂Si₂O₇→Na₂Ca₃Si₂O₈. As the most stable sodium calcium silicate compound, Na₆Ca₃Si₆O₁₈ forms by the solid-state reaction of preformed Na₂SiO₃ with CaO and SiO₂, while increasing the calcination temperature and holding time can promote its crystal stability. The decomposition of Na₆Ca₃Si₆O₁₈ in sodium aluminate solution follows the mixed control of the film diffusion and chemical reaction, and the corresponding activation energy is between 40 and 41 kJ/mol.     

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 0.1 MPa. 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 200 nm 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.     

Combustion synthesis of Ca1−xCuxAl2O4 (x = 0.0, 0.4 and 0.8) copper doped calcium aluminate

We report the effect of Cu²⁺ ion on CaAl₂O₄ with different molar concentrations of 0.0, 0.4 and 0.8 M prepared by simple combustion method. The materials have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) and scanning electron microscopy (SEM). DC electrical conductivity has also been measured to study the electrical behavior of the materials. The XRD patterns confirm the formation of single-phase CaAl₂O₄ along with some impurity phases like CaAl₄O₇, CaAl₁₂O₁₉ and Ca₁₂Al₁₄O₃₃. The FT-IR spectra show the stretching and bending vibrations of the synthesized compounds. DC electrical conductivity of the Ca_1−xCu_xAl₂O₄ is found to vary from 26.46 × 10⁻⁴ to 515.68 × 10⁻⁴ S cm⁻¹ for x = 0.0 to x = 0.8 at the measuring temperature of 1000 °C. SEM images show the morphological features of the compounds.     

Liquidus Relations in the Quaternary Subsystem CaAl2O3-CaAl4O7-Ca2Al2SiO7-MgAl2O4

Equilibrium diagrams for the joins CaAl₄O₇-MgAl₂O₄, CaAl₄O₄-Ca₂Al₂SiO₇-MgAl₂O₄, and Ca₂Al₂O₄–Ca₂Al₂SiO₇-MgAl₂O₄ were determined by the classical quenching method and hot stage microscopy. Liquidus relations in the quaternary subsystem are discussed. Two univariant curves are located within the composition tetrahedron. The quaternary invariant point is a peri-tectic. Technological application of results is indicated.     

Chemical zoning of calcium aluminoferrite formed during melt crystallization in CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₂(Al_xFe_1−x)₂O₅, with x<0.7 (ferrite), Ca₂SiO₄ (belite), Ca₃Al₂O₆ (C₃A) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇), were crystallized out of a complete melt during cooling at 8.3 °C/min. Upon cooling to 1370 °C, both the crystals of ferrite with x=0.41 and belite would start to nucleate from the melt. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.7. At ambient temperature, the ferrite crystals had a zonal structure, the x value of which successively increased from the cores toward the rims. The value of 0.45 was confirmed for the cores by EPMA. The chemical formula of the rims was determined to be Ca_2.03[Al_1.27Fe_0.68Si_0.02]_Σ1.97O₅ (x=0.65). As the crystallization of ferrite and belite proceeded, the coexisting melt would become progressively enriched in the aluminate components. After the termination of the ferrite crystallization, the C₃A and belite would immediately crystallize out of the melt, followed by the nucleation of C₁₂A₇. The C₁₂A₇ accommodated about 2.1 mass% Fe₂O₃ in the chemical formula Ca_12.03[Al_13.61Fe_0.37]_Σ13.98O₃₃, being free from the other foreign oxides (SiO₂ and P₂O₅).     

Oxidative Destruction of Hydrocarbons on Ca12Al14-x Si x O33+0.5x (0 ≤ x ≤ 4) with Radical Oxygen Occluded in Nanopores

Three types of calcium aluminosilicate, Ca₁₂Al₁₄O₃₃ (C₁₂A₇), Ca₁₂Al₁₄Si₂O₃₄ (C₁₂A₆), and Ca₁₂Al₁₀Si₄O₃₅ (C₁₂A₅), were prepared by calcining the respective hydrothermally synthesized hydrogarnet, Ca₃Al₂(OH)₁₂, Ca₃Al₂(SiO>₄)_1/3(OH)_32/3, and Ca₃Al₂(SiO₄)_0.8(OH)_8.8. Different amounts of superoxide (O₂^-) and peroxide (O₂^2-) were occluded in the lattice of these calcium aluminosilicates. No activity improvement was observed for the oxidation of propylene and benzene by increasing the amounts of (O₂^-) and (O₂^2-).     

Effects of calcium fluoride mineralization on silicates and melt formation in portland cement clinker

The mineralizing effect of 0.5% F^? as CaF₂ on the high-temperature reactions in the quaternary system CaOAl₂O₃Fe₂O₃SiO₂ was studied in the presence of minor amounts of MgO and K₂SO₄. The partitioning of fluorine between the calcium aluminate and calcium silicate phases was determined in relation to the burning temperature and the presence of MgO.     

The role of sulphates in cement clinkering: Subsolidus phase relations in the system CaO-Aℓ2O3-SiO2-SO3

Experimental studies are reported on the CaO-rich portions of the system CaO-A?₂O₃-SiO₂-SO₃ in the temperature range 950°–1150°C. Six four-phase and 18 three-phase essemblages have been found. These define the equilibria between CaO, Ca₂SiO₄, Ca₃A?₂O₆, Ca₁₂A?₁₄O₃₃, CaA?₂O₄, calcium silicosulphate (Ca₅Si₂(SO₄)O₈) and aluminosulphate, Ca₄A?₆(SO₄)O₁₂. The equilibria above 1175°, where Ca₃SiO₅ becomes stable, are predicted. Possible departures from equilibrium during clinkering are discussed.     

Quantification of oxygen capture in mineral matter during gasification

It has been observed that during the transformation of minerals at higher temperatures (>1000 °C), mineral species are formed containing a high number of oxygen molecules, i.e. gehlenite (Ca₂Al₂SiO₇), mullite (Al₆Si₃O₁₅), margarite (CaAl₄Si₂O₁₀(OH)₂) and almandine (Fe₃Al₂Si₃O₁₂).Results of the coal sources evaluated in this investigation indicated significant differences in mineral elemental composition, i.e. the CaO content varied between 5 mass % and 10 mass %, the Fe₂O₃ content varied between 1.6 mass % to more than 5 mass %, as well as differences in the TiO₂, P₂O₅ and MgO content. The coal sources producing the highest concentration of Ca-Al-Si species (CaAl₂Si₂O₈ anorthite and CaAl₄Si₂O₁₀(OH)₂ margarite), which crystallized from the slag-liquid phase during the combustion stage, also contained the highest amount of acidic components or highest percentage of kaolinite. The highest concentration of mullite and free SiO₂ after the gasification reaction (before the combustion zone), also resulted in the highest concentration of Ca-Al-Si compounds forming during the oxidation phase. The free-SiO₂ in the mineral structure of the coal sources resulted then in the formation of mineral structures with Mg, Na or Ca when present in the mineral structure, to form new mineral compounds such as KAl₃Si₃O₁₀(OH)₂ (muscovite), Mg₅Al₂Si₃O₁₀(OH)₈ (clinochlore), or other high oxygen molecule-containing mineral compounds. Thus, if free-SiO₂ was not present after the gasification phase, and mostly taken up in the form of anorthite (due to high or higher CaO contents or Fe-contents in high Fe-containing coal sources), the concentration of Si-oxygen capture compounds are relatively low.An acceptable linear correlation between oxygen capture tendencies (increase in mineral matter content during the combustion phase) versus CaO-content was obtained with the South African coal sources evaluated. This confirmed the observations obtained based on HT-XRD and FactSage modelling. It can be concluded that the linear model to predict oxygen capture behavior from CaO-content is acceptable and can be used as a predictive tool. The SiO₂ content, for example, has an inverse affect on oxygen trends up to a specific concentration of CaO in the coal. However, this model is only valid for the coal types tested (South African Highveld coal sources), and additional test work will have to be conducted for other coal types, i.e. northern hemisphere coal.     

Preparation of calcium aluminate cement by combustion synthesis and application for corundum‐based castables

Calcium aluminate cement was prepared by combustion synthesis with CaO₂, Al, and Al₂O₃ as raw materials. The effects of CaO/Al₂O₃ (C/A) molar ratios in raw materials on the phase compositions and morphologies of calcium aluminate were investigated in detail. It was found that when the C/A reduced from 1.1 to 0.74, the content of CaO·2Al₂O₃ (CA₂) in products increased, whereas contents of CaO·Al₂O₃ (CA) and 12CaO·7Al₂O₃ (C₁₂A₇) decreased; when the C/A was 0.8, the phase composition of product (CS71) was equal to that of Secar71. Additionally, the crystallines of CA and CA₂ in the product were reduced when the C/A molar ratio was decreased. And then, the bulk density, apparent porosity, permanent linear change, cold crushing strength (CCS), and cold modulus of rupture (CMOR) of the corundum‐based castables bonded with CS71, Secar7 were compared. The castables bonded with CS71 demonstrate obviously improved CCS, CMOR, and volume stability.     

Formation of calcium aluminates in the lime-sinner process Part I. Qualitative studies

A study of the formation of several calcium aluminates, CaO.Al₂O₃, 12CaO.7Al₂O₃ and 3CaO.Al₂O₃, from pure components in the lime-sinter process was undertaken. The qualitative aspects of the formation of these aluminates were determined by X-ray diffraction analysis, EMPA, SEM, DTA and high-temperature X-ray diffraction analysis.