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This paper describes the characteristics of akermanite-gehlenite and diopside bioceramics derived from the CaO-MgO-Al2O3-SiO2 system as a potential bone substitute material. The akermanite-gehlenite and diopside bioceramics were synthesised for 4 h at 500 rpm via the wet mechanochemical process. The resultant powder was sintered at 1050 and 1150 °C for 1 h to obtain the akermanite-gehlenite and diopside bioceramics. The bioceramic’s microstructural, physical, and mechanical properties were investigated comparatively. XRD diffraction, FTIR and scanning electron microscopy, with energy dispersive spectroscopy (SEM-EDS) analysis, were conducted to characterise the samples. The bioceramic samples’ linear shrinkage, density, porosity, and water contact angle were analysed to determine their physical properties. The mechanical properties of the samples were determined using compressive tests before and after undergoing in vitro bioactivity. The in vitro bioactivity of the bioceramic was investigated by analysing its apatite-forming ability in a simulated body fluid (SBF) at 37 °C. XRD patterns, SEM micrographs, and FTIR spectra for pellets sintered at 1050 °C indicated the presence of apatite particles on their surfaces. The pellets’ compressive strength before being immersed in SBF ranged between 24.42 ± 1.5 and 24.93 ± 1.7 MPa. The compressive strength of the pellets noticeably decreased to 34.4% and 40.7% after 14 days of immersion in SBF. In conclusion, the akermanite-gehlenite bioceramic prepared using the wet mechanochemical method can potentially become a bone substitute.

Heat flow curves measured in differential scanning calorimetry (DSC) may contain overlapping peaks corresponding to multiple-step processes. To analyze the individual steps in such a process, a deconvolution of the overlapping peaks into the individual peaks is necessary. Using the Frazer-Suzuki function to describe asymmetric peaks, we apply a nonlinear least squares analysis to perform the decomposition of heat flow curves measured for a mixture of an illite/smectite clay with 19.6 wt.% of CaCO3. The curves contain two overlapping peaks associated with the crystallizations of gehlenite and anorthite in the temperature interval from 900 °C to 1050 °C. Several versions of the decomposition analysis may be used, depending on the number of optimized parameters. These may range from four to all eight parameters (four for either peak). We applied the versions with seven and eight fitting parameters, yielding results that are in very good agreement with the experimental data.

Ellinaite, a natural analog of the post-spinel phaseβ-CaCr2O4, was discovered at the Hatrurim Basin, Hatrurim pyrometamorphic formation (the Mottled Zone), Israel, and in an inclusion within the super-deep diamond collected at the placer of the Sorriso River, Juína kimberlite field, Brazil. Ellinaite at the Hatrurim Basin is confined to a reduced rankinite–gehlenite paralava, where it occurs as subhedral grains up to 30 µm in association with gehlenite, rankinite and pyrrhotite or forms the rims overgrowing zoned chromite–magnesiochromite. The empirical formula of the Hatrurim sample is (Ca0.960Fe0.0162+Na0.012Mg0.003)0.992(Cr1.731V0.1833+Ti0.0683+Al0.023Ti0.0034+)2.008O4. The mineral crystallizes in the orthorhombic system, space group Pnma, unit-cell parameters refined from X-ray single-crystal data: a 8.868(9), b 2.885(3), c 10.355(11) Å, V 264.9(5) Å3 and Z=4. The crystal structure of ellinaite from the Hatrurim Basin has been solved and refined to R1=0.0588 based on 388 independent observed reflections. Ellinaite in the Juína diamond occurs within the micron-sized polyphase inclusion in association with ferropericlase, magnesioferrite, orthorhombic MgCr2O4, unidentified iron carbide and graphite. Its empirical formula is Ca1.07(Cr1.71Fe0.063+V0.06Ti0.03Al0.03Mg0.02Mn0.02)Σ1.93O4. The unit-cell parameters obtained from HRTEM data are as follows: space group Pnma, a 9.017, b 2.874 Å, c 10.170 Å, V 263.55 Å3, Z=4. Ellinaite belongs to a group of natural tunnel-structured oxides of the general formula AB2O4, the so-called post-spinel minerals: marokite CaMn2O4, xieite FeCr2O4, harmunite CaFe2O4, wernerkrauseite CaFe23+Mn4+O6, chenmingite FeCr2O4, maohokite MgFe2O4 and tschaunerite Fe(FeTi)O4. The mineral from both occurrences seems to be crystallized under highly reduced conditions at high temperatures (>1000 ∘C), but under different pressure: near-surface (Hatrurim Basin) and lower mantle (Juína diamond).

Shagamite, KFe11O17 (IMA 2020-091) was discovered in the ferrite zone of gehlenite hornfels from the Hatrurim Complex exposed near Mt. Ye’elim, Hatrurim Basin, Israel. The mineral occurs in outer zones of gehlenite rock blocks that were heterogeneously altered by high-temperature (>1200 °C) ferritization. Ferritization was induced by K-bearing fluids or melts, generated as a by-product of late combustion processes. Shagamite crystallized from a thin melt that formed on the rock surface during cooling to approximately 800–900 °C. It is mainly associated with minerals of the magnetoplumbite group like barioferrite, Sr-analog of barioferrite, and gorerite but also with magnetite, maghemite, harmunite, devilliersite and K(Sr,Ca)Fe23O36 hexaferrite. Shagamite is a modular compound with a β-alumina-type structure (P63/mmc, a = 5.9327 (5), c = 23.782 (3) Å, γ = 120°, V = 724.91 (13) Å3, Z = 2), and it is isostructural with diaoyudaoite, NaAl11O17, and kahlenbergite, KAl11O17. Its structure is also closely related, though non-isotypic, to those of the magnetoplumbite-group minerals. Shagamite is dark brown with a semi-metallic luster and forms platy crystals flattened on (001). Its mean empirical formula is: (K1.00Ca0.15Mn2+0.05Na0.04Rb0.01)Σ1.25(Fe10.36Mn2+0.15Al0.14Mg0.12Zn0.10Ni0.07Cu0.03Cr3+0.02Ti4+0.01)Σ11.00O17. The Vickers microhardness VHN25 = 507 kg/mm2 corresponds to a Mohs hardness of ~5. The calculated density, based on the empirical formula and unit-cell parameters, is 4.12 g·cm−3. The main bands in the Raman spectrum of shagamite occur at 685 and 715 cm−1 and are assigned to ν1(FeO4)5− tetrahedral vibrations.

Naomaohu coal from the Santanghu Basin, Xinjiang, is characterized by anomalously high Na and Ca contents, which strongly affect its gasification behavior and slagging tendency. However, the genetic linkage between geological alkali enrichment and their transformation during thermal processes remains insufficiently constrained. In this study, an integrated investigation combining coal seam profile analysis, coal petrography, mineralogical characterization, and fixed-bed gasification experiments was conducted to elucidate the enrichment mechanisms and transformation pathways of alkali and alkaline earth metals (AAEMs). A total of forty six samples were collected along a vertical seam profile to determine the depositional control of alkali and alkaline earth metals (AAEMs), and seven representative samples were further subjected to pressurized fixed-bed gasification. Alkali migration and mineral phase evolution were systematically analyzed using XRD, XRF, and SEM-EDS. The results indicate that Na enrichment is mainly controlled by groundwater infiltration and weak paleoweathering, while Ca accumulation reflects deposition in humid, Ca-rich mire environments. During gasification, Na volatilizes and recondenses as Na-feldspars (NaAlSi2O6) and NaCl, whereas Ca decomposes into gehlenite (Ca2Al2SiO7) and brownmillerite (Ca2AlFeO5). The formation of these low-melting Na–Al–Si phases and Ca–Fe–Al phases dominate the ash fusion and slagging behavior. This study establishes a coupled geological–thermal transformation model for AAEMs in high-Na coal, providing mechanistic insight into mineralogical inheritance and offering guidance for mitigating alkali-induced slagging during gasification.

The hydrogen reduction of bauxite residue lime pellets at elevated temperatures was carried out to recover iron and alumina from the bauxite residue in a new process route. Prior to the H2 reduction, oxide pellets were initially prepared via the mixing of an industrial bauxite residue with fine calcite powder followed by calcination and high-temperature sintering. The chemical, compositional, and microstructural properties of both oxide and reduced pellets were studied by advanced characterization techniques. It was found that iron in the oxide pellets is mainly in the form of brownmillerite, and calcium–iron–titanate phases, while upon reduction they are converted to wüstite and shulamitite intermediate phases and further to metallic iron. Moreover, it was found that the reduction at lower temperature of 1000 °C is faster than that at higher temperatures of 1100 °C and 1200 °C. The slower rate and extent of reduction at the higher temperatures is attributed to the porosity loss and reduction mechanism change to a diffusion-controlled process step. In addition, it was found that Al-containing phases in the raw materials are converted mainly to gehlenite in sintered pellets and further to the leachable mayenite phase. The alkaline leaching of selected reduced pellets by a sodium carbonate solution yielded up to 87% Al recovery into the solution, while the metallic iron was not affected.

This present study evaluates the effect of silica modulus (Ms) and curing temperature on strengths and the microstructures of binary blended alkali-activated volcanic ash and limestone powder mortar. Mortar samples were prepared using mass ratio of combined Na2SiO3(aq)/10 M NaOH(aq) of 0.5 to 1.5 at an interval of 0.25, corresponding to Ms of 0.52, 0.72, 0.89, 1.05 and 1.18, respectively, and sole 10 M NaOH(aq). Samples were then subjected to ambient room temperature, and the oven-cured temperature was maintained from 45 to 90 °C at an interval of 15 °C for 24 h. The maximum achievable 28-day strength was 27 MPa at Ms value of 0.89 cured at 75 °C. Samples synthesised with the sole 10 M NaOH(aq) activator resulted in a binder with a low 28-day compressive strength (15 MPa) compared to combined usage of Na2SiO3(aq)/10 M NaOH(aq) activators. Results further revealed that curing at low temperatures (25 °C to 45 °C) does not favour strength development, whereas higher curing temperature positively enhanced strength development. More than 70% of the 28-day compressive strength could be achieved within 12 h of curing with the usage of combined Na2SiO3(aq)/10 M NaOH(aq). XRD, FTIR and SEM + EDX characterisations revealed that activation with combined Na2SiO3(aq)/10 M NaOH(aq) leads to the formation of anorthite (CaAl2Si2O8), gehlenite (CaO.Al2O3.SiO2) and albite (NaAlSi3O8) that improve the amorphosity, homogeneity and microstructural density of the binder compared to that of samples synthesised with sole 10 M NaOH(aq).

Karlleuite, ideally Ca2MnO4, is a newly approved accessory mineral found in the xenolith sample within the basaltic lava from the Caspar quarry, Bellerberg volcano, Eifel, Germany. It usually occurs as thin tabular/plate crystals, which range from 40 to 80 μm in diameter, and is associated with other members of the perovskite supergroup such as srebrodolskite, brownmillerite, sharyginite, perovskite, and lakargiite distributed within rock-forming minerals represented by reinhardbraunsite, fluorellestadite, fluorapatite, larnite, gehlenite, and several hydrated Ca aluminosilicates. Karlleuite crystals are brown with sub-metallic lustre, a light brown streak, and a good cleavage along (001). It is non-fluorescent, brittle and has an uneven fracture, a Mohs hardness of 3.5 and calculated density Dx = 3.79 g/cm3. The empirical formula of the holotype karlleuite calculated based on O = 4 atoms per formula is (Ca1.97Ce3+0.06)2.03(Mn4 + 0.39Ti0.36Fe3+0.19Al0.09)1.03O4, which shows that it is a multicomponent phase characterised by various substituents at the octahedral site. Karlleuite is tetragonal I4/mmm (no. 139), with a = 3.7683(2) Å, c = 11.9893(8) Å, V = 170.254(17) Å3, and Z = 2. The calculated strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 5.995 (43), 2.742 (100), 2.665 (91), 2.023 (25), 1.998 (28), 1.884 (61), 1.553 (38), 1.371 (24). The new mineral is the first natural phase which exhibits a first order of Ruddlesden-Popper type structure, which indicates a modular nature and consists of Ca(Mn, Ti, Fe, Al)O3 perovskite layers, packed between CaO rock-salt layers arranged along the c-axis. Raman spectroscopy supports the interpretation of the chemical and structural data. Mineral association, structural data, as well as the study of the synthetic Ca-Mn-O system suggest that karlleuite could form under high-temperature conditions, above 1000˚C.

The utility of recycling some intensive industries’ waste materials for producing cellular porous ceramic is the leading aim of this study. To achieve this purpose, ceramic samples were prepared utilizing both arc furnace slag (AFS) and ceramic sludge, without any addition of pure chemicals, at 1100 °C. A series of nine samples was prepared via increasing AFS percentage over sludge percentage by 10 wt.% intervals, reaching 10 wt.% sludge and 90 wt.% AFS contents in the ninth and last batch. The oxide constituents of waste materials were analyzed using XRF. All synthesized samples were investigated using XRD to detect the precipitated minerals. The developed phases were β-wollastonite, quartz, gehlenite, parawollastonite and fayalite. The formed crystalline phases were changed depending on the CaO/SiO2 ratio in the batch composition. Sample morphology was investigated via scanning electron microscope to identify the porosity of the prepared ceramics. Porosity, density and electrical properties were measured; it was found that all these properties were dependent on the composition of starting materials and formed phases. When increasing CaO and Al2O3 contents, porosity values increased, while increases in MgO and Fe2O3 caused a decrease in porosity and increases in dielectric constant and electric conductivity. Sintering of selected samples at different temperatures caused formation of two polymorphic structures of wollastonite, either β-wollastonite (unstable) or parawollastonite (stable). β-wollastonite transformed into parawollastonite at elevated temperatures. When increasing the sintering temperature to 1150 °C, a small amount of fayalite phase (Fe2SiO4) was formed. It was noticed that the dielectric measurements of the selected sintered samples at 1100 °C were lower than those recorded when sintering temperatures were 1050 °C or 1150 °C.

This paper elucidated the potential of electron backscatter diffraction analysis for ground granulated blast furnace slag geopolymers at 1000°C heating temperature. The specimen was prepared through the mechanical ground with sandpaper and diamond pad before polished with diamond suspension. By using advanced technique electron backscatter diffraction, the microstructure analysis and elemental distribution were mapped. The details on the crystalline minerals, including gehlenite, mayenite, tobermorite and calcite were easily traced. Moreover, the experimental Kikuchi diffraction patterns were utilized to generate a self-consistent reference for the electron backscatter diffraction pattern matching. From the electron backscatter diffraction, the locally varying crystal orientation in slag geopolymers sample of monoclinic crystal observed in hedenbergite, orthorhombic crystal in tobermorite and hexagonal crystal in calcite at 1000°C heating temperature.