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This paper presents the experimental results on the impact of modified coal fly ash on mechanical performance of cement materials. Fly ash used in the experiments was received from Burshtyn thermal power plant (Ukraine). The surface modification was carried out by the acidic treatment in order to increase its pozzolanic activity. The impact of raw fly ash and surface-modified fly ash on the cement mortar strength have been compared. It was found in the experiments that the replacement of cement by 5% (wt) of modified fly ash increase the compressive strength value from 61.3 MPa to 63.2 MPa compared to unmodified fly ash at age 28 days. The mineral phase formation, structure and chemical compositions of the modified fly ash/cement composites were analyzed by XRD, SEM and EDS. This research is of great importance and opens a new way for the future investigation and utilization of coal fly ash.

The aim of this study is to investigate in-depth the effect of Co 2+ ions doping on ZnFe 2 O 4 nanoparticles in term of morphology, magnetic and optical properties. Zn 1-x Co x Fe 2 O 4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) powders were prepared by co-precipitation method. X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) confirmed the incorporation of Co 2+ ions within ZnFe 2 O 4 host lattice, and an increase of the crystallite size from 37 to 51 nm in agreement with TEM analysis. FTIR revealed a slight change in two main bands υ 1 and υ 2 in the frequency range of 400–4000 cm −1 , which caused by stretching vibrations at the A- and B-sites respectively. The elastic moduli such as stiffness constant, Young's modulus, rigidity modulus, bulk modulus, Poisson's ratio, wave velocity and Debye temperature have been calculated using FTIR data. VSM showed transformation of magnetic behavior from paramagnetic to ferromagnetic as a result of Co 2+ ions doping. With higher Co 2+ (x = 0.2 to x = 0.5) content, the saturation magnetization (Ms) increased rapidly from 5.7 emu/g to 82 emu/g respectively. The remanance magnetization (Mr) and coercivity (Hc) were influenced by Co 2+ content too and this was mainly related to the change in crystallite size. The optical band gap ( Eg ) of Co-Zn ferrite samples was determined by means of diffuse reflectance spectra. The optical band gap value estimated from Tauc plots was found to decrease from 1.89 to 1.31 eV with increasing Co 2+ content.

The cation distribution of the spinel system Zn1-xCoxFe2O4 (with x = 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; 1.0) has been investigated by means of X-ray diffraction (XRD), Mossbauer spectroscopy and Fourier transform infrared spectroscopy (FTIR). In the present work, ferrites were prepared by co-precipitation method. XRD diffraction patterns show that all compositions have a pure single-phase cubic spinel structure over the whole composition range. It is found that both the lattice parameter and the average crystallite size decrease with increasing Co2+ content; from 8.4392 to 8.3536 A and from 37 to 25 nm for ZnFe2O4 and CoFe2O4, respectively. Morphological observations (SEM, TEM) reveal that the crystallinity decreases significantly with increasing Co2+ content meanwhile the particle size becomes more uniform. The elemental compositional stoichiometry have been carried out by means of energy dispersive analysis (EDS). The force constants Kt and Ko for the two sites have been deduced from IR band frequencies and compared with the trend of bond lengths: KT increases (from 2.084 to 2.144 dyn/cm2) because the bond length MA-O decreases, while KO decreases (from 1.043 to 0.948 dyn/cm2) because the bond length MB-O increases with increasing Co2+ content. For the first time antistructural modeling of zinc-cobalt ferrites considered. It shown that the acceptor in crystall latice is tetrahedral iron FeA , while the donor is octahedral cobalt Сo B ′ . With increasing Co2+ content, the concentration of active centers in tetrahedral and octahedral sublattice of zinc-cobalt ferrites are increases. The nature of the active centers are affecting on the chemical, electrical, magnetic, optical and catalytic properties of ferrites.

Nanostructured Zn-doped cobalt ferrites (Co1−xZnxFe2O4, where x ranges from 0.0 to 1.0 with a step of 0.1) were studied in order to elucidate the relation between their cationic distribution within the spinel sublattices and catalytic properties. The thermal transformation of the precursors (metal hydroxides) obtained through the hydroxide co-precipitation method was studied by DTA, TGA and FTIR. The thermal behavior of the precursors showed that cobalt ferrite was formed at a lower temperature (376 °C) in comparison to zinc ferrite (478 °C). FTIR analysis revealed vibrational bands around 400 cm−1 and 600 cm−1 related to (MO4)6− and (MO6)9− groups, respectively. The cationic distribution was determined from Mossbauer spectra analysis and the results showed that Zn ions occupy the A-sites, while Co and Fe ions are located in the A- and B-sites. The spectra indicate spinel magnetic ordering in samples with x = 0.0…0.5. The increase in Zn content influences the difference in the Pauling electronegativities, distances between magnetic ions (hopping length) and polaron radius of Co1−xZnxFe2O4 ferrites, inducing changes in the ionic bond strength for the A- and B-sites. Photoelectrochemical measurements demonstrated that the introduction of Zn2+ into the spinel structure modifies the ability of the materials to reduce O2. Whereas photoinduced reduction of dioxygen at CoFe2O4 competes successfully with anodic photocurrent generation (observed as a decrease of photocurrent), for ZnFe2O4 this process is negligible. The relation between the cationic distribution (analyzed in terms of antistructure modeling) and the catalytic activity of spinel ferrites was demonstrated for the example of the soot combustion process. It was shown that the octahedral cobalt centers exhibit a higher catalytic activity than the tetrahedral ones.

The effect of Zn-doping in CoFe2O4 nanoparticles (NPs) through chemical co-precipitation route was investigated in term of structural, optical, and magnetic properties. Both XRD and FTIR analyses confirm the formation of cubic spinel phase, where the crystallite size changes with Zn content from 46 to 77 nm. The Scherrer method, Williamson-Hall (W-H) analysis, and size-strain plot method (SSPM) were used to study of crystallite sizes. The TEM results were in good agreement with the results of the SSP method. SEM observations reveal agglomeration of fine spherical-like particles. The optical band gap energy determined from diffuse reflectance spectroscopy (DRS) varies increases from 1.17 to 1.3 eV. Magnetization field loops reveal a ferromagnetic behavior with lower hysteresis loop for higher Zn content. The magnetic properties are remarkably influenced with Zn doping; saturation magnetization (Ms) increases then decreases while both coercivity (HC) and remanent magnetization (Mr) decrease continuously, which was associated with preferential site occupancy and the change in particle size.