•Effect of the aggregate contents of quartz and K-feldspar on the compressive and bending strength is studied.•Determine the optimum modulus of alkaline activator at different proportions of quartz and K-feldspar.•Microstructure and microanalysis of the region between the aggregate and matrix are studied.Geopolymer appears to be an alternative to ordinary Portland cement as a new sustainable material. The effect of the aggregate contents of quartz and K-feldspar on the microstructure and mechanical properties of Geopolymer samples was investigated in the present study. It was found that the aggregate contents influence the compressive strength and bending strength of geopolymeric bricks. Also, The K-feldspar/quartz mass ratio (F/Q) has a linear relation with the modulus of alkaline activator at the peak value of bending strength. This is due to the fact that the quartz reacts with the sodium hydroxide and forms [H2SiO4]2− which is the main component of sodium silicate, and that the K-feldspar has the ability to provide additional alkaline. The results of electron-microprobe analysis indicated that the matrix phase is composed of more amount of [SiOAlOSi]n polymerization monomer with increasing alkaline activator modulus ratio when K+ is not involved in the reaction. However, the polymerization monomer of [SiOAl]n was only favored despite of variation of alkaline activator modulus when the K+ exists.Download high-res image (195KB)Download full-size image

•Ammonioleucite was synthesized using (NH4)2CO3 solution and analcime by hydrothermal method for the first time.•Nitrogen atoms occupy the sites by water molecule in analcime.•The exchange fraction of Na is up to 99.08% at 130 °C for 24 h.•Abundant N contents in ammonioleucite after exchange are beneficial to the application in agriculture.The NH4+NH4+/Na+ ions exchange process of analcime in (NH4)2CO3 solution using hydrothermal method and its kinetics were studied. X-ray diffraction (XRD), Fourier transformation infrared spectrometry (FTIR), and scanning electron microscopy (SEM) were applied to characterize the as-exchanged samples. Rietveld refinements were utilized to investigate crystallographic occupancy of NH4+NH4+ based on the XRD results and the crystal chemistry rules, showing that there exists isostructural crystal between analcime and ammonioleucite after ion-exchange although they belong to different crystal systems and nitrogen atoms surrounded by six oxygen atoms in ammonioleucite are located in the sites of water molecule in analcime. FTIR results indicated that disappearance of water molecule in the region of 3400–3500 cm−1 is caused by the escape of zeolite water from analcime (NaAlSi2O6·H2O) to ammonioleucite (NH4AlSi2O6) with the hydrothermal-treated temperature increased and time prolonged. The lower frequency of vibration at 3270 cm−1 as compared to buddingtonite indicates there exists relatively strong hydrogen bonding between ammonium ions and the framework environment. It turned out that the reaction rate was controlled by chemical reaction at lower temperatures (70 and 90 °C) and by product layer diffusion at higher temperatures (110 and 130 °C) from the experiments and analysis of kinetics according to shrinking core model.

The sintering preparation of K2MgSi3O8 at 700–950 °C for 2 h using biotite acid-leaching residues, K2CO3, and Mg(OH)2 as the starting materials was investigated in this research. The sintered samples were characterized by X-ray diffraction, X-ray fluorescence, Fourier transformation infrared spectrometry, and scanning electron microscopy. The results indicated that the hexagonal kalsilite-like K2MgSi3O8 with an irregular blocky morphology was prepared at 900 °C for 2 h. The solubility of K2MgSi3O8 in HCl and citric acid solution shows that the extraction of K2O is 38.94% in 0.50 M HCl and 23.58% in 0.10 M citric acid solution, respectively. For the optimal sample dispersed in distilled water, the accumulative release of K2O is 6.06% in the first day and 44.90% in 28 days, which reaches the Chinese national standard of slow-release fertilizer. All the results indicate that the as-prepared K2MgSi3O8 is appropriate for application as slow-release fertilizers supplying nutritional components K, Mg, and Si for crops.

The use of a small amount of calcium fluoride as an additive greatly accelerated the reduction of magnesia during the preparation of magnesium from magnesia using the vacuum carbothermic reduction method. At 1573 K (1300 °C), the magnesia reaction rates of the samples with 1, 3, and 5 pct CaF2 were all approximately 26 pct, three times that of free CaF2, and they were arranged in order of the calcium fluoride weight percentages at 1673 K (1400 °C). The residues were analyzed using chemical analysis, XRD, SEM, EDS, and XRF. The possible acceleration mechanism was discussed. Calcium fluoride combined with magnesia and silicon dioxide to form a eutectic that melted as a channel to aid the solid–solid reaction between carbon and magnesia at approximately 1573 K (1300 °C). Calcium fluoride in the molten state offered free calcium ions and fluorine ions. Fluorine ions entered and distorted the magnesia crystal lattice. The structural strength and chemical stability of the magnesia crystal lattice decreased, which facilitated the magnesia reduction by carbon. Calcium ions were employed to generate the calcium and magnesium silicate. The easyly evaporating fluorides, including magnesium fluoride and silicon tetrafluoride, were regarded as the main reason for the loss of fluorine.

•Analcime is synthesized for the first time using quartz syenite by alkali-hydrothermal method.•Quartz syenite is the starting material for synthesis of analcime and leaching of potassium.•The isometric trapezohedral analcime ranges in uniform particle size from 15 to 20 μm.•Conversion mechanism is attributed to dissolution–precipitation process.Analcime was successfully synthesized for the first time via a hydrothermal treatment of quartz syenite powder and NaOH solution. The experiments were carried out at the temperature from 240 °C to 260 °C for 2–8 h using different concentrations of NaOH solution and mass ratio of NaOH/syenite. The as-prepared samples were characterized by X-ray fluorescence, X-ray powder diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The results indicate that the crystalline analcime can be prepared using natural quartz syenite as an economic raw material.

The properties of aluminosilicate kalsilite have attracted the interest of researchers in chemical synthesis, ceramic industry, biofuels, etc. In this study, kalsilite was hydrothermally synthesized from microcline powder in a KOH solution. The microcline powder, rich in potassium, aluminum, and silicon, was collected from Mountain Changling in Northwestern China. The effects of temperature, time, and KOH concentration on the decomposition of microcline were investigated. The kalsilite and intermediate products were characterized by means of wet chemistry analysis, X-ray Diffraction (XRD), infrared spectrometry (IR), 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR), 27Al MAS NMR, and scanning electron microscope (SEM). With increasing temperature, the microcline powder transforms into a metastable KAlSiO4 polymorph before transforming further into pure kalsilite. A mixture of both kalsilite and metastable KAlSiO4 polymorph is obtained when the hydrothermal reaction is carried out within 2 h; but after 2 h, kalsilite is the predominant product. The concentration of KOH, which needs to be larger than 4.3 M, is an important parameter influencing the synthesis of kalsilite.

•Potassium merlinoite was synthesized using K-feldspar as raw material.•Microwave-assisted hydrothermal method was proved to be simple and economical.•The mixed heating strategy was appropriate for merlinoite formation.•K-feldspar powder was converted to merlionite-type eco-fertilizer by the simple way.Potassium merlinoite was successfully synthesized from K-feldspar powder by microwave-assisted as well as conventional hydrothermal methods. Effects of aging, H2O/Al2O3 molar ratio, crystallization temperature, crystallization time, and mixed heating strategy on potassium merlinoite formation were systemically studied. It was observed that aging was not a necessary factor to the formation of merlinoite, and merlinoite was the unique crystalline product at a wide range of H2O/Al2O3 molar ratio, crystallization temperature and time. The reaction rate of microwave-assisted hydrothermal synthesis was 2–3 times as much as conventional method. The acceleration effect of microwave was proved to work only at the initial period of irradiation, and a mixed heating method was considered as an appropriate strategy to prepare merlinoite. A simple and economical chemical process has been proposed to convert the natural K-feldspar powder into merlinoite-type eco-fertilizer in this paper.

•Efficient and clean utilization of potassic quartz syenite was introduced.•Hydrothermal extraction of potassium from potassic quartz syenite is 93.2%.•Acicular wollastonite was synthesized using the K2SiO3–Na2SiO3 solution.•Potassium carbonate with purity of 96% was prepared.•Al(OH)3 was prepared by low-lime sintering using the hydroxycancrinite.Potassic quartz syenite, composed of microcline and quartz, is a promising source for the recovery of potassium, alumina, and silica values. Hydrothermal extraction of potassium by dissolution of the potassic quartz syenite powder in NaOH solution and utilization of alumina and silica in the potassic quartz syenite powder for preparation of acicular wollastonite and aluminum hydroxide were explored in this research. It mainly contains alkali-hydrothermal process, causticization process, and low-lime sintering process. The samples were characterized by using wet chemical analysis, X-ray diffraction, and scanning electron microscopy. The results indicated that the optimal extractions of K2O and SiO2 from the potassic quartz syenite powder were 93.2% and 62.5%, respectively. The purities of as-prepared acicular wollastonite, potassium carbonate, and aluminum hydroxide were 93.8%, 96%, 99%, respectively. The approach presented in this work is a promising process of comprehensive utilization of potassic silicate minerals such as K-feldspar.Download full-size image