Influence of calcination temperature of kaolin on the structure and properties of final geopolymer

In this paper, the structure of two types of metakaolins from kaolin calcined at 800 and 900 °C, respectively, and the obtained geopolymer were systematically characterized. It was found that calcination temperature had little effect on the environment of silicon atoms but had great effect on that of aluminum ones. ²⁷Al NMR analysis showed that tetrahedral aluminums in the metakaolin from kaolin calcined at 800 and 900 °C were in different environment, of the type AlQ₃(3Si) and AlQ₄(4Si), respectively, leading to different environment of aluminum atoms in the resulted geopolymer. Aluminum atoms in the geopolymer based on metakaolin from kaolin calcined at 800 °C were in the types of tetrahedral and octahedral, and silicon atoms were in the types of tetrahedral Q₄(3Al) together with a small amount of Q₄(0Al). However, geopolymer based on metakaolin from kaolin calcined at 900 °C consisted of Q₄(4Si) unit aluminum and Q₄(3Al) unit silicon. The results revealed that the calcination temperature had a great effect on environment of the aluminum atoms of the metakaolin, thus led to the different structure and properties including mechanical strength and thermal conductivity of the post obtained geopolymer.     

One pot fusion route for the synthesis of zeolite 4A using kaolin

A new process route for obtaining zeolite 4A from kaolin was realized by subjecting all raw materials needed for hydrothermal synthesis to thermal activation in one pot method. For that purpose, different routes of activation steps for kaolin, which are metakaolinization, alkali fusion and one pot fusion were investigated for a wide range of kaolin sources in order to obtain zeolite 4A product with highest crystallinity. The influence of Si/Al ratio in addition to the chemical composition of raw kaolin, i.e. quartz and iron impurity content, on the product 4A was investigated following all three activation routes. It was found that alkali fusion process allows incorporation of quartz as silicon source during the hydrothermal synthesis of zeolite 4A, which may result in a variety of products such as zeolite P and zeolite 13X. In this study, a novel approach was developed by thermal activation of a mixture of kaolin, alkali and alumina sources at once in one pot route, which was followed by hydrothermal synthesis. The current study indicates the possibility of adjusting Si/Al ratio adding Al(OH)₃ during the activation step in one pot fusion process, which resulted in synthetic zeolite 4A of higher than 90% crystallinity using low quality kaolin.     

Mechanochemical synthesis of granulated LTA zeolite from metakaolin

LTA zeolite can be prepared from dry mixes in a vibratory mill with an impact-shear loading conditions. For the synthesis of LTA zeolite, it is necessary to use the anhydrous ingredients (Al₂Si₂O₇–metakaolin, γ-Al₂O₃). The process of synthesis is controlled by X-ray diffraction, IR-spectroscopy, and atomic-force microscopy. The presence of structural water in the initial ingredients (Al₂Si₂(OH)₄–kaolin or Al(OH)₃) leads to the formation of feldshpatoids (nepheline, sodalite). It was established that the process of mechanical activation requires the synthesis of sodium aluminate of cubic and/or tetragonal crystal systems with a lattice parameters close to those for LTA zeolite. These sodium aluminate acts as a steric template for an “assembly” of the zeolite. The presence of sodium aluminate with some other crystal structure results in the formation of sodalite. There is an optimal time of mechanochemical activation, which is determined by the synthesis of sodium aluminate with cubic or tetragonal crystal structure. A model of the mechanochemical synthesis of LTA zeolite was proposed.     

Structure characterization of hydration products generated by alkaline activation of granulated blast furnace slag

The hydration mechanism and mineral phase structures by waterglass activation of granulated blast furnace slag (GBFS) are investigated in detail by means of XRD and FTIR. The results show that the network structures of glassy phases are disintegrated and there is not any new material phase formed in the early stage of hydration processes. With evolution of hydration, the polycondensation reaction takes place between [SiO₄]⁴⁻ and [AlO₄]⁵⁻ species and some new mineral phases are produced. A hydration mechanism for the formation of geopolymer by waterglass activation of GBFS is proposed in detail.     

Formation of aluminosilicate geopolymers from 1:1 layer-lattice minerals pre-treated by various methods: a comparative study

Materials resembling aluminosilicate geopolymers have been prepared from the kaolinitic 1:1 layer lattice aluminosilicate clay mineral halloysite by reaction with sodium silicate solution under alkaline conditions. The effect on geopolymer formation of pretreating the clay mineral reactant by heating, high-energy grinding or exposure to acid or alkali was monitored by the ability of the samples to cure and harden, and by XRD, ²⁷Al and ²⁹Si solid-state MAS NMR spectroscopy. Only samples prepared from the fully thermally dehydroxylated clay showed the typical XRD and NMR geopolymer characteristics. Less complete reaction was found in samples pretreated by highly energetic grinding, whereas samples exposed to chemical pretreatment with dilute acid did not react to form viable geopolymers. Pretreatment with dilute alkali produced a zeolite which reacted with sodium silicate, but the hardened sample was not X-ray amorphous and showed subtle differences in its NMR spectra. These results are discussed in terms of the vital role played in the early stages of the reaction sequence by the presence of labile aluminium. The efficacy of the various pretreatment methods is related to their ability to render the aluminium source (the solid aluminosilicate clay) sufficiently soluble in alkali.     

Synthesis and characterization of 13X zeolite from low-grade natural kaolin

In this study, 13X zeolite was successfully synthesized from low-grade natural kaolin via alkali fusion followed by hydrothermal treatment, without extra Si source or dealumination. Fusion with NaOH, followed by hydrothermal reaction, kaolinite, illite and trace of quartz in kaolin sample were converted into zeolite. The effects of various factors during the synthesis process such as NaOH addition amount, crystallization time and temperature on the crystalline products were studied. The optimum synthesis conditions to get purity 13X zeolite were found to be alkali fusion of kaolin with the weight ration of NaOH/kaolin = 2.0 at 200 °C for 4 h, and crystallized at 90 °C for 8 h after homogenization by agitated at 50 °C for 2 h. The product was characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and N₂ adsorption–desorption. The BET surface area of the product was found to be 326 m² g⁻¹. It can be concluded that the study provides the basic data and the process for extensive and efficient utilization of low-grade natural kaolin.     

In situ preparation of self-bonded zeolite MCM-22 bodies by vapor-phase transport method

Self-bonded bodies of zeolite MCM-22 were prepared by vapor-phase transport method. The resultant materials were characterized by means of X-ray diffraction, scanning electron microscope, mercury porosimetry, and nitrogen porosimetry. Self-bonded MCM-22 bodies were in situ prepared at pH 10.0 with the molar composition of 0.05Na₂O:SiO₂:0.033Al₂O₃. It was found that the bodies, prepared by aluminosilicate gel, had been transformed into zeolite MCM-22. The MCM-22 bodies of which the mechanical resistance was 126 N/cm avoided binder accession. By adding auxiliary chemical–PEG20000 to the aluminosilicate gel, the pore size distributions of MCM-22 bodies could be adjusted. The average pore radius of MCM-22 bodies reached in the 149.41–653.64 nm range when AC/SiO₂ ratio was 1.5 × 10⁻⁴–9.0 × 10⁻⁴.     

Conventional hydrothermal synthesis of Na-A zeolite from cupola slag and aluminum sludge

Na-A type zeolites were prepared from two industrial wastes: the solid by-product of cupola slag and aluminum sludge from an aluminum plating plant. Two preparation methods using the same starting material compositions were carried out. In the first method, alkaline fusion was introduced, followed by the hydrothermal treatment to obtain sodium aluminosilicate which was then crystallized in NaOH solution under the condition of 90 ± 3 °C for 1–9 h with different H₂O/SiO₂ ratios. The result shows that higher H₂O/SiO₂ ratio increases the rate of crystallization. The largest amount of crystallinity for Na-A was found at 3 h. In the second method, alkaline hydrothermal treatment without fusion was carried out in the same condition as the first method. No Na-A zeolite was obtained by this method. The changes of the dissolved amounts of Si⁴⁺ and Al³⁺ in 3 M NaOH were investigated during the hydrothermal reaction.     

Hydrothermal synthesis of NaA zeolite membrane together with microwave heating and conventional heating

Uniform and dense NaA zeolite membrane was prepared by hydrothermal synthesis method together with microwave heating and conventional heating. The properties of the as-synthesized zeolite membrane were investigated by XRD, SEM and pervaporation evaluation for dehydration of 95 wt.% isopropanol/water mixture at 343 K, respectively. After microwave heating, the α-Al₂O₃ support surface was covered with homogeneous zeolite nuclei, which facilitated to form uniform, pure and dense NaA zeolite membrane in the following conventional heating process. High quality NaA zeolite membrane, i.e., with a separation factor (water/isopropanol) of 10,000 and a flux of 1.44 Kg/(m² h), could be hydrothermal synthesized together with microwave heating and conventional heating.     

Synthesis of pure NaA zeolites from coal fly ashes for ammonium removal from aqueous solutions

The pure NaA zeolites used as adsorbents for ammonium removal were successfully synthesized from coal fly ash by means of alkaline fusion followed by hydrothermal treatment. The experiment samples were characterized in terms of X-ray diffraction, X-ray fluorescence, Scanning electron microscopy, Fourier transform infrared spectroscopy, and Automated specific surface area and pore size analyzer. The results showed that the NaOH solution’s concentration, reaction temperature, and reaction time had significant effects on the species of the zeolite phases. Well-ordered cubic NaA zeolite with Brunauer–Emmett–Teller specific surface areas of 41.6 m²/g was prepared in 3 M NaOH solution at 353 K for 3 h. The adsorption behavior of NH₄ ⁺ from aqueous solution onto NaA zeolites was investigated at different initial concentrations, pH values, and contact times. The adsorbent equilibrium could be reached within 60 min, and the maximum removal efficiency of NH₄ ⁺ was achieved at pH 7.0. The Langmuir and Freundlich adsorption models were also applied to describe the equilibrium isotherms. The obtained results show that the experimental data have a better (R ² = 0.99) fit to Freundlich model and the adsorption of NH₄ ⁺ ions using synthesized NaA zeolite is favorable.     

Selective extraction of extraframework aluminum from USY zeolite

The extraction of extraframework aluminum from USY zeolite prepared by a hydrothermal treatment at 780°C was investigated. A series of extracted samples was prepared by the addition of concentrated hydrochloric acid to the suspension of the zeolite in a solution of NH₄NO₃. X-ray data and chemical analysis were used for the characterization of the samples. Results showed that the presence of competitive NH₄⁺ cations to the H⁺ ions in the solution increased the stability of framework aluminum toward acid. The removal of extraframework Al species was very selective at pH as low as 0.85. The discrepancy observed between the pH and the selectivity of the extraction indicated that the pH is not the decisive factor influencing the dealumination of the zeolite framework in this reaction system. It was found that the degree of framework dealumination depends on the equivalent fraction of protons S_H⁺ in the solution, and to obtain the high process selectivity, the extraction had to be carried out at as low S_H⁺ as possible. Results also showed that the supplementary dealumination of the hydrothermally treated zeolite framework during the extraframework aluminum extraction led to the decrease of the crystallinity of the USY zeolite sample.     

Preparation of granular X-type zeolite/activated carbon composite from elutrilithe by adding pitch and solid SiO2

The preparation of granular X-type zeolite/activated carbon composites from a locally available elutrilithe by adding pitch powder and solid SiO₂ was studied, and the variations in the synthesis process of zeolite X were investigated. The preparation steps of the composite involved (1) calcination of pre-shaped mixture (2) activation of the carbonaceous material from elutrilithe and pitch to prepare activated carbon and (3) hydrothermal conversion (zeolitisation) of aluminosilicate in elutrilithe and additional SiO₂ to zeolite X in alkaline medium. The adding of additional SiO₂ in the reaction system to adjust SiO₂/Al₂O₃ ratio of the reaction mixture was necessary for the formation of zeolite X. The characterization of XRD, SEM and N₂ adsorption of the resulting composites had a hierarchical pore structure, which shows that pure X-type zeolite phase with high crystallinity could be obtained regardless of the content of carbon in the composites.     

Potential of fusion technique in production of mesoporous zeolite A powder from poor kaolin through modification by boehmite: Effect of clay mineralogy on particle morphology

The aim of present study is to investigate the modification of poor kaolin by boehmite to produce zeolite A via fusion techniques. The processed kaolins containing considerable content of quartz, as a major impurity, were employed, and alkalinity, boehmite content, fusion temperature, and ageing time were changed to achieve zeolite A. Regardless of the starting material composition, the sodium hydroxide/kaolin ratio should be fixed in the range of 2.00–2.33 to delete quartz. Boehmite was added to material mixtures for compensating aluminum deficit followed by thermal treatment at 700, and 800 °C. The presence of boehmite in the precursors is very crucial for the formation of zeolite A. The maximum relative crystallinity was obtained by control of the boehmite/kaolin ratio at the level of 0.26. The zeolite A produced from clay containing illite + kaolinite + pyrophyllite indicated the semi-cubic particles with rounded edges, and lower surface area, 17.5 m².g^?1 while the application of kaolinite-rich clay resulted in the mesoporous product with an average pore size of 8 nm, and larger surface area of about 59.6 m².g^?1.     

Alkali activation of metakaolins: parameters affecting mechanical,structural and microstructural properties

The composition, structure and properties of the reaction product resulting from the alkali activation of metakaolin (MK) are directly impacted by the specific surface and composition of the initial kaolin and the type, concentration and relative amount of alkali activator used. This study aimed to analyze the effect of these parameters on the flexural strength, degree of reaction, porosity and chemical and mineralogical composition of alkali-activated metakaolin pastes. Two types of metakaolin with different specific surfaces were activated under hydrothermal conditions (85 °C, 2 h) using solutions consisting of waterglass and Na(OH) as activators (Na concentrations = 6, 8, 10, 12, 15, 18, 20 M) and two metakaolin/solution ratios. Regression analysis was used to quantify the effect of the parameters tested (activator concentration and MK/solution ratio) on flexural strength. Mathematical models were likewise built to relate the degree of reaction and amount of sodium fixed in the polymer structure to the synthesis parameters. According to ²⁹Si and ²⁷Al MAS NMR, XRD, FTIR, DTA/TG data and chemical analysis, the material obtained by activating two MKs with waterglass plus NaOH was an amorphous hydrated sodium aluminosilicate in which a Q⁴ Si (3Al) type three-dimensional structure predominated, i.e., a structure where three Al atoms are connected to SiO₄ tetrahedra. The alkaline ions in the structure maintain the electrical balance. The general formula obtained for this inorganic polymer was Na₂O·3SiO₂·Al₂O₃·3H₂O.     

Synthesis of zeolite Li-ABW from fly ash by fusion method

The zeolite Li-ABW was synthesized by fusion method using fly ash as raw material. It comprised alkaline fusion followed by hydrothermal treatment in LiOH·H₂O medium. Crystallinity of zeolite as high as 97.8% was attained under the following conditions: LiOH·H₂O concentration, 3 M; hydrothermal temperature, 180 °C; the corresponding aging time, 12 h. The content of Li-ABW increased at the expense of lithium aluminum silicate or quartz with an increase of LiOH·H₂O concentration. With increasing hydrothermal temperature and aging time, the soluble species re-crystallized and crystalline phase transformation between different zeolites was observed. Scanning electron microscopy (SEM) observation revealed that the obtained zeolite Li-ABW was a rod-like crystal. The pore size distribution curve indicated the presence of mesopores.     

The status of chromium in aluminum-free chromosilicate with the ZSM-5 structure

It is shown that the hydrothermal synthesis of aluminum-free chromosilicate results in the formation of crystalline zeolite with the ZSM-5 structure and some Cr³⁺ ions substitutes isomorphically for Si⁴⁺ ions in the lattice of as-made samples. After oxidative calcination most chromium ions aggregate in α-Cr₂O₃ microcrystals on the outer surface of the zeolite. However, some chromium remains stabilized strongly in the zeolitic structure as isolated low coordinated Cr(V) ions in two discrete coordinative states. It may be supposed that Cr(V) ions chemically bonded with zeolite lattice also have out of lattice ligands and are capable of interacting specifically with different molecules.     

Silicalite-1 zeolite membranes on unmodified and modified surfaces of ceramic supports: A comparative study

Silicalite-1 zeolite membranes were prepared hydrothermally on the porous ceramic supports, both unmodified and modified with 3-aminopropyl triethoxysilane (APTES) as a coupling agent following ex situ (secondary) crystal growth process. The microstructure of the membranes was examined by scanning electron microscopy (SEM). The permeation study with a single gas, nitrogen (N₂) was performed through the membranes. For the surface modified support, a more surface coverage of the seed crystals on the porous support was observed resulting in a relatively higher dense packing of the crystals during secondary crystal growth process compared to that obtained from the unmodified support. The membrane developed on surface modified support rendered lower permeance value i.e. 9 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ of N₂ compared to that formed on the unmodified support which gave permeance value of 20 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ of N₂.     

Characterisation of class F fly ash geopolymer pastes immersed in acid and alkaline solutions

Acid and alkaline resistance of class F fly ash based geopolymer pastes has been investigated. As prepared geopolymers showed high solubility in both strong alkali and acid solutions. Calcination of the fly ash based geopolymers at 600 °C resulted in a decrease of amorphous component from 63.4 to 61.6 wt.%. However, the solubility of the Al, Si and Fe ions in 14 M NaOH and 18% HCl after 5 days immersion decreased from 1.3 to 16-fold in comparison to as prepared geopolymer samples. Calcination of the geopolymers also resulted in a 30% reduction in compressive strength. Acid and alkali resistance of the geopolymers investigated strongly depends on mineralogical composition change of the calcined geopolymer. Partial crystallisation of non-reacted fly ash particles in the geopolymer decreases its solubility in acid and alkali solutions.     

Evolution of the pozzolanic activity of a thermally treated zeolite

Zeolites generally show pozzolanic activity due to their structural characteristics. The utilisation of pozzolans as additions to cements results in added technical advantages of the construction materials. In this study, the pozzolanic activity of a thermally treated natural mordenite-type zeolite from the Palmarito open-air deposit (Cuba) was evaluated with respect to a non-treated zeolite. Initially, a thermal treatment of the zeolite was performed at different temperatures within the range 300–1000 °C for 5 h in order to evaluate the better temperature of treatment. Afterwards, the pozzolanic activity was determined for each temperature after 7 days of reaction with a saturated Ca(OH)₂ solution that simulates the release of lime from ordinary Portland cement during the hydration reaction. The higher pozzolanic activity was achieved with the thermal treatment performed at 300 °C. Therefore, a further study studied the evolution of the mineralogical phases produced during pozzolanic reaction up to 90 days, carried out with the zeolite treated at that temperature. Consumption of Ca²⁺ in solution and formation of C–S–H-like phases with low Ca/Si ratio were experimentally observed as the main pozzolanic products. The thermodynamic study confirms high reactivity of the zeolite at short-term and chemical stability of the reaction products after 28 days. The zeolite thermally treated at 300 °C confirmed an increase in the pozzolanic activity with respect to the non-treated zeolite.     

The effect of zeolite on the properties and hydration of blended cements

In this paper the properties and the hydration of cements containing natural zeolite, coming from the Metaxades area, Thrace, Greece, are studied. The experimental part comprises three stages. In the first stage a complete mineralogical characterization of the zeolite was performed. In addition, the pozzolanic reactivity of the zeolite was evaluated on the basis of the Chapelle test. In the second stage, the mechanical and physical properties of blended cements, incorporating 0%, 10% and 20% per weight of fine zeolite were determined. Finally, the hydration rate and products were studied by means of X-ray diffraction and FTIR spectroscopy, in combination with thermoanalytical methods (TG/DTG and DTA). As it is concluded, the examined zeolite consists mainly of heulandite type-II and is a pozzolanic material that contributes to the strength development of zeolite-cement mixtures, the consumption of Ca(OH)₂ formed during the hydration of Portland cement and the formation of cement-like hydrated products. Finally, the addition of zeolite up to 20% w/w does not significantly affect the physical and mechanical properties of the blended cements.