Formation Of Zeolite Research Articles

Statistical optimization of process conditions for pyrvinium pamoate elimination using UV/zeolite‐based nanostructures/H 2 O 2

Statistical optimization of process conditions for pyrvinium pamoate elimination using UV/zeolite‐based nanostructures/H ₂ O ₂

Waste-derived glass as a precursor for inorganic polymers: From foams to photocatalytic destructors for dye removal

Synthetic alumino-silicate glasses may yield inorganic polymers, through activation with alkali hydroxide solutions. In this framework, we formulated a glass prepared by the melting of red mud from bauxite refinement, combined with coal combustion fly ash, discarded pharmaceutical glass and a minor addition of sodium carbonate. The activation with 6 M NaOH aqueous solution allowed for the manufacturing of highly porous foams, by gas generation at the early stages of gelation. These foams featured an extensive formation of zeolite at cell walls which, combined with the presence of magnetite formed upon cooling of the melt, favoured the application of the foams as sorbents for dye removal from contaminated water. The powders prepared by crushing the highly porous foams showed an excellent water purification ability documented by efficient removal of methylene blue used as a model contaminant. The specific iron oxide polymorph facilitated both magnetic recovery of dispersed powders and photocatalytic destruction of the dye under UV irradiation.

Firing and post-firing dynamics of Mg- and Ca-rich bricks used in the built heritage of the city of Padua (northeastern Italy)

Abstract. Diverse types of bricks from monuments in the city of Padua (northeastern Italy) were studied using a multi-analytical approach based on spectrophotometry, X-ray fluorescence (XRF), X-ray powder diffraction (XRPD), polarized-light optical microscopy (POM) and/or high-resolution scanning electron microscopy with coupled energy-dispersive X-ray spectroscopy (HRSEM-EDS). The most representative bricks were yellow or beige and in well-preserved condition. The results showed that they were made of Mg- and Ca-rich illitic clays, were fired at high temperatures (from 900 to over 950 ∘C), and achieved an incipient vitrification. Two main processes took place during firing: (i) the development of a Ca-aluminosilicate amorphous phase where very abundant pyroxene-type crystals were nucleated and (ii) the transformation of the pristine Mg-rich clayey grains into Mg-silicate mineral phases. The analyses suggest a firing dynamic within a highly reactive and supersaturated unstable system, particularly rich in calcium and magnesium. There are also signs of the rapid heating and/or soaking of the bricks and the irregular heat distribution and/or different residence times inside the kilns. The formation of zeolite and calcite secondary phases was also observed. The former was largely promoted by the high calcium content of the bodies and the very humid conditions, while the latter was mainly precipitated from Ca-rich solutions. The preservation of the bricks was enhanced by processes that took place both during and after firing. Firstly, the significant development of a Ca-rich amorphous phase and of high-temperature pyroxene-type crystals has provided strength to the bricks. Secondly, the porosity yielded by the firing of the carbonate-rich clays was almost filled by secondary calcite, which acted as a cementing agent. The information attained has increased the knowledge of (i) the mineralogical and microstructural changes that take place during the firing over 900 ∘C of Ca- and Mg-rich illitic clays and (ii) the formation of secondary phases within highly calcareous bricks laid in very humid environments and affected by Ca-rich solutions. The key role of the Ca- and Mg-rich raw clays and of the high firing temperatures, in producing high-quality bricks, and of the secondary calcite, which increased their durability, is highlighted. All these factors have contributed to the better preservation of the built heritage of the city.

Nitrogen- and Halogen-Free Multifunctional Polymer-Directed Fabrication of Aluminum-Rich Hierarchical MFI Zeolites.

Aluminum-rich hierarchical MFI-type zeolites with high acidic-site density exhibit excellent activity and selectivity in bulky molecule-involved reactions. However, it is challenging to develop a facile and environmentally benign method for fabricating them. Herein, we employ a polymer that does not contain nitrogen and halogen elements to successfully synthesize aluminum-rich hierarchical ZSM-5 zeolite with a Si/Al ratio of 8 and a significant number of mesopores comprised of oriented-assembled nanocrystals. It is demonstrated that the nitrogen- and halogen-free polymer is instrumental in the formation of the ZSM-5 zeolite by serving as a template for constructing the hierarchical micro/mesoporous structure. Moreover, this polymer also acts as a crystal growth modifier to form a single-crystalline zeolite. Notably, the resultant zeolite shows a better catalytic performance in converting waste plastic into hydrocarbons than a commercial one. Our work enables the synthesis of high-quality hierarchical zeolites without requiring quaternary ammonium templates.

Impact of residual sodium cations in azonia-spiro templates on the formation of large and extra-large pore zeolites

Azonia-spiro compounds have been reported as organic templates for synthesizing many extra-large pore high silica zeolites, often isomorphically substituted by germanium. However, these templates are usually obtained in the presence of sodium hydroxide, leading to persisting sodium ions in the final products. Sodium cations may compete with azonia-spiro organics as templates during zeolite synthesis. Therefore, effect of any residual sodium cations on the synthesis results needs to be investigated. This work documents the effect of sodium ions on zeolite formation using four structurally similar azonia-spiro compounds as templates. Different silicogermanate zeolites (UTL, an unknown zeolitic phase, etc.) were formed in the absence of sodium cations. Notably, the syntheses of silicogermanate UTL and siliceous MTW zeolites using azonia-spiro[4,6] and [6,6] are reported for the first time. Controlled amounts of sodium chloride were added to zeolite crystallization mixtures without residual sodium hydroxide. In the presence of 0.005–0.1 mol/L Na+, silicogermanate zeolites were the dominant phases. Further increasing the Na+ concentration to 0.152 and 0.317 mol/L, formation of UTL and an unknown silicogermanate phase were inhibited in favor of a dense sodiumgermanate (Na4Ge9O20). Moreover, sodium ions next to azonia-spiro organic templates also influence the morphologies of the MTW zeolites. The observed high sensitivity of zeolite formation in the presence of even traces of sodium therefore has impact on synthesis protocols of large pore silicogermanate zeolites. This work highlights how small, often overlooked, factors in inorganic material synthesis can drastically change the outcome.

Zeolite and associated mineral occurrences in high-sulphur coals from the middle Miocene upper coal seam from underground mines in the Çayirhan coalfield, (Beypazarı, Central Turkey)

Three sampling profiles (BA, BB, and BC) from, respectively, the A, B, and C sectors of the Çayırhan coalfield representing the working Tv and Tb splits of the Middle Miocene-age upper seam were investigated for mineralogical and elemental compositions. The Tv and Tb splits are mainly separated by a 70–100-cm thick tuffaceous clastic rock/marl parting in the study area, and the thickness of the Tv and Tb splits varies from 1.50 to 1.70 m, and 1.55 to 1.80 m, respectively. In both the Tv and Tb splits, Cr, Ge, As, and Zr are enriched, while Th and U are enriched only in the Tv split in all sampling profiles. This study indicates that the most significant difference between the Tv and Tb splits is in mineralogical composition. In contrast, zeolites (analcime and clinoptilolite) and carbonate minerals are detected as dominant to abundant phases in the Tv and Tb splits in the BA and BB profiles, while zeolite minerals are only identified as accessory phases in the BC profile. In addition, plagioclase grains are commonly identified in the BA and BB sampling profiles, whereas authigenic K-feldspars are more common in the sampling profile BC. The abundance of analcime and clinoptilolite, and their co-existence with syngenetic carbonate minerals in the BA and BB sampling profiles could imply that the alteration of volcanic inputs along with Ca-rich water influx into palaeomires seems to have resulted in favourable alkaline conditions and ion support for natural zeolite formation in the palaeomires. In contrast, the accessory presence of zeolite minerals and the common occurrence of authigenic K-feldspar grains in the sampling profile BC could imply that the semi-open hydrogeological system seems to allow transferring necessary ions for natural zeolite formation to other parts of palaeomires. Overall, the variations in mineralogical and elemental composition seem to be controlled by a combination of water chemistry and hydrogeological conditions within palaeomires and the geochemical properties of volcanic inputs into palaeomires.

SINTESIS ZEOLIT A BERBAHAN DASAR ABU TERBANG (FLY ASH) LIMBAH PT. INDONESIA CHEMICAL ALUMINA (ICA) MENGGUNAKAN METODE ALKALI HIDROTERMAL (SYNTHESIS OF ZEOLITE A BASE ON FLY ASH WASTE PT. INDONESIA CHEMICAL ALUMINA (ICA) USING HYDROTHERMAL ALKALINE METHOD)

Fly ash is one of the combustion residues from coal-fired power plants which is produced in large quantities. The main elemental composition of fly ash is silica and alumina with content of 48.055% and 14.653%, respectively. This study aims to determine the characteristics of zeolite A synthesized from fly ash with various ratios of SiO2 and Al2O3. Zeolite A was synthesized using the alkaline hydrothermal method by varying the Si/Al molar ratio of 0.8 1 and 1.2. The initial process begins with the leaching process of fly ash waste and is followed by alkaline smelting. Furthermore, zeolite A was synthesized by adding sodium silicate and sodium aluminate to form a white gel. These steps were followed by a hydrothermal process using a hydrothermal autoclave at a temperature of 100֯C for 5 hours. Qualitative results on XRD showed that zeolite A synthesized at a ratio of Si/Al 1,2 had a 2ϴ position closer to standard zeolite A, which was seen based on the highest peaks at 2ϴ=12,420, 16,070, 20,360, 21,590, 23,960, 26,070, 27,080, 29,910, 32,500, 41,480, 44,090, 52,540, 54,230 and 66,660 and the structure formed is in cubic form with a crystallinity of 76.36%. Analysis using XRF showed that the Si/Al 1.2 ratio was the best for the formation of zeolite A. In this variation the silica and alumina content increased by 54.412% and 35.054%, respectively

СИНТЕЗ ЦЕОЛІТУ З ПРИРОДНОЇ ГЛИНИ І ЗОЛИ РИСОВОГО ЛУШПИННЯ В ЯКОСТІ ПЕРСПЕКТИВНОГО ФОТОКАТАЛІЗАТОРА ДЛЯ ЗМЕНШЕННЯ ЗАБОРОНЕНОЇ ЗОНИ ДІОКСИДУ ТИТАНУ

In this study, we have utilized clay from Pariaman, Indonesia enriched with silica from rice husk ash to produce zeolite through a hydrothermal process. The resulting zeolite is then composited with the most common semiconductor photocatalyst, an anatase titanium oxide, to increase the semiconductor efficiency in terms of particle distribution and light sources activation. From X-Ray Fluorescence (XRF) measurement, it can be seen that the Si/Al mole ratio in the clay has been successfully increased from 1.8 to 2.0. These data are strengthened by the results of X-Ray Diffraction (XRD) analysis which shows the formation of zeolites of several types consisting of zeolite faujasite, P1, sodalite, X, and nu-6. When the synthesized zeolite is mixed with titania anatase, a composite is formed as evidenced by FTIR analysis with the appearance of Si-O-Si and Si-O-Al absorption bands for zeolite and Ti-O-Ti from titania. This zeolite has been shown to reduce the bandgap energy of titanium oxide after the two materials have been composited. Measurements with Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis DRS) showed that the TiO2-anatase band gap decreased by about 20 %, from 3.20 to 2.56 eV allowing theoretically the composite to be considered as a promising photocatalyst.

Fabrication of highly mesoporous ZSM-48 zeolite by anionic surfactant-organosilane system for catalytic conversion of methanol to gasoline

A highly mesoporous ZSM-48 zeolite has been prepared using low-cost anionic surfactant sodium dodecylsulphate (SDS) as mesopore-generating agent and short-chain 3-aminopropyltrimethoxysilane (APTMS) as co-structure directing agent. The mesoporosity formation took place through the interaction between the silica species/APTMS and SDS molecules existing in the gel composition. In the synthesis solution with a pH slightly less than ten, the methoxysilyl moiety in APTMS was hydrolysed to the –SiO3 units and became incorporated inside the zeolite crystals while the positively charged ammonium site of APTMS interacted with the negatively charged head group of the SDS. Aggregation of aluminosilicate/APTMS species around the SDS micelles during the crystallization process, led to the formation of a mesoporous zeolite. In this study, a conventional ZSM-48 and seven ZSM-48 zeolites from different mixtures containing SDS, APTMS and SDS-APTMS with different APTMS/SiO2 molar ratios were synthesized. Changes in the properties of the synthesized samples were investigated by XRD, FTIR, SEM, TEM, nitrogen adsorption/desorption isotherms, NH3-TPD and TGA. The optimum ZSM-48/Sur/0.008OS zeolite prepared with APTMS/SiO2 molar ratio of 0.008 exhibited both bundled needle-like and spherical morphologies. The catalytic conversion of methanol to gasoline (MTG) was evaluated in a fixed-bed reactor, and the results revealed that mesoporosity development using Sur-OS system enhanced catalytic activity of ZSM-48/Sur/0.008OS catalyst compared to the other sample. The optimum zeolite showed long stable phase of 40 h and liquid hydrocarbon yield of 42% was achieved for 100% methanol conversion in 22 h of reaction. The proposed method opens a new and simple route for the synthesis of hierarchically porous ZSM-48 with tunable porosity using low-cost anionic surfactants.

Effect of zeolite type, temperature, and pH on Stage III glass alteration behavior for two nuclear waste glasses

Long-term laboratory-scale static alteration tests (up to 1500 d) were conducted on two low-activity waste glasses (LAWA44 and IDF18-A161) at S/V = 2000 m–1. Tests were seeded with either analcime, clinoptilolite, zeolite P1, or zeolite P2 to induce Stage III glass alteration behavior. The effect of several parameters on glass Stage III behavior was studied with consideration of several variables, including temperature (22 °C–90 °C) and pH (floating or initially set using KOH at values from 9.5 to 11.5). Temporal solution data and solids characterization at the end of the experiments are reported. Solution data demonstrated the occurrence of a sustained rate acceleration at temperatures down to 40 °C and the activation energy for these Stage III rates was determined for both glasses. Three major secondary phases were identified at the end of the experiments for both glasses: zeolite P1, zeolite P2, and/or analcime (along with minor phases: zeolites, clays, carbonates). The identity of the crystalline phases at the end of the experiment often differed from the zeolite seed phase. In addition, the use of the Avrami equation showed unimpeded zeolite growth (3-dimensional) in certain conditions, suggesting that in certain conditions the glass dissolution rate is controlled by zeolite formation kinetics whereas in other conditions the glass corrosion process impeded zeolite formation kinetics. The morphological evolution of the samples revealed the growth of the zeolites which completely fill the intergranular space in the powder bed. These results are discussed with regards to previous seeded and unseeded Stage III dissolution rate experiments performed on low- and high-activity nuclear waste glasses.

Frontiers in zeolites: towards establishing a new discipline of condensed matter chemistry-an interview with Ruren Xu.

In recent decades, the application of zeolite has been extended to many sustainable processes. Professor Ruren Xu (徐如人) of Jilin University is a leader within Chinese, Asian and worldwide zeolite communities, as well as the founder of the inorganic synthesis discipline in China and the first person in the world to propose the scientific discipline of modern inorganic synthetic chemistry. Professor Xu started his scholarly research on zeolites in the mid-1970s. He focused initially on crystallization and mechanisms of zeolite formation. In the 1980s, he gradually shifted his research to the exploration of microporous materials with novel frameworks and compositions. In 1984, he outlined new directions in the synthesis of zeolites and placed emphasis on the ‘heteroatom concept’, which turned out to be very influential and fruitful for the subsequent development of heteroatom-containing zeolite catalysts. In the following years, he and his group systematically developed new solvothermal routes for zeolite synthesis. In the late 1990s, Xu started to think about the rational synthesis of zeolites, a major challenge for zeolite as well as inorganic synthesis in general. His group developed several effective strategies for the rational design and synthesis of zeolitic materials. He is the chairman of the 15th International Zeolite Conference (15th IZC) held in 2007 for the first time in China. Because of his significant contribution to zeolite science in China, he received the National Zeolite Lifetime Achievement Award of China in 2017. NSR recently interviewed Professor Xu about the current status and future prospects of zeolites and related porous materials. This interview is dedicated to Professor Xu on his 90th birthday, in recognition of his seminal contribution to zeolite science, modern inorganic synthetic chemistry and the new discipline of condensed matter chemistry, which was first suggested by Professor Xu in 2018.

Precipitated silica, alkali silicates and zeolites from construction and demolition waste materials

The construction and demolition industry generates a large volume of wastes, which are used in the same industry to elaborate new cementitious materials. However, the use of the mentioned wastes to prepare other type of silicate materials has been little reported. In the present work, silica, sodium silicate, potassium silicate, SAPO-34 zeolite and zeolite X, phillipsite, and sodalite were prepared using local sand waste, concrete, ceramic sanitary ware waste and building block. First, the wastes were characterized to know the elemental composition and mineralogical phases. Further alkaline fusion was carried out with NaOH and KOH, respectively. The products obtained in this step were neutralized with H2SO4 solution to form precipitated silica with high purity. For the zeolite experiments, SAPO-34 zeolite was synthesized using the precipitated silica, sodium aluminate, and H3PO4. Low Si/Al ratio zeolites were synthesized using the sodium silicate and like potassium silicate. The results of XRD technique show the formation of amorphous sodalite and phillipsite zeolites. Pure zeolite X was obtained only with the sodium silicate prepared from the ceramic ware waste; however, the use of an alternative aluminum source could be used with the other wastes to promote the formation of pure zeolites. With the non-conventional use of construction and demolition industry wastes, new technical opportunities were demonstrated herein to obtain functional materials promoting the circular economy and sustainable development.

Controlling the competitive growth of zeolite phases without using an organic structure-directing agent. Synthesis of Al-rich *BEA

The synthesis of zeolites without using an organic structure-directing agent (OSDA) offers great environmental and economic advantages; however, the zeolites being formed are not stabilized by the organic molecules and kinetic control of the synthesis ensuring high yield and purity of the phases is challenging for many zeolites. For OSDA-free synthesis of an Al-rich *BEA zeolite, this study shows that the zeolite is formed as a metastable phase that can undergo transformation immediately after completion of its crystallization to form a thermodynamically more stable MOR zeolite according to Ostwald's rule of stages. The OSDA-free zeolite synthesis is a dynamic process in which crystallization and dissolution occur simultaneously. As soon as a thermodynamically more stable MOR zeolite begins to form in the synthesis mixture, its formation causes depletion of the Al and Si from the synthesis mixture, and, because of the desaturation of the synthesis gel, the original *BEA zeolite begins to dissolve rapidly under hydrothermal conditions. The formation of targeted and parasitic zeolite structures is fundamentally affected by the Al and Si sources. Sources of Al and Si with suitable solubility and the presence of stable zeolite *BEA seeds allow kinetic control to direct the exclusive formation of *BEA zeolite up to a relatively high yields. Then, however, the MOR zeolite phase appears in the product and its formation becomes predominant. This study demonstrates the crucial importance of kinetic control for the selective formation of metastable zeolites in the OSDA-free synthesis.

Synthesis of beta nanozeolite aggregates with hierarchical pores via steam-assisted conversion of dry gel and their catalytic properties for Friedel-Crafts acylation

Modulation of the porosity and acidity of zeolites is a very attractive subject for designing highly efficient solid-acid catalysts. In this work, Beta nanozeolite aggregates with hierarchical porous structure were synthesized via steam-assisted conversion (SAC) of dry gel in presence of seeds and a cationic polymer of polydiallyldimethylammonium chloride (PDAD). The influence of various synthesis parameters, such as the alkalinity, the contents of PDAD and zeolite seeds, and the crystallization conditions (temperature, water/dry gel ratio and time) on the formation of Beta zeolites was studied by combining a variety of characterization techniques. The SAC-synthesized Beta zeolites (S-Beta) are assembled from ultra-small nanocrystals around 10 nm to form self-sustaining micron-sized zeolitic aggregates, and possess abundant interconnected mesopores and relatively mild acidity. Compared with the conventional Beta zeolite, S-Beta aggregates show much higher catalytic activity for the Friedel-Crafts acylation of anisole with acetic anhydride. The excellent catalytic performance of S-Beta should be mainly attributed to the enhanced mass transport ability for reagents as well as the mild acidity that can avoid the rapid deactivation of the catalyst under the test conditions.

Exfoliation of lamellar SAPO-34 zeolite to nanosheets and synthesis of thin SAPO-34 membranes by a nanosheet-seeded secondary growth approach

High-aspect-ratio SAPO-34 nanosheets were fabricated by exfoliation of lamellar SAPO-34 zeolite using a solvent-mediated freeze-thaw method for the first time. A novel biomass-based organic structural directing agent (OSDA) together with triethylamine was used to induce the formation of lamellar SAPO-34 zeolite. These nanosheets had a thickness of several nanometers and aspect ratio as high as 230–560. A thin nanosheet layer with thickness less than 100 nm was deposited on the surface of tubular alumina support. High-quality SAPO-34 thin membranes were reproducibly synthesized after nanosheet-seeded secondary growth. Four SAPO-34 membranes prepared under the optimized synthesis conditions displayed average CO2 permeance of (2.5 ± 0.40) × 10-6 mol/(m2 s Pa) [equals to 7500 ± 1200 GPU, 1 GPU = 3.34×10-10 mol/(m2 s Pa)] with average CO2/CH4 selectivity of 102 ± 12.6 in an equimolar CO2/CH4 mixture at room temperature and 0.2 MPa feed pressure.

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

In the pursuit of controlling the propensity of Cu-mordenite (MOR) for the selective oxidation of CH4, we take a closer look at intrinsic zeolite parameters. Via synthesis design, we vary the relative proportion of Al situated near the 8-rings and 12-rings of MOR zeolite. This is accomplished using different Al sources impacting the local degree of silica dissolution and zeolite formation as evidenced by crystallization times and morphological differences. Interrogating the crystalline system with steric probe molecules in conjunction with spectroscopic techniques such as 1H magic angle spinning (MAS) NMR, infrared spectroscopy, as well as temperature-programmed desorption confirms discrete changes of the Al within the unit cell. The subsequent copper exchange allows for the generation of Cu-MOR materials of different inclinations for the activation of methane in the stepwise formation of MeOH. Here, an increasing degree of acid sites in more easily accessible locations (e.g., 12-ring) correlates with increasing maximum productivity toward MeOH at moderate exchange degrees. X-ray absorption spectroscopy supports this notion, finding a higher concentration of self-reduction-resistant framework-associated Cu2+ species, previously established as the active sites in the selective oxidation of CH4.

Recent strategies for synthesis of metallosilicate zeolites

Incorporation of catalytically active metallic sites into the framework of silica-based zeolites is highly efficient for design of active, selective, and stable heterogeneous catalysts, but there are still many challenges for this kind of preparation such as the presence of aggregated metallic sites due to the alkaline media in the synthesis, relatively low concentration of these metallic sites in the zeolite frameworks, and difficulty for morphological control of metallosilicate zeolites. To overcome these issues, it has been developed new strategies for synthesis of metallosilicate zeolites such as introduction of additives to adjust crystallization rate, addition of chelating agents to form heteroatom complexes, growth of zeolite crystals along with targeted directions, zeolite transformation, and heteroatom replacements. In this review, the recent progresses for syntheses of metallosilicate zeolites are concisely summarized. In addition, the fundamental principles are briefly discussed for the formation of the metallosilicate zeolites. This review should be helpful for design and preparation of the heteroatom-substituted silica-based zeolites as highly efficient heterogeneous catalysts.

Design of a dual electrochemical quartz crystal microbalance with dissipation monitoring

Abstract. The design and fabrication of a dual electrochemical quartz crystal microbalance sensor unit with dissipation monitoring (EQCMD) for in situ monitoring of crystallization processes, such as the formation of zeolites from liquid media, is reported. The integrated temperature unit is based on Peltier elements and precision temperature sensors with accurate and fast temperature control. In this design, two thickness-shear mode quartz disk resonators are oppositely arranged, enabling the application of an electric field through the sample while concurrently being able to monitor the resonance frequencies and quality factors of both resonators. As demonstrated experimentally, this allows for the characterization of the sample by means of the viscosity, via the acoustic impedance, and the electrical conductivity. Monitoring zeolite formation based on these parameters, however, turned out to be challenging, mainly because the electrodes suffered from severe corrosion. Despite the use of chemically resistant materials and insulating coatings, the electrodes were attacked by the reaction medium, presumably due to surface defects. Furthermore, air bubbles, which developed over time and adhered persistently to the quartz surfaces, also had a negative influence on the measurement. Despite the encountered issues, we want to communicate our sensor design, as its basic functionality, including the dedicated electronics and software perform well, and reporting the observed issues will enable further progress in this field.

Role of Aluminum and Lithium in Mitigating Alkali-Silica Reaction—A Review

Effective mitigation of alkali-silica reaction (ASR) is critical for producing durable concrete. The use of alumina-rich supplementary cementitious materials (SCMs) and chemical admixtures such as lithium salts to prevent expansion caused by ASR was first reported 70 years ago, shortly after the discovery of ASR in 1940s. Despite numerous investigations, the understanding of the mechanisms of Al and Li for mitigating ASR remain partially inexplicit in the case of Al, and hardly understood in the case of Li. This paper reviews the available information on the effect of Al and Li on ASR expansion, the influencing factors, possible mechanisms and limitations. The role of Al in mitigating ASR is likely related to the reduction of dissolution rate of reactive silica. Moreover, the presence of Al may alter the structure of crystalline ASR products to zeolite or its precursor, but such effect seems to be not that significant at ambient conditions due to the slow kinetics of zeolite formation. Several mechanisms for the lithium salts in mitigating ASR have been proposed, but most of them are not conclusive primarily due to the lack of knowledge about the formed reaction products. Combination of Al-rich SCMs and lithium salts may be used as an economic solution for ASR mitigation, although systematic studies are necessary prior to the applications.

Синтез цеолита типа LTA на основе природных минералов Нахчывана: влияние различных факторов на процесс кристаллизации

The process of crystallization of LTA type zeolite promising for practical use has been studied, the influence of various factors on the rate and selectivity of synthesis using natural minerals from Nakhchivan has been considered. The influence of temperature in the range of 80 – 150 °C, the weight ratio of the initial components of halloysite (G) and obsidian (O) G:O, equal to 1:1, 1:2, 1:3, 2:1 and 3:1, the concentration of the used thermal NaOH solution in the range of 5 – 30 % and the process duration in the range of 10 – 100 hours for the crystallization of LTA were studied. It has been determined that the optimal conditions for the synthesis of a single-phase zeolite of the LTA type with 100 % crystallinity are: temperature 100 ± 10 °C, concentration of thermal NaOH solution — 10 – 15 %, the ratio of the initial components of halloysite (G) and obsidian (O) G:O, equal to 1:1, processing time — 20 hours. Synthesis of zeolite LTA using natural minerals of Nakhchivan was carried out in the presence of tetramethylammonium hydroxide (TMAOH). The effect of temperature, concentration of thermal NaOH solution, ratio of initial components, duration of aging on the process of LTA crystallization has been studied. It was shown that violation of the optimal synthesis conditions leads to a decrease in the selectivity of the process and the formation of other zeolites (X), aluminosilicates (sodalite, anorthite, sanidine) or their associations, as well as quartz. The process of aging was studied in the duration interval from 1 to 3 days. It has been determined that an increase in the aging duration leads to an increase in the linear rate of crystallization of the LTA zeolite.