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Alkali leaching is an effective desilication method for improving the alumina-silica mass ratio (A/S) of bauxite. The paper aims to study the kinetics of kaolinite dissolution and hydrosodalite precipitation during alkali leaching of kaolinite-rich diasporic bauxite. Alkali leaching of bauxite in NaOH solutions was studied at Na 2 O concentrations of 200–260 g/L, temperatures of 90–105 °C, and times of 30–150 min. The leached bauxites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques. The rate equations describing kaolinite dissolution and hydrosodalite precipitation were derived by fitting the Avrami model. The results show that kaolinite with finer particle sizes was preferentially dissolved during alkali leaching, providing a material basis for the precipitation of hydrosodalite. Hydrosodalite was heterogeneously nucleated on the dissolved edges of some kaolinite and grew in both one- and two-dimensions, presenting acicular and lamellar morphology. The dissolution rate of kaolinite and the crystallization mechanism of hydrosodalite were primarily influenced by Na 2 O concentration and temperature. Both kaolinite dissolution and hydrosodalite precipitation were controlled by chemical reactions with activation energies of 70.152 (± 1.429) kJ/mol and 289.089 (± 2.063) kJ/mol, respectively, and the orders of reaction with respect to Na 2 O of 0.733 (± 0.070) and 7.165 (± 0.047), respectively. The kinetic equations describing kaolinite dissolution, hydrosodalite precipitation and even the leaching of SiO 2 were eventually modeled with Na 2 O concentration, temperature and time as variables. • The preferential dissolution of kaolinite occurs on particles with finer sizes. • Hydrosodalite is heterogeneously nucleated on the dissolved edges of kaolinite. • Hydrosodalite presents both one- and two-dimensional growth. • The kinetic equation for the leaching of SiO 2 is derived using the Avrami model. • The Avrami exponent is linearly related to temperature and Na 2 O concentration. [ABSTRACT FROM AUTHOR]

Managing clay-containing slurries during drying process remains a persistent challenge in various industries. Despite challenges of drying clay-containing-slurries, limited information is available. The aim of this study is to explore the drying performance of slurries containing kaolin and bentonite and gain insights into the underlying drying mechanisms. The research presented integrates drying experiments, rheology measurements, settling experiments, zeta potential measurements, FTIR, TGA/DTA, and SEM analysis. Bentonite-containing slurries retained more moisture due to their high-water adsorption capacity, with higher bentonite percentages extending drying times. The addition of Ca2+ ions reduced moisture content by replacing Na+ ions with smaller Ca2+ ions, making the slurries less viscous. The addition of Ca2+ disrupted the gel-like structure of bentonite as confirmed by SEM and FTIR. In contrast, kaolin-containing slurries maintain lower moisture levels owing to the non-swelling structure of kaolinite. SEM showed the formation of agglomerates for kaolin when Ca2+ was added The addition of Ca2+ ions had a subtle impact on drying rates, despite a slight increase in slurry viscosity probably due to the agglomeration of kaolinite particles. Both slurries exhibited three drying phases: rapid drying due to high moisture, a moderate phase with reduced rates, and a final phase of slowed drying as tightly bound moisture was harder to remove. This paper demonstrated the significance of understanding the drying processes of clay-containing slurries to enhance the overall drying efficiency. • Detrimental effect of clays on drying of clay-containing ores was observed. • Three different drying regions were observed for kaolin and bentonite slurries. • The addition of Ca2+ disrupted the gel-like structure of bentonite. • SEM showed the formation of agglomerates for kaolin when Ca2+ was added. • Ca2+ ions significantly improved drying of bentonite slurries but not kaolin slurries. [ABSTRACT FROM AUTHOR]

Coal gangue (CG) contains abundant rare earth elements (REEs), which is a potential REE resource. REEs are mainly present in clay minerals as hydrates. The adsorption, desorption, and diffusion of REEs-hydrates on the surface of clay minerals are the key to regulating the leaching effect of REEs. The paper focuses on the typical clay minerals kaolinite and REEs-hydrates and combines quantum mechanics and molecular dynamics methods to systematically study the adsorption, desorption, and dynamic diffusion behavior of REEs-hydrates in acid solution. The research results indicate that stable REEs-hydrates La(H 2 O) 9 and Y(H 2 O) 7 are formed, with ion bonds between REEs and water molecules. La(H 2 O) 9 and Y(H 2 O) 7 mainly interact with the kaolinite (001) surface through hydrogen bonding. Competitive adsorption is formed between water molecules, hydrated hydrogen ions, and REEs-hydrates. Water molecules and hydrated hydrogen ions occupy the adsorption active sites of REEs-hydrates and form a dense water molecular layer on the surface. The distance between H 3 O+ and REEs-hydrates and the number of H 3 O+ in the solvent layer affect the desorption and diffusion behavior of hydrates. The bonding law, dynamic desorption, and diffusion mechanism of REEs-hydrates can provide meaningful references for selective leaching of REEs from CG. [Display omitted] • Adsorption, desorption, dynamic diffusion of REEs-hydrates are investigated by DFT and MD simulations. • REEs-hydrates are adsorbed in pores of the Al O six-membered rings. • Hydrogen bonds are formed between REEs-hydrates and kaolinite surface. • Competitive adsorption occurs between H 2 O, H 3 O+, and REEs-hydrates. • Number and distance of H 3 O+ both affect adsorption and desorption of hydrates. [ABSTRACT FROM AUTHOR]

The development of thermal insulation materials with superior comprehensive properties is crucial to building energy conservation. This paper presents a kaolinite-based porous geopolymer type composite thermal insulation material through alkali activation, room temperature foaming technology, and the rock wool fiber reinforcement. The effects of rock wool fiber contents and heat treatment on the comprehensive properties of the kaolinite-based porous geopolymer were investigated. The addition of rock wool fiber can enhance the mechanical properties of kaolinite-based porous geopolymer, while heat treatment can generate a hierarchical porous structure, resulting in a dramatical improvement in thermal insulation performance. With a rock wool fiber content of 15 wt%, the sample exhibits a porosity of ∼90%, density of 0.118 g/cm3, thermal conductivity of 0.057 W/(m·K), and a maximum compressive strength of 0.66 MPa, increased by ∼22% compared to samples without addition of rock wool. Additionally, the optimized thermal insulation material followed waterproofing treatment exhibits excellent hydrophobicity with a contact angle of up to 98.5°. Therefore, this novel thermal insulation material not only has a simple manufacturing process and eco-friendly properties, but also showcases properties including lightweight construction, high strength, and low thermal conductivity. Consequently, it holds immense potential in the area of building energy conservation. [Display omitted] • Rock wool fibers were firstly used to reinforce the strength of kaolinite based porous geopolymer. • Optimized material exhibits a low thermal conductivity of 0.057 W/(m·K) • Compressive strength improved by 22% with addition of rock wool. • Simulations to evaluate the thermal insulation effects were compared. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of sands and clays subject to "normal" (plastic) compression are very similar, albeit at different stress levels, despite the differences in inter-particle interactions (Hertzian normal contact forces or similar for sand) and electro-chemical forces for clays. However the hysteretic behaviour of clay is very different to sand with clays exhibiting "springy" hysteresis loops and ratchetting volumetric compaction. This paper aims to show that whereas the "plastic" compression arises from degradation of macro particles, the "quasi-elastic" unload-reload behaviour is consistent with an increase in the number of platelet edge contacts as clay "stacks" rearrange and compact. [ABSTRACT FROM AUTHOR]

For over a century, zeolite has been synthesized and widely employed in fields such as environmental protection, petrochemical production and petroleum refining, owing to its unique structure and exceptional physical/chemical properties. In light of modern production and green chemistry, the trend is to develop green approaches for zeolite production. Natural clay, featuring abundant reserves, diverse properties and low cost, possesses the required elements and comparable structural units necessary for zeolite synthesis. As a result, natural clay holds distinct advantage as a raw material for green zeolite production. This paper introduces recent advances in the synthesis of zeolites using natural clays including kaolinite, bentonite, illite, attapulgite, and diatomite. The activations of clays and the synthesis methods of zeolites are discussed, and the future development trends for clay-based zeolite are anticipated. To advance the field, scientists must delve deeper into the synthesis mechanism and discover more energy-efficient, effective and environmentally sustainable solutions, which can be achieved through the continuous optimization of the clay activation and zeolite synthesis methods. • Natural clays are promising in eco-friendly zeolite production. • A comprehensive overview of common clays for zeolite synthesis. • Analyzing the impact of activation and synthesis methods on zeolite properties. • Identifying issues and potential future directions for clay-based zeolite synthesis. [ABSTRACT FROM AUTHOR]

This paper evaluates the potential of water treatment sludge ash (WTSA) as high-reactivity pozzolan using strength tests in mortars and techniques to evaluate the calcium hydroxide consumption in pastes with hydrated lime. The sample was dried in an oven and then calcined at 600, 700, and 800 °C in a muffle. A high-reactivity metakaolin sample was used as a pozzolan reference. The properties of the samples assessed were fineness (Blaine), density (Le Chatelier), and chemical and mineralogical composition using X-ray diffraction (XRD), X-ray fluorescence (XRF), and thermogravimetric analysis (TG). The assessment of the pozzolanic properties employed standardized tests such as compression tests with hydrated lime (NBR 5751) and Portland cement (NBR 5752) in mortars. Also, non-standardized techniques, such as XRD, TG, and electrical conductivity, were utilized to evaluate the consumption of calcium hydroxide in pastes and solutions with hydrated lime and pozzolan. The standardized tests showed that mortars with metakaolin (MK) presented higher compressive strength values, while mortars with ash calcined at 600 °C (SA600) obtained the lowest values among WTSA. This behavior occurred because the preparation of mortars was strongly influenced by their consistency, affecting the compressive strength. However, analyzing the consumption of calcium hydroxide in pozzolan-hydrated lime pastes, SA600 presented higher values. Unlike tests on mortars, consistency is not affected in preparing the pastes, allowing a better development of pozzolanic reactions. Based on Ca(OH) 2 consumption, it was concluded that the samples of WTSA studied could be used as high-reactivity pozzolans. • The kaolinite content in raw WTS measured by Rietveld refinement was 66.4% • Temperature calcination range from 600 to 800 °C is suitable to obtain high-reactivity pozzolan from water treatment sludge. • WTSA samples presented higher calcium hydroxide consumption than metakaolin. • Calcium aluminosilicate hydrates were produced from the reaction of WTSA and calcium hydroxide. [ABSTRACT FROM AUTHOR]

Per - and polyfluoroalkyl substances (PFAS) have been detected in ecosystems globally and pose a serious threat to human health. Clay minerals are promising adsorbents for PFAS removal from contaminated water, thus it is crucial to understand the interactions between PFAS and clay minerals. This paper performed molecular dynamics simulations to reveal the complex behaviors and fundamental mechanisms of the adsorption of perfluoroalkyl sulfonic acids (PFSA) on the siloxane and hydroxyl external basal surfaces of kaolinite. The results demonstrate that adsorption is favorable on both the surfaces; however, their mechanisms are distinct. On the hydroxyl surface, PFSA anions exhibit a steady "point adsorption" mode, with the diffusion coefficients considerably lower than those in bulk aqueous solution. The terminal sulfonate groups form hydrogen bonds with the surface hydrogen atoms like a "three-pin plug", with a matching molecular geometry. The molecular force and molecular electrostatic potential analyses suggest that the Coulomb electrostatic attraction force from kaolinite to the terminal sulfonate groups of PFAS anions is the predominant driving force of adsorption for the hydroxyl surface. The potential of mean force (PMF) calculations show that the PFSA anions have to go over an energy barrier, which is induced by hydration of the hydroxyl surface, to reach the stable adsorption position. On the siloxane surface, PFSA anions exhibit a mobile "surface adsorption" mode, with a relatively high freedom along the plane parallel to the surface. The diffusion coefficients of the PFSA anions adsorbed on the siloxane surface are slightly lower than those in bulk aqueous solution. The non-polar CF chains lie flat above the siloxane surface due to the hydrophobic interaction, while the terminal sulfonate groups stay relatively far away from the siloxane surface. A greater van der Waals repulsive force from the outer water molecules to the CF chains is the predominant driving force of adsorption, and pushes PFSA anions to the siloxane surface as PFSA anions approach kaolinite. The minimum values of the PMF of adsorbed PFSA anions on the siloxane surface are lower than those on the hydroxyl surface, indicating that adsorption stability is higher for the siloxane surface. [Display omitted] • PFSA exhibits a point adsorption mode on kaolinite hydroxyl surface via electrostatic attraction. • PFSA exhibits a surface adsorption mode on kaolinite siloxane surface via hydrophobic interaction. • Adsorption stability of PFSA on kaolinite siloxane surface is higher than on hydroxyl surface. • Diffusion coefficient of the adsorbed PFSA on kaolinite is lower than in bulk aqueous solution. • Van der Waals repulsive force from outer water pushes PFSA towards kaolinite siloxane surface. [ABSTRACT FROM AUTHOR]

The defected kaolinite is common in sedimentary kaolin, but its structure and disorder mechanism remains unclear. In this paper, three different size distributions of kaolinite particles: 0–300 nm, 200–1000 nm, and 500–2000 nm, were separated and classified from high-defect sedimentary kaolin, collected from Quaternary sedimentary kaolin in Guangxi Zhuang Autonomous Region of China. The three particle sizes of kaolinite have been monitored by particle size analyzer, high-resolution magic-angle spinning NMR (MAS NMR), small and wide-angle X-ray scattering (SAXS and WAXS), X-ray diffraction (XRD), BET surface analyzer, nitrogen gas-adsorption, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the smallest particle size of kaolinite exhibited high degrees of broken Al-O-Al and Si-O-Si bonds, more disordered structure, and elevated specific surface area. The degree of the structural disorder of kaolinite increases as the particle size decreases, which can be attributed to the more aluminum substituted for silicon in tetrahedron sheet in the finer kaolinite crystal, compared to the coarser kaolinite crystals. • Kaolinite particles were separated into three parts, <300 nm, 200–1000 nm, and 500–2000 nm. • The degree of the disorder increases as the kaolinite particle size decreases. • The smallest particle size of kaolinite showed high degrees of broken Al-O-Al and Si-O-Si bonds. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate the effects of co-calcining kaolinite and montmorillonite clay blends with alkali and alkali earth metal (AAEM) hydroxides as a route to enhance the reactivity of the clay blends. NaOH, KOH, and Mg(OH) 2 were added in different dosages (0–20%) to the clay blend and calcined together at 750 °C. When NaOH and KOH added at a dosage <10%, this co-calcination promoted amorphization, increased non-bridging oxygen per tetrahedron (NBO/T), increased dissolution of Si, and therefore improved reactivity. At a higher dosage, these additives decreased reactivities. These clay blends were utilized as a precursor in a seawater-cured lime-pozzolana binder system. The evolution of the microstructures was monitored using XRD, FTIR, 29Si solid-state NMR, 27Al solid-state NMR, ICP-OES, and SEM-EDS for up to 90 days. The maximum compressive strength came from the 5% Mg(OH) 2 batch which was 22.1 MPa at 90 days. The 5% dosages of all the AAEM hydroxides showed better performance than that of the control batch. The mineralogy showed the presence of hydrocalumite, ettringite, gehlenite, hydrogarnet, and calcium aluminum silicate hydrate (C-A-S-H) in seawater cured paste samples. The SEM-EDS revealed the presence of plate and needle like C-A-S-H in the seawater cured samples. • Co-calcination with NaOH & KOH at 5% dosage increased the amorphous content. • Co-calcination with NaOH & KOH reduced polymerization of the aluminosilicate. • Co-calcination with Mg(OH) 2 did not alter the aluminosilicate network. • Co-calcination with NaOH & KOH changed Al coordinate in aluminosilicate network. • The maximum compressive strength was from the 5% Mg(OH) 2 sample. [ABSTRACT FROM AUTHOR]

This paper reported a new method to enhance adsorption performance of La(III) and Y(III) on kaolinite by enlarging the interlayer space of kaolinite. The intercalation method could enlarge the interlayer space of kaolinite effectively in previous research literatures, whereas there were few studies on utilizing further reaction of intercalators to expand and exfoliate kaolinite by generating gas. The expanded kaolinite (EKH) involved in this study could enhance adsorption performance of La(III) and Y(III) significantly. The EKH in a state between extended and exfoliated was obtained undergoing two steps. Firstly, H 2 C 2 O 4 was inserted into the kaolinite layers through the secondary substitution liquid phase intercalation method. Secondly, the Kaol-H 2 C 2 O 4 (KH) intercalation complex was rapidly calcined with H 2 C 2 O 4 and Na 2 CO 3 , the power provided by the neutralization reaction promoted the thermal decomposition of H 2 C 2 O 4 between the kaolinite layers to produce a large amount of gas. XRD showed that the interlayer space of the KH intercalation complex expanded from 0.717 nm to 1.112 nm. The specific surface area and total pore volume of the EKH were 1.5 and 2.2 times higher than unmodified kaolinite respectively. The surface of EKH possessed higher zeta potential with more negative charges. The results of batch adsorption experiments showed that the adsorption capacity (101.5 mg/g and 78.9 mg/g) of EKH to La(III) and Y(III) was much higher than that of kaolinite (17.1 mg/g and 9.8 mg/g). Pseudo-second-order model and Langmuir model manifest the adsorption kinetics and isotherm. Therefore, the EKH prepared by the intercalation expansion method could be prospectively applied to the treatment of La(III) and Y(III) in industrial wastewater. [Display omitted] • Expanded kaolinite is obtained by intercalation-expansion method. • H 2 C 2 O 4 was inserted into the kaolinite layers and generated gas to expand kaolinite. • The adsorption performance of La(III) and Y(III) on EKH were up to 101.5 mg/g and 78.9 mg/g. • Pseudo-second-order kinetics and Langmuir model could manifest the adsorption kinetics and isotherm. [ABSTRACT FROM AUTHOR]

Granite residual soil (GRS) contains a large amount of kaolinite, which is highly hydrophilic and thus easily disintegrated by water. These characteristics may cause severe landslides and debris flows in some hillslopes composed of GRS. This paper describes an investigation of possible solutions to this problem: the addition of fiber and/or alkaline solutions to GRS to reinforce GRS. The effects of the silica-to‑sodium oxide (SiO 2 -to-Na 2 O) molar ratios of alkaline solutions on the static mechanical properties of reinforced GRS and the effects of fiber type on the impact resistance of reinforced GRS are discussed. The reinforcement mechanism, characterized by a combination of techniques, namely X-ray powder diffraction, Fourier-transform infrared spectroscopy, microcomputed tomography, and scanning electron microscopy, is also discussed. Static load test results indicated that the alkaline solution–reinforced GRS with a SiO 2 -to-Na 2 O molar ratio of 0.5 exhibited the highest compressive strength (4550 kPa), which is attributable to this reinforced GRS having the lowest porosity (2.3%). The alkaline solution reacted with GRS to form cementitious materials, thereby improving the compressive strength of GRS. Drop weight impact testing showed that fiber-reinforced GRS had dynamic mechanical properties superior to those of pristine GRS, and that in combination with a given alkaline solution, glass fibers reinforced GRS more effectively than basalt fibers. This is because the silicon atoms on the surface of glass fibers participated in the reaction with the alkaline solution, thus forming more cementitious materials. These materials were evenly distributed on the surface of the glass fibers, and thus particles of glass fiber–reinforced GRS were more closely bound to each other than particles of basalt fiber–reinforced GRS particles. These findings suggest that waste GRS, once appropriately reinforced, could be used for the construction of high-strength foundations and embankments. • Alkaline solution and fiber are used to reinforce granite residue soil (GRS). • Solution with a SiO 2 /Na 2 O ratio of 0.5 shows best effect on static mechanical property. • Glass fiber shows better effect on dynamic mechanical property than basalt fiber. • The consumption of kaolinite and formation of gels favor the improved properties. [ABSTRACT FROM AUTHOR]

The shuttling effect of polysulfide ions is an important reason that hinders the commercial application of lithium‑sulfur batteries. The natural clay mineral halloysite has good adsorption performance and hollow tubular structure, which can absorb and accommodate enough sulfur, while the flake kaolinite can roll to form a halloysite-like structure. The effect of acid-modified halloysite and kaolinite nanoscrolls as sulfur carriers on the performance of lithium‑sulfur batteries was studied in this paper. As a result, both acid-modified halloysite and kaolinite nanoscrolls could effectively adsorb polysulfide ions, while kaolinite nanoscrolls were more conducive to the migration and diffusion of Li+ due to its thinner wall (about 5 nm), thus exhibiting faster reaction kinetics and higher sulfur utilization, and had better rate performance and cycle stability. The application of clay minerals in the field of energy has always been unpromising. This research work makes full use of the characteristics of clay minerals and broadens its application, providing a basis for the design of clay minerals and a possible modification direction for their application in lithium‑sulfur batteries. Compared with the thin-walled halloysite nanotubes by acid treatment, the kaolinite nanoscrolls present high capacity and cycle stability, which is attributed to the thinner tubewall accelerating the migration of lithium ions. [Display omitted] • Kaolinite nanoscroll with larger inner lumen is suitable for loading sulfur. • Kaolinite nanoscroll sulfur cathode has good rate performance and cycling stability. • Thinner tube walls effectively enhance the Li+ diffusion coefficient in battery. • Hollow tubular structure and silicate component synergistically inhibit shuttle effect. [ABSTRACT FROM AUTHOR]