Calcined Kaolin Research Articles

Solar-driven calcination of clays for sustainable zeolite production: CO2 capture performance at ambient conditions

This study presents the environmentally sustainable synthesis of zeolites from solar-calcined kaolin and halloysite, emphasizing their application in CO2 capture due to their distinctive porous structures and chemical attributes. Expanding upon prior research that utilized solar energy for kaolin calcination, we now explore halloysite as an alternative clay mineral for zeolite production and CO2 capture. Employing a solar simulator, halloysite was calcined at temperatures ranging from 700 to 1000 °C, resulting in the synthesis of zeolites 4A and 13X via hydrothermal methods. The synthesized zeolites were characterized using X-ray diffraction (XRD), low angle XRD (LA-XRD), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FE-SEM), and Brunauer–Emmett–Teller (BET) surface area measurements. Notably, the presence of Al-Si spinel, which crystallizes at elevated solar calcination temperatures, persisted within the zeolite 13X matrix, inducing a secondary mesoporous phase. The observed hysteresis in 13X samples, rather than confirming the mesoporous character of zeolite 13X, indicates a tandem effect of mesoporous Al-Si spinel with microporous zeolite 13X, exemplifying systems known as micro/mesoporous zeolitic composites (MZCs). The correlation obtained between the interplanar distances calculated from LA-XRD and pore size distributions acquired from the BJH desorption branches highlights LA-XRD as an alternative analysis method for assessing mesoporosity. While the microporosity of Al-Si spinel possessing 13X samples positively correlates with CO2 capture performance, mesoporosity appears to have minimal impact. Among the zeolites synthesized using solar energy, zeolite 4A (LTA) demonstrates superior CO2 capture capability, achieving an adsorption capacity of 2.15 mmol/g at 25 °C and 1 bar. This study highlights the potential of solar energy in producing eco-friendly zeolites from kaolin and halloysite for improved CO2 capture, advancing sustainable environmental solutions.

The catalytic wet air oxidation of pharmaceutical wastewater with alkali-activated Mn and Cu composites: preparation of precursors by calcination of kaolin with Mn and Cu

In recent decades, the concentration of pharmaceutical residues and narcotics has increased in municipal wastewater. Decomposing these toxic organic chemicals is challenging and requires new techniques and advanced catalytic materials. Precursors of metal composites were prepared by calcining an aqueous suspension of natural clay–based kaolin with Mn and Cu, binding chemically the active metals to the aluminosilicate frame structure of the precursor. The specific surface area of Mn and Cu composite was 67 m2/g and 81 m2/g, respectively. The mechanical durability was determined in terms of compressive strength, and 3.3 MPa and 3.6 MPa were obtained, respectively. In the CWAO of pharmaceutical wastewater, Mn composite gave the highest conversions of 54% and 46% of the chemical oxygen demand (COD) and total organic carbon (TOC), respectively. Metal composites were mechanically and chemically highly durable, inducing only 1.2 wt.% and 1.4 wt.% mass loss. In CWAO, Mn and Cu composite increased the biodegradation of organic species in the wastewater by 65% and 75%, respectively.

Castable refractory materials from magnesium oxychloride cement-bonded cordierite-mullite

Cordierite-mullite ceramics are well-suited for use as kiln furniture due to their superior mechanical strength and thermal shock resistance. This study explores the development of cordierite-mullite refractory castables bonded with magnesium oxychloride (MOC) cement. These castables were created using calcined kaolin aggregates and a matrix comprised light-burned magnesia, calcined alumina, milled sand, and magnesium chloride hexahydrate. Interaction between light-burned magnesia and magnesium chloride hexahydrate produced a needle-like MOC cement phase, significantly enhancing the green strength of the castables. After firing at 1350 °C for 4 h, the MOC crystals decomposed, forming robust cordierite and mullite phases. The resulting sintered castables demonstrated a modulus of rupture of 14.55 MPa, a bulk density of 1.95 g/cm3, a porosity of 24.84 %, and a thermal expansion coefficient of 2.89x10−6°C−1. These findings indicate the effectiveness of MOC cement bonding for manufacturing high-strength cordierite-mullite refractory castables, making them ideal for kiln furniture applications.

Durable Self-Cleaning Radiative Cooling Coatings for Building Energy Efficiency.

Passive daytime radiative cooling (PDRC) is an energy-saving technology without an additional energy supply or environmental pollution. At present, most PDRC coatings for buildings are only aiming at high solar reflectivity (RS) and high mid-infrared emissivity (EMIR) while ignoring practicalities such as adhesion strength, scalability, and durability. In this work, modified calcined kaolin/(ethylene trifluorochloroethylene copolymer-polydimethylsiloxane) (MK/(FEVE-PDMS)) coating with super practicability is prepared by using MK as a filler, FEVE as an adhesive, and PDMS as a hydrophobic modifier. The RS and EMIR of the coating are 92.5 and 94.6%, respectively. The MK/(FEVE-PDMS) coating exhibits superhydrophobicity, with an advancing contact angle (ACA) of 160.2° and a hysteresis contact angle of 7.3°. At an average solar irradiance of 742.78 W m-2, the coating achieved a temperature drop of 13.12 °C (shielded with PE film) and 3.09 °C (without shielding), respectively, relative to the environment. The coating adheres firmly to the substrate with an adhesion strength of class 2. The superhydrophobicity of the coating provides excellent durability and ease of repair, which can resist UV aging and mechanical damage. The durable superhydrophobicity gives the coating long-term stability in PDRC performance. Additionally, the cheap raw materials and the preparation process, consistent with the production of existing paints, show excellent scalability. Moreover, the energy consumption simulation results show that the energy saving ratio of the coating is more than 10% in the densely populated Yangtze River Delta and Pearl River Delta. The durable self-cleaning radiative coating developed in this work has potential application prospects in areas where the demand for cooling in summer is large and the demand for heating in winter is small.

Enhancement of sorption of the azoic dye (Azucryl Red) by natural and calcined hyper-aluminous kaolins

Purpose. To remove of basic textile dye Azucryl Red (AR) from aqueous solutions using hyper-aluminous kaolins from Charente deposits (France) in natural and calcined states. Methodology. Batch interactive parameter pH, pHpzc contact time, dye concentration, adsorbent loading and temperature are taken to obtain optimums for the AR adsorption process in natural and calcined kaolin named respectively Kca, Kcm, Ckca, and Ckcm. Findings. The adsorption equilibrium was established in 7 min while the second-order kinetic model better described the adsorption kinetics for all kaolins with the chemisorption process. The adsorption isotherm of the results obtained corresponds better to the Langmuir model. The maximum quantity retained was 67.97 and 73.38 mg/g respectively for Kcm and Kca samples. Moreover, in the calcined state, the maximum quantity retained was 76.66 and 75.64 mg/g respectively for the calcined kaolin CKcm and CKca samples for a temperature of 298 K and pH = 6. The thermodynamic nature of the adsorption process was determined by calculating H, S and G° values. The positive value of H° confirms the fact that adsorption is endothermic spontaneous which is enhanced at higher temperatures. Originality. The heat treatments of the different kaolins at 400 °C enhance the adsorption process. Therefore, the results indicate that Ckcm performed with the highest adsorption capacity in the removal of the dye, followed by Ckca. Practical value. Adsorption processes of toxic Azucryl Red dye in the aqueous medium using natural and calcined hyper-aluminous kaolins at 400 °C were investigated. Optimization and modeling of the adsorption parameters with the theories by Langmuir, Freundlich and Elovich allowed us to find the optimal experimental conditions for sorption. Our results therefore indicate that the adsorption process of Azucryl red dye from aqueous solutions was enhanced when calcined kaolin at 400 °C containing organic matters was used.

Effect of kaolin calcined temperature on the preparation and crystallization mechanism of SAPO-34 molecular sieve for methanol-to-olefins performance

SAPO-34 molecular sieves synthesized using calcined kaolin as partial raw material under three-stage of high-low-high temperature hydrothermal condition are explored systematically. Especially, the effect of kaolin calcination temperature on the crystallization mechanism of SAPO-34 and the methanol to olefin performance are investigated thoroughly. It is found that the sample (SP-500) synthesized by using kaolin calcined at 500 °C (MK-500) demonstrates clean cubic structure, large specific surface area and mild acidity, providing the initially activated Si and Al source for the synthesis of SAPO-34 molecular sieve due to the well scattered structure of MK-500. From the crystallization mechanism consideration, it can be proposed that the SAPO-34 molecular sieves will be controlled by the low degree of Si aggregation in metakaolin and quantity of Si (3Al,1Si) species. As a result, SP-500 displays excellent catalytic lifetime during methanol to olefin reaction for its special Si coordination environment, showing the highest stable light olefin yield of 80.6 % and a methanol conversion above 90 % for more than 720 min. In contrast, SP-800 sample with the most strong acid sites is only 390 min, even if showcasing the highest selectivity of light olefins (84.5%).

Preparation of amphoteric polycarboxylate superplasticizer at low temperature and its application in cement-calcined kaolin blended system

As one of supplementary cementitious materials (SCMs), calcined kaolin (CK) has received increasing attention in the development of green and sustainable SCMs. When CK was applied into cement, the regular anionic polycarboxylate superplasticizer (PCE) is insufficient to disperse the cement-calcined kaolin blended system (CMT-CK system). Herein, a novel amphoteric polycarboxylate superplasticizer (VDA-1) was synthesized with hydroxybutyl vinyl polyoxyethylene ether (VPEG), acrylic acid (AA) and dimethyl diallyl ammonium chloride (DMDAAC) at low temperature (10 °C). Among a series of synthesized PCE samples, VDA-1 (nVPEG:nAA:nDMDAAC = 1:2.25:0.75) exhibits the best performance in the CMT-CK system. Results showed that the fluidity of the CMT-CK mixed slurry with VDA-1 increased by 127.9 % compared with pure CMT-CK slurry. Meanwhile, the mixed slurry with VDA-1 had good flow retention. The great fluidity of the mixed slurry with VDA-1 is attributed to that VDA-1 can be adsorbed on the surface of cement and calcined kaolin, which was demonstrated by total organic carbon (TOC) adsorption amount analysis. Besides, compared with pure CMT-CK system, the compressive and flexural strength of blended cement with VDA-1 was enhanced 2.26% and 10.60%, respectively. Split Hopkinson Pressure Bar (SHPB) test showed that the CMT-CK system containing VDA-1 exhibited large stress value before instability stage and the maximum absorption energy in the whole process.

Low-cost waterborne radiative cooling paint for large scale production and application in buildings

Passive daytime radiative cooling (PDRC) is a cooling technology with no energy consumption and no pollution generation. It emits self-generated heat to outer space using thermal radiation, thus reducing its temperature. Radiative cooling technology aims to the energy consumption problem of buildings and environmental pollution caused by the use of Freon in compressor air conditioners. Many kinds of radiative cooling coatings have been attempted, but the cost and large-scale practical application have been challenging. This work prepared a composite coating with a radiative cooling function using calcined kaolin (CK) powder and waterborne fluorocarbon (WF). The inexpensive CK has a wide particle size distribution and exhibits high solar reflectivity (RS) and infrared emissivity (EIR). The excellent adhesion and chemical stability of waterborne fluorocarbon (WF) provides a good guarantee for the practical use of the coating. As a result, the CK/WF coating achieves an RS of 91% and an EIR of 95% in the transparent atmospheric window (ATW) of 8–13 µm. The coating demonstrated a temperature decline of 3.8 °C under direct sunlight in the field. The coating meets the conditions for large-scale applications. When used as a roofing material for simulation, it can save up to 30% of energy consumption throughout the year. Overall, the CK/WF coating has outstanding energy saving and pollution reduction performance, which will further advance the radiant cooling material into real-world applications.

Characterisation and Performance of Nigerian Kaolin and Metakaolin in Geopolymer Synthesis

This study is aimed at characterising and understanding the mechanical and microstructural behaviour of natural and calcined kaolin clay for geopolymer applications. The clay samples obtained from Kaduna State, Northern area of Nigeria, were calcined within the temperature range of 700 and 900oC and are represented by MK7, MK8 and MK9. The raw kaolin and metakaolin were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM). To produce geopolymer binders, the precursors were reacted with a 1:1 combination of sodium hydroxide and sodium silicate solution and a precursor to activator ratio of 1.2 was suitable for preparation. The compressive strength of the samples was determined at three testing age of 7, 14 and 28 days. From the results obtained, the kaolin-based geopolymer generally had a long setting time and testing was impossible at the 7-day testing age. The compressive strength of other samples however, increased with increase in the calcination temperature. The values obtained at the maximum testing age of 28 days for all samples fall between 5.1 -14.9 MPa. The scanning electron microscope morphology of the produced binder, show the presence of air trapped in the paste which may have reduced the strength value. Vibrating samples before setting can improve the performance of the geopolymer binders by removing most of the trapped air. Geopolymers produced from this study based on strength obtained, can be used in less critical areas of the construction industry.

Solarization of the zeolite production: Calcination of kaolin as proof-of-concept

The current production of zeolites from natural sources requires high temperatures to achieve a phase change in the feedstock materials. Thermal processes such as calcination are energy-consuming and industry relies on fossil fuels to generate the necessary heat, which is neither sustainable nor environmental. This study presents for the first time the treatment of aluminosilicate clay kaolin in a solar reactor to produce metakaolins that are eligible as precursors for zeolite synthesis. Kaolin was loaded in a rotary kiln and a solar simulator was used for calcination at temperatures of 700–1000 °C for 2 h. After the solar calcination process, solar-produced metakaolins were used to synthesize zeolite 4 A and zeolite 13X by hydrothermal route. The kaolin, metakaolin, and product samples obtained were analyzed by XRD, XRF, FTIR, and FE-SEM. Results show that solar-calcined kaolin was as good and efficient as the conventionally calcined one in terms of supplying the metakaolin for zeolite synthesis. Often disregarded by researchers, high-temperature calcination of the kaolin was proved to be an alternative way for the synthesis of zeolite 13X without using an extra silica source, which was achieved for the first time. The overall results suggest that solar calcination is a good alternative to mass produce meta forms of the minerals to be used in the synthesis of value-added products such as zeolites.

Daytime radiative cooling performance and building energy consumption simulation of superhydrophobic calcined kaolin/poly(vinylidene fluoride-co-hexafluoropropylene) coatings

Daytime Radiative Cooling (DRC) is a non-consumptive and non-polluting cooling technology that transfers heat through thermal radiation. Radiative cooling technology can effectively solve the problems of non-renewable energy consumption (e.g., oil) and environmental pollution. This work uses calcined kaolin (CK) modified poly(vinylidene fluoride-co-hexafluoropropylene) (P) to prepare a self-cleaning coating with a radiative cooling function. The self-cleaning property of CK/P coating allows rainwater to easily remove dust adhering to the coating surface, without any influence on its radiative cooling effect. The coating exhibits excellent abrasion resistance and ultraviolet aging resistance to ensure long-term effectiveness of the coating. The extinction coefficient of P is close to zero, which can scatter most of the solar energy. CK has an ultra-high IR emissivity in the 8–13 µm atmospheric transparent window. Thus, the CK/P coating has a solar reflectance of nearly 91% in the wavelength range of 200 to 2500 μm and an IR emissivity of 96% in the transparent atmospheric window. The coating exhibits a cooling effect of −1.95 °C in direct outdoor sunlight. In addition, the coating reduces energy consumption throughout the year as a roof coating for simulated houses, demonstrating the material's feasibility in addressing energy consumption and environmental pollution.

Role of extreme vertex design approach on the mechanical and morphological behaviour of residual soil composite

This research work reports the usability of binary additive materials known as tile waste dust (TWD) and calcined kaolin (CK) in ameliorating the mechanical response of weak soil. The extreme vertex design (EVD) was adopted for the mixture experimental design and modelling of the mechanical properties of the soil-TWD-CK blend. In the course of this study, a total of fifteen (15) design mixture ingredients’ ratios for water, TWD, CK and soil were formulated. The key mechanical parameters considered in the study showed a considerable rate of improvement to the peak of 42%, 755 kN/m2 and 59% for California bearing ratio, unconfined compressive strength and resistance to loss in strength respectively. The development of EVD-model was achieved with the aid of the experimental derived results and fractions of component combinations through fits statistical evaluation, analysis of variance, diagnostic test, influence statistics and numerical optimization using desirability function to analyze the datasets. In a step further, the non-destructive test explored to assess the microstructural arrangement of the studied soil-additive materials displayed a substantial disparity compared to the corresponding original soil material and this is an indicator of soil improvement. From the geotechnical engineering perspective, this study elucidates the usability of waste residues as environmental friendly and sustainable materials in the field of soil re-engineering.

Deacidification of Acidic Oil Catalyzed by Sulfonated Mesoporous Silica for Biodiesel Production: Optimization, Kinetic, and Enhancement by Using Calcined Kaolin

Acidified oil is characterized as high acid value. Hence, deacidification is indispensable in biodiesel production from acidic oil. Heterogeneous catalysts have many advantages over homogeneous counterparts in deacidification of acidic oil. Therefore, in the present work, sulfonated mesoporous silica (SMS) was prepared and characterized, followed by the systematical investigation on the effect of reaction variables on the deacidification rate (DR). Furthermore, response surface methodology (RSM) was employed to optimize the reaction parameters. Results showed that the optimal studied reaction variables for the deacidification process were as follows: reaction temperature 65 °C, methanol to oleic acid ratio 35:1, catalyst loading of 2%, in which, deacidification rate of 97.78% could be achieved. DR of 62.52% could still be obtained after catalyst had been used for four consecutive cycles. Finally, significant enhancement of DR could be achieved by using calcined kaolin to assist the deacidification of acidified oil with high water content, indicating that there is great potential in deacidification of acidic oil catalyzed by SMS with the assistance of calcined kaolin for biodiesel production.

29Si and 27Al NMR study of the structural transformation of calcined kaolin residue-based geopolymer using low alkali activator content for sustainable construction materials

Geopolymer commercialization efforts in parallel with unconventional research are the only pathway to achieve the large-scale deployment of geopolymer technology. Geopolymers with a low quantity of alkali activator produced by the compression process are one way to bring this technology to industry because of the higher strength and lower efflorescence of the product. In this work, calcined kaolin residue-based geopolymers with a low alkali activator content were synthesized by various Si/Al mole ratios with soluble silicate from Na2SiO3 to adjust the Si ratio. The effects of Si/Al mole ratios on the structural transformation, microstructure, and compressive strength of geopolymers containing low alkali activator content were studied. The structural transformation and microstructure of geopolymers were characterized by NMR, SEM, and BET, and the compressive strength was assessed using a mechanical testing machine. The results present that geopolymer synthesized with various Si/Al mole ratios of precursor materials impacted the change in polymerization degree of silicate species (Qn(mAl)) and aluminate species. The Q4(4Al) and Q4(3Al) structures were transformed into the Q4(2Al) structure, and the content of the AlIV structure decreased when the Si/Al mole ratio of geopolymer was increased to 1.58. However, the contents of Q4(4Al) and AlIV structures increased again when the geopolymer had a Si/Al mole ratio of 1.62. SEM/EDX and BET results of geopolymers presented denser matrices, more amorphous aluminosilicate gels, and higher Si/Al ratios in aluminosilicate gels, while the pore size and pore volume were quite small when the Si/Al mole ratio of precursor materials was increased. In contrast, the highest Si/Al mole ratio of 1.62 showed different properties. The compressive strength of geopolymer was maximized at a Si/Al mole ratio of 1.58, and the decline in compressive strength of geopolymer with higher Si/Al mole ratio was related to the contents of Si–O–Si, Si–O–Al, and Al–O–Al bonds, content of aluminosilicate gels, and properties of the microstructure. The geopolymer synthesis using calcined kaolin residue and low alkali activator content is considered an eco–friendly process, which uses waste materials and less chemicals, and obtains high-strength products.

Adsorption of diazo dye direct red-28 and tetra-azo dye direct black-22 using calcined kaolin in aqueous solutions

ABSTRACT. In this study, the removal of diazo and tetra-azo dyes using calcined kaolin were compared. Direct red-28 (DR-28) and Direct black-22 (DB-22) was chosen as diazo and tetra-azo dyes, respectively. The kaolin obtained from Turkiye and calcined at 200 °C was found to be the most efficient adsorbent. Natural and calcined kaolins were characterized using SEM-EDS, FTIR, TGA, and XRD. The parameters such as, calcination temperature of the kaolin, adsorbent amount, temperature, initial pH, contact time, and initial concentration were studied. It was found that the removal rate of DR-28 was higher than that of DB-22 at the studied conditions. The removal of DR-28 and DB-22 were found to be 90.1% and 46.2% under the following conditions: adsorbent amount: 1.2 g/200 mL, initial concentration: 20 mg/L, original pH, temperature: 20 °C and contact time 120 min. The adsorption of the dyes was fitted to Langmuir isotherm model. The maximum adsorption capacity of calcined kaolin at 200 °C was determined as 5.39 and 3.5 mg/g for DR-28 and DB-22, respectively. The adsorption of DR-28 and DB-22 followed a pseudo-second order model. The thermodynamic study showed that the adsorption of DB-22 onto calcined kaolin was endothermic while that of DR-28 was exothermic. 
 
 KEY WORDS: Adsorption, Diazo dye, Tetra-azo dye, Kaolin
 
 Bull. Chem. Soc. Ethiop. 2023, 37(3), 593-610. 
 DOI: https://dx.doi.org/10.4314/bcse.v37i3.5

Synthesis of calcined kaolin-based geopolymer foam: Assessment of mechanical properties, thermal insulation, and elevated temperature stability

Calcined kaolin-based geopolymer foam was prepared, characterized, and evaluated in this paper. The different hydrogen peroxide foaming agent dosages were added to the geopolymer paste at 2.5%, 5%, 7.5%, and 10 wt%. The specimens were cured at temperatures of 40 °C, 60 °C, and 80 °C. The influence of hydrogen peroxide and different curing temperatures on the physical properties, mechanical characteristics, and pore distribution were studied. Correlations between these properties were drawn. The results indicated that geopolymer foam can be synthesized using calcined kaolin with acceptable compressive strength values (2.35–11.23 MPa). The geopolymer foam had a bulk density of (0.46–1.25 g/cm3), a porosity of 45.2%–87.4%, and thermal conductivity of (0.14–0.33 W/mK). The estimated cost of the geopolymer foam developed in this investigation was 156.12 ($/m3). Compared to common thermophysical materials, the developed geopolymer foam is considered a cost-effective option for insulating construction materials.

Influence of Mixing Order on the Synthesis of Geopolymer Concrete.

Geopolymers are high-performance, cost-effective materials made from industrial waste that ideally fit the needs of 3D printing technology used in construction. The novelty of the present work lies in the investigation of methods to mix geopolymer concrete from fly ash (FA) class F, ground granulated blast furnace slag (GGBS), and raw calcined kaolin clay (RCKC) to determine the mixing procedure which provides the best mechanical strength and structural integrity. The experimental results show that aluminosilicates with different reaction parameters when mixed one after another provide the optimal results while the geopolymer concrete possesses the highest compressive strength and the denser structure. The results demonstrated that the reactivity of GGBS, FA, and RCKC increased for different depolymerization speeds of the selected aluminosilicates. This research will provide results on how to improve the mixing order for geopolymer synthesis for 3D printing demands. The highest compressive strength and denser structure of geopolymer concrete is achieved when each type of aluminosilicate is mixed with an alkaline medium separately.

Fluoride adsorption enhancement of Calcined-Kaolin/Hydroxyapatite composite

The use of waste or natural resources is an interesting approach to preparing adsorbent materials. Most adsorption materials are powder-based, making them impractical for a variety of applications. In this work, the natural kaolin clay and hydroxyapatite synthesized from biogenic waste were studied as defluoridation materials. The point of zero charge (pH PZC ), the fluoride adsorption capability and the adsorption isotherm of calcined kaolin and mixed calcined kaolin/hydroxyapatite in both powdered and moulded forms were investigated. The hardness of the moulded (post-formed) samples was tested before and after in immersion in a fluoride solution. The maximum hardness was 15.8 kilo-pounds for the post-formed calcined kaolin sample. Sample hardness values slightly decreased after immersion in a fluoride solution due to the formation of micro-cracks. Most samples presented high pH PZC values, implying that these materials are suitable for the capture of fluoride anions. The adsorption properties varied with the ratio of calcined kaolin to hydroxyapatite. These properties for post-formed samples were different from those in powdered form. Post-formed samples showed higher fluoride adsorption. The maximum fluoride adsorption capacity and efficiency of the post-formed samples (calcined kaolin) at pH 3 were 1.74 F − mg/g and 87%, respectively. The sorption of fluoride of hydroxyapatite and mixed calcined kaolin/hydroxyapatite powders was found to have the form of the Langmuir isotherm, which indicates a monolayer adsorption on the adsorbent surface. Isotherms of calcined kaolin powder, post-formed calcined kaolin and mixed calcined kaolin/hydroxyapatite samples followed the Freundlich isotherm, which indicates multilayer adsorption on a heterogenous adsorbent surface.

Sintered glass-ceramic foams from fluorite tailings and waste glass with calcium phosphate addition

Lightweight glass-ceramic foams as novel building and decoration materials exhibit great advantages. Herein, fluorite tailings and waste glass were recycled to sinter glass-ceramic foams by introducing calcined kaolin and calcium phosphate. The effects of calcium phosphate addition and sintering temperature on the crystalline phase, microstructure evolution, fluoride leaching performance and physical properties of the glass-ceramic foams were systematically investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectra, computed tomography, and differential scanning calorimetry. The obtained glass-ceramic foams sintered at 1110 °C exhibited relatively homogeneous pore sizes and distributions, low densities and thermal conductivities, and high compressive strengths. Moreover, the addition of calcium phosphate promoted the formation of fluorapatite, which benefited the toxic fluorine stabilization in the glass-ceramic foams. Thermodynamic data were calculated using Factsage 8.1 and verified through experiments to establish the physical structure transition during sintering. This study provides an efficient utilization route to convert fluorite tailings and waste glass into stable and valuable lightweight glass-ceramic foams.

Influence of GGBFS and silica fume on characteristics of alkali-activated Metakaolin-based concrete

Alkali-activated or geopolymer composites have been introduced as a promising green alternative to ordinary Portland cement concretes and mortars. Metakaolin (MK) is a common source material produced from the calcination of kaolin, which has been widely employed for geopolymer synthesis. The present study involves testing thirteen alkali-activated concrete mixes divided into two groups to investigate their fresh and mechanical properties. The two groups attempted to study the effect of partial replacement of MK by ground granulated blast furnace slag (GGBFS) and silica fume (SF) (ranging from 5% to 30% by MK weight). The effect of the water-to-solids ratio for MK-GGBFS and MK-SF concretes was also studied. The investigated fresh and mechanical properties were slump, density, compressive strength, compressive strength rate of development, splitting tensile strength, water absorption, abrasion resistance, and ultrasonic pulse velocity. The partial replacement of MK by GGBFS of up to 10% caused about 10% reduction in the compressive strength of geopolymer but enhanced early strength development. The conclusions derived from the study are supported by XRD analysis and SEM images. Models are proposed for estimating the mechanical properties of geopolymer concrete as a function of either the molar ratios and the aggregate content or compressive strength.