Formation Of Aluminosilicate Gel Research Articles

Insights into microstructural alterations in alkali-activated materials incorporating municipal solid waste incineration fly ash

Municipal solid waste incineration fly ash (MSWI FA) is categorized as a hazardous waste with significant implications on environmental safety and human health. This study tailored three types of alkali-activated materials (AAMs) by using ground granulated blast-furnace slag (GGBS) and metakaolin (MK) for the low-carbon stabilization/solidification (S/S) of MSWI FA. The intricate interactions between MSWI FA and AAMs synthesized from Al-rich precursors and Ca-rich precursors were investigated to elucidate the microstructural alterations of AAM-MSWI FA systems. Comprehensive characterization techniques were conducted, including isothermal calorimeter, thermal gravimetric analysis, Fourier-transform infrared (FTIR), X-ray diffraction (XRD), and 27Al and 29Si nuclear magnetic resonance (NMR). The results revealed that the incorporation of MSWI FA significantly delayed alkali activation reactions, hindering the formation of aluminosilicate gel due to high contents of Cl and potentially toxic elements (PTEs) in the MSWI FA. High volume of dissolved Cl from MSWI FA would react with available Ca from MSWI FA and GGBS as well as Al from GGBS and MK to form Friedel’s salt, and this reaction was found to be more dominant in the Al-rich environment. Meanwhile, the C-(N-)A-S-H gel exhibited longer chain length in the Al-rich environment than in the Ca-rich environment. Overall, this study delivers microstructural insights into AAM-MSWI FA systems utilizing GGBS and MK as precursors. The results provide scientific groundwork for diverse practical applications of AAM-MSWI FA materials, thereby supporting the development of resource-efficient and low-carbon hazardous waste treatment strategies.

Corrosive effect of HCl and H2SO4 exposure on the strength and microstructure of lithium slag geopolymer mortars

Resistance of geopolymer against aggressive effects of strong acid governs its service life. This research investigates the effect of silica fume incorporation on the strength degradation of lithium slag geopolymer (LSG) mortars after exposure to strong acids and presents the relationship between microstructure, calorimetry, and residual strength. Silica fume incorporated LSG mortars containing sodium (Na) and potassium (K) based alkaline activators were exposed to 5% sulphuric and hydrochloric acid solutions separately for 60 days. The sulphuric acid deterioration of LSG in terms of maximum percentage reduction in compressive strength was 37.42% upon incorporating 40% silica fume in geopolymer with a Na-based alkaline activator. LSG mixes activated by Na-activators demonstrated higher acid resistance compared to those activated by K-based activators. Leaching of aluminium (Al) from the aluminosilicate gel matrix occurred, which subsequently deteriorated aluminosilicate gel matrix when exposed to hydrochloric acid, whereas the leaching of Al occurs alongside the crystallization of calcium sulfate when specimens were exposed to sulphuric acid solution. The incorporation of silica fume reduces the initial calorimetric heat evolution and higher degree of geopolymerization marked by higher heat evolution between 5–48 h than the control LSG. The intense evolution of heat in control LSG at the initial stage of geopolymerization yielded a poor microstructure. Thus, acid-induced strength degradation is lower in silica fume-incorporated gel due to its denser microstructure and higher quantity of aluminosilicate gel formation. Hence, the incorporation of silica fume significantly improved its acid resistance, thus LSG may have industrial applications concerning acid exposure.

Optimizing the Fly Ash/Activator Ratio for a Fly Ash-Based Geopolymer through a Study of Microstructure, Thermal Stability, and Electrical Properties

Fly ash (FA)-based geopolymer was prepared using sodium hydroxide and sodium silicate (in 2.5ratio) as an alkali activator liquid (AL). The condition of FA/AL was optimized for achieving 1.00, 1.25, 1.5, and 1.75 ratios by varying the alkali concentrations, which are referred to as GP1, GP2, GP3, and GP4, respectively. The influence of slight variations in the FA/AL ratio on microstructure, morphology, functional groups, and composition was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray fluorescence (XRF), and Fourier transform infrared spectroscopy (FTIR). FESEM detected a homogeneous fused matrix of fly ash and alkali activator solution up to 1.5 ratios; GP3 showed a dense morphology. FTIR confirmed that the formation of aluminosilicate gel induced a shift in the T–O (T = Si or Al) asymmetric stretching band, nearing a lower frequency. XRD showed an amorphous structure with phases, including quartz, mullite, hematite, and sodalite. The thermogravimetry and differential thermal analysis (TGA–DTA) indicated that the geopolymer samples were thermally stable. The electrical study concluded that the geopolymer possessed insulating properties.

A comprehensive micro-nano investigative approach to study the development of aluminosilicate gel in binary blends of lithium slag geopolymer

This study extensively investigates the formation of aluminosilicate gel in binary blends of lithium slag geopolymer (LSG) containing fly ash and silica fume. A comprehensive analysis of the aluminosilicate gel was performed at the micro-nano scale, employing techniques such as mineralogical, crystallographic, micromorphological, and micromechanical analysis. C-(N)-A-S-H and N-(C)-A-S-H gels were found between Si/Al ratios of 2.41–3.04 and 3.88 to 4.33, respectively. Fly ash incorporated LSG at 45% alkaline activator yielded the highest average nano-indented modulus of 22.54 GPa. Statistical Gaussian deconvolution was employed to categorize mineral phases in nano-indentation of LSG, revealing substantial distinctions indicated by the low p-values. Carbonation in silica fume incorporated LSG was affirmed by selected area electron diffraction (SAED) patterns of calcite crystallites. The microstructure of LSG containing fly ash was stiffened by the growth of needle crystals of hydroxy-sodalite and aluminosilicate gel. Thus, the formation of C-(N)-A-S-H gel and hydroxy-sodalite governs the strength development of LSG binary blends.

Investigation of un-calcined coal gangue together with ground granulated blast furnace slag and fly ash to ambient-curing production high-strength geopolymer

Aim of this work is to exploit an ambient-curing method to produce high-strength geopolymers by applying un-calcined coal gangue (G). An optimized alkaline activator solution with a mixture proportion of 1.2 M, 8% AE, and 0.32 liquid-solid ratio was adopted to stimulate the mixture of G, ground granulated blast furnace slag (S), and fly ash (A). The geopolymer totally composed of S displayed the highest strength, which was more than 70 MPa at 28 d. Compressive strengths dramatically decreased from 55 MPa to 20 MPa when the content of G changed from 25% to 55% in S-G binary geopolymers. The strength of S-A-G ternary geopolymers did not significantly influence by the content of G until 17%, and the strengths decreased with the increasing G content as it exceeded 22%. Appropriate contents of G resulted in aluminosilicate gel formation in the ternary S-A-G geopolymers, and compressive strengths of the ambient-curing ternary geopolymers can reach ∼65 MPa at 28 d.

Development of construction materials from the geopolymerization of red clay and coal fly ash

Red clay contains solid aluminosilicate has been shown to be reactive in the presence of an alkaline activator. The addition of coal fly ash and lime has shown improvements in their mechanical and physical properties of the geopolymer products. FTIR analysis and SEM images of the product have shown the formation of aluminosilicate gel in the geopolymeric product. The maximum compressive strength of the geopolymer products GP-RFL was achieved to be 15.92 N/mm2 having water absorption of 10.47 % and bulk density 2.81 g/cm3. These results indicated that geopolymer mortars made from red clay, coal fly ash and lime could be used as an alternative construction material./p> BIBECHANA 19 (2022) 119-126

Geopolymerization Behaviour of Red and White Clays

Construction is one of the most important activities increasing the demand for Portland cement resulting significant amount of CO2 emission, natural resources degradation, and a high amount of energy consumption. The use of geopolymer has been studied as a potential substitute for Portland cement. Geopolymers are environmentally-friendly binding materials that are produced by the polymerization of alumino-silicates in presence of alkali polysilicates forming Si-O-Al bonds, which are used for several building applications. In this study, red and white clays which contain solid alumino-silicate have shown reactive in presence of an alkaline activator. The addition of lime has shown improvement in the mechanical and physical properties of the geopolymer products. The FTIR analysis and SEM images of the product have shown the formation of aluminosilicate gel in the geopolymeric product. The maximum compressive strength of the geopolymer products RCW and RWL were achieved to be 15.91 and 20.30 MPa, respectively. Such geopolymer products are in good agreement with cementitious products and can be used in building applications.

Benefits of pre-treating MSWI fly ash before alkali-activation

The significant amount of heavy metals and soluble salts contained in municipal solid waste incineration fly ash (MSWI FA) and the corresponding risk of leaching make them classified as hazardous waste. Nowadays, the ashes are usually landfilled, but another application can be the inertization and valorization of MSWI FA in building materials. Geopolymer can be a potential material because it can naturally entrap heavy metals physically and chemically inside its matrix. This research investigated how the use of electrodialytic treatment compared to a water washing procedure, considered as pre-treatments for MSWI FA, influenced a metakaolin-based geopolymer as an encapsulation matrix for MSWI FA. Geopolymers with metakaolin and 5, 10, 20 wt% of raw or pre-treated MSWI FA were compared with a sample containing only metakaolin for environmental, mechanical, and physical properties. The formation of aluminosilicate gel was confirmed for all geopolymers, although with varying degrees of impurities. The results show low compressive strength for the sample with washed ash but good mechanical strength for geopolymers with raw and electrodialytic treated ash compared to metakaolin-based geopolymer or with geopolymer with washed ash. A decrease in leaching for all the metals in the samples confirms the encapsulation capacity of geopolymers. The electrodialytic pre-treatment decreases, particularly the leaching of Cr, Zn and chloride in geopolymers, confirming the potential use of this pre-treatment to improve the environmental and mechanical characteristics of geopolymers with MSWI FA.

Silty Clay Stabilization Using Metakaolin-Based Geopolymer Binder

Geopolymer binders are adjudged as the latest wave of sustainable alkali-activated materials for soil stabilization due to their excellent bonding properties. This study applied metakaolin as a precursor for synthesizing the geopolymer binder by employing the mixture of quicklime and sodium bicarbonate as an alkali activator. The optimal mass mixing ratio of the alkali activator, metakaolin, and silty clay was determined by unconfined compression tests. The stabilization mechanisms of the geopolymer binder were measured by x-ray diffraction and Fourier transform infrared spectroscopy. The microstructural characteristics of the geopolymer-stabilized silty clay were observed by scanning electron microscopy with an energy dispersive x-ray spectroscopy and mercury intrusion porosimetry test for understanding the strengthening mechanism of the silty clay after the treatment. Results indicate that the optimal mass mixing ratio of the alkali activator, metakaolin, and silty clay is 1:2:17, and the unconfined compressive strength of the geopolymer-stabilized silty clay reaches the maximum value of 0.85 MPa with adding 15 wt% of the geopolymer binder. Diffraction patterns show an insufficient polymerization of the geopolymer binder in the silty clay in the early days but a rapid synthesis of aluminosilicate gels after that. The new asymmetrical stretching vibration peaks signified the formation of aluminosilicate networks and are responsible for the strength improvement of the silty clay. Microstructural analyses further confirm the formation of aluminosilicate gels and their positive impacts on the structure of the silty clay over curing age.

Mechanical Properties of Geopolymers Synthesized from Fly Ash and Red Mud under Ambient Conditions

Aluminosilicate gels form geopolymers and nanocrystalline zeolites which have comparable strength properties, making them a potential replacement for ordinary Portland cement. The study explores the use of two untreated industrial wastes, Class-F fly ash and red mud, for synthesizing geopolymeric material at ambient synthesis conditions. The high alkalinity present in the red mud was exploited for the dissolution of silica in the fly ash and red mud. The mechanical, mineralogical, microstructural, and pore characteristics were analyzed and the contributions of curing period, Si/Al, Na/Al, and liquid-to-solid (L/S) ratios on the compressive strength of the end products were also investigated. The alkalinity of the system due to the red mud was adequate for the dissolution of raw fly ash and the subsequent formation of aluminosilicate gels. The strength of the end product was directly proportional to the initial Si/Al ratio and the specimens with highest fly ash content exhibited highest compressive strength values after 28 days of curing. Furthermore, fly ash contributed to the formation and distribution of interstitial and capillary pores in the aluminosilicate matrix. The lowest L/S ratio of the initial mix resulted in the end product with the highest unconfined compressive strength.

Synthesis and characterization of one-part geopolymers for extrusion based 3D concrete printing

Interest in innovative construction processes such as 3D concrete printing (i.e. digital construction), is growing rapidly both in academia and industry. Processing conventional geopolymer mixes, in which alkaline solutions are used for activation, could be troublesome in concrete printing due to the high viscosity of the alkaline solution. One-part geopolymers offer one possible solution to this challenge as they involve the use of a solid activator with solid aluminosilicates precursors. In this work, a printable one-part geopolymer mix was developed, which could be extruded through the nozzle of a 3D printer and stacked together without deforming the bottom layers. Flow properties such as yield stress, viscosity and thixotropy of the developed geopolymer were assessed along with its strength development curve. Printed specimen showed anisotropic behaviour in mechanical properties when compared to the mould casted samples. Microstructural characterization revealed the formation of alumino-silicate gel with high tetrahedrally coordinated Al and interlayer K ions in its structure. When compared to OPC-based mixes, the developed geopolymer mixes revealed a lower environmental impact, which could be even further reduced with the use of alternative activators.

Microstructure and phase evolution of alkali-activated steel slag during early age

This work aims to investigate the early age evolution of alkali-activated steel slag under curing condition of 60 °C. Microstructural characterization and reaction degree of alkali-activated steel slag (Isothermal calorimetric, scanning electron microscope, Transmission electron microscopy, Fourier Transform Infrared Spectrometer, and 29Si nuclear magnetic resonance) were carried out. The results showed that the densifying microstructure of aluminosilicate gel significantly increases, attributing to the increasing of reaction degree due to the formation of Si-O-T, in the early evolution of alkali-activated steel slag at 60 °C curing condition. The 29Si structure of Q0 in steel slag gradually converted into Q1 and Q2 in the intermediate products which represented the formation of aluminosilicate gels. The evidence indicated that the steel slag can be an alternative raw material in alkali activated application.

PRODUCTION OF FLY ASH BASED GEOPOLYMERS USING ACTIVATOR SOLUTION WITH DIFFERENT Na2O AND Na2SiO3 COMPOSITIONS

In this work, samples of geopolymers were produced by the activation of fly ash with different alkaline solutions (NaOH and NaOH + Na2SiO3). The mechanical strength and microstructural characterization were employed to study the influence of the geopolymerization process in the final proprieties of the binder material. The results showed that the geopolymers have high mechanical strength in the first 24 hours of curing (28 MPa) increasing to 48 MPa after 28 days. The physical proprieties such as water absorption, apparent porosity, and density changed when different solutions were used. The SEM images showed a denser morphology with aluminosilicate gel formation on the surface which is responsible by an increase of mechanical strength of fly ash based geopolymers.

Enhanced methods of crystallization: The crossover synthesis from gel to melt flow - A case study on sodalites

A new crystallization pathway composed of a combination of gel synthesis and crystallization in a melt flow is demonstrated using the sodalite system as a case study. The specific characteristics of this enhanced method of crystallization persists in a controlled heating of a pressed pellet of a mixture of selected solids, containing a “water donator” as well as a “fluxing agent”. The reaction of zeolite 13 X (“water donator and SiAl-source”) NaOH granulate (“fluxing agent”) and a template salt like NaCl (or others) is shown to yield suitable products already after a few hours of heating the mix-pellet under open conditions.In a first synthesis series experiments were performed to investigate the details of the reaction mechanism. Beside a study of the role of the hydrate water, the NaOH granulate and certain template salts, even the total decomposition of the zeolite 13 X framework structure could be demonstrated under the conditions of the crossover from gel to flux growth. According to the results it could be claryfied that the mechanism consists of alkaline aluminosilicate gel formation and crystallization in the early period but shifts into crystal growth in NaOH melt at elevated temperature.In a second series examples of tailored sodalites and the characterization of these products will show the effectiveness of the new method. From SEM invertigations could be further derived that the crystal size and morphology depends on Tmax. the holding period at Tmax. and nano- or micro-crystalline sodalites can be obseved.

Improved Reactivity of Fly Ash-Slag Geopolymer by the Addition of Silica Fume

This study investigates the improved reactivity of a geopolymer based on a combination of fly ash and blast furnace slag (BFS) by the addition of silica fume. The geopolymer was synthesized by activating a mixture of fly ash, BFS, and three different types of silica fume with alkali activator. X-ray diffraction (XRD) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) were utilized to characterize the reaction. The silicate structure was also analyzed by nuclear magnetic resonance (NMR) spectroscopy. From these results, it was found that the replacement of fly ash with the silica fume led to a significant decrease in theQ4(1Al) and an increase in theQ4(2Al),Q4(3Al), andQ4(4Al). The Si/Al ratio of the aluminosilicate gel was relatively constant, ranging from 2.0 to 2.6, while the Si/Al ratio of the C-S-H gel increased with the addition of silica fume. Therefore, some of the Al dissolved from the slag contributed to the formation of aluminosilicate gel, and the remnant slag particles mostly participated in the formation of the C-(A)-S-H gel with a decrease in theQ2(1Al). The increase in the reactivity of slag caused by the addition of silica fume was attributed to the reaction of the Al in the slag with the silica fume.

Effect of ethanol as an additive on the morphology and crystallinity of LTA zeolite

In the present work detailed investigation was made to ascertain the effect of ethanol addition on morphology, phase purity and crystallinity of LTA zeolite. The effect of addition time of ethanol during the reaction mixture preparation was also studied. The synthesis of LTA zeolite was made using inorganic sources of aluminium and silicon in an ethanol–Al2O3–SiO2–Na2O–H2O system. Micron sized distorted cubic shape LTA zeolite along with sodalite disc like clusters were attained on the addition of ethanol before mixing of sodium aluminate with sodium silicate whereas the knobbed surface spheres were obtained when ethanol was added after mixing of same. The addition of ethanol just after the sodium aluminosilicate gel formation (i.e. after the addition of alkali solution to the resultant mixture of sodium aluminate and sodium silicate) also leads to the formation of LTA zeolite crystals along with sodalite but at later stage of crystallization. Thus, the present study reveals that the addition of ethanol makes predominant impact on the LTA zeolite cubic crystal formation and it also provides better understanding of nucleation, crystallization and morphology of LTA zeolite synthesized in the presence of ethanol.

Influence of fluorite on the isothermal leaching and rheological behaviours of chlorite mineral pulps at low pH

Chlorite clay gangue mineral leaching at low pH may produce viscous slurries, which deleteriously impact on value metal extraction and recovery in metallurgical plant operations. The presence of fluorite as a minor or an accessory mineral phase with chlorite may markedly exacerbate the pulp rheology and reduce the leaching dramatically. In the present work, isothermal leaching and rheological behaviours of chlorite were investigated in the presence of fluorite (chlorite/fluorite mass ratios of 20, 10 and 5 to 1) at pH1 and 70°C. Fluorite was found to dramatically enhance the kinetics of chlorite leaching, the mechanism of which was incongruent. The leach dispersion rheology showed a distinctly non-Newtonian behaviour characterized by thixotropy, shear thinning and time and fluorite concentration-dependent shear yield stress. The highest, maximum shear yield stress and apparent viscosities were observed at chlorite/fluorite ratio of 10:1, below and above which weaker gels formed. Solution-state 27Al and 29Si NMR studies indicated that polycondensation reactions involving solution Al(III) six to four fold coordination and a plethora of Si(IV) tetrahedra complexes led to the formation of aluminosilicate gels. The chlorite/fluorite ratio had a striking impact on the formation and development of the strongly viscous gel structures. The findings suggest that the resulting maximum yield stresses (40–200Pa) and apparent viscosity (10–25Pa·s) are highly detrimental to plant chlorite clay–fluorite rich, low grade laterite ores' leaching and handling operations.

The Effect of Dissolved Humic Acids on Aluminosilicate Formation and Associated Carbon Sequestration

Allophane and imogolite neogenesis in soils may occur in the presence of organic matter. To understand this process under conditions relevant to soils, the influence of dissolved organic carbon (DOC) as humic acid (HA), on aluminosilicate formation was studied at , pH 6, and low-DOC concentrations. For solutions with initial Al/Si ratios of 1–2.1, and 0–6 mg/L DOC, precipitates recovered after 20 h had Al/Si ratios of 2.2–2.7. The formation of allophane, imogolite-like material, and aluminosilicate gel was confirmed by XRD, FTIR, and NMR. The effect of DOC was to produce a small, but systematic increase in imogolite-like Si in the precipitate, and a decrease in the formation of aluminosilicate gel. Results suggest that the presence of DOC as HA slows the otherwise rapid polymerization of Al and Si at low temperature, and may also promote the formation of imogolite. The high C content of these precipitates indicates that this process may facilitate the sequestration of organic matter, slowing C cycling in soils.

The role played by the reactive alumina content in the alkaline activation of fly ashes

This study explores the relationship between the chemical composition of fly ashes and the microstructural characteristics and mechanical properties of the cementitious materials resulting from the alkali activation of fly ashes (AAFA). Three reactive systems were prepared by mixing three F ashes, with an 8 M NaOH solution and stored at 85 °C. The main reaction product formed in three systems is an amorphous alkaline aluminosilicate gel. This gel ( zeolite precursor), has a three-dimensional framework, with Al occupying Al(4Si) and Si occurring in a variety of environments Q 4( nAl). After short thermal activation periods (2–5 h), an Al-rich gel was formed with silicon in tetrahedral Q 4(4Al) units ( intermediate phase), yielding low mechanical strengths. When the curing time increase (7 days) the gel changes into a more stable Si-rich phase with a greater mechanical strength. Finally, it has been shown that the amount of the reactive aluminium plays an important role in the aluminosilicate gel formation, from a kinetic point of view.

Effects of Anions on the Formation of Aluminosilicate Gel in Geopolymers

This work demonstrates that an increase in soluble silicate concentration over ∼200 mM in an aqueous alkaline solution (pH = ∼13.95) can significantly increase the reactivity of a Class F fly ash at room temperature. The greatly increased dissolution of the fly ash primary phase(s) and the subsequent secondary precipitation are responsible for the formation of an aluminosilicate gel similar to the major binding phase of a geopolymer. On the basis of this observation, a reaction model was developed to simulate aluminosilicate gel formation in a real geopolymeric system. In this work, potassium salts of chloride, carbonate, oxalate, and phosphate (KCl, K2CO3, K2C2O4·H2O, and K2HPO4) were used as solution contaminants. It was found that the anions (Cl-, CO32-, C2O42-, and PO43-) significantly affected the formation kinetics as well as the nature of the aluminosilicate gel formed in the reaction model. This observation could be successfully used to mechanistically explain the retardation effects of the various salts on the gel solidification time of a geopolymer. Aluminosilicate crystallization in the chloride-affected and the oxalate-affected reaction models implies that similar processes could also occur in geopolymers. This could affect the chemical stability and durability of geopolymers and thoroughly deserves a careful future investigation.