Synthesis and alkali activation of Magnesium-rich aluminosilicates

Abstract Alkali-activated materials (AAMs) are alternative cementitious materials with lower carbon footprints compared to traditional Portland cement (PC). In addition to Ca, Si, and Al, the precursors used in the preparation of AAMs can sometimes include considerable amounts of Mg, so that Mg significantly influences the structure and properties of AAMs. When compared to Ca, Si, and Al, relatively few studies have focused on the role of Mg in AAMs. This thesis deals with alkali activation of Mg-rich aluminosilicate precursors with the following objectives: 1) preparation, characterization, and estimation of alkaline reactivity of Na-Mg aluminosilicate glasses; 2) synthesis of AAMs from Na-Mg aluminosilicate glasses and their detailed characterization to understand the fate of Mg; and 3) estimation of the potential of phlogopite as a Mg-rich raw material for alkali activation. The structural study of Na-Mg aluminosilicate glasses indicates that the higher cationic field strength (CFS) of Mg than Na makes Mg preferable as a network modifier, whereas Na acts as a charge compensator. Alkaline reactivity studies of Na-Mg aluminosilicate glasses reveal that as Mg replaces Na in glasses, the reactivity of the glasses increases initially, attains a maximum, and then drops. This trend can be explained by the interplay between glass depolymerization and optical basicity: depolymerization dictates the glass reactivity initially, while the effect of optical basicity dominates at later stages. Detailed structural study of AAMs prepared from Na-Mg aluminosilicate glasses indicates that Mg in AAMs exists as an amorphous magnesium silicate (AMS) phase, but the existence of this phase is not well documented in the literature. The driving force for AMS formation is the high CFS of Mg, which leads to effective stabilization of the depolymerized silicate species. The absence of hydrotalcite-group phases from these AAMs is due to the depletion of Al by zeolite production. The assessment of phlogopite mineral as a Mg-rich precursor for alkali activation indicates that untreated phlogopite is highly inert. However, thermal treatment could enhance the alkaline reactivity of phlogopite.Tiivistelmä Alkali -aktivoidut materiaalit (AAM) ovat vaihtoehtoisia sementtimateriaaleja, joilla on pienempi hiilijalanjälki verrattuna perinteiseen portland -sementtiin (PC). Ca: n, Si: n ja Al: n lisäksi esiasteissa (joita käytetään AAM: ien valmistukseen) voi joskus olla huomattava määrä Mg: tä, ja tämä johtaa siihen, että Mg vaikuttaa merkittävästi AAM: ien rakenteeseen ja ominaisuuksiin. Verrattuna Ca, Si ja Al, on ollut suhteellisen vähän tutkimuksia, joissa keskitytään Mg: n rooliin AAM: issä. Tämä opinnäytetyö käsittelee Mg-rikkaiden alumiinisilikaattiesiasteiden alkaliaktivaatiota seuraavilla tavoitteilla: 1) Na-Mg-alumiinisilikaattilasien alkalisen reaktiivisuuden valmistelu, karakterisointi ja arviointi; 2) synteesi AAM: istä Na-Mg-alumiinisilikaattilasista ja niiden yksityiskohtainen karakterisointi Mg: n kohtalon ymmärtämiseksi; 3) Flogopiitin potentiaalin arviointi Mg-rikkaana raaka-aineena alkalin aktivoimiseksi. Na-Mg-alumiinisilikaattilasien rakennetutkimus osoittaa, että Mg: n korkeamman kationisen kentänvoimakkuuden (CFS) takia Na: n vuoksi Mg on edullinen verkon muokkaajana, kun taas Na toimii varauksen kompensoijana. Na-Mg-alumiinisilikaattilasien alkaliset reaktiivisuustutkimukset paljastavat, että kun Mg korvaa Na: n lasissa, lasien reaktiivisuus kasvaa aluksi ja saavuttaa maksimin, minkä jälkeen se laskee. Tämä suuntaus voidaan selittää lasin depolymeroinnin ja optisen emäksisyyden välisellä vuorovaikutuksella: depolymerointi sanelee aluksi lasin reaktiivisuuden, kun taas optisen emäksisyyden vaikutus hallitsee myöhemmässä vaiheessa. Yksityiskohtainen rakenteellinen tutkimus AAM: ista, jotka on valmistettu Na-Mg-alumiinisilikaattilasista, osoittaa, että Mg AAM-yhdisteissä esiintyy amorfisena magnesiumsilikaatti (AMS) -faasina, jonka olemassaoloa ei ole hyvin dokumentoitu kirjallisuudessa. AMS: n muodostumisen liikkeellepaneva voima on Mg: n korkea CFS, mikä johtaa depolymeroitujen silikaattilajien tehokkaaseen vakautumiseen. Hydrotaltsiittiryhmän faasien puuttuminen näistä AAM: ista on havaittu johtuvan Al: n ehtymisestä zeoliittituotannolla. Flogopiittimineraalin arviointi Mg-rikkaana esiasteena alkalin aktivoitumiselle osoittaa, että käsittelemätön flogopiitti on erittäin inertti. Lämpökäsittely voi kuitenkin parantaa flogopiitin emäksistä reaktiivisuutta

Structural collapse in phlogopite mica-rich mine tailings induced by mechanochemical treatment and implications to alkali activation potential

Abstract The alkali activation of mine tailings is of interest to diminish the impoundment storage of large waste stream from mining industry. However, most of the mine tailings, like phosphate mine tailings generated in Finland, are rather inert and need pre-treatment to induce the reactivity for alkali activation. In this work, mechanochemical treatment was conducted to improve the reactivity of phosphate mine tailings. The ground specimens were subjected to the crystal structural analysis of X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermal analyses by Thermogravimetry/Differential Thermal Analysis (TG/DTA), showing the increment of amorphous content and structural degradation as a function of grinding time. The subsequent alkaline reactivity test illustrated incremental alkaline dissolution of Si, Al and K and a schematic diagram of altered phlogopite structure (sources of aluminosilicates in tailings) was also proposed. Additionally, the mineralogical composition of individual particles was carried out by mineral liberation analysis (MLA), thereby evaluating the influence of pre-treatment on the mineralogy of tailings. The results indicate profound micromorphological changes and structural cleavage of the precursor due to grinding, which strongly increase alkaline reactivity

One-part geopolymers from mining residues – effect of thermal treatment on three different tailings

Abstract Use of mine tailings as an aluminosilicate precursor in alkali activation is becoming an interesting alternative to manage the high-volume of waste generated from mining industries. However, very few tailings have so far been studied for their mineralogical properties and alkali activation potential. This study aims at understanding the ability of mine tailings from phosphate, kaolinite and lithium mines for their efficient participation in alkali activation. Biotite, muscovite, kaolinite, albite, and quartz were found to be the major minerals present in them. The impact of variation in mineralogy on silica and alumina solubility of these tailings was analyzed. The solubility was found to be high in impure kaolinite compared to the other two. Effectiveness of thermal treatment (750 °C and 900 °C) on improving the reactivity of these tailings in alkaline condition was also investigated. It was observed from the results that the effect of thermal treatment on the crystalline structure and solubility of an aluminosilicate material mainly depends on the mineral structure of the material, as well as the treatment temperature. Interestingly, thermal treatment reduced the solubility of lithium tailings with albite and quartz mineral. Effort has been made to relate the strength attained by alkali activation of mine tailings to their solubility values. However, despite of the higher solubility offered by impure kaolinite, phosphate tailings gives the maximum strength improvement by 62%. This can be due to the presence of calcium compounds in phosphate tailings that resulted in additional hydration products

Immobilization of sulfates and heavy metals in gold mine tailings by sodium silicate and hydrated lime

Abstract Gold mining produces hazardous tailings wastes with elevated sulfur content and high levels of heavy metals including oxyanion elements such as V and As. This research investigated activation of these tailings with calcium hydroxide and sodium hydroxide/sodium silicate as a way to stabilize the material and limit leaching of harmful components. The effects of thermal treatment on the reactivity of the tailings and the use of different activating solutions on the physical properties, microstructure and leaching of harmful components are reported. The effect of adding ground granulated blast furnace slag to the tailings is also assessed. The use of 5 wt % Ca(OH)₂ activating solution produces optimum performance increasing the immobilization efficiency of sulfates, arsenic and the other harmful elements. Heat-treating mine tailings at 900 °C slightly improves the reactivity but did not improved the immobilization efficiency. Microstructural analysis by TEM and XRD confirmed that stabilization is based on calcium sulfate and/or ettringite formation during alkali-activation. All materials achieved reasonable compressive strength after 28 days of curing and the potential for using alkali activation as a method to treat tailings from mining is discussed

On the carbonation of brucite:effects of Mg-acetate on the precipitation of hydrated magnesium carbonates in aqueous environment

Abstract The role of organic ligands on the formation of hydrated magnesium carbonates (HMCs) has been remaining unclear. This work reports insights into the effects of Mg-acetate on the carbonation of brucite including the kinetics of reaction, the precipitation of different HMCs, and reaction mechanisms. We found that the organic ligand increases the kinetics of brucite's carbonation and alter the formation and conversion of HMCs. A relatively unknown phase (i.e., giorgiosite) precipitates in the presence of Mg-acetate with nanowire morphology. With the presence of acetate ligand, nucleation sites formed after the breakdown of Mg-acetate complexes and be replaced by the Mg–CO₃ bonds. These sites act as a sink for Mg²⁺ to grow crystals and prevent the passivation layer of HMCs on brucite's surface. Findings reported here can enable an approach to steer pore solution chemistry in the HMC-based binder for better reaction degree, durability, and mechanical properties

Ag- or Cu-modified geopolymer filters for water treatment manufactured by 3D printing, direct foaming, or granulation

Abstract In this work, we compared the main characteristics of highly porous geopolymer components for water treatment applications manufactured by 3D printing, direct foaming, or granulation. Furthermore, different approaches to impregnate the materials with Ag or Cu were evaluated to obtain filters with disinfecting or catalytic properties. The results revealed that all of the investigated manufacturing methods enabled the fabrication of components that possessed mesoporosity, suitable mechanical strength, and water permeability, even though their morphologies were completely different. Total porosity and compressive strength values were 28 vol% and 16 MPa for 3D-printed, 70–79 vol% and 1 MPa for direct-foamed, and 27 vol% and 10 MPa for granule samples. Both the filter preparation and the metal impregnation method affected the amount, oxidation state, and stability of Ag and Cu in the filters. However, it was possible to prepare filters with low metal leaching between a pH of 3–7, so that the released Ag and Cu concentrations were within drinking water standards

Study of synthetic titania slags demonstrating characteristics similar to high titania ilmenite slag

Abstract The upgradation of the ilmenite ore, using a pyrometallurgy method, is performed using a carbothermic reduction of the ilmenite. A high titania slag is obtained which is used as a feedstock for the TiO₂ pigment production. The slag is cooled after tapping in big molds and can take ten days to cool. This cooling method has remained the same since the inception of ilmenite smelting and recently rapid cooling through granulation has been utilized. The work presented in this paper focuses on the microstructural study of the slags that were prepared using different techniques and cooled at different cooling rates. Various analytical techniques, such as X-ray powder diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS) were used to exhibit the similarity of these synthetic slags to the properties of high titania ilmenite slag. The slag consisted mostly of pseudo-brookite phase with a M₃O₅ stoichiometry and smaller amounts of silicate and rutile phase. A glassy phase of silica was observed and most of the impurities were found to be present in the silicate phase. These silica phases were observed to be separate from the pseudo-brookite phase and along the phase boundaries. Micro-cracking of the slag surface, which is the characteristic of the M₃O₅ phase formed in the ilmenite slag, were observed under the SEM analysis. The XPS analysis revealed that faster cooling does result in lower amount of oxidation but the difference in the TiO₂ and Ti₂O₃ composition can have larger impact on oxidation than the cooling speed

Hydration of blended ladle slag and calcium aluminate cement

Abstract Partial replacement and co-hydration of calcium aluminate cement (CAC) with ladle slag was investigated in this study as a pathway in valorizing the slag and reducing the economic cost of CAC. The impact of this replacement on the physical and microstructural properties were analysed using different techniques such as mechanical strength test, freeze-thaw, sulfate attack, XRD, SEM etc. Thermodynamic modelling was used to predict the phase assemblages of the blended cement using the hydration kinetics of the system. Experimental results showed the reference CAC mortar and the substituted mortar exhibited comparable strength gain at 7 and 28 days, and durability as measured by sulfate attack, abrasion, and freeze-thaw resistance. A low water-to-binder ratio (0.35) lessened the effect of conversion on the hydrates, as XRD showed metastable CAH₁₀ and C₂AH7.5 as the hydrates at 7, 28 and 60 days. This however can convert later to the thermodynamically stable phase C₃AH₆. Thermodynamic modelling suggests these two metastable phases as major binding phases, while Si-hydrogarnet and FeOOH appeared a minor trace in the binder. *Cement chemistry notation used, where C = CaO, A = Al₂O₃ and H = H₂

Preparation and characterization of binary Mg-silicate glasses via Sol-Gel route

Abstract Sol-gel processing allows synthesis of low-energy glasses. In this work, binary magnesium silicate glasses with various MgO contents are synthesized using a modified sol-gel route. TGA and XRD analyses indicate that amorphous glasses with up to 50 mol% MgO can be obtained at 500°C. The reactivity of the glasses is evaluated to assess the use of the sol-gel technique in the large-scale synthesis of alternative cementitious materials. Reactivity tests show that, as MgO content increases, reactivity of glasses increases, reaches an optimum and then declines. This trend doesn’t agree with the theoretical one estimated by NBO/T value, which is generally used for the evaluation of glass reactivity. Mg²⁺ ions play a role as the network modifier when first introduced to silicate glasses. This leads to the depolymerization of the networks, causing an increase in reactivity. Then the magnesium partly behaves as a network former, bonding with oxygens to form MgOₓ polyhedron when there are insufficient primary glass-forming oxide SiO₂, resulting in the polymerization of networks, hence the decrease in reactivity

Longitudinal single-sided NMR study:silica-to-alumina ratio changes the reaction mechanism of geopolymer

Abstract Characterizing and understanding the mechanisms underlying geopolymerization are critical in achieving the use of sustainable construction material, geopolymer, for widespread commercial production. Non-destructive ¹H NMR relaxometry can provide novel information about geopolymerization as it allows simultaneous detection of where the water goes and how the pore structure changes. Coupled with the development of single-sided NMR devices, NMR measurements are not limited by the specimen size and are therefore able to observe in-situ conditions of geopolymer synthesis. Here, the curing process of metakaolin-based geopolymers was monitored by ¹H relaxometry on a single-sided NMR device. The silica-to-alumina ratio (Si/Al) was found to affect reaction stages of the geopolymerization. After the dissolution of aluminosilicate precursor, the low Si/Al of 1 was found to generate three gelation/polymerization stages as well as a water-binding stage, and two gel phases appeared. When Si/Al varied in 1.5–2.5, two gelation/polymerization stages and only one gel phase was observed