Produzione e caratterizzazione di schiume inorganiche a base geopolimerica

Scopo di questa tesi di laurea sperimentale (LM) è stata la produzione di geopolimeri a base metacaolinitica con una porosità controllata. Le principali tematiche affrontate sono state: -la produzione di resine geopolimeriche, studiate per individuare le condizioni ottimali ed ottenere successivamente geopolimeri con un’ultra-macro-porosità indotta; -lo studio dell’effetto della quantità dell’acqua di reazione sulla micro- e meso-porosità intrinseche della struttura geopolimerica; -la realizzazione di schiume geopolimeriche, aggiungendo polvere di Si, e lo studio delle condizioni di foaming in situ; -la preparazione di schiume ceramiche a base di allumina, consolidate per via geopolimerica. Le principali proprietà dei campioni così ottenuti (porosità, area superficiale specifica, grado di geopolimerizzazione, comportamento termico, capacità di scambio ionico sia delle resine geopolimeriche che delle schiume, ecc.) sono state caratterizzate approfonditamente. Le principali evidenze sperimentali riscontrate sono: A)Effetto dell’acqua di reazione: la porosità intrinseca del geopolimero aumenta, sia come quantità che come dimensione, all’aumentare del contenuto di acqua. Un’eccessiva diluizione porta ad una minore formazione di nuclei con l’ottenimento di nano-precipitati di maggior dimensioni. Nelle schiume geopolimeriche, l’acqua gioca un ruolo fondamentale nell’espansione: deve essere presente un equilibrio ottimale tra la pressione esercitata dall’H2 e la resistenza opposta dalla parete del poro in formazione. B)Effetto dell’aggiunta di silicio metallico: un elevato contenuto di silicio influenza negativamente la reazione di geopolimerizzazione, in particolare quando associato a più elevate temperature di consolidamento (80°C), determinando una bassa geopolimerizzazione nei campioni. C)Effetto del grado di geopolimerizzazione e della micro- e macro-struttura: un basso grado di geopolimerizzazione diminuisce l’accessibilità della matrice geopolimerica determinata per scambio ionico e la porosità intrinseca determinata per desorbimento di N2. Il grado di geopolimerizzazione influenza anche le proprietà termiche: durante i test dilatometrici, se il campione non è completamente geopolimerizzato, si ha un’espansione che termina con la sinterizzazione e nell’intervallo tra i 400 e i 600 °C è presente un flesso, attribuibile alla transizione vetrosa del silicato di potassio non reagito. Le prove termiche evidenziano come la massima temperatura di utilizzo delle resine geopolimeriche sia di circa 800 °C

Geopolymers with tailored porosity

Geopolymers are synthetic materials formed by alkali-activation of aluminosilicate particles. They have attracted increasing attention as sustainable materials, being obtained from different raw materials, including industrial by-products, and by production processes at low temperature. Thanks to the good properties showed by these materials (thermal stability, fire-resistance, etc.), and the intrinsic mesoporosity, geopolymers have been studied as new materials for applications in many industrially relevant fields. To achieve full advantage of their porous structure, it is necessary to control its formation. The geopolymer production process in aqueous medium allows to tailor the porosity from nanometric to millimetric range since water acts as pore former. Moreover, ultra-macroporosity may be induced in the materials exploiting different techniques, commonly used for the production of porous ceramics, determining the possibility to obtain materials with different architectures, pore size and shape, etc. Hierarchical pore systems, where the mesopores of the geopolymer skeletal materials are directly connected to macro- and finally to ultra-macropores, may be constructed in this way. The main goal of this research project was to investigate the use of different process techniques applied to geopolymer matrices to generate porous structures characterized by peculiar porosities able to determine specific properties and functionalize the materials. In detail, the porosity was induced by direct foaming or addition of lightweight aggregates. Furthermore, geopolymers with main unidirectional anisotropic macropores were produced, for the first time, using a freeze-casting technique. All the materials produced were deeply investigated to optimize the production processes and evaluate the final properties, many of which arising from the intrinsic and induced porosity generated, in order to address the materials for potential applications as, for example, thermal insulating panels or heat transfer devices

Achievement and exploitation of porous geopolymer-based spheres

Several spherification processes were applied to obtain porous geopolymer spheres, different in term of porosity, specific surface area and adsorption properties. The processes are based on the formulation of metakaolin-based geopolymer slurries, produced using a potassium- or sodium-based alkaline activating solution, and exploiting an injection-solidification method in different mediums, i.e. polyethylene glycol (PEG), liquid nitrogen or calcium chloride, to produce the spheres. When liquid nitrogen was used, the geopolymer slurries underwent a maturation step (several hours at room temperature) to trigger the geopolymerization without reaching a complete chemical consolidation. Spheres were obtained injecting in liquid N2 the mixture as it is or mixed with water, to modulate the final porosity (ice-templating process). The spheres were then freeze dried to remove the solidified water and complete the chemical consolidation of the geopolymer. Please click Additional Files below to see the full abstract

Lightweight insulating geopolymer material based on expanded perlite

Expanded perlite, owing to its lightweight and excellent thermal insulating properties, has been extensively used in different industrial sectors to produce self-standing insulating boards bonded with various organic polymers or calcium-silicates. In order to improve the high temperature behavior and mechanical performances of such materials inorganic binders, such as geopolymers, can be regarded as a promising alternative. Please click Additional Files below to see the full abstract

Geopolymer-zeolite composites for CO2 adsorption

Geopolymer-zeolite composites were produced mixing different geopolymer matrices with a synthetic commercial Na13X zeolite, to combine the functional microporosity of the zeolite with the mesoporosity of the geopolymer matrix, with the further possibility to consolidate the zeolite powder. The new materials were designed and produced in forms of monoliths to be used as adsorbents for low temperature CO2 capture applications. A potassium or sodium silicate activating solution was used to produce the metakaolin-based geopolymer matrices, then mixed with the synthetic zeolite used as a filler. As geopolymers can be regarded as the amorphous counterpart or precursor of crystalline zeolites, it is important to underline the chemical affinity between these two constituents. As a matter of fact, the morphological characterization evidenced the presence of geopolymer nanoprecipitates covering zeolite particles for the K-based composite, while in the Na-based composite the formation of a NaA zeolite phase was evidenced (Fig. 1). Please click Additional Files below to see the full abstract

Ammonium recovery from municipal wastewater by ion exchange: Development and application of a procedure for sorbent selection

Ion exchange represents one of the most promising processes for ammonium recovery from municipal wastewater (MWW). However, most previous studies on ammonium ion exchange did not optimize the process or evaluate its robustness under real operational conditions. This experimental study aimed at (i) developing a procedure for the selection of a sorbent for selective ammonium removal/recovery from MWW, (ii) validating the procedure by applying it to several sorbents, (iii) performing a preliminary optimization and robustness assessment of ammonium removal/recovery with the selected sorbent. The application of the procedure to natural and synthetic zeolites and a cation exchange resin confirmed that batch isotherm tests need to be integrated by continuous-flow tests. The selected sorbent, a natural mixture of Chabazite and Phillipsite, resulted in high performances in terms of cation exchange capacity (33 mgN gdry resin-1), ammonium operating capacity (5.2 mgN gdry resin-1), ammonium recovery yield (78-91%) and selectivity towards ammonium. The process performances resulted stable during 7 adsorption/desorption cycles conducted with MWW treatment plant effluents in a 60-cm column. The switch to a highly saline effluent produced in a hotspot of seawater intrusion did not determine significant changes in performances. Contact time was reduced to 6 min without any decrease in performances. Potassium – well tolerated by crops – was selected as the regenerating agent, in the perspective to produce a desorbed product to be re-used as fertilizer. The study shows that Chabazite/Phillipsite has a high capacity to recover ammonium from MWW in a circular economy approach

Geopolymers adsorbents: Production and use

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Synthesis and properties of new geopolymeric foams

Geopolymers are innovative, versatile and cheap inorganic materials with a wide number of industrial applications, having, furthermore, obtained in environmentally friendly conditions [1]. In previous papers, the synthesis and thermal stability of geopolymers were deeply investigated [2,3]; the aim of this study was to develop new geopolymeric foams with tailored porosity in the nano-ultramacro range, in the view of potential applications in the thermal insulation, catalysis, filtration, biomaterials, etc. Geopolymers have been prepared starting from metakaolin and potassium silicate; the process conditions were varied to change the intrinsic nano-micro-porosity of the material and study their influence on the geopolymerization degree. Optimum geopolymerization conditions were selected to develop porous 3D networks by inducing interconnected ultra-macro-porosity (up to millimetric range) in the material, exploiting the ability of Si powder to generate H2 by reaction with H2O (Fig. 1). The in situ foaming was strongly dependent on H2O content of the precursors and the successive process of H2O elimination. The H2 formation is in fact a H2O consuming process, thus increasing the viscosity, as consolidation occurs. The geopolymeric inorganic resins and the related foams were fully characterized in term of microstructure, intrinsic and induced porosity size distribution, specific surface area, geopolymerization degree and surface accessibility. The thermal behavior of the materials was also deeply investigated. The experimental findings highlighted the versatility of these foams, that may be properly tailored as a function of the possible final applicatio

Alkali-bonded inorganic polymers: environmentally friendly and sustainable materials with various applications

Alkaly-aluminosilicate binders are synthetic inorganic polymers set at low temperature to produce ceramic type materials using an easier process than that usually required for ceramic production. Alkali-bonded devices can be produced by moulding while machining and finishing are generally not required. Additions of different inert fillers allow to reinforce or functionalize the materials for specific applications, such as thermal insulation and heat resistance in the field of building and infrastructures or transportation, etc. Actually, alkali-bonded composites have a high heat tolerance because they do not burn or ignite, they do not release gases or smokes and they do not "explode" because they do not incorporate waters of hydration within the structure as hydraulic cements do. Industrial wastes and by-products can be recycled as raw materials or inert fillers for this kind of application. An overview of composite materials prepared by exploiting the binder as a glue for other phases (SiC, Al2O3, non hazardous wastes) and showing porosity in the range nano-micro - ultramacro are presented for different application

Pressurized Steam Conversion of Biomass Residues for Liquid Hydrocarbons Generation

Biomass residues are often considered as a resource if conveniently converted in fuel and alternative feedstock for chemical processes, and their conversion into valuable products may occur by different pathways. This work is focused on the thermochemical conversion at moderate temperature and in steam atmosphere, a mild process in comparison to hydrothermal liquefaction, followed by extraction of soluble products in a solvent. Such process has been already applied to various residues and here extended to the case of marc, the residual pomace from wine making, largely produced worldwide. A pressurized batch reactor was used for the quantitative determination of produced solid and liquid fractions, and their qualitative characterization was performed by instrumental analyses. The pressurized steam conversion of marc was effective, providing a yield in liquid fraction, upon extraction in solvent, up to 30% of the raw dried biomass. The use of polar and nonpolar solvent for the extraction of the liquid fraction was inspected. Applied operating conditions, namely residence time in the batch reactor and extraction modality, showed a significant influence on the process performance. In particular, long residence and extraction times and use of nonpolar solvent substantially improved the yield in liquid fraction