Synthesis and characterization of the fly ash based geopolymer

Istražena je sinteza geopolimera temeljenih na alkalnoj aktivaciji letećeg pepela iz domaće termoelektrane ložene ugljenom (TE Plomin 2, Plomin) u cilju razvoja novih, ekološki prihvatljivih, kvalitetnih i ekonomski povoljnijih anorganskih mineralnih veziva, kao alternative portlandskom cementu, odnosno kao adsorbensa u procesima završne obrade otpadnih voda opterećenih ionima teških metala (bakra). Uzorcima geopolimera (serije A, B, T1 i T2), sintetiziranih aktivacijom letećeg pepela s alkalno-silikatnom otopinom (Na2SiO3, molarni omjer SiO2/Na2O, Ms = 1,2-1,4) istraživana su strukturna, mehanička i sorpcijska (ravnoteža i kinetika sorpcije bakra u šaržnom reaktoru) svojstva. Pripremljeni uzorci geopolimera su imali promjenjiv sastav obzirom na zamjenske dodatke letećem pepelu i na omjer aktivacijska otopina/suha tvar. Kao zamjenski dodaci letećem pepelu korišteni su mulit, amorfni SiO2 (mikrosilica), Al(OH)3, Al2(SO4)3, NaAlO, Al2O3, Ca(OH)2, i Na3PO4·12H2O. Za pripremu geopolimernih mortova kao agregat je korišten kalcijev karbonat (CaCO3). Metodama Ramanove spektroskopije (RS), rendgenske difrakcije (XRD), pretražne elektronske mikroskopije s energijsko-disperzijskom spektroskopijom (SEM-EDS) te Fourierove transformirane infracrvene spektroskopije (FTIR) praćen je razvoj vezivne faze i izvršena je karakterizacija reakcijskih produkata. Uočeno je da staklasta faza iz letećeg pepela reagira s otopinom aktivatora, a glavni produkt reakcije geopolimerizacije je amorfni alkalijski alumosilikat. Njegovo nastajanje potvrđuju apsorpcijske vrpce na FTIR spektrima uzoraka karakteristične za asimetrično istezanje veze T-O (T = Si ili Al) uočene na nižim valnim brojevima (1010-1020 cm-1) u odnosu na one prisutne u izvornom uzorku letećeg pepela (1074 cm-1). Strukturna analiza nastalih faza u uzorcima geopolimera provedena je primjenom Ramanove spektroskopije. Radi određivanja stupnja polimerizacije i prisutnih tipova SiO2 tetraedara u sintetiziranim uzorcima geopolimera, rezultati analize ispitivanih uzoraka su uspoređeni s odgovarajućim analizama srodnih kristaličnih i amorfnih silikatnih materijala. Dobiveni rezultati ukazuju na staklu slično ponašanje sintetiziranih geopolimera i otvaraju novu perspektivu u istraživanjima mehanizma geopolimerizacije. Rezultati također upućuju na postojanje reakcija polimerizacije bez prisustva aluminija. Na temelju dobivenih rezultata Ramanovog spektra razvijen je i predložen koncept i izračun Indeksa Depolimerizacije Geopolimera (GDI), kao približna mjera sadržaja depolariziranih SiO4 tetraedara u geopolimerima na bazi letećeg pepela. Iako je nuklearna magnetska rezonancija (NMR) u čvrstom stanju široko prihvaćenametoda za određivanje tipova polimerizacije SiO4 tetraedara prisutnih u geopolimerima, dobiveni rezultati potvrđuju i predlažu Ramanovu spektroskopiju kao jednako korisnu metodu. Uzorci geopolimera, sintetizirani pri 70º/1 h, su korišteni za ispitivanje tlačne čvrstoće te ravnoteže i kinetike sorpcije bakra u šaržnom reaktoru. Za određivanje utjecaja omjera sastojaka na tlačnu čvrstoću i sorpcijske karakteristike sintetiziranih geopolimera primijenjen je Taguchijev pristup u planiranju eksperimenta. Rezultati pokazuju gotovo linearan utjecaj dodatka Ca(OH)2 geopolimerizacijskom sustavu na tlačnu čvrstoću uzoraka, a negativan utjecaj dodatka Al2(SO4)3 na tlačnu čvrstoću i na sorpcijske karakteristike uzoraka. Freundlichova izoterma pokazuje najbolje slaganje s ekspermentalnim podacima za ispitivani uzorak geopolimera pripremljen uz dodatak kalcitnog agregata, ali ta izoterma ne daje mogućnost određivanja ravnotežnog kapaciteta. Sipsova i Langmuirova izoterma pokazuju najbolje slaganje s dobivenim eksperimentalnim rezultatima za geopolimer pripremljen bez dodatka agregata. Kinetički model sorpcije bakra na uzorcima geopolimera koji najbolje opisuje eksperimentalno dobivene podatke je jednadžba pseudo-drugog reda za male koncentracije sorbata te Elovichev model za više koncentracije sorbataThe synthesis of geopolymers based on the alkaline activation of fly ash from the domestic coal-fired Thermal Power Plant (TPP Plomin 2, Plomin) has been investigated. The primary objective was to explore an effective way to the synthesis and development of new, environmentally friendly, sufficiency performing and cost effective inorganic mineral binders as an alternative to Portland cement, i.e., as adsorbents in the final treatment of wastewater charged with ions of heavy metals (copper). Geopolymer samples (series A, B, T1, T2) synthesized by activation of the coal fly ash with alkali-silicate solution (Na2SiO3, molar ratio SiO2/Na2O, Ms = 1.2-1.4) were used for investigation of the structural, mechanical and sorption (equilibrium and kinetics of copper sorption in batch reactor) properties of these materials. The prepared geopolymer samples had a variable composition with respect to the fly ash replacement additions and to the ratio of the activation solutionto-dry substance. As the fly ash replacement additions, mullite, amorphous SiO2 (microsilica), Al(OH)3, Al2(SO4)3, NaAlO, Al2O3, Ca(OH)2, i Na3PO4·12H2O were used. Calcium carbonate (CaCO3) was used as an aggregate for the preparation of geopolymer mortars. Analytical techniques including Raman Spectroscopy (RS), X-ray diffraction (XRD), scanning electron microscopy with energy dispersion spectroscopy (SEM-EDS), and Fourier's Transformed Infrared Spectroscopy (FTIR) were used to investigate the binding phase development and the reaction product characterization. It was observed that the glass phase of the fly ash reacts with activator solution, and that the main product of the geopolymerization reaction is an amorphous alkali aluminosilicate. Its formation was confirmed by the characteristic absorption bands on the FTIR spectra of the synthesized samples belonging to the asymmetric stretching of the T−O (T = Si or Al) at lower wave numbers (1010-1020 cm-1) on the FTIR spectra compared to the same ones present in the original fly ash sample (1074 cm-1). Structural analysis of the phases produced in the samples of the geopolymer was carried out using Raman's spectroscopy. To determine the degree of polymerization and the present types of SiO2 tetrahedra in the synthesized samples of the geopolymer, the results of the analyzes of the examined samples were compared with the corresponding analyzes of related silicate materials, both crystalline and amorphous. The obtained results suggest glass-like behavior of the investigated samples and introduce a new perspective in research of geopolymerization mechanism. The results also suggest the existence of the aluminum-free polymerization reactions. Based on the obtained results ofthe Raman’s spectra, concept and calculation of the Geopolymer Depolymerization Index (GDI), as an approximate measure of the content of the depolymerized SiO4 tetrahedra in a fly ash-based geopolymer have been development and proposed. Although solid state nuclear magnetic resonance (NMR) is a widely accepted method for the determination of SiO4 tetrahedron polymerization types present in the geopolymer, the obtained results confirm and suggest Raman's spectroscopy as equally useful method. Geopolymer samples, synthesized at 70º/1h, were used for determination of the compressive strength, and for determination of the equilibrium and kinetics of copper sorption in the batch reactor. To determine the influence of component ratios on the compressive strength and sorption properties of the synthesized geopolymer, Taguch's approach to experiment design was applied. The results show the almost linear influence of the Ca(OH)2 addition to the geopolymerization system on the compressive strength of the samples, and the negative influence of the Al2(SO4)3 addition on both the compressive strength and the sorption characteristics of the samples. Freundlich's isotherm shows the best fitting with the experimental data obtained for the analyzed geopolymer samples prepared with the addition of calcite aggregate, however, this isotherm does not provide the ability for the equilibrium capacity calculation. Sips’s and Langmuir’s isotherms show the best fitting with the obtained experimental results for the geopolymer samples prepared without the addition of calcite aggregate. The kinetic model of copper sorption on the geopolymer samples that best describes the experimental data is the pseudo-second order equation for the low concentrations of sorbate, and Elovichev's model for higher concentrations of sorbate concentratio