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Carbonation of industrial wastes rich in earth-alkali oxides is found to have a significant potential for CO2 sequestration. This process opens new perspectives not only for carbon dioxide mitigation, but also for the valorization and new applications of industrial waste materials from coal-burning power 2 plants. In this study, mineral carbonation of high-calcium fly ash is investigated under dry and moist conditions in a continuous flow reactor during up to 2 hours, at temperatures ranging from 160 to 290 ºC and CO2 pressures between 1 and 6 bar. A comprehensive charaterization of treated and untreated samples was carried out before and after carbonation using X-ray diffraction, X-ray fluorescence spectroscopy, thermogravimetric analysis, infrared spectroscopy and scanning electron microscopy. The maximum sequestration capacity achieved was 117.7 g CO2/kg fly ash (48.14 % carbonation efficiency) under dry conditions. Results showed that increasing the pressure and temperature enhances the process of carbonation, as well as the presence of moderate amounts of water vapor in the CO2 gas flow. Newly formed carbonates were always present in the treated samples. This study shows that about 21% of all CO2 emissions of a coal-burning power plant could potentially be sequestered as carbonates.

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Limited utilization possibilities of high-calcium fly ashes (HCFA) are a serious issue not only in Europe, but also worldwide. The properties of such waste from coal-fired power plants could be conveniently treated in order to make their compositions compliant with national regulations and allow their use in a variety of industrial applications. This work reports on an investigation of mineral carbonation of HCFA from Greece, Poland and Spain with total CaO contents between 10 and 15 wt.%. Two types of experiments, batch and continuous flow, with and without the addition of water vapor, were performed. Best carbonation efficiency obtained was 47 % of the bulk CaO content. The free lime content of the samples was found to be the controlling factor. After treatment, the amount of free lime was reduced 2 to values suitable for their utilization as supplementary cementitious materials. The addition of water to the system played also an important role in the progress of the carbonation reactions. Our results strongly suggest that a carbonation treatment of HCFA could contribute to the circular economy of such waste materials and potentially increase their utilization in the construction industry, as well as make a significant contribution to lowering of the CO 2 emissions in coal-burning industrial facilities.

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The utilization of high-calcium fly ashes (HCFA) from coal-fired power plants in the construction industry is problematic, since their high free lime contents can lead to durability problems. In this research, the carbonation of a high-CaO fly ash has been carried out using simulated flue gas and concentrated CO2, with the aim to assess the valorization potential of such materials in the construction industry. The results show that, at 7 bars total pressure, an up to 36% carbonation efficiency can be achieved in just 30 min when pure CO2 is used; a comparable result with flue gas requires about 4 h of reaction. On the other hand, experiments carried out at atmospheric pressure show significantly different carbonation efficiencies depending on the CO2 concentration of the gas used. All experiments resulted in a substantial reduction in the original free lime content, and after reaction times of 4 h (at atmospheric pressure) and pressures of 7 bars (for any reaction time >30 min), the final free lime values were low enough to comply with the requirements of European Standards for their utilization as additions in cement.

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This work discloses new insights into the formation and evolution of Mg -carbonates as well as carbonation processes of Mg-rich oxides and silicates with the aim of providing a safe and permanent anthropogenic CO2 storage, helping to tackle the worst effects of climate change. Carbonation reactions were carried in a purpose -built steam-mediated carbonation system at temperature and pressure ranges between 50-205ºC and 1 to 10 bar, respectively. A hydrated amorphous Mg-carbonate was identified upon carbonation of Mg-rich silicates and oxides at 50ºC. Such material might have similar composition and thermal dehydration behavior as nesquehonite. Results from this thesis provided new insights into the enigmatic and yet unconstrained transition of such phase to less hydrated phases. The evolution of Mg - carbonate phases is mainly controlled by the slow dehydration kinetics of Mg2+ rather than evolving to a thermodynamically more stable or structurally similar phase. Despite it has been predicted that such material could straightforward transform into magnesite, it is strongly argued that such transition is greatly inhibited due to preferential nucleation pathways to le ss hydrated Mg-carbonate phases. Phases within the group Mg5(CO3)4(OH)2·XH2O (11=X=4) allows a progressive dehydration whereas the MgCO3·nH2O (n=0) seemingly not. It is proposed that the transition between hydrated amorphous Mg-carbonate to highly disordered dypingite-like phases could occur progressively as it dehydrates and crystallizes, forming dypingite-like phases. The progressive evolution of dypingite-like phases is controlled by the removal of molecular water, inducing cell -shrinkage as well as ordering the internal structure heterogeneity, resulting in a crystalline hydrated structure with the name of hydromagnesite. This might explain the inconsistencies in the solubility and decomposition behavior data reported in the literature for such carbonate phases. No further dehydration is allowed within this group, entailing a significant kinetic barrier in order to allow the transition from hydromagnesite to magnesite. Results from this work shed light into the yet enigmatic evolution of Mg -carbonate phases. The understating of such processes is of paramount importance to accelerate the transition and/or dehydration kinetics among such phases and possibly unlocking preferential nucleation pathways. Brucite carbonation was observed to occur at feasible conversion rates even under simulated flue gas conditions, highlighting the potential of mineral carbonation processes for direct combined CO2 capture and storage/utilization. Carbonation of Mg-rich silicates remained a challenging field under the studied conditions, even for activated serpentine, despite its partial high-reactivity attributed to the presence of a highly-reactive amorphous Mg-rich phase. It was also found the presence of a poorly-reactive Mg-rich amorphous phase (formed upon activation of lizardite) which remained seemingly unreacted upon carbonation. Such observation might provide new insights into the yet unanswered low carbonation efficiencies for direct-carbonation of activated serpentine. Similar carbonation yields were observed for brucite-bearing serpentinized dunite when compared to activated lizardite. Strategically sourcing serpentinized rocks with higher brucite contents will potentially increase the carbonation potential of such materials. Coexisting lizardite and/or forsterite were also observed to be partially carbonated. Carbonation of enstatite and forsterite were also individually studied under conditions relevant to localized early Martian conditions. Enstatite dissolution was observed by the formation of a Si -rich passivating layer, where Si and Mg are heterogeneously distributed. Such observation is consistent with morphological changes within this layer, strongly suggesting an intergrowth of nucleating and growing Mg-carbonates with Si-rich phases. Aquest treball mostra noves percepcions sobre la formació i l’evolució dels carbonats de Mg, així com els processos de carbonatació diòxids i silicats rics en Mg amb l’objectiu de proporcionar un emmagatzematge segur i permanent d’antropogènic CO2, ajudant a fer front als pitjors efectes del canvi climàtic. Les reaccions de carbonatació es van dur a terme en un sistema de carbonatació dissenyat específicament per al des envolupament d’aquesta tesi. Els experiments s’han realitzat a intervals de temperatura i de pres s i ó entre 50-205 ºC i 1 a 10 bar, respectivament. Un carbonat de Mg amorf hidratat es va identificar després de la carbonatació de silicats i òxids rics en Mg a 50 ºC. Aquest material pot tenir una composició i un comportament de deshidratació tèrmica similars a la de la nesquehonita. Els resultats d'aquesta tesi van aportar una visi ó nova de la transició enigmàtica d'aquesta fase amorfa a fases menys hidratades. L’evolució, via deshidratació, de les fases de carbonat de Mg és controlada principalment per la lenta deshidratació de Mg2+ en lloc d’evolucionar cap a una fase termodinàmicament més estable o estructuralment similar. Tot i que s'ha previst que aquest material es podria transformar directament en magnesita, es defensa que aquesta transició està inhibida a causa de les vies de nucleació preferents a les fases de Mg-carbonat menys hidratades. Les fases del grup Mg5(CO3)4(OH)2 ·XH2O (11=X=4) permeten una deshidratació progressiva mentre que MgCO3·nH2O (n=0) aparentment no. L'evolució progressiva de les fases semblants a la dypingita es controla mitjançant l'eliminació d'aigua molecular. A mesura que es deshidrata es produeix la contracció de l’estructura interna, donant com a resultat una estructura hidratada cristal·lina amb el nom d’hidromagnesita. Això podria explicar les incoherències en les dades de solubilitat i de descomposició tèrmica per a aquestes fases. Els resultats d’aquest treball van donar llum a l’encara enigmàtica evolució de les fas es de carbonat de Mg. L’enteniment d’aquests processos és d’importància per accelerar la cinètica de transició i/o deshidratació entre aquestes fases i possiblement desbloquejar vies de nucleació preferents. S’ha observat que la carbonatació de la brucita succeeix ràpidament, fins i tot en condicions de gasos de combustió simulades, recalcant el potencial dels processos de carbonatació mineral per a la captació i l'ús d'emmagatzematge de CO2. La carbonatació de silicats rics en Mg és encara un camp difícil en les condicions estudiades, fins i tot per a la serpentina activada, malgrat la seva alta reactivitat atribuïda a la presència d'una fase rica en Mg amorfa altament reactiva. No obstant això, també es va observar la presència d'una fase amorfa poc reactiva, formada després de l'activació de la lizardita. Aquesta observació podria aportar noves visions sobre la baixa carbonatació directa de la serpentina activada, la qual encara és desconeguda. Es van observar rendiments similars de carbonatació per a dunites serpentinitzades en comparació amb la lizardita activada. L’obtenció estratègica de dunites serpentinitzades amb un contingut més elevat de brucita augmentarà el potencial de carbonatació d’aquests materials. La carbonatació de cristalls d’enstatita i forsterita també es va estudiar individualment a condicions rellevants a l’antic Mars. La dissolució de l’enstatita es va observar inequívocament mitjançant la formació d’una capa passivadora rica en Si, on Si i Mg es distribueixen de forma heterogènia. Aquesta observació és coherent amb els canvis morfològics dins d'aquesta capa