>Magister Scientiae - MSc / Fly ash is the most abundant of the waste materials generated from coal combustion in coal-fired power stations. South Africa uses more than 100 million tonnes of low grade bituminous coal annually to produce cheap electricity thereby generating huge amounts of fly ash each year. The disposal of fly ash has been a major concern to the world because of its potential environmental impact due to the possible leaching of the toxic elements contained in fly ash. This study centres on the chemical characterization and leaching behaviour of the fly ashes generated from SASOL Synfuels and ESKOM power station at Secunda and Tutuka in South Africa respectively. The aim is to understand the composition of the fly ashes and to determine the leachability of species from the ashes in order to predict the environmental effect of the different ash handling system of the coalfired stations (wet disposal system in Secunda and dry disposal system in Tutuka). Several leaching methods were employed in this study in order to develop a methodology for evaluating and modelling ash system and were able to discriminate between ash types and model ash handling system. Fly ashes from the two South African coal-fired stations were subjected to total acid-digestion and XRF analyses in order to determine the total amounts of major and minor species contained in the fly ashes. The total acid-digestion test and the XRF analysis revealed that the major species such as Al, Si, Ca, Na, Mg, K, Sr, Ba and S04, and minor species such as Fe, Ti, V, Mn, Cr, Ni and Cu were present in both fly ashes in fairly similar concentrations. The mineralogical characterization by XRD of Secunda and Tutuka fly ashes revealed mullite and quartz as the major mineral phases with minor peaks of CaO and calcite. Several leaching tests and different leaching conditions were employed in this study in order to develop a standardized replicable methodology for environmental impact assessment and for modelling the impact of different ash handling scenarios. The fly ashes were exposed to these different leaf leachant of different pHs on the leachability of species from the fly ashes. To achieve this, DIN-S4, TCLP and ANC tests were employed. The natural pH of the fly ash leachates were very high ranging between 12.56 and 13.08. The DIN-S4 leaching test revealed that the easily soluble species of the fly ashes include Ca, Mg, Na, K and S04 and various toxic elements. The leachates from the TCLP test recorded higher concentrations of Ca, Mg, Na, K and S04 which was attributed to the slight decrease in the pH due to the addition of a acidic leachant with a pH of 2.88. Comparison of the amount leached (DIN-S4) from the fly ashes with the total concentrations of each of the components of the fly ashes (determined by the total acid-digestion), the percentage of each of the readily soluble species ranged from 15-24.23% for Ca, 0.23-0.45% for K, 0.58-0.82% for Na, 0.0047-0.007% for Mg, 0.96-3.33% for Ba and 0.012-1.51 % for S04 per dry mass of each component in the fly ash. The ANC test revealed the effect of a leachant of specified pH on the release of species from the fly ashes with concentrations of the major and minor species leached out of the fly ashes found to be higher than the concentrations released into the leachates when DIN-S4 and TCLP test were considered at specific pH and showed the pH dependence of the solubility and release of species. These tests also showed the effect of the liquid to solid ratio upon leachability of species. In addition to the batch leaching tests mentioned above, dissolution kinetics and up-flow percolation tests were carried out on the fly ashes to determine the leaching behaviours of the fly ashes over time and the factors controlling the release of species from the fly ashes in the long term. The dissolution kinetics test was done for an extended period of 60 days with recycle of the leachant and the up-flow percolation test was carried out with constant leachant renewal until a liquid/solid ratio of 20 was attained (:::::;9d0ays). The geochemical computer code PHREEQC and MINTEQ database was used for geochemical modelling of the leachates at various reaction times and LIS ratios. The geochemical modelling results revealed that the release of the species from the fly ashes is controlled by the solubility of mineral phases in many case except for Na. The release of Ca, S04, Mg, Ba and Sr in the leachates of the fly ashes were predicted to be controlled by portlandite, gypsum, brucite, barite and celestite respectively while birnessite, magnetite, BaCr04, CaMo04 and Ba(As04h were predicted to be the mineral phases controlling the release of Mn, Fe, Cr, Mo and As respectively. The pH of the leachates plays a significant role in the leaching of both major and minor species from the fly ashes. The concentrations of species leached into solution at low pH (ANC and TCLP) were higher than the concentrations released at high pH (DIN-S4, dissolution kinetics and up-flow percolation tests). The amounts of the toxic elements such as As, Se, Cd, Cr and Pb that leached out of the fly ashes when in contact with demineralized water (DIN-S4) were very low and below the target water quality range (TWQR) of South African Department of Water Affairs and Forestry (DWAF), but the amounts of As and Se leached out by acidic leachant applied in the TCLP test and at lower pH ranging between 8 and 10 the case of the ANC test were slightly higher than the TWQR, which is an indication that the pH of the leaching solution and the contact time playa significant role on the leaching of species out of the fly ashes. This study revealed that the leaching of species from the fly ashes depends on various factors which include: physical and chemical characteristics and mineralogical composition of the fly ashes, the total concentrations of species in the ash, the rate of flow through the ash system and more importantly the pH of the leachant to which the ash system is exposed to. The results of different experiments and analysis carried out on the two South African fly ashes (Secunda and Tutuka fly ashes) showed that, despite the high concentrations of soluble species or leachable elements in the fly ashes, the leaching of major, minor and trace elements into the soils and the groundwater could be minimized if certain conditions such as avoiding acidic precipitation that could reduce the pH of the ash system are adhered to. The leaching trends of the species and the geochemical modelling data also showed that the formation of secondary mineral phases could reduce the release of toxic elements, the release of which would require aggressive low pH leachants, high flow rate, high recharge and long-term leaching for the dissolution of the formed mineral phases. In conclusion, the combination of the leaching tests employed in this study gives information on the leaching behaviour of the Secunda and Tutuka fly ashes and the factors controlling the leaching of the elements from the fly ashes. This study has been able to show that elements are leached out of the fly ashes at both alkaline and acidic pH. It is also revealed in the study that the disposal techniques employed by the coal-fired stations which were simulated by using the dissolution kinetics and up-flow percolation tests are adequate methods for modelling of the ash disposal scenario. These two methods show that the dry disposal system at Tutuka will encourage equilibration of the ash/water system thereby facilitating the precipitation of mineral phases that could control the release of both major and minor species from the fly ash, whereas the wet ashing system at Secunda may expose the ash to sufficient flow to rapidly leach species out into the environment.

Ground granulated blast furnace slag is a by-product of the steel industry and is often used in combination with ordinary Portland cement as a binder in concrete. When concrete is exposed to high temperatures, physical and chemical transformations lead to significant loss of mechanical properties. This study aims to investigate the effect of high temperatures and rehydration on the mechanical properties, microstructure and phase composition of alkali activated slag. The results of the research could make an important contribution to decisions made concerning the reconstruction of structures affected by fire. In suitable cases it would be possible to regenerate parts of a structure instead of totally rebuilding it.

Aufgrund ihrer speziellen Eigenschaften können sogenannte eisnukleierende Partikel die Bildung von Eis in Wolken katalysieren. Laboruntersuchungen zum Gefrierverhalten dieser Partikel haben sich als wertvoll erwiesen, wenn es um das Verständnis zugrunde liegender Prinzipien und Mechanismen geht. Eine Spezies, die in früheren Untersuchungen vernachlässigt wurde, ist Flugasche aus Kohleverbrennung. Kohle-Flugasche (KFA) wird aufgrund ineffizienter Filterung submikroner Partikel über die Schornsteine von Kraftwerken emittiert und kann, in Abhängigkeit der meteorologischen Bedingungen, die Vereisung von Wolken in der Nähe der Quelle und darüber hinaus beeinflussen. In dieser Arbeit wurde das Immersionsgefrierverhalten, d.h. der Einfluss eingeschlossener Partikel auf das Gefrieren unterkühlter Tropfen, für vier verschiedene KFA-Proben aus deutschen Kohlekraftwerken untersucht. Dabei wurden einerseits Tropfen untersucht, die ein einzelnes submikrones Partikel enthielten. Andererseits wurde das Gefrierverhalten von Suspensionstropfen, die eine Vielzahl verschieden großer Partikel beinhalteten, quantifiziert. Zusätzlich wurden die Proben, sowohl in ihrer Gesamtheit als auch in Form einzelner submikroner Partikel, bezüglich ihrer chemischen Zusammensetzung, Morphologie und Kristallographie analysiert. Es wurde festgestellt, dass die Gefriereffizienz der Proben innerhalb von Minuten abnimmt, sobald diese in Berührung mit Wasser kommen. Immersionsgefriermessungen mit purem Anhydrit (CaSO4 ), das in den Proben nachgewiesen wurde, zeigten einen ähnlichen Trend, d.h. eine abnehmende Effizienz mit zunehmender Suspensionszeit. Diese Beobachtung, und die Übereinstimmung von Messungen mit KFA-Suspensionspartikeln und Gips (CaSO 4 * 2H2O, ein Hydrat des Anhydrits), weisen darauf hin, dass Hydratation die Ursache für die Abnahme der Gefriereffizienz sein könnte. Dieser Einfluss von Probeneigenschaften und Methodologie auf das Immersionsgefrierverhalten von KFA-Partikeln muss bei der Abschätzung der Relevanz der Partikel für die atmosphärische Eisnukleation unbedingt berücksichtigt werden.:1. Introduction 2. Fundamentals 2.1 Ice nucleation theory 2.2 Properties of CFA particles 3. Materials and Methods 3.1 Materials 3.2 Methods 4. Results 4.1 Physicochemical sample characterization 4.2 Immersion freezing behavior of CFA 5. Discussion 5.1 Comparison to literature results 5.2 Physicochemical particle properties and immersion freezing behavior 5.3 Atmospheric implications 6. Summary and Conclusions 7. Outlook / Due to their specific properties, atmospheric ice-nucleating particles are able to catalyze ice formation in clouds. Laboratory studies investigating the freezing behavior of these particles have proven to be of unmatched value when attempting to understand underlying principles and mechanisms. One species that has almost entirely been neglected in previous ice nucleation studies is fly ash from coal combustion (CFA: coal fly ash). Emitted through the stacks of power plants due to inefficient filtering of submicron particles, CFA has the potential to influence cloud glaciation in source regions and beyond, depending on the meteorological conditions. In this thesis, the immersion freezing behavior, i.e., the influence of particles immersed in supercooled cloud droplets on ice nucleation, of four samples from German power plants was determined with the help of several single particle and bulk instruments. In parallel, single particles and bulk CFA were investigated with respect to their chemical composition, morphology, and crystallography. It was found that the immersion freezing efficiency of the CFA particles decreases in contact with water on the time scale of minutes. Hydration products, that were found in both single particles and in the bulk after suspension, could be responsible for this unique behavior. Immersion freezing measurements with pure anhydrite (anhydrous CaSO4 ), which is known to occur at the surface of CFA particles, showed the same qualitative trend, i.e., a decreasing efficiency with increasing suspension time. This observation, and the agreement between measurements with suspended CFA particles and gypsum (CaSO4 * 2H2O, a hydrate of anhydrite), support the hypothesis that hydration causes the observed decrease in immersion freezing efficiency. This influence of sample properties and methodology on the immersion freezing behavior of CFA must be taken into consideration when assessing the relevance of these particles for atmospheric ice nucleation.:1. Introduction 2. Fundamentals 2.1 Ice nucleation theory 2.2 Properties of CFA particles 3. Materials and Methods 3.1 Materials 3.2 Methods 4. Results 4.1 Physicochemical sample characterization 4.2 Immersion freezing behavior of CFA 5. Discussion 5.1 Comparison to literature results 5.2 Physicochemical particle properties and immersion freezing behavior 5.3 Atmospheric implications 6. Summary and Conclusions 7. Outlook

Vattenbyggnadsbetong används som en samlande beteckning på betongkonstruktioner relaterade till vattenkraftanläggningar, dammar och tyngre anläggningar. Med stor framgång har man använt betong under mycket lång tid för dessa typer av konstruktioner. Dessa konstruktioner ställer höga krav på betongens kvalitet och konstruktionsutformning eftersom de förväntas ha en livslängd på ett hundra år eller mer.   Vid gjutning så utsätts betongen för en temperaturstegring som kan leda till sprickbildning i den nygjutna konstruktionen. Eftersom temperaturstegringen är den primära orsaken till sprickrisken så kan man använda olika metoder för att minska detta. Ett verksamt sätt för att minska temperaturstegringen är att försöka hålla cementhalten i betongen så låg som möjligt, eftersom man vid adiabatiska förhållanden kan säga att den totala temperaturstegringen är direkt proportionell mot cementhalten i betongen. Ett verksamt sätt är att minska denna temperaturstegring är att använda sig utav en del flygaska istället för cement. Flygaska är en pulvermassa som fås vid tillverkning av el- och värmeproduktion på kolkraftverken och kraftvärmeverken. Flygaska är ett puzzolant material vilket innebär att det reagerar med kalciumhydroxid och vatten, och kan på så vis delvis ersätta klinker i cement.   Syftet med denna rapport är att ge en ökad förståelse i hur flygaska påverkar vattenbyggnadsbetong. Genom att läsa denna rapport så får man till en början grundläggande kunskaper om vad betong, vattenbyggnadsbetong och flygaska är för något. Vidare avsnitt som behandlas är sprickbildning i vattenbyggnadsbetong, allmän kunskap följt utav orsaker och åtgärder. En jämförelse har gjorts mellan en typisk vattenbyggnads konstruktionsdel, med respektive utan flygaska. Det som jämförts är hur konstruktionen påverkats med respektive utan flygaska med avseende på hållfasthet, beständighet och risk för sprickbildning. Vidare så har temperatursprickberäkningar utförts med programmet HACON. Syftet med beräkningarna var att visa hur olika parametrar med respektive utan flygaska påverkar risken för sprickbildning i en typisk vattenbyggnadskonstruktion.   Resultatet av temperaturberäkningarna visar att man får en lägre temperaturutveckling i en monolit gjuten med flygaska och anläggningscement jämfört med en monolit gjuten med anläggningscement utan flygaska. I och med den reducerade temperaturutvecklingen så uppstod det lägre spänningar i flygaskemonoliten. I undersökningen som utförts i denna rapport visar resultatet att det uppstår dragspänningar som överskrider draghållfastheten i monoliten utan flygaska och därmed spricker konstruktionen. I monoliten som undersökts med flygaska som sprickförebyggande åtgärd uppstår dragspänningar som är lägre än draghållfastheten och därmed spricker inte konstruktionen. Resultatet visar att sprickrisken i en typisk vattenbyggnadskonstruktion kan reduceras med flygaska som sprickförebyggande åtgärd. / Engineering Concrete is used as a collective term for concrete structures related to the hydropower plants, dams and heavier plants. With great success, concrete has been used for a very long time for these types of structures. These constructions make high demands on the concrete quality and construction design as they are expected to have a lifetime of a hundred years or more.   In casting such concrete is exposed to a temperature which can lead to cracking of the newly cast structure. Because the temperature rise is the primary cause of cracking, you can use various methods to reduce this. An effective way to reduce the temperature rise is to try to keep the cement content in the concrete as low as possible, because at the adiabatic conditions may say that the total temperature rise is directly proportional to the cement content in the concrete. An effective way to reduce the temperature rise is to use out some fly ash instead of cement. Fly ash is a powder mass obtained in the production of electricity and heat in coal-fired plants and cogeneration plants. Fly ash is a puzzolanic material which means that it reacts with calcium hydroxide and water, and can thus partially replace cement clinker.   The purpose of this report is to provide a better understanding of how the fly ash affecting hydraulic concrete. By reading this report you will get at first a basic knowledge of what the concrete, hydraulic concrete and fly ash are. Furthermore, the section that dealt with the cracking of the hydraulic concrete, general knowledge followed out causes and remedies. A comparison has been made between a typical water building structural component, with and without fly ash. What has been compared are how the construction affected with and without fly ash on strength, resistance and the risk of cracking. Furthermore, the temperature dot calculations performed with the program HACON. The purpose of the calculations was to show how the different parameters with and without fly ash affects the risk of cracking of a typical hydraulic structures.   The result of the temperature calculations show that you get a lower temperature development in a monolith cast with the fly ash and the construction cements compared to a monolith molded with construction cement without fly ash. With the reduced temperature development arose lower tensions in the flyashmonolith. In the survey carried out in this report, the results show that there is tension that exceeds the tensile strength of the monolith without fly ash and thereby bursting the structure. The monolith examined with fly ash as crackreducing action occurs tensile stresses which are lower than the tensile strength and thus does not crack structure. The results show that the cracking in a typical hydraulic structures can be reduced with fly ash.

>Magister Scientiae - MSc / The waste products acid mine drainage (AMD), formed during coal mining and fly ash (FA) from coal burning power generation, pose substantial environmental and economic problems for South Africa. Eskom has developed a remediation system employing alkaline FA to neutralize and precipitate heavy metals from toxic acidic AMD streams. The aim of this study was to assess the microbial diversity in and microbial impact on this remediation system. The total microbial diversity was assessed by well-established molecular phylogenetic analyses using 16S rDNA gene sequences. The results obtained from the AMD confirmed the presence of acidophilic organisms, such as Acidithiobacillus ferrooxidans (At. ferrooxidans). After co-disposal of FA and AMD, microbial cell growth was not detected and microbial genomic DNA could not be extracted. The absence of microbial communities in the co-disposal phase is beneficial to the continuation of the development of such a co-disposal process. Results of this project will assist in the effective implementation of FA-AMD co-disposal systems, which may improve water quality in effected regions of South-Africa.

Coal plays a significant role in meeting the world's need for energy and will continue to do so for many years to come. Economic, environmental, and public opinion are requiring coal derived energy to be cleaner and operate in a more narrow window of operating conditions. Fouling and slagging of heat transfer surfaces continues to be a challenge for maintaining boiler availability and expanding the use of available fuels and operating conditions. The work incorporates existing information in the literature on ash deposition into a User-Defined Function (UDF) for a commercial comprehensive combustion and CFD code. Results from the new submodel and CFD code is are then compared to deposition measurements in on a simulated boiler tube where particle mass deposited and ash size distribution are measured. Several model components governing various aspects of ash deposition have been incorporated into the UDF which has been implemented in a quasi-unsteady Computation Fluid Dynamics (CFD) simulation. The UDF consists of models governing ash particle impaction and sticking, thermal and physical properties of ash deposits, unsteady growth of the ash deposits, and the effects of the insulating ash layers on the combustion processes. The ash layer is allowed to transition from an accumulation of individual particles, to a sintered layer, and finally to a molten or frozen slag layer. The model attempts to predict the deposit thickness, thermal conductivity, and emittance. Measurements showed fly ash particle sizes that were much smaller than predicted under a non-fragmentation assumption. Use of a fragmentation model matched mean particle diameters well but did not match the upper tail of the particle sizes where inertial impaction takes place. Assuming 100% capture efficiency for all particles provided reasonably good agreement with measured deposition rates. The observed trend of lower deposition rates under reducing conditions was captured when the gas viscosity was calculated using the probe temperature.

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<p>The study focused on examining the effects of using nanoadditives (nano-TiO2 and colloidal silica) on the hydration kinetics, microstructure, mechanical properties, and durability of concretes, especially those containing fly ash and slag and cured at low (4°C) temperature. </p> <p>The results of the Vicat and isothermal calorimetery (IC) tests suggest that the addition of nano-TiO2 accelerates the hydration process of pastes. In addition, the results of the thermogravimetric analysis (TGA) indicated that the addition of nano-TiO2 increased the amount of hydration products in the pastes, with more notable increases observed in fly ash pastes. Moreover, X-ray diffraction (XRD) results revealed that the addition of nano-TiO2 reduced the mean size of calcium hydroxide (CH) crystals.  </p> <p>The interfacial transition zone (ITZ) of concretes with nano-TiO2 was found to be less cracked and less porous when compared to that of concrete without nano-TiO2. Furthermore, the energy dispersive X-ray (EDX) analyses of the outer hydration products around partially hydrated cement particles in fly ash concretes with nano-TiO2 revealed reduction in the values of Ca/Si atomic ratios when compared to the reference fly ash concrete. The image analysis results of the concrete air void system indicated slightly reduced air content, increased specific surface area (SSA), and decreased spacing factor (SF) in concretes with added nano-TiO2. </p> <p>The addition of nano-TiO2 was also found to enhance the compressive and flexural strengths of mortars and concretes. Nano-TiO2 also improved the resistivity and formation factor values of concretes containing fly ash. Moreover, the total volume of pores, as well as the values of water absorption, were also reduced as a result of addition of nano-TiO2. This was true for all types of concretes (i.e., with or without SCMs). Finally, the use of nano-TiO2 seemed to be more beneficial with respect to improving the scaling and freeze-thaw resistance of fly ash concretes compared to cement-only and slag concretes. </p> <p>Concretes with added nanosilica (colloidal silica) also developed higher compressive and flexural strengths when compared to reference concrete. Moreover, the total pores and permeability of concretes decreased due to the addition of nanosilica while the improvement in scaling resistance of these concretes was only slight. Furthermore, concretes with nanosilica were found to have higher percentage of finer air voids compared to reference concretes. Finally, the ITZ of concretes with nanosilica was found to have fewer defects and cracks compared to the reference concrete. </p> <p>In summary, this dissertation presents the results of a study on the multi-scale behavior of nanomodified concretes with and without SCMs cured at both room and low temperatures. Knowledge gained from this study would be helpful in developing concretes with denser and less porous microstructure, a more refined and better-distributed air void system, improved strength, reduced permeability, and enhanced scaling and freeze-thaw resistance, especially in cases when involving the use of SCMs and exposure to low early-age temperatures.</p>