Pozzolanic reactions play a key role in improving the compressive strengths of compacted fly ash-lime specimens. Based on studies performed with cement amended fly ash (FA), activation of fly ash-lime pozzolanic reactions should accelerate the rate of strength development and mobilize larger compressive strengths facilitating improved engineering performance of fly ash amended materials. Further, use of phosphogypsum (PG) is a cause of environmental concern as the material is acidic (pH < 3.0) and contains considerable amounts of fluoride (0.86%). The main research objectives of the thesis are to activate lime-fly ash reactions by thermal and chemical activation process and examine the efficacy of fly ash pozzolanic reactions in controlling fluoride release by phosphogypsum. A comprehensive laboratory experimental program was performed to examine the influence of curing temperature (thermal activation) and calcined PG addition (chemical activation) on lime-fly ash reactions. The kinetics of fly ash-lime reactions are examined by monitoring the reacted lime content as function of curing period and temperature. The influence of variations in fly ash/lime content and dry density on the compressive strength developed by specimens is evaluated. The thermodynamic parameters for the fly ash-lime (FA-L) reactions have been delineated. Fly ash-lime-phosphogypsum (FA-L-G) mixes in slurry and compacted states were monitored for fluoride released as function of curing period The influence of curing temperature in activating fly ash-lime reactions is first examined. Specimens were cured at 25°C (termed RTC or room temperature cured) and at 80° (termed SC or steam cured) to understand thermal activation of fly ash-lime reactions. The rate of lime consumption by SC specimens classified as 2 stage process. The robust increase during stage 2 of steam curing suggested that the lime-solidification reactions did not reach equilibrium even after 4 days of curing at the elevated temperature. While only 3.1 to 3.3 % of added lime was consumed after 28 days of curing at room temperature, much larger amounts of lime ( 8.6-9.3%) were consumed after 4 days of steam-curing. Further, the lime-fly ash reactions were accelerated by 6 to 7 folds on curing the specimens at elevated temperature. The results indicated that activation of lime-fly ash reactions by curing at elevated temperature besides accelerating the rate of strength development also facilitated development of larger strength. Analysis of the free energy change values (ΔG°) indicated that the lime solidification reaction alters from dis-favored (less spontaneous) to favoured (spontaneous) state on curing at 80oC. The positive ΔH° (enthalpy change) values for the fly ash-lime reactions indicated that the reactions are endothermic in nature and are facilitated by increase in curing temperature. Gypsum activation was achieved by addition of 2.5 to 5% calcined phosphogypsum to fly ash lime mixes and curing the compacted specimens at room temperature (FA-L-G specimens). The rate of lime consumption by FA-L-G specimens appeared to be three stage process. The mass of lime consumed by FA-L-G specimens was about 1.5 to 3 folds higher than values of the RTC and SC specimens. Additional lime is consumed by FA-L-G specimens in ettringite formation. A similarity existed between rate of lime consumed and rate of strength developed by the FA-L-G specimens. It is proposed that besides lime solidification reactions, densification of the matrix by filling up of voids by fine gypsum particles and compaction of matrix by the growth of ettringite crystals also contribute to compressive strength of FA-L-G specimens; this additional mechanism of strength development accounts for their higher compressive strength in comparison to the SC and RTC specimens despite similar initial lime addition values. The trend of results suggests that activation of FA-L reactions by calcined PG addition is more effective than steam curing. Comparison of ΔG° values of RTC, SC and FA-L-G specimens revealed that the spontaneity of the lime solidification reactions is least for RTC specimens and improves with addition of phosphogypsum and further improves on curing at elevated temperature. Fly ash-lime pozzolanic reactions substantially reduced the fluoride released from the FA-L-G specimens. The marked reduction in fluoride released by PG amended with fly ash and lime is ascribed to entrapment of PG particles in the cemented matrix formed by fly ash-lime pozzolanic reactions together with consumption of fluoride in formation of insoluble fluoride bearing compounds. The thesis brings out that activation of fly ash-lime reactions leading to quicker and larger compressive strength development is achieved by curing the compacted fly ash-lime specimens at 80°C for 24 hr or by addition of 2.5 to 5% of calcined PG to fly ash-lime mix and curing the compacted specimens at room-temperature. As larger strengths are developed by PG addition than by curing at 80oC, it is recommended that FA-L-G technique be adopted for manufacture of building materials in the civil engineering industry. This technique is also sustainable as it does not require energy for heating which is needed in the steam-curing technique.

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2005. / 192 leaves on CD format, preliminary i-xii pages and numbered pages 1-135. Includes bibliography, list of figures and tables. / ENGLISH ABSTRACT: As a fibre reinforced material, engineered cementitious composite (ECC) has tough, strain-hardening behaviour in tension despite containing low volumes of fibres. This property can be brought about by developments in fibre, matrix and interfacial properties. Poly Vinyl Alcohol (PVA) fibre has been developed in recent years for ECC, due to its high tensile strength and elasticity modulus. However, the strong interfacial bond between fibre surface and matrix is a challenge for its application. This study focuses on the tailoring of matrix and fibre/matrix interfacial properties by cement replacement with fly ash (FA) and Ground Granulated Corex Slagment (GGCS). In this study the direct tensile test, three point bending test, micro-scale analysis, such as X-Ray Fluorescence Spectrometry analysis (XRF), Scanning Electron Microscope (SEM), are employed to investigate the influence of cement replacement, aging, Water/Binder (W/B) ratio, workability on ECC behaviour. This study has successfully achieved the aim that cement replacement by FA and GGCS helps to improve the fibre/matrix interfacial properties and therefore enhances the ECC tensile behaviour. Specifically, a high volume FA-ECC has stable high tensile strain capacity at the age of 21 days. This enables a constant matrix design for the investigation of other matrix influences. The Slag-ECC has a higher tensile strength but lower tensile strain capacity. The combination of FA and GGCS, moderate tensile strength and strain capacity is achieved Both tensile tests and Micro-scale analyses infer that the high volume FA-ECC has an adhesive type fibre/matrix interfacial interaction, as opposed to the cohesive type of normal PVA fibre-ECC. The different tensile behaviour trend of steel fibre-ECC and PVA fibre-ECC with the FA content is presented and discussed in this research. The investigations of aging influence indicate that the high volume FA-ECC has a beneficial effect on the properties of the composite at an early stage. However, at a high age, it has some difficulty to undergo multiple cracking and then leads to the reduction of tensile strain capacity. The modified mix design is made with the combination of FA and GGCS, which successfully increases the interfacial bond and, thereby, improves the shear transfer to reach the matrix crack strength. Therefore, an improved high age tensile behaviour is achieved. The W/B and fresh state workability influence investigations show that the W/B can hardly affect the tensile strain at early age. However, the workability influences on composite tensile strain significantly, because of the influence on fibre dispersion. Other investigations with regard to the hybrid fibre influences, the comparison of bending behaviours between extruded plate and cast plate, the relation between bending MOR and tensile stress, and the relation between compression strength and tensile strength contribute to understand ECC behaviour. / AFRIKAANSE OPSOMMING: As ‘n veselversterkte materiaal, het ontwerpte sementbasis saamgestelde materiale, taai vervormingsverhardingseienskappe in trek, ten spyte van lae veselinhoud. Hierdie eienskap word bewerkstellig, deur ontwikkelings in vesel, matriks en tussenveselbindingseienskappe. Poli-Viniel Alkohol (PVA) vesels is ontwikkel vir ECC, as gevolg van die hoë trekkrag en hoë modulus van hierdie veseltipe. Die sterk binding tussen die PVA-veseloppervlak en die matriks is egter ‘n uitdaging vir sy toepassing. Hierdie studie fokus op die skep van gunstige matriks en vesel/matriks tussenvesel-bindingseienskappe deur sement te vervang met vlieg-as (FA) en slagment (GGCS).In hierdie navorsing is direkte trek-toetse, drie-punt-buigtoetse, mikro-skaal analise (soos die X-straal ‘Fluorescence Spectrometry’ analise (XRF) en Skanderende Elektron Mikroskoop (SEM))toegepas. Hierdie metodes is gebruik om die invloed van sementvervanging,veroudering, water/binder (W/B)-verhouding en werkbaarheid op die meganiese gedrag van ECC te ondersoek.Die resultate van hierdie navorsing toon dat sementvervanging deur FA en GGCS help om die vesel/matriks tussenveselbindingseienskappe te verbeter. Dus is die ECC-trekgedrag ook verbeter. Veral ‘n hoë volume FA-ECC het stabiele hoë trekvervormingskapasiteit op ‘n ouderdom van 21 dae. Dit bewerkstellig ‘n konstante matriksontwerp vir die navorsing van ander matriks invloede. Die Slag-ECC het ‘n hoër treksterkte, maar laer trekvervormingskapasiteit. Deur die kombinasie van FA en GGCS word hoë treksterkte, sowel as gematigde vervormbaarheid in trek verkry. Beide trektoetse en mikro-skaal analise dui aan dat die hoë volume FA-ECC ‘n adhesie-tipe vesel/matriks tussenvesel-bindingsinteraksie het, teenoor die ‘kohesie-tipe van normale PVA vesel-ECC. Die verskille in trekgedrag van staalvesel-ECC en PVA vesel-ECC ten opsigte van die FA-inhoud is ondersoek en word bespreek in die navorsing. Die navorsing toon verder dat die hoë volume FA-ECC goeie meganiese eienskappe het op ‘n vroeë ouderdom. Op hoër ouderdom word minder krake gevorm, wat ‘n verlaging in die trekvervormingskapasiteit tot gevolg het. Met die kombinasie van FA en GGCS, word die vesel-matriksverband verhoog, waardeur ‘n verbetering in die skuifoordrag tussen vesel en matriks plaasvind. Verbeterde hoë omeganiese gedrag word daardeur tot stand gebring. Navorsing ten opsigte van die invoed van die W/B en werkbaarheid dui daarop dat die W/B slegs geringe invloed het op die trekvormbaarheid, terwyl die werkbaarheid ‘n dominerende rol speel in hierdie verband.Verdere studies sluit in die invloed van verskillende vesels, die vergelyking van die buigingsgedrag van geëkstueerde plate en gegote plate, die verhouding tussen buigsterkte en treksterkte, en die verhouding tussen druksterkte en treksterkte dra by tot beter begrip van die gedrag van ECC.

>Magister Scientiae - MSc / Coal combustion accounts for over 40 % of the world's energy production and this figure is projected to increase with increasing human population and industrialization. The combustion of coal leads to the generation of waste products such as fly ash (FA), brine from water treatment, bottom ash, slag, flue gas desulphurization products (FGD) and gas emissions such as N20, and C02. The emissions contribute to air pollution and global warming, while FA, brines, and FGD are possible soil and water pollutants. In order to minimize the environmental impact of coal combustion, mitigation of the effects of coal burning processes such as the waste products (FA, brine, bottom ash, slag and FGD) and gas emissions is required. This study investigated utilization of the Secunda FA (class F) and reverse osmosis (RO) Tutuka brine to sequester C02 in an attempt to make coal power production more environmentally sustainable. It was hypothesized that South African FA and brine could sequester C02 through mineral carbonation. A statistical approach was undertaken to optimize the % CaC03 formed from FAlbrine/C02 interaction with input parameters of temperature, pressure, particle size and solid/liquid ratio (S/L) being varied. The ranges adopted for the input parameters were: temperature of 30°C or 90 °C; pressure of 1 Mpa or 4 Mpa; four particle sizes namely bulk ash, > 150 11m, < 20 11m and 20 urn- 150 11m particle size range; S/L ratios ofO.1, 0.5 or 1. The FA! brine dispersions were carbonated in a high pressure reactor varying the above mentioned input parameters. The fresh Secunda FA of various size fractions was characterized morphologically using scanning electron microscopy, chemically using X-ray fluorescence and mineralogically using qualitative X-ray diffraction. The carbonated solid residues on the other hand were characterized using quantitative X-ray diffraction, scanning electron microscopy, thermal gravimetic analysis and Chittick tests. The raw brine from Tutuka together with the carbonation leachates were characterized using inductively coupled mass spectrometry and ion chromatography. Total acid digestion was carried out to evaluate the differences in the total elemental content in both the fresh ash and the carbonated solid residues. The results suggested that South African FA from Secunda belongs to class F based on the CaO content as well as the total alumina, silica and ferric oxide content, while the RO brine from Tutuka were classified as NaS04 waters. Mineral carbonation occurred and ranged between 2.75 % and 6.5 % of CaC03 depending on the input parameters. Two polymorphs of CaC03 were identified in the carbonated residues i.e. calcite and aragonite. The carbonated ash/brine leachates were cleaner with respect to major and trace element concentration compared to raw brine thus the carbonation process could be used to improve the quality of brines generated in the power industry. Removal of the major elements from brine was as follows Ca-74.8 %, Na- 28.7 %, Mg- 98 %, K- 82.9 %, S04- 20.8 %. Hundred percent removal was observed for traces of Fe, Al, Mn, Cu, Zn, Pb, Ni, As, Ti, Sr, Se, Si and N03. However Mo, V, B, and Cl concentrations increased by 72.5 %, 94 %,48.2 % and 7.2 % respectively after carbonation at 90°C, 4 Mpa, S/L ratio of 1 using the bulk ash. The parameters found to be of most significance in the carbonation process were the main effects of temperature, particle size and S/L ratio while the interactions of temperature and particle size as well as the interaction of temperature with S/L ratio were also found to be significant. The statistical approach led to a clear understanding of the effect of each input parameter as well as the ansmg interactions. The conditions of 90°C, 4 Mpa, using bulk ash at a S/L ratio of 1 resulted in the highest yield of % CaC03 with a value of 6.5 %. Theoretically one ton of Secunda FA containing 9.2 % of CaO could sequester 0.083 tons of C02. With the optimized protocol developed in this study bearing in mind that the carbonation efficiency is 75.54%, 1 ton of Secunda FA could sequester 0.062 tons of CO2. This translates to 0.65 % of CO2 produced annually at Secunda plant being sequestered in the FAlbrine dispersions. In other words, 16 tons of FA are required to sequester a ton of C02 annually. It was also observed that carbonation using brine resulted in higher carbonation efficiency than carbonation using water as the Ca2+ component in the brine contributed towards the Ca 2+concentration.