Geopolymer concretes synthesised from composite class F fly ashes and a mixed alkaline activator were optimised by use of Taguchi orthogonal design method. The optimised mix achieved a compressive strength at the age of 28 days of 70 and 58 MPa after initial curing at 70??C for 12 hours and at 23??C for 24 hours, respectively. The resultant Geopolymer has an amorphous aluminosilicate structure. Efflorescence and the potential risk of alkali-silica reaction for the Geopolymer used in this study are both very low. The research confirmed that the Geopolymer concrete developed in this study is far superior to Portland cement concrete when exposed in a sulphuric acid environment. The standard immersion tests finally selected for this research were in 10% sulphuric acid for 56 days and in 1% sulphuric acid for one year. Geopolymer concrete samples retained their shape without softening though they experienced a mass loss of about 5% and a strength loss of some 30%. Portland cement concrete recorded a mass loss of some 40% in a 10% sulphuric acid for 28 days. The penetration rate of sulphuric acid into the Geopolymer concrete was found to approximately follow Fick’s first law of diffusion and a linear relationship between the neutralisation depth and the square root of immersion time (in day) was established. The degradation processes of Geopolymer concrete in sulphuric acid environments were intensively studied. The first stage involved the preferential liberation of alkali ions. The tetrahedral aluminium in the Si-O-Al configuration was removed and converted to octahedral aluminium. Consequently, the original units of Si(1Al) degraded to a silica polymorph structure in the corroded Geopolymer, which continued to serve a cementitious role. In contrast, in the case of Portland cement concrete, the acid solution dissolved the hydration products of the cement paste. The residual reaction products were found to be soft and have no structural strength. Geopolymers with alkaline activators of mixed sodium hydroxide and sodium silicate did not exhibit any cracking problems. Class F fly ash with low calcium content was found to be suitable for developing a Geopolymer binder able to withstand sulphuric acid attack.

Geopolymer concretes synthesised from composite class F fly ashes and a mixed alkaline activator were optimised by use of Taguchi orthogonal design method. The optimised mix achieved a compressive strength at the age of 28 days of 70 and 58 MPa after initial curing at 70??C for 12 hours and at 23??C for 24 hours, respectively. The resultant Geopolymer has an amorphous aluminosilicate structure. Efflorescence and the potential risk of alkali-silica reaction for the Geopolymer used in this study are both very low. The research confirmed that the Geopolymer concrete developed in this study is far superior to Portland cement concrete when exposed in a sulphuric acid environment. The standard immersion tests finally selected for this research were in 10% sulphuric acid for 56 days and in 1% sulphuric acid for one year. Geopolymer concrete samples retained their shape without softening though they experienced a mass loss of about 5% and a strength loss of some 30%. Portland cement concrete recorded a mass loss of some 40% in a 10% sulphuric acid for 28 days. The penetration rate of sulphuric acid into the Geopolymer concrete was found to approximately follow Fick’s first law of diffusion and a linear relationship between the neutralisation depth and the square root of immersion time (in day) was established. The degradation processes of Geopolymer concrete in sulphuric acid environments were intensively studied. The first stage involved the preferential liberation of alkali ions. The tetrahedral aluminium in the Si-O-Al configuration was removed and converted to octahedral aluminium. Consequently, the original units of Si(1Al) degraded to a silica polymorph structure in the corroded Geopolymer, which continued to serve a cementitious role. In contrast, in the case of Portland cement concrete, the acid solution dissolved the hydration products of the cement paste. The residual reaction products were found to be soft and have no structural strength. Geopolymers with alkaline activators of mixed sodium hydroxide and sodium silicate did not exhibit any cracking problems. Class F fly ash with low calcium content was found to be suitable for developing a Geopolymer binder able to withstand sulphuric acid attack.

Four sphalerites (synthetic, high grade natural, moderately impure flotation concentrate and highly impure flotation concentrate) were leached in acid sulphate media without and with ferric ions present under the following conditions :- Case (i) [Fe3+]o : [H2S04]o = 0,0 Case ( ii) [Fe3+]o : [H2S04]o = 1,8 Case (iii) [Fe3+]o : [H2S04]o = 0,1 Extensive data for leaching under these conditions are tabulated. Kinetic mechanisms based on Langmuir-Hinschelwood adsorption theories were proposed, and leaching models were developed for different assumed rate limiting steps. The initial rate and overall forms of the models were tested using experimental data.Leaching under case (i) conditions Non-oxidative dissolution took place with Zn2+ and H2S the predominant reaction products. The H2S partial pressure was monitored continuously and solution samples were taken for analysis at discrete time intervals. Vibratory (i.e. attrition) milling eliminated very large differences observed in the leaching characteristics of course size fractions of the natural sphalerites. The initial rate form of a model based on a dual site reaction mechanism and on either H+ adsorption or reaction product desorption rate control was found to fit the data for the synthetic and vibratory milled forms of sphalerite. The most impure vibratory milled sphalerite adsorbed Zn2+ and H2S very strongly, and this resulted inproduct desorption rate control. Vibratory milled forms of the high grade natural sphalerite and the moderately impure flotation concentrate, exhibited virtually identical initial rate dissolution kinetics, despite large differences in their chemical compositions. Leaching under case (ii) conditions Oxidative dissolution took place with Zn2+ and elemental sulphur the predominant reaction products. Scanning electron microscope photographs of leached and unleached particles showed the sulphur present on the particle surface. These photographs, and optical microscope photographs of etched polished sections, showed that dissolution took place in a complex way. A model based on ferric ion adsorption as the rate limiting step was proposed and confirmed experimentally. The model demonstrated a proportional dependency of the rate on the area and ferricion concentration, and an inverse dependency on the hydrogen ion concentration. For a -90,0 + 63,0 um size fraction, the three natural sphalerites exhibited virtually identical dissolution rates per unit area. The effect of ball milling or vibratory milling the sphalerites fine, was to increase the rate per unit area for the most impure natural sphalerite but decrease the rate per unit area for the high grade natural sphalerite.It was shown that for course size fractions of sphalerite, the most impure sphalerite which leached slowest under case (i) conditions (i.e. adsorbed H+ poorly) leached fastest under case (ii) conditions (i.e. adsorbed Fe3+ strongly). The reverse was true for the high grade natural sphalerite. Except in the case of synthetic sphalerite leaching under case (i) conditions, no correlation was shown to exist between the way the B.E.T. measured area changed, and the way the calculated active area changed during leaching. Leaching under case (iii) conditions Oxidative and non-oxidative dissolution, as well as H2S oxidation by Fe3+ occured simultaneously. The extents to which oxidative or non-oxidative dissolution occured could be explained in terms of the hydrogen ion and ferric ion adsorption characteristics of the sphalerites.The ferric ion oxidation of H2S was studied in the absence and presence of solids, and the presence of sphalerite or activated charcoal catalysed this reaction. No advantage was gained by leaching in the presence of activated charcoal with or without Fe3+ present, unless conditions were such that H2S was formed as a product of reaction. / Thesis (Ph.D.)-University of Natal, Durban, 1977.

Please read the abstract in the section 00front of this document. / Dissertation (MSc (Chemistry))--University of Pretoria, 2007. / Chemistry / unrestricted

Elemental sulphur (S⁰) is removed from sour gas deposits (high H₂S) during refinement. The resulting S⁰ is often stored onsite when the costs of shipping S⁰ to market exceeds the costs of storing it in large above ground blocks. With the aid of acidiphilic bacteria, atmospheric air and water oxidize S⁰ to sulphate (SO₄^2-). Long term storage is under consideration; however, oxidation rates and the role of each oxygen source (O_2(g) and H₂O) is not clear. S⁰ oxidation experiments were conducted over a range of temperatures (6-32¡ãC) to investigate reaction rates and isotopic fractionation of O and S isotopes during oxidation. The experiments also investigated the effect of integrating S⁰ oxidizing microorganisms and available nutrients on both the reaction rates and isotope fractionation. Results indicated > 95% of total SO₄^2- generated can be attributed to autotrophic microbial activity. Experiments conducted in a nutrient rich mineral solution showed rates increase with temperature from 0.16 (6¡ãC) to 0.98 (32¡ãC) ¦Ìg S⁰ cm^-2 d^-1 (Q₁₀ ¡Ö 1.7 - 1.9). Experiments conducted in a nutrient poor solution (deionized water) showed oxidation rates did not increase with temperature (0.06 to 0.08 ¦Ìg S⁰ cm^-2 d^-1) between 12 and 32¡ãC. Oxygen isotope analysis of the generated SO₄^2- indicated essentially all oxygen incorporated into the SO₄^2- originated from H₂O. In addition, effluent samples obtained from S⁰ block effluent at SCL indicated ¦Ä¹⁸O_(SO4) generally reflected the ¦Ä¹⁸O_(H2O) in the system at the time of oxidation. While covering the S⁰ blocks with an impermeable cover would undoubtedly minimize total SO₄^2- accumulation in block effluent, the results of this study suggest ¦Ä¹⁸O_(SO4) can also be used to track water movement through the block.

Elemental sulphur (S⁰) is removed from sour gas deposits (high H₂S) during refinement. The resulting S⁰ is often stored onsite when the costs of shipping S⁰ to market exceeds the costs of storing it in large above ground blocks. With the aid of acidiphilic bacteria, atmospheric air and water oxidize S⁰ to sulphate (SO₄^2-). Long term storage is under consideration; however, oxidation rates and the role of each oxygen source (O_2(g) and H₂O) is not clear. S⁰ oxidation experiments were conducted over a range of temperatures (6-32¡ãC) to investigate reaction rates and isotopic fractionation of O and S isotopes during oxidation. The experiments also investigated the effect of integrating S⁰ oxidizing microorganisms and available nutrients on both the reaction rates and isotope fractionation. Results indicated > 95% of total SO₄^2- generated can be attributed to autotrophic microbial activity. Experiments conducted in a nutrient rich mineral solution showed rates increase with temperature from 0.16 (6¡ãC) to 0.98 (32¡ãC) ¦Ìg S⁰ cm^-2 d^-1 (Q₁₀ ¡Ö 1.7 - 1.9). Experiments conducted in a nutrient poor solution (deionized water) showed oxidation rates did not increase with temperature (0.06 to 0.08 ¦Ìg S⁰ cm^-2 d^-1) between 12 and 32¡ãC. Oxygen isotope analysis of the generated SO₄^2- indicated essentially all oxygen incorporated into the SO₄^2- originated from H₂O. In addition, effluent samples obtained from S⁰ block effluent at SCL indicated ¦Ä¹⁸O_(SO4) generally reflected the ¦Ä¹⁸O_(H2O) in the system at the time of oxidation. While covering the S⁰ blocks with an impermeable cover would undoubtedly minimize total SO₄^2- accumulation in block effluent, the results of this study suggest ¦Ä¹⁸O_(SO4) can also be used to track water movement through the block.

No description available.

No description available.

The prediction of the expected long-term performance of concrete exposed to sewage and similar materials can be difficult as it is affected by a large number of parameters. In addition, the deterioration process in concrete is generally slow. The focus of the study was to ascertain the life expectancy of concrete septic tanks located in rural Victoria. In developing the accelerated test method, ASTM C 192, ASTM C 452, ASTM C 1293 and ASTM C 109 standard procedures were adopted wherever possible. From the analysis of mass change data after 350 days, it was found that the mass change rate for concrete in sulphate solutions increases with the increase of concentration of Na2SO4 solution. The weights of the samples in Na2SO4 solution increased with time, whereas the control specimens lost weight as a result of heating cycles. The probable reason for weight increase in Na2SO4 solution was hypothesized as the formation of gypsum (CaSO4.2H2O) and ettringite (3CaO.Al2O.3CaSO4.32H2O), which is confirmed from microstructural analysis. The rate of weight gain was higher at the beginning and reduced with time. The stronger the concentration of Na2SO4 the stronger was the weight gain or expansion of mass. All the samples in sulphate solutions attained their maximum weight at around 250 days. For the specimens in sulphuric acid solutions weight loss was observed to be higher for higher concentrations. The lower the pH of the acidic solutions, the larger was the weight loss. The weight loss of specimens in acidic solutions exceeded the control specimen after 250 days. The reason for the loss of weight of the samples in sulphuric acid may be the decalcification of C-S-H gel within the concrete, and as a consequence the loss of cementitious structure. Comparison of the corrosion of concrete and also microstructural examination of field samples confirmed that the deterioration mechanism is similar to that observed in the laboratory. The accelerated testing adopted here offers a realistic method of predicting the deterioration of septic tanks under biogenic sulphuric acid corrosion. Two equations have been proposed to predict deterioration due to sulphate attack and sulphuric acid attack as mass loss (or gain) with time.

The present study investigates the effect of fluorine species during anodizing of aluminium and AA2024-T3 alloy in sulphuric acid and tartaric-sulphuric acid (TSA) electrolytes. The investigation comprises four main parts; (i) Effects of fluoride on barrier film formation on aluminium. (ii) Effects of fluoride and fluorozirconic acid (FZ) on porous film growth on aluminium in sulphuric acid. (iii) Effects of FZ on porous film growth on aluminium and AA 2024-T3 alloy in sulphuric acid and TSA. (iv) Effects on anodizing of other fluoroacids (fluoroboric (FB), fluorosilicic (FS) and fluorotitanic acid (FT)). The anodic films were examined by analytical scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, Rutherford backscattering spectroscopy, nuclear reaction analysis and glow discharge optical emission spectroscopy. The behaviour of fluoride ions during the growth of barrier-type films on aluminium was investigated in ammonium pentaborate solution with added sodium fluoride. Additions of up to 3.5 x 10-3 M sodium fluoride had a negligible influence on the film growth. In contrast, 3.5 x 10-2 M sodium fluoride reduced the efficiency to 60% as fluoride ions promoted field-assisted ejection of Al3+ ions from the film. Incorporated fluoride ions migrated inwards at a rate about twice that of O2- ions, forming a fluoride-rich layer at the film base. The study of the influence of FZ on formation of porous anodic films in sulphuric acid and TSA employed a range of anodizing voltages, electrolyte temperatures and anodizing times. Fluoroacid increased the growth rate, with a reducing influence as the temperature increased. The films contained fluoride and sulphate ions, zirconium was not detected. The fluoride concentration decreased with increasing temperature, whereas the sulphate concentration was unaffected. Anodizing aluminium and AA 2024-T3 alloy in other fluoroacids resulted in similar influences on the anodizing behaviour as FZ. The differences in growth rate, film composition and film morphology were comparatively small and did not show a systematic dependence on the type of fluoroacid employed. Boron, silicon and titanium were not detected in the films.