Browsing by Author "Pienaar, Jacoline"
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- ItemDegradation characterisation of various geo-chemical matrices during leaching(Stellenbosch : Stellenbosch University, 1999-12) Pienaar, Jacoline; Lorenzen, L.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH SUMMARY: Immobilisation is currently a well-known and cost-effective method in which solid waste can be treated to render hazardous elements less harmful to the environment, by converting the contaminants into their least soluble, mobile or toxic form. The stabilising interaction between the wastes and the solidifying reagents may be either chemical or mechanical. In the latter case, the elements are encapsulated into a monolithic solid. This would evidently require a material of high structural integrity, able to withstand long-term exposure to water infiltration in disposal sites conditions, while continuing to limit the leaching of contaminants into ground or surface waters. Laboratory scale leaching tests are generally used for testing the stability of such materials. Characterisation of these tests is of major importance in clarifying the leaching phenomena, for valid extrapolations of leaching behaviour in a few years’ time (ageing). The impetus of this study was on characterising the degradation behaviour of immobilised matrices during leaching tests. Two specific waste forms (fly ash and jarosite) were stabilised and additives were added to produce two geo-chemical matrix types: (a) a pozzolanic and (b) a geopolymeric matrix. A 5% acetic acid solution was used during batch leaching tests, where the pH was held constant throughout the tests. Regarding crack patterns, the physical degradation appeared somewhat different for the two matrix types. Larger cracks were found in some pozzolanic samples, and the outer layers (of about 5mm) of some samples, appeared detached from the rest of the blocks after leaching. The geopolymeric matrix exhibited smaller cracks and the layers rather flaked off, compared to the larger layers that cracked off the pozzolanic samples. The main degradation mechanisms emerged to be acid attack and the alkali silica reaction for both matrices. The leaching of the main matrix elements was further considered as an indication of the degradation performance. These elements were primarily Si, AI, Fe, Na and Ca. Typically during leaching tests the behaviour is governed by an initial fast surface reaction, followed by much slower diffusion and a slow mobilising chemical reaction and/or corrosion or structural breakdown of the waste matrix. However, a substantial percentage of the elements that leached into the leaching solution were precipitated. A simple semi-empirical model, that considered the effect of degradation, was used to characterise these typical reactions that govern the leaching behaviour of each element. In both matrix types, Ca and Na (and some of the other minor elements) showed a clear reaction front in the leaching phenomenon (difference in leaching rate from the deeper layers of the samples than from surface samples). This is contradictory to the leaching of Si, Fe, Al and Ti. The leaching behaviour of Ca, Na, Mn, Mg and P were very closely related, while in turn, Si, Fe, Al and Ti showed similar behaviour. These similarities in performance indicate that they might be bound into the matrix structure in the same way. The percentages that were leached out after 600 hours were in the same order of magnitude for both matrices, but Si, AI, Ca, and Fe were slightly more stable in the geopolymeric matrix, while Na and Mg were more stable in the pozzolanic matrix. Overall, the geopolymeric matrix seemed to be more structurally stable at extended leaching times.