Sulfuric acid-induced corrosion of portland cement-based concrete has been a well-known problem for years, causing immense damage to concrete building components in biogas plants, sewage plants, sewage systems and in cooling towers of power plants. In the case of sulfuric acid attack, the extremely low pH results in a combination of dissolving and expanding attack of concrete, which most concretes have no significant resistance to.
An alternative to concrete with portland cement as a binder, which has been researched for many years, are so-called geopolymers. This special type of alkali-activated binder consists of a reactive, aluminosilicate, powdery raw material and an alkaline activating solution, whereby the contact of the two precursors leads to the formation of the inorganic, aluminosilicate polymer typical of geopolymers. The increased acid resistance compared to portland cement binders has been demonstrated by various publications in the past. In many cases, reference was made to the Si/Al ratio of the aluminosilicate polymer, since higher Si/Al ratios, equivalent to higher proportions of silicon in the geopolymer, lead to an increased acid resistance. The evaluation of the Si/Al influence on the acid resistance of the geopolymer has been researched by a wide variety of analytical methods as well as by using various geopolymer precursors or geopolymer formulations. What has hardly been considered in this context, however, are the chemical-physical changes at certain depths of the corroded surface layer at different durations of exposure to sulfuric acid.
The present dissertation takes up precisely this still open question. The elementary basis of this work is therefore a general research on the state of the art in the field of geopolymer technology, and in particular on the subject of the acid resistance of these special alkali-activated binders. The experimental program includes the detailed characterization of the powdery precursors metakaolin and silica fume, which were activated in varying proportions with a potassium silicate solution. Silica fume was specifically incorporated into the geopolymer formulations to increase the Si/Al ratio. In order to qualitatively and quantitatively assess the intensity of sulfuric acid induced corrosion of the geopolymers, a wide variety of geopolymer formulations were stored in sulfuric acid with a pH of 1.0 for a period of up to 84 days. The corrosion resistance and the destruction of the corroded surface layer caused by the sulfuric acid at different durations of exposure were analysed using XRD (X-ray diffraction), FTIR (Fourier-transform infrared spectroscopy), SEM (scanning electron microscopy) and EDX (energy dispersive X-ray spectroscopy), among other methods.
The main focus of the experimental test program results from EDX analysis of the geopolymers before and after sulfuric acid exposure. Thereby, the material composition of the geopolymers, starting from the corroded surface layer of the samples to the core of the test specimens, was semi-quantitatively recorded by EDX mappings. The subdivision of those EDX mappings into segments enabled the generation of EDX depth profiles. With these, the chemical composition could be detected in 100 µm steps at any depth of the corroded specimen. Based on these data, it was possible to see how the Si/Al ratio affects corrosion resistance, which elements were leached from the specimens at various durations of exposure, and which proportions of silica fume can be applied to achieve a significantly increased erosion and corrosion resistance.
In the final chapter of the dissertation, a regression analysis of the EDX depth profiles shows, how deviating Si/Al ratios of the geopolymers, resp. deviating proportions of silica fume in the formulations, affect the corrosion rate and the corrosion progress. | English |