A Metallurgical Approach Toward Bloating of Canal-Dredging Sediments

The similarity of the fine fraction of dredged freshwater sediments to natural clays stimulates the valorization in a product similar to expanded clay aggregates, obtained by bloating at high temperature. The impact of the ratios between different flux elements (Fe2O3 + CaO + MgO + K2O + Na2O) on the melting behavior is quantified using thermodynamic modeling and validated with experiments. The total content of alkalis is found to be key in controlling melting and bloating. Increased alkali content triggers the formation of alkali feldspars that start to melt at low temperature and give rise to a gradual increase of the amount of melt with temperature. The ratio (CaO + MgO + K2O + Na2O)/Al2O3 should preferably be smaller than or close to 1 to avoid excessive pyroxene formation, which causes complete meltdown of the granules around 1150 °C. The chemical composition has a complex influence on the melting behavior, even when on the same location on the SiO2–Al2O3–flux diagram. The conventional approach of trying to modify the composition towards a certain range in the ternary diagram does not work for all materials, and therefore, the understanding of the melting behavior using the phase composition at high temperature (= the metallurgical approach) provided in this work is shown to be crucial to produce lightweight aggregates from materials with a higher chemical variability.