No abstract

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 (Fe 2 O 3 + CaO + MgO + K 2 O + Na 2 O) 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 + K 2 O + Na 2 O)/Al 2 O 3 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 SiO 2 -Al 2 O 3 -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.

The development of an optimal low-calcium alkali-activated binder for high-temperature stability based on ferronickel slag, silica fume, potassium hydroxide, and potassium silicate was investigated based on Mixture Design of Experiment (Mixture DOE). Mass loss, shrinkage/expansion, and compressive and flexural strengths before and after exposure to a high thermal load (900 °C for two hours) were selected as performance markers. Chemical activator minimization was considered in the selection of the optimal mix to reduce CO2 emissions. Unheated 42-day compressive strength was found to be as high as 99.6 MPa whereas the 42-day residual compressive strength after exposure to the high temperature reached 35 MPa (results pertaining to different mixes). Similarly, the maximum unheated 42-day flexural strength achieved was 8.8 MPa, and the maximum residual flexural strength after extreme temperature exposure was 2.5 MPa. The binder showed comparable properties to other alkali-activated ones already studied and a superior thermal performance when compared to Ordinary Portland Cement. A quantitative X-ray diffraction analysis was performed on selected hardened mixes, and fayalite was found to be an important component in the optimal formulation. A life-cycle analysis was performed to study the CO2 savings, which corresponded to 55% for economic allocation.