Herbestemming van bauxiet residu tot bouwmaterialen door middel van hydrothermaal en carbonatatie uitharding

One major issue concerning the valorization of the alumina waste bauxite residue towards construction materials is its low reactivity. In literature, several options have been suggested for valorization, e.g. by transforming the residue into a reactive binder precursor using a combined chemical and thermal treatment. A significant increase in reactivity and satisfactory properties were reached in alkali-activated binder systems. All proposed processes so far require significant capex and opex and/or incorporate a relatively low fraction of BR (as the critical component is blast furnace slag, the fly ash, or the cement) and/or deliver end-products of low- added value with substantial environmental footprint. Also importantly: there are no demonstrated cases at high technology readiness level. Without additional techno-economic and environmental assessments and especially pilot plant scale experimentation, any BR upgrading process remains restricted to an academic level for now. In contrast, the AVAREST project is approaching the BR valorization challenge from a different perspective. The process itself involves activation of the solids (high percent of BR, and low level of additional minerals that are trivial, of low cost, and often in the vicinity/within a bauxite mine) with an alkaline solution, press-shaping and finally hydrothermal curing in an autoclave, at a temperature range between 220 to 260 °C and a steam pressure of up to 50 bar. The materials produced resemble bricks. The main technological objective is the development of a material satisfying lowest carbon footprint at best possible performance (in terms of durability and compressive strength) and cost. The first task comprises a screening (including a design of experiments approach, DOE) to test the influence of multiple parameters on the final properties of the monolithic construction (dense) materials. On the formulation side, the raw materials' chemical composition, the type and quantity of the additives, as well as alkali additions play a vital role. The influence of the variation in molding pressure to form weakly-bound green bodies is evaluated. In the autoclave processing, heating rate, temperature, steam pressure, residence time and cooling rate influence the phase assemblage, microstructure and porosity that determine the final properties, such as mechanical performance, water absorption, hardness and susceptibility to efflorescence. The chemical, microstructural and crystallographic nature of the composites will be analyzed to better understand the interplay between raw materials, processing and properties. In an upscaling campaign (task 2), selected sets of experiments will be performed based on the DOE findings and the screening tests using the pilot plant facility of the SREMat group. The produced samples will be characterized by standard and novel testing procedures, including compressive and flexural strength, evaluate shrinkage and leaching behavior, water absorption, hardness and susceptibility to efflorescence. For the durability assessment, tests on carbonation and freeze-thaw will be performed. A life cycle assessment will be performed in close collaboration with project partners for the environmental analysis of the process, with the aim to highlight the main environmental hotspots and the potential benefits.