Valorization of Industrial Waste by CO2 supercritical curing: Toward Sustainable Building Materials and Circular Economy

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The cumulative carbon emission from worldwide fossil-fuel-based energy infrastructure is expected to top 850 Gt by mid-century. At this rate, our society will surpass the prescribed carbon budget set to limit the mean global temperature increase below 1.5°C. Current carbon emission levels, and climatic call for the development of new technologies in the capture, storage, and utilization of carbon dioxide. In this context, rapid CO2 mineralization offers an attractive direction in the immobilization of carbon dioxide (captured from industrial processes) through valorization of industrial waste, and creation of sustainable building materials for construction industry. A significant portion of the solid industrial waste such as slags, cement-kiln dust, fly and incineration ash, demolition concrete, construction gypsum sheetrock, contain natural and synthetic divalent cations, which show strong chemical affinity toward CO2, and offer a potential for rapid mineralization through carbonation. As a result, calciumbearing building materials can be manufactured for reuse in the construction e.g., concrete aggregates, CO2-cured structural and non-structural products. Per recent estimates, the volume of construction waste generated worldwide every year is expected to reach 2.2 billion tons by 2025. In case of constructional gypsum and demolition waste, ≈90% of bulk waste is directly disposed. In both cases, waste storage represents non-negligible environmental hazard resulting from inadequate waste isolation, or structural stabilization. A lot of alkaline waste carbonation techniques with the use of either gaseous or liquid state of carbon dioxide have been proposed yet they have their shortcomings due to their intrinsic nature. In this work, the author proposes a novel method of ex-situ mineral carbonation, defined here as super-critical curing of waste materials, and aiming at the extension of the method to different kinds of waste materials along with the plan to bypass the shortcomings associated with the other methods of carbon curing. The proposed technique relies on curing of the waste materials from the municipalities and industries with carbon dioxide (CO2) in a super critical state which allows the rate and extent of carbonation to exceed the limitations that the curing techniques using gas and liquid state carbon dioxide bear. First, the industrial and municipal waste materials having potential for carbon dioxide sequestration is explored and selected through an extensive literature review. The parametric study targeting packing density, degree of saturation and cement chemistry is designed after analyzing the data from ad-hoc test runs. The experimental outcome is assessed in terms of CO2 efficiency, which is in fact the weight-increase due to carbon dioxide intake divided by the theoretical weight increase potential, using weight increase method which is cross validated through CO2 efficiency achieved through results obtained from Thermogravimetric Analysis coupled with Image analysis results. Along the process, the achieved degree of saturation and porosity prior and post carbonation is reported with the error associated with it. It is demonstrated, the porosity has a pronounced inverse effect in the rate and extent of carbonation while the effect of moisture content could not infer any contrasting results. Furthermore, the fly ash sample with same parameters were cured in different conditions: normal curing, hydrothermal curing and super-critical CO2 curing, and the samples were then tested with the ASTM testing in uniaxial compression. Finally, microstructure of a sample was studied, secondary electron imaging and electron dispersive x-ray spectroscopy, with the help of Scanning Electron Microscopy. The qualitative analysis for elemental composition is observed and a rough estimate of quantitative elemental composition is also observed.

supercritical carbon dioxide, alkaline waste materials, carbon curing, Thermo gravimetric analysis, indicator test