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Stabilization of sulfates and molybdenum by using alternative binders

Abstract : Excavation operations produce several tons of soil generally contaminated by the presence of pollutants. Excavated soil is considered as waste and it can be either sent to landfill or destined for reuse depending on the level of pollution. In any case, soil should be properly treated in order to: (i) decrease the release of pollutants into the environment, and (ii) minimize the problems involved in civil engineering applications due to the reactions between cementitious phases and pollutants. In the context of this thesis, we focused on sulfates and molybdenum (Mo). Concerning sulfates, we considered two main issues: (i) external sulfate attack of concrete structures, which are in direct contact with sulfate-rich soil (e.g. dams, foundations), and (ii) the release of sulfates into solution in addition to the swelling and mechanical strength loss in sulfate-rich soil intended for valorization (e.g. reuse in road construction). In the case of Mo, its release into solution is also a serious concern as it can lead to significant risks for the environment. Therefore, in this thesis, we investigated the reaction of concrete in contact with sulfates, and the stabilization of sulfates by using alternative binders for pollution reduction and for reuse of soil. Additionally, we studied the interaction of Mo with alternative binders and their capacity to stabilize Mo. First, we studied the capacity of seven different concretes to resist external sulfate attack under similar experimental conditions. It was found that ordinary Portland cement had high expansions (>0.1%) due to the formation of ettringite in excess caused by the reaction between aluminates and sulfates. Portland cement without C_3A presented lower expansions but gypsum was found to be responsible of cracking at later ages. Meanwhile, alternative binders had low expansions in the range of 0.01-0.03% explained by the absence of C3A and portlandite, in addition to the formation of ettringite during hydration (case of ettringite binders) and the absence of calcium (case of the geopolymer-based metakaolin). Second, we compared the capacity of four different binders to stabilize sulfates in a sulfate-spiked soil. Binders having high C_3A content led to high volume expansions (>5%) caused by the formation of ettringite in excess. These binders also released heavy metals into solution due to their high clinker content. In contrast, binders containing ground granulated blast furnace slag (GGBS) led to low expansions (<2%), sulfate retention was about 89% and lower heavy metals contents were detected in solution. Sulfate solubility was controlled by ettringite, which did not lead to expansion probably due to the low kinetics of precipitation in addition to the absence of portlandite, which is often related to expansive ettringite.[...]
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Submitted on : Tuesday, April 27, 2021 - 3:18:08 PM
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Laura Diaz Caselles. Stabilization of sulfates and molybdenum by using alternative binders. Civil Engineering. Université Paul Sabatier - Toulouse III, 2020. English. ⟨NNT : 2020TOU30255⟩. ⟨tel-03209902⟩



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