Direct valorisation of convertor-slag as binder in mortars

Cement production, accounting for approximately 4.6 billion tons annually, is responsible for 5% of Green House Gas (GHC) emissions.The objective of this work is to contribute to the reduction of anthropogenic CO2 emissions by exploring the viability of direct valorization of a by-product from the steel industry as a hydraulic binder, specifically as a substitute for Portland Cement in non-structural mortar applications.The by-product in question, known as Convertor-Slag or Basic Oxygen Furnace (BOF) Slag, is generated during the oxidation process (conversion) of pig iron into steel. Currently, it is only partially utilized in applications with low technical requirements, limited added value, and minimal reduction in CO2 emissions. However, if this by-product could be employed as a hydraulic binder without the need for complex or energy-intensive post-processing, it would have the potential to significantly reduce construction industry CO2 footprint. Therefore, the objective is to explore ways of a direct valorization, i.e. as it exists and without additional processing steps than grinding.A multi-characterization approach, combined with thermodynamic modeling, reveals that BOF-Slag is a highly complex material with disordered crystalline phases and a high concentration of foreign elements. The evaluation of the hydration behavior of the phases present, while similar to those found in traditional cements, demonstrates a low intrinsic hydraulic character due to identified and detailed reasons. This low reactivity represents the primary obstacle to its utilization as a binder.In addition to the low hydration potential, the presence of free lime in BOF-Slag leads to significant dimensional instability (swelling) when mixed with water. This poses a second significant barrier to practical usage in the construction industry, even for non-structural applications.Based on the identification of the mechanisms responsible for the low hydraulic properties and swelling behavior, this study proposes and evaluates strategies to overcome these barriers. Firstly, to control the risk of dimensional instability, and secondly, to maximize the hydraulic properties by selecting specific additives that facilitate the engineering of hydration mechanisms, sequence, as well as nature and morphology of hydrates.The study investigates the enhancement of Layered Double Hydroxides (LDH) precipitation through two compatible approaches: 1) the introduction of additives providing anions into the system, and 2) the introduction of complexing agents inducing a ligand-promoted dissolution of the constitutive phases of BOF-Slag.Furthermore, the study explores the surface modification of the initial precipitated hydrates (LDH) by adsorption of high density of charge salts, enabling the heterogeneous precipitation of secondary hydrates on their surfaces.This two-step hydration process, utilizing the initial hydrates as a germinative surface for secondary hydration products, results in a distinctive network microstructure. The high level of solid bridging, percolating network, and high compactness of the hydrates enable strength development comparable to other slag materials, such as Ground Granulated Blast-Furnace Slag (GGBS), which have been widely used as supplementary cementitious materials for several decades.The study then describes how the combination of the identified hydration engineering strategies with other minerals having reduced CO2 footprints, such as Low Temperature Calcined (dehydroxylated) or Raw Clays, can allow the formulation of BOF-Slag based binders with performance characteristics that make them viable as standalone full substitutes for traditional cements.Finally, the practical feasibility of using these binders is logically evaluated and demonstrated through real industrial mortar formulations used for high volume applications, i.e. contributing to cement high consumption, thus high fraction of CO2 emissions.