Early Age hydration of low-CO₂ footprint binders containing iron and aluminum : AFm phases' formation and stability

The increase in anthropogenic CO₂ emissions and their impact on the greenhouse effect has led to increased social awareness and a shift in business paradigms. As a result, the CO₂ footprint has become a decisive factor in product selection. This concern is particularly relevant to OPC, given that the cement industry contributes to approximately 8% of global CO₂ emissions. In this context, various types of low-CO₂ hydraulic binders such as high Fe-bearing materials - including Basic Oxygen Furnace (BOF) slag, or else clays - with better environmental credentials have been investigated and proposed as potential OPC substitutes. Some of these alternatives are composed, in part, of iron and aluminum oxides, for example the brownmillerite (i.e. C₄A₂₋ₓFₓ - with 0.67 ≤ x ≤ 2 - in cementitious notation; noted C₄AF below). As Calcium-based layered hydrates (i.e. AFm phases) can be formed during the hydration process of brownmillerite, the general objective of this thesis is to enhance knowledge on Fe-containing AFm phases, and to complete recent research that enabled direct valorization of BOF slag as binder in mortars. The AFm phases are part of the group of layered double hydroxides (LDH) with the specific composition:[Ca₂Mᴵᴵᴵ(OH)₆]⁺[A⁻·zH₂O] (Mᴵᴵᴵ=Al³⁺ or Fe³⁺). The anions A- generally encountered in hydraulic binders are CO₃²⁻, SO₄²⁻, Cl⁻, and NO₃⁻. The present work focuses on the AFm-NO₃, AFm-Cl, and AFm-CO₃ phases for following end-use oriented reasons: Ca(NO₃)₂ and CaCl₂ based additions are known as accelerators of C₄AF hydration and favor the rapid formation of respectively the AFm-NO₃ and AFm-Cl phases. Regarding AFm-CO₃; it can form during the hydration of C₄AF-bearing binder or its ageing due to the atmospheric CO₂. Al to Fe solid solutions may occur for AFm phases due to the close ionic radii for trivalent Al³⁺ and Fe³⁺ cations. Many authors investigated the behavior of Al-pure AFm phases, while fewer studied Fe-pure AFm phases and investigations into mixed Al/Fe-containing AFm phases are rare.This thesis aims thus to enhance the general knowledge of mixed Al/Fe-containing AFm phases through: (1) synthesis of composition-controlled AFm phases, (2) BOF slag hydration study, and (3) combining results of characterization done on both type of materials: (1) AFm phases synthesized were thoroughly characterized. The existence of solid solutions between the two pure end members Fe-AFm and Al-AFm was demonstrated for the three types of synthetic AFm series intercalated with NO₃⁻, CO₃²⁻, and Cl⁻ anions, with a similar distance between trivalent cations regardless of the intercalated anion. Vegard's laws for the a lattice parameter of these three phases were determined. Additionally, the thermal behavior, hydration levels, and anion exchange capabilities of these AFm phases were investigated. (2) The behavior of BOF slag during early hydration was examined to understand the reaction kinetics and to improve the reactivity of anhydrous phases using various additives. The impact of these additives on BOF slag hydration was tested, with a particular focus on their effect on AFm phase formation. (3) The long-term hydration behavior of BOF slag was also studied, focusing on the behavior of AFm phases. Mechanical strength, dimensional variation, mineralogical composition, morphology, and porosity were monitored over a year on various formulations of BOF slag pastes under different curing conditions. This research improved the understanding of mixed Al/Fe-containing AFm phases, facilitating better characterization of hydrated low-CO₂ binders. AFm phase formation contributes to early mechanical strength, providing support until the precipitation of CSH hydrates permitting final mechanical resistance. Thus, AFm phases are beneficial during early hydration but do not cause any damage as they degrade over time. Additionally, AFm phases can delay corrosion by trapping chloride anions within their interlayers.