Soutenance mémoire
To date, concrete is the most commonly used construction material on earth. The hydraulic binder connecting the aggregates contained in the concrete is mainly Ordinary Portland Cement (OPC). Every year, about 1.7 billion tons of OPC are produced worldwide, which corresponds to the amount required to produce annually more than 6 km3 of concrete [1]. OPC cement is a splendid building material not only because of its mechanical properties but also because it is accessible, easy to use and affordable. The concrete embodied energy is lower than that related to the production of other building materials, as shown Figure Intro-1 [2]. However, OPC cement is produced in large quantities in comparison with other building materials . Thereby, OPC cement industry is responsible for almost 5% of total anthropogenic CO2 emissions. In OPC cement production, the clinkerisation stage (which corresponds to the thermal treatment at high temperature in order to synthesize what is called ‘clinker’) is the most energy-intensive phase, and therefore the largest contributor to CO2 emissions and costs of energy production . During the clinkerisation stage, large quantities of emitted CO2 come mainly from limestone decarbonation, according to the following reaction: CaCO3 → CaO + CO2↑ .
To reduce CO2 footprint of OPC cement industry, a great deal of current research projects is focusing on alternative environmental friendly binders. Recently, calcium sulfoaluminate (CSA) cement has been regarded as a sustainable binder to replace OPC cement, since the production of the CSA cement involves less calcium carbonate and a lower clinkering temperature compared to OPC. This entails that the CSA cement is a more sustainable cement than OPC leading to less CO2 emissions during its production process. In addition, CSA cement can reduce energy consumption during clinker grinding due to its more friable nature [1,5,6]. CSA cements are of interest in applications such as shrinkagecompensating concrete and self-stressing concrete. They exhibit key features including high early strength and rapid setting, which are mainly attributed to the presence of the « ye’elimite (Ca4(AlO2)6SO4 i.e. C4A3S̅in cementitious notation) » cementitious phase. CSA cement named (Komponent®) was used as a shrinkage-compensating cement additive to minimize or eliminate drying shrinkage cracking of concrete during the rehabilitation project of the Lokern Road Bridge, California, USA .
The CSA cement contains besides ye’elimite, other phases such as calcium aluminates, belite (dicalcium silicate) and calcium aluminoferrite, and they are generally blended with calcium sulfate [8]. According to recent surveys about the different CSA compositions [9], it is clear there are several types of CSA cements. Because of the chemical and mineralogical nature of the CSA clinkers, it is not possible to standardize under EN 197-1 which only covers Portland compositions. This standardization, which is a necessary condition for the use of these cements in the building sector, will need the development of a completely new standard different from EN 197-1. In this respect, the composition of a cement has to be described precisely and thoroughly, together with its chemical and physical characteristics. At the present time, to our knowledge, CSA cements have not been normalized according to European standardization procedure because neither the composition nor the characteristics have been described precisely. Therefore, there is a need to identify the composition of each phase together with its chemical and physical behaviors. To examine the hydration behavior of calcium sulfoaluminate cements, individual and pure components should be prepared, one being ye’elimite. As commercially available CSA cements are a mix of several phases, it is impossible to separate ye’elimite from the other phases. Therefore, a need exists to synthesize ye’elimite phase in a pure state at the lab scale.
Besides the fundamental comprehension of pure ye’elimite hydration, the investigation of ye’elimite synthesis conditions and the study of its formation mechanisms are of utmost importance. In fact, this is helpful, on one hand, for industrial cement researchers to understand and to optimize the clinkerisation process of CSA cements, and, on the other hand, for academic researchers to synthesis pure ye’elimite for fundamental hydration studies, crystallographic structure characterization and other investigation experiments.
CSA cement hydration is one subject of interest to cement researchers; the main conclusion is that ye’elimite phase is the main reacting phase at early ages and the silicate phases are contributor to the microstructure development and the durability of CSA cements at later ages [10]. The principal research objectives of our PhD thesis are the following: (i) the synthesis of highly pure ye’elimite powder prepared by two ways consisting in solid-state reaction method and sol-gel routes, (ii) the fundamental comprehension of ye’elimite formation mechanisms during the clinkerisation, and finally, (iii) the hydration study of CSA model cement consisting of the lab-made pure ye’elimite blended with gypsum. In this last part of the work, the influence of an additive, namely citric acid, and ye’elimite finesse on hydration rate will be examined.
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Table des matières
Introduction
•Background
•Research objectives
•Outline of the thesis
Chapitre I. Literature review
Introduction
Calcium Sulfo-Aluminate (CSA) cements
I.2.1. CO2 emission reduction in the manufacturing process of CSA cements
I.2.2. Mineralogical composition of CSA cements
I.2.3. Crystallographic structure of ye’elimite
Lab-synthesis methods for ye’elimite and other cement phases
I.3.1. Solid-state synthesis of ye’elimite
I.3.2. Sol-gel synthesis of anhydrous cement phases
Hydration of ye’elimite phase
I.4.1. Effect of amount and nature of the added calcium sulfate
I.4.2. Effect of ye’elimite crystallographic structure
Parameters influencing cementitious phases hydration at early age
I.5.1. Parameters influencing the dissolution of cementitious phases
I.5.2. Parameters influencing the crystallization of cementitious hydrates
I.5.3. A focus on the influence of fineness and citric acid addition
Chapitre II. Characterization methods
X-Ray Diffraction (XRD) and Quantitative Rietveld Analysis (QRA)
Chemical analysis by X-ray fluorescence (XRF)
Thermogravimetric analysis (TGA) coupled to differential thermal analysis (DTA)
Thermogravimetric analysis (TGA) coupled to Mass-spectroscopy (MS)
Particle Size Distribution by Laser Diffraction (PSD-LD)
Particle Size Distribution by Image Analysis (PSD-IA)
BET specific surface area (SSA)
Thermal Differential Dilatometry (TDD)
Open porosity estimation by Archimedes principle (EN 623-2:1993)
Porosity estimation from backscattered electron image analysis (BSE-IA)
Scanning electron microscopy (SEM)
Ionic conductivity and pH measurements
Zeta potential measurement
Free-lime determination by volumetric dosage method
Conclusion
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