Physical assumptions on air cavities morphology, expected mechanism and modeling description

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Heat released associated to cement hydration

Hydration reactions are exothermic, Figure 12 is a summary of the main stages of hydration, it starts with the dissolution peak, then the dormant period where there is no apparent or reversible change in rheology but hydration is continuing. Then, there is an acceleration of kinetics, followed by a deceleration (stages 4 and 5). The durations of these different stages depend on the cement and mix-design of the cementitious material. The different stages with the chosen cement for this work are detailed, analysing the conductivity curve in section 4.1 of the present chapter.

Strength of hardened mortar

The mortar mix-design is composed of a significant part of paste, this induces a compressive strength at 28 days of 120MPa (1 day: 43MPa, 7 days: 99MPa). The flexural strength at 28 days is 17MPa (1 day: 8MPa, 7 days: 16MPa).

Hydration stoppage principle

Hydration stoppage is one of the ways to stop the cement hydration. The resultant sample is a powder. Among the other techniques there is also, the freeze-drying process or heating the sample in an oven (105° in order to evaporate water). Whatever the stoppage process, the hydrates microstructure can be altered. Solvent exchange is used in this study because the microstructure alteration is limited, chemically bound of water in the hydrated phase are preserved. It is important to note that the best way to characterize the microstructure is a continuous and nondestructive characterization, but some techniques of analysis require stopped samples in order to provide an instantaneous picture of the ongoing process.
Solvent exchange is performed on fresh mortar or paste with two washes with acetone (boiling point: 56°C) and one with ether (boiling point: 35°C) on a regular Buchner filtration system. The solvent removes water because of the better affinity of the solvent in the pores compared to water thus hydration is stopped because there is no more water available for the cement. Then the solvent is evaporated before the analyses. In the following, different techniques are used in order to characterize cement hydration and especially hydrates formation.

DTA: Differential thermal analysis

The DTA technique permits to characterize the chemical composition by observing the thermal degradation of the sample when heated. It is based on the energy transfers due to chemical reactions (such as absorption and emission). The analysis is performed on stopped samples of mortar. The temperature of a sample is compared to the one of the reference inert material while temperature is increasing. The reference sample is a material close to the cementitious material in terms of calorific properties and density but is not supposed to have any thermal variation due to chemical reactions. The sample is placed in a specified atmosphere in a furnace. The evolution of the temperature is defined inside the program. A differential thermocouple measures the difference between the reference temperature and the sample temperature; the two samples are placed symmetrically inside the furnace (Figure 23). Figure 23. Scheme of DTA principle, the sample is placed inside a furnace chamber, equipped with thermocouple [23].
The thermocouple measures the furnace, the reference and the sample temperatures. For each material, maximum of temperature differences are recorded (Figure 24). Those differences are characteristic of pure compound, and comparing the recorded results to a reference materials database which make possible to identify the sample composition. Usually α-alumina is used as reference.

Negative water pressure effect principle

Before to explain the origin of water suction in concrete, the water pressure phenomenon can be illustrated with Figure 43 considering a box filled of water. In the first case, removing water without air in an open container induces a reduction of the volume of water. If air content is null, a removal of water can push beyond liquid-vapor equilibrium, forming bubbles: this is cavitation [37]. For the last case, water removal from water with air in a closed container leads to an increase of the air bubbles and a reduction of the global pressure. Similar phenomenon appends during cement hydration, with three phases: solid, air and water. The following chapter deals with those volume changes.

Linking pore pressure to volume and microstructure evolution: objectives of the chapter

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At initial time, the three phases of the mortar are solid fraction (cement and sand), water (with in-solution admixtures) and air. Fresh mortar under atmospheric pressure behaves as a dense suspension [38] of inert aggregates and entrained air bubbles which are embedded in a deformable matrix of cement paste [39]. In this fresh state, the behavior of the material is governed by colloidal interactions [40] [41] under a critical sand content as shown by Yammine et al. [42] which is close to 80% the packing fraction. From fresh state to setting time, the total volume reduces because of cement hydration inducing Le Chatelier contraction [43]. Small attractive forces reduce the average distance between particles, and the system becomes flocculated. As the cement hydration continues, the particles are bonded [5] [44] and gradually interlocked (hydrates formation) to finally form a solid structure (a solid skeleton) which can therefore support and transfer high stresses [45] [46]. The air volume remains almost unchanged, until the percolation of the solid contact network (Figure 44), corresponding to the “hardened state”.

Table of contents :

Chapter 1: Hydrates identification (type and formation time) and adhesion consequences of the reference mix-design
1. Introduction
2. Theory of hydration
2.1 Cement grain composition and process
2.2 Hydration reaction
2.3 Hydrates formation mechanisms
2.4 Heat released associated to cement hydration
3. Materials
3.1 Materials and procedures
3.2 Raw materials analysis
3.3 Strength of hardened mortar
3.4 Hydration stoppage principle
4. Hydrates identification
4.1 Conductimetry
4.2 Cristallography with X-ray diffraction
4.3 TGA: Thermogravimetric analysis
4.4 DTA: Differential thermal analysis
4.5 NMR Relaxometry: Nuclear Magnetic Resonance
5. Mechanical consequences
5.1 Yield stress evolution
5.2 Hydrates adhesion
6. Conclusion of chapter 1
Chapter 2: Origin and modeling of suction
1. Introduction
2. Physical background
2.1 Terzaghi equation
2.2 Negative water pressure effect principle
2.3 Linking pore pressure to volume and microstructure evolution: objectives of the chapter3
2.4 Theoretical volume evolution
2.5 Negative pore water pressure measurement
2.6 Young-Laplace pressure
3. Experimental procedures
3.1 Mixing procedures and materials
3.2 Physical parameters
3.3 Hydration study and chemical parameters
3.4 Stress and volume evolution
3.5 Pore water pressure sensor
4. Experimental results
4.1 Physical parameters
4.2 Chemical parameters
4.3 Stress and volume evolution
4.4 Pore water pressure measurement
5. Modeling
5.1 Physical assumptions on air cavities morphology, expected mechanism and modeling description
5.2 Analysis of the mechanism on reference mix-design
5.3 Pore size estimation with different air content
5.4 Comparison of the methods and pore pressure prediction
6. Conclusion of chapter 2
Chapter 3: Controlling water pressure of mineral suspensions – and shear behavior characterization
1. Introduction
2. Water pressure-controlled device
3. Physical background
3.1 Bulk yield stress measurement: rheology
3.2 Thixotropy
3.3 Shear stress measurements on the device
3.4 Rigidity of the device
3.5 Air behavior in cement-based material
4. Materials and mixing procedure
5. Experimental procedure and assessment of the device
5.1 Protocol
5.2 Device verifications and possible drawbacks
5.3 Water distribution characterization
5.4 Compressibility measurements
5.5 Total pressure and stress transfer
6. Results
6.1 Device verifications and calibration
6.2 Total and effective stress
6.3 Detailed mechanisms of stress transmission phenomena- Yield stress results
6.4 Friction coefficient
7. Discussion
7.1 Mechanisms
7.2 Water pressure participation to yield stress increase and modeling
8. Conclusion of chapter 3
Chapter 4: Experimental approach of a moving formwork
1. Introduction
2. Physical background and state of the art
2.1 Concrete interfacial behavior
2.2 Interaction between concrete and formwork
2.3 Effect of concrete mix-design on interfacial behavior
3. Experimental procedures
3.1 Material characteristics
3.2 Cement hydration kinetics characterisation
3.3 Device equipment
3.4 Metallic surface roughness characterization
3.5 Interfacial shear stress
3.6 Influence parameters
3.7 Verification of the device
4. Experimental results
4.1 Total tangential shear stress measurement and local shear stress comparison
4.2 Tangential shear stress vs effective stress
4.3 Mix-design comparison in function of degree of hydration
5. Discussion and modelling approach on the reference mix-design
6. Conclusion and perspectives
General conclusion and perspectives
Appendice 1: Spectrometer Kea² 20MHz parameters
6.1 Magnetic impulsion (M0 deviation)
6.2 Inversion-recovery
6.3 T2 determination
6.4 CPMG: Carr Purcell and Melboom Gill Pulse Sequence


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