Christian Gault

Application of ultrasonic testing to describe the hydration of calcium aluminate cement at the early age

Abstract Nondestructive and in situ characterisation techniques, such as ultrasonic measurements, permit to follow cement hydration at the early age from a few minutes to a few hours after mixing. The technique reported in this paper is based on measurements in reflection modes. Results concerning an aluminous cement, Secar71, are presented (water-to-cement weight ratio (W/C): 0.3 and 0.4; temperature of measurement: 20°C; duration: 0–24 h). Information deduced from ultrasonic measurements (longitudinal wave velocity, reflection coefficient) associated with other data obtained from X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric (TG) measurements enable to propose a qualitative description of hydrations chronology. The sensitivity of these ultrasonic parameters to hydrates formation and structuring is underlined.

Application of X-ray computed tomography to characterise the early hydration of calcium aluminate cement

Abstract Techniques that monitor in situ the setting and hardening of calcium aluminate cements (CACs) are of interest to the engineering community. This paper focuses on one non-destructive technique, X-ray computed tomography (CT). The early hydration of the cement can be followed from a few minutes to a few hours after mixing. This technique is based on both X-ray absorption measurements and qualitative observations in chosen axial cross-sections. Results concerning an aluminous cement, Secar 71, are presented (water-to-cement weight ratio: 0.33; duration: 0–24 h). Information deduced from these measurements clearly show that a strong variation of X-ray absorption values occurs during the early age of hydration. Both quantitative and qualitative analyses of CT data allow a chronology of cement setting to be proposed.

Temperature dependence of Young's modulus in Si3N4-based ceramics: roles of sintering additives and of SiC-particle content

Abstract The temperature dependence of Youngs modulus has been investigated by ultrasonic-echography in the 20–1400 °C temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si 3 N 4 particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920 °C. The transition observed in ceramics is located between 1080 and 1150 °C and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor ( b ) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques.

Ultrasonic measurement of Young’s modulus MgO/C refractories at high temperature

Abstract The paper deals with the elastic behavior of MgO/C refractories used in BOF at temperatures up to 1400°C in air or inert atmosphere. Measurements have been made by the way of a high temperature ultrasonic technique. Heating-cooling cycles and long time aging in the range 700–1400°C show strong variations of Young’s modulus which have been interpreted with the aid of XRD analysis, SEM observations and EDS analysis. Carbon oxidation and sintering of MgO particles are found to be responsible of the major parts of the measured evolutions. ©

Time delay and phase shift measurements for ultrasonic pulses using autocorrelation methods

At the interface between two media, a monochromatic ultrasonic pulse separates into two waves, a reflected one and a transmitted one. A simple hypothesis is to consider the phase shift, introduced by the reflection, as equal to 0 or π. But, in many cases, it becomes necessary to know the phase shift precisely. This is done by a Fourier analysis of the different signals. This article deals with mathematical methods that can be used for time and/or phase shift measurements. A new method, based on an analysis of the analytic signal of the autocorrelation function, is described and compared to existing phase slope and autocorrelation methods. An application of this method is given in the case of a ‘‘long bar’’ mode propagation at the interface between alumina and glass.

Ultrasonic Non Destructive Evaluation of Microstructural Changes and Degradation of Ceramics at High Temperature

This paper deals with the use of pulsed ultrasonic waves to monitor microstructural changes and degradation of ceramics in the field of high temperature. Two types of devices are described. One is a low frequency system for the measurement of Youngs modulus at temperatures up to 1800°C. The second is an ultrasonic spectroscopy system for the evaluation of damage in structural ceramics during thermal fatigue experiments. Applications concern phase changes, porosity evolution or microcracking induced by thermal stresses when heating monolithic or composite ceramics.

Characterisation of early stage calcium aluminate cement hydration by combination of non-destructive techniques: acoustic emission and X-ray tomography

Abstract Techniques that monitor in real time the setting and hardening of calcium aluminate cements (CAC) are now of interest to the engineering community. This paper focuses on the association of two in situ techniques, acoustic emission (AE) and X-rays computed tomography (CT). These non-destructive techniques make it possible to follow the cement hydration in the early stage from a few minutes to a few hours after mixing. Results concerning a calcium aluminate cement paste, Secar71, are presented (water to cement weight ratio: 0.33; duration: 0–24 h). Both the acoustic emission (AE) signals and X-rays absorption values were recorded and analysed. Information deduced from experiments clearly show that a close correlation exists between the variations of the X-ray absorption values and the AE activity during the early stage of hydration. Based on the experiments, the combined analysis (quantitative and qualitative) of both AE and CT data make it possible to propose a relationship between the experimental characteristics recorded during the tests and the different mechanisms taking place during the early hydration of a cement paste. A chronology of cement setting is also proposed.

Young’s Modulus of Dry-pressed Ceramics: The Effect of the Binder

Abstract Measurements of Young’s modulus of green compacts prepared from spray-dried alumina powders containing two binders: polyvinyl alcohol or polyethylene glycol are reported. The variations of Young’s modulus with forming pressure are compared to those of mechanical strength and discussed in terms of glass transition temperature ( T g ). When the T g of the polymer is lower than the pressing temperature (case of PEG and moisture-plasticized PVA), the variation of the Young’s modulus is related to the evolution of the binder film covering the surface of primary particles inside the granules. Microcracking of the brittle external polymer-rich layer of granules seems to be responsible for a different evolution of Young’s modulus of green compact in the case of a binder with a T g higher than the forming temperature (dry PVA).

The effect of binders on the strength and Young's modulus of dry pressed alumina

Abstract The strength of green samples prepared from spray-dried powders of alumina is investigated versus forming pressure. The binders used are polyethylene glycol and polyvinyl alcohol (3 wt% with respect to alumina). The variation of strength is discussed in terms of, (i) the binder distribution in granules and in the pressed compacts, (ii) the glass transition temperature of the polymer and (iii) the adhesive properties of the external part of spray-dried granules. A transition from intra- to intergranule fracture mode is observed when the glass transition temperature of the binder increases. Finally, the results of a preliminary investigation concerning the variation of the Young’s modulus versus forming pressure are also presented and discussed.

Demonstration of grain growth induced Microcracking and its role in the electrical response of YBa2Cu3O7 − δ

Abstract The effect of grain size on the electrical properties of polycrystalline YBa 2 Cu 3 O 7 − δ has been investigated. A strong decrease in the critical current density has been observed when an average grain size of 15 μm is exceeded. Large grains promote microcracks due to anisotropic volume changes during cooling from the sintering temperature. This causes a reduction in the effective current-carrying cross section of the material. Ultrasonic measurements were used to confirm a significant increase in the microcrack density for large grained samples.