In this paper, preliminary results about both the chemical and physical behaviours of hemp fibres in calcium rich medium are presented. The influence of hemp fibres introduction in Portland cement ...pastes on the setting time has been investigated. It seems that pectin contained in the fibres can form complex molecules with calcium ions and could be responsible for the observed delay. The role of fibres surface treatment has been also studied. Chemical attack of the fibres surface by an alkaline and calcium rich solution degrades hemicelluloses contained in the fibres and seems to roughen the surface. This surface modification seems to play a major role in the strengthening of the cement/fibres interface. These results show that hemp fibres introduced in cement pastes exhibit a typical composite behaviour compared to cement sample, which lead to an improvement of the mechanical properties.
In the last few years, eco friendly materials have become an important part of the building materials market. Natural fibres are already used in various types of materials, like plastics, concrete ...and lime-based products. They demonstrate different attributes like the combination of good mechanical, thermal and acoustic properties that allow these types of materials to be used for different applications. The main drawback associated with plaster is its brittleness, especially under tensile stress. Therefore, it is interesting to investigate different methods that could potentially enhance the mechanical properties of plaster. Adding fibres to gypsum to obtain a composite material is one way to improve the behaviour of the product, especially after the failure of the matrix. The aim of this work was to the study the effects of adding natural fibres, namely hemp and flax fibres, on the setting time of plaster and the mechanical properties of the composite matrix. It was shown that hemp delayed the setting of plaster, unlike flax. The initial and final setting times almost doubled when hemp was added in a plaster matrix, whereas flax fibres did not drastically change them. Different chemical treatments of hemp were tested and the impact on the setting time was measured. The setting times of both composites made with hemp and flax were reduced once the fibres were treated (25–40% reduction), compared to the setting time of the calcium sulphate hemihydrate alone. The mechanical properties of the composite materials are also discussed. The behaviour of plaster was modified from brittle to a non-linear one when fibres were added, and even at small levels of addition, flax fibres allowed slightly higher values of flexural strength to be reached.
Stable glasses are successfully synthesized in the TeO2-GeO2-ZnO system at 850 °C by the melt-quenching method and the glass forming domain is determined in the TeO2-rich part of the diagram. The ...thermal study, carried out using differential scanning calorimetry, reveals that the glass transition temperature, as well as the thermal stability, increases with the addition of ZnO or GeO2. Bulk glass samples are elaborated within two series of compositions, corresponding to fixed concentrations in GeO2 (respectively 5 or 10 mol. %), and to various contents in ZnO. Structural changes caused by the ZnO addition are discussed based on Raman spectroscopy data. A progressive but very moderate network depolymerization is shown with increasing amount of ZnO. However, two different regimes can be identified, depending on the ZnO content. It is believed that ZnO acts as a network modifier for compositions below 20 mol. %, and starts to participate as a glass network former over such concentration. It is well evidenced that GeO2 contributes to the increase in Young's modulus E, evaluated from ultrasonic echography measurements. In addition, this oxide favors the network reticulation detected by the decrease of the Poisson ratio and the increase of the fractal bond connectivity. However, the role of ZnO is more complicated and will be extensively discussed. The decrease in the atomic packing density Cg probably explains the global evolution of E as a function of ZnO content. The refractive indices and optical band gap energies are extracted from UV-Visible-NIR optical transmission data. For the studied glasses, it is found that the transmission threshold decreases with larger ZnO contents, reflecting the increase in the optical band gap value. Refractive index is finally seen to decrease as a function of both ZnO and GeO2 contents. Such variation is explained by the decrease of the molar electronic polarizability, and by the lower optical basicity values known for TeO3 entities in comparison to TeO4 units.
Display omitted
•Large glassy domain within the TeO2-rich part of the TeO2-GeO2-ZnO ternary system.•Thermal stability increases with the addition of ZnO and/or GeO2.•Progressive but slight network depolymerization with increasing the amount of ZnO.•GeO2 causes decrease of Poisson ratio and increase of fractal bond connectivity.•ZnO improves the optical transmission level and the optical band gap value.
Many refractory materials exhibit high thermal shock resistance, which is often mostly due to their high flexibility. Understanding the microstructure key points allowing to develop a non-linear ...mechanical behaviour is of great relevance for future material improvements. The present work aims at optimising the processing of magnesia–spinel refractory materials close to industrial ones with simplified microstructures. The final goal is the investigation of the relationship existing between microstructure evolutions and induced thermomechanical properties. The thermal expansion mismatch which exists between the two phases (spinel inclusions and magnesia matrix) is expected to generate, during cooling, radial microcracks around the inclusions. The development of such microcracks network, closely related to the inclusions content, has been studied and the damage occurrence has been confirmed by several high temperature characterisation techniques. The influence of this thermal micro damage on the evolution of stress–strain law in tension of such materials has also been investigated.
Andalusite is a well known alumino-silicate mineral largely employed in the refractory industry in order to provide high mechanical properties and thermal shock resistance. This work is devoted to ...the study of the thermal expansion behaviour of andalusite aggregates used in refractory castables. Anisotropic thermal expansion behaviour has been underlined by dilatometric measurements on andalusite single crystals according to their crystallographic axes. This behaviour can be magnified by the presence of quartz, even in small quantity, as impurities in the andalusite particle. The anisotropy of dilation is preserved after mullitisation of andalusite. These important results make it possible to explain the origin of the diffuse damage observed in andalusite-based castables which is most probably responsible for the enhancement of the strain to rupture required for the good thermal shock performance of such material.
Refractory materials containing cordierite (2MgO·2Al
2O
3·5SiO
2) and mullite (3Al
2O
3·2SiO
2) are used as support in furnaces, because of their low thermal expansion properties (coefficient of ...thermal expansion (CTE) ≈3–4
×
10
−6
K
−1) which confer them a very good ability to thermal shock resistance. Composed of two phases presenting very different CTE (1.5–3
×
10
−6 for cordierite and 4–6
×
10
−6
K
−1 for mullite), these materials can develop damage during thermal cycling due to internal stresses. This paper is devoted to the characterisation of the damage generated by this CTE mismatch, thanks to the application of ultrasonic techniques like long bar mode echography and acoustic emission (AE). The combination of these two techniques allows, during the applied thermal cycle (20–1200
°C), to continuously follow both the evolution of the elastic properties (Young's modulus) and the acoustic emission activity generated within the material. The analysis of these two characteristics, which are closely related to the damage evolution, makes it possible to propose a chronology of the mechanisms (damage, expansion) acting during the heating and the cooling stages.
This paper reports results concerning the oxidation of silicon carbide of pure SiC powder (98% of SiC) and the oxidation of two silicon carbide castables, used in waste-to-energy plants (WTE), and ...containing 60% and 85% of SiC, respectively. The investigated temperature range (800–1200
°C) corresponds to typical service conditions in WTE. Thanks to thermogravimetric and thermal expansion tests, kinetics of oxidation of the powder of SiC and of the castables has been investigated. According to these tests, several important points have been underlined. Firstly, the oxidation of SiC aggregates has a high influence on the thermal expansion and on the weight gain of SiC castables. Secondly, the grain size distribution of SiC aggregates within castables plays a dominating role (especially enhanced for the fine particles) in castable expansion behaviour induced by oxidation. This is a key point according to the evolution of thermal expansion of such materials.
Fused-cast refractory materials are widely used in the glass industry, especially in the building of superstructures and side walls of fusion furnaces. The HZFC (High Zirconia Fused Cast) products ...are especially used as tank blocks for the fusion of highly corrosive glasses melted at very high temperature (such as LCD glass), due to their high corrosion resistance and their low generation of glass defects generation.
The presence of this high amount of pure ZrO
2 in the refractory can be responsible for microdamage occurrence during the cooling step after melt casting (annealing), associated to the martensitic transition of zirconia.
Acoustic emission (AE) analysis is well known as a reliable tool to investigate microstuctural evolution at a very small scale. In this work, a fused-cast ZrO
2 refractory has been investigated using a AE unsupervised pattern recognition procedure and a frequency-energy coupled analysis. Data gathering during thermal cycles at high temperature (typically 1500
°C) has been done thanks to an innovative self-developed testing device. The analysis of frequency and energy parameters makes it possible to detect and to characterize the occurrence and the chronology of microdamage in specific range of temperature. Hypothesis concerning different ways of microdamage formation below the temperature of the martensitic transformation of ZrO
2 during the cooling stage can be proposed related to thermo-mechanical properties and the microstructure of the material. In particular, intergranular and intragranular microcracks due to CTE mismatches occurring in the material have been also investigated.
Large strain to rupture behavior is essential, for refractory materials, to improve their thermal shock resistance. The non-linear comportment under loading of specific developed ceramics associated ...to their type of microstructure (micro-cracked) leads to the possibility to increase their strain-to-rupture level. Aluminum titanate (AT: Al2TiO5) ceramics are one of these materials and are characterized by a mechanical behavior strongly dependent on their microstructure. Indeed, this behavior can vary from a fragile one to a large non-linear one according to the degree of microcracking present within the material. The paper here presented is devoted to the study of this nonlinear behavior thanks to four-points bending test associated with digital image correlation technique to determine kinematics fields. Results highlight the asymmetric character of the mechanical behavior of a microcracked aluminum titanate. A comparison between the Young moduli and fracture strength obtained using conventional and ones identified by digital image correlation will be done.
Plaster or gypsum (CaSO4·2H2O) is obtained from a suspension of β calcium hemihydrate (CaSO4·1/2H2O). The effect of two additives, known to retard setting, namely citric and malic acids, on the ...electrical conductivity and on the acoustic emission (AE) has been studied. This work presents the correlation obtained between these two techniques and their ability to differentiate between the crystallisation of a plaster with or without an organic additive. In particular, the surface adsorption of these carboxylic acids on solid surfaces has been evidenced by a specific acoustic emission activity.
