Microstructure and physico-chemical transformation of some common woods from Cameroon during drying

Journal of Thermal Analysis and Calorimetry - The influence of drying on the microstructure, physical and chemical properties of some tropical wood species has been investigated using...     

Non-contact dilatometry of hard and soft porcelain compositions

Two different porcelain compositions were studied: a soft and a hard one. DTA, optical non-contact dilatometry and DSC were used to evaluate the thermal behaviour of the porcelain compositions with the aim to master the suitable thermal cycle for each. Results were interpreted on the basis of mineralogy and microstructure: the amorphous phase, abundant in soft porcelain, plays an important role on thermal expansion (8·10^–6 K^–1 for soft and 6·10^–6 K^–1 for hard at 1000°C). Thermal expansion behaviour as function of firing time was also studied. Non-contact dilatometry characterisation of porcelain bodies enable to master the suitable thermal cycle for the sintering.     

Use of noncontact dilatometry for the assessment of the sintering kinetics during mullitization of three kaolinitic clays from Cameroon

Noncontact dilatometry, compared to differential scanning calorimetry (DSC), was used together with scanning electron microscopy and densification behavior studies to investigate the parameters that govern the kinetics of transformation of kaolin to mullite during sintering. Three kaolinitc clays from Cameroon, with different SiO₂/Al₂O₃ molar ratio, were examined. The temperatures of mullite nucleation were 973, 979, and 984 °C at 5 °C/min heating rate, respectively, for values of SiO₂/Al₂O₃ molar ratio equal to 4.22, 2.22, and 2.08. At 20 °C/min heating rate, the temperatures are shifted to higher values, 992, 997, and 1,001 °C. The mullitization phenomenon, which includes a first step of nucleation and a second one of crystal growth, presented activation energy in the range of 650–730 kJ/mol, depending on the nature of the sample investigated. These values, obtained by noncontact dilatometer measurements, were comparable to those obtained by means of DSC and are in agreement with literature values. The difference in sintering kinetics for the three kaolinitic clays could explain the different morphologies obtained for the mullite grains.     

Role of ɣ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on the mechanical and microstructural properties of metakaolin-based geopolymer cements

The main target of this work is to investigate the influence of ?-Al₂O₃ on the properties of metakaolin-based geopolymer cements. The kaolin used as starting material for producing geopolymer cements contains approximately 28 and 64% of gibbsite and kaolinite, respectively. This kaolin was transformed to metakaolins by calcination at 500, 550, 600, 650, and 700?°C for 1?h. Gibbsite contained in kaolin was transformed to γ-Al₂O₃ during the calcination process. The hardener was obtained by mixing commercial sodium silicate and sodium hydroxide solution (10?M) with a mass ratio sodium silicate/sodium hydroxide equal to 1.6:1. Geopolymer cements, GMK-500, GMK-550, GMK-600, GMK-650, and GMK-700, were obtained using the prepared hardener with a mass ratio hardener/metakaolin equal to 0.87:1. It could be seen that the specific surface area of metakaolins decreases with increasing the calcination temperature of kaolin owing to the formation of the particles of γ-Al₂O₃. The compressive strengths 18.21/29.14/36.61/36.51 increase in the course GMK-550/GMK-600/GMK-650/GMK-700. The X-ray patterns and micrograph images of geopolymer cements, GMK-600, GMK-650, and GMK-700, indicate the presence of γ-Al₂O₃ in their structure. It was typically found that γ-Al₂O₃ remains largely unaffected during the geopolymerisation, and therefore could act as an inert filler and reinforce the structure of geopolymer cements.

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Enhancing the crystallization phenomena and strength of porcelain stoneware: the role of CaO

Journal of Thermal Analysis and Calorimetry - Limestone was used to modify the fluxing action of two potash feldspars (a pure potash feldspar and a soda-potash feldspar) labeled P and C,...     

Sintering behaviors of two porcelainized stoneware compositions using pegmatite and nepheline syenite minerals

Semi-vitreous bodies, with flexural strength of ~78 MPa and water absorption of ~0.4 % for the optimal maturated specimens, were successfully prepared with solid solutions of pegmatite and nepheline syenite, respectively, for the full dense (P) and relatively porous composition (G). Despite the similarity on the bulk chemical composition of the two formulations, it was found that their thermal behaviors were significantly affected by the action of CaO on the flux system considered leading to earlier vitrification of P specimens with high flexural strength as from 1,175 °C. Conversely, the flexural strength of G specimens increases progressively with soaking time and temperature development up to 1,225 °C. The positive action of 5 mass% of CaO in the vitrification range of pegmatite solution, the relative low temperature of maturation of the semi-vitreous matrices, and the results of water absorption allow us to describe the formulations studied to offer promising potential in the production of sustainable and low-cost porcelainized stoneware. The high strength of P at relatively low temperature could be explained by the matrix-strengthening theory, while the extent of crystallization of G could be ascribed to the mullite theory.     

Insulating behavior of metakaolin-based geopolymer materials assess with heat flux meter and laser flash techniques

Thermo physical behavior of metakaolin-based geopolymer materials was investigated. Five compositions of geopolymers were prepared with Si/Al from 1.23 to 2.42 using mix of sodium and potassium hydroxide (~7.5?M) as well as sodium silicate as activator. The products obtained were characterized after complete curing to constant weight at room temperature. The thermal diffusivity (2.5?C4.5?×?10^?7m²/s) and thermal conductivity (0.30?C0.59?W/m?K) were compared to that of existing insulating structural materials. The correlation between the thermal conductivity and parameters as porosity, pore size distribution, matrix strengthening, and microstructure was complex to define. However, the structure of the geopolymer matrix, typical porous amorphous network force conduction heat flux to travel through very tortuous routes consisting of a multiple of neighboring polysialate particles.     

Pore network and microstructure in the prediction of heat flux transport in sponge-like geopolymers for thermal insulation

Journal of Thermal Analysis and Calorimetry - A microstructural approach to the prediction of heat flux transport in porous geopolymers is used experimentally to understand the effects of the pore...