Chunling Liu

Pore structures of fly ashes activated by Ca(OH)2 and CaSO4 · 2H2O

The nature of the pore structure which develops when low-lime fly ash reacts with Ca(OH)2 and CaSO4 · 2H2O under hydrothermal treatment has been investigated. The nitrogen adsorption-desorption isotherms of hydrothermally treated samples of fly ash and activated fly ash were analyzed. X-ray diffractometry was used to characterize the hydration products and SEM was used to analyze microstructure. The shapes and sizes of the hysteresis loops of isotherms and the pore size distribution data indicated that the pore structures of samples were comprised primarily of wedge-shaped pores with open ends. The surface area obtained when fly ash reacted with Ca(OH)2 under hydrothermal treatment at 100 °C was 33.4 m2/g, while that of untreated fly ash was only 1.3 m2/g. The surface area of fly ash after reaction with CaSO4 · 2H2O was 2.9 m2/g. For fly ash reacted with Ca(OH)2, the volumes of the pores with radii of 19Aincreased with increasing temperature of thermal treatment. Depending on the temperature, calcium silicate hydrate, calcite and anhydrite formed. Because the pozzolanic reaction produces calcium silicate hydrate with a very large surface area, it controls the pore structures in which fly ash is activated by Ca(OH)2. Therefore, a realistic assessment of the pore structure of activated fly ash is needed to understand those important physical and mechanical properties of concrete.

Porous Silicon Oxycarbide Glasses from Organically Modified Silica Gels of High Surface Area

High surface area alkyl-substituted silica aerogels and xerogels were successfully prepared by sol-gel processing and supercritical drying. The gels were further heat treated in inert atmosphere to temperatures as high as 1000°C. Surface areas and pore structure of the gels and gels pyrolyzed at high temperatures were determined by multipoint BET surface area measurement. The aerogels and xerogels exhibited surface areas of about 1100 m2/g. No significant effect of pH was found on the surface areas of gels in the two step sol-gel process, but gels of low pH showed smaller pore diameter and higher density. Xerogels showed smaller surface area, pore size, and pore volume compared to aerogels. Upon pyrolyzing in inert atmosphere, the surface areas of all the gels decreased with temperature as a result of collapse of micropores and shrinkage of mesopores. Unlike pure silica gel, which loses almost all surface area and densifies at 1000°C, the advantage of the alkyl-substituted gels is that they maintained a high surface area of 400 m2/g at 1000°C.

High Surface Area SiC/Silicon Oxycarbide Glasses Prepared from Phenyltrimethoxysilane-Tetramethoxysilane Gels

Phenyltrimethoxysilane (PhTMS) was hydrolyzed or cohydrolyzed with tetramethoxysilane (TMOS) to make aerogels and xerogels. Porous SiC/silicon oxycarbide glasses were prepared by further pyrolyzing these gels in inert atmosphere up to 1500°C. The pore structure and chemical nature were studied by nitrogen and water sorption measurement, chemical analysis, X-ray diffraction and scanning electron microscopy. It has been found that the addition of PhTMS into TMOS gels decreased the surface area and porosity of TMOS gels, but enhanced their hydrophobicity and thermal stability. Pyrolyzing 25 mole% PhTMS-75 mole% TMOS aerogel in argon resulted in porous silicon oxycarbide glass which has a surface area of 581 m2/g at 1000°C. Pyrolyzing pure PhTMS gels at 1400°–1500°C produced porous SiC/C/silicon oxycarbide composites having surface areas in the range of 400–500 m2/g.

Carbon-silica xerogel and aerogel composites

High surface area carbon-silica xerogels and aerogels were prepared by adding various amounts of activated carbon particles during gelation of tetramethyoxysilane (TMOS). Surface areas and pore structure of the gels were determined by nitrogen and water adsorption and desorption measurements. Carbon increased the surface area of silica gel and the carbon-silica gel composites were found to be hydrophobic. The pore structure of xerogel composites was dominated by carbon whereas that of aerogel composites was apparently controlled by silica component.

Nanocomposite route to the stabilization of porous ϑ-alumina

Stabilization of ϑ-alumina phase by silica was studied in nanocomposite (diphasic) alumina-silica gels by XRD and BET surface areas measurements. Five wt.% of silica (22 nm particles) increased the crystallization temperature of ϑ to α-alumina by about 100°C from boehmite (10 nm particles) derived alumina. Stabilization of ϑ-alumina was caused by the formation of intimate contact (Al-O-Si) between components by diffusion of silica into the defect alumina structure.

Porous mullite ceramics through diphasic gels of large boehmite and small silica particles

Mullite formation process has been studied in stoichiometric mullite (3Al2O3·2SiO2) diphasic gel containing large boehmite (∼1 μm) and small silica (∼10 nm) particles. It has been found that initial mullitization did not take place inside the silica phase (cristobalite), but took place in the defect θ-alumina phase. θ-alumina was stabilized by silica when the temperature was below 1350°C. At temperatures above 1350°C, mullite crystallized directly. It was suggested that silica diffused into the pores (<1.8 nm) of θ-alumina and prevented the collapse of θ-alumina pore structure. On the other hand, when silica was not present, the pore structure of θ-alumina collapsed and α-alumina crystallized at ∼1100°C. Porous mullite ceramics were obtained by using special diphasic gels containing large boehmite and small silica particles.

Pore structures of fly ashes activated by Ca(OH)[sub 2] and CaSO[sub 4] [center dot] 2H[sub 2]O

The nature of the pore structure which develops when low-lime fly ash reacts with Ca(OH)2 and CaSO4 · 2H2O under hydrothermal treatment has been investigated. The nitrogen adsorption-desorption isotherms of hydrothermally treated samples of fly ash and activated fly ash were analyzed. X-ray diffractometry was used to characterize the hydration products and SEM was used to analyze microstructure. The shapes and sizes of the hysteresis loops of isotherms and the pore size distribution data indicated that the pore structures of samples were comprised primarily of wedge-shaped pores with open ends. The surface area obtained when fly ash reacted with Ca(OH)2 under hydrothermal treatment at 100 °C was 33.4 m2/g, while that of untreated fly ash was only 1.3 m2/g. The surface area of fly ash after reaction with CaSO4 · 2H2O was 2.9 m2/g. For fly ash reacted with Ca(OH)2, the volumes of the pores with radii of 19Aincreased with increasing temperature of thermal treatment. Depending on the temperature, calcium silicate hydrate, calcite and anhydrite formed. Because the pozzolanic reaction produces calcium silicate hydrate with a very large surface area, it controls the pore structures in which fly ash is activated by Ca(OH)2. Therefore, a realistic assessment of the pore structure of activated fly ash is needed to understand those important physical and mechanical properties of concrete.

Refereed paperPore structures of fly ashes activated by Ca(OH)2 and CaSO4 · 2H2O

The nature of the pore structure which develops when low-lime fly ash reacts with Ca(OH)2 and CaSO4 · 2H2O under hydrothermal treatment has been investigated. The nitrogen adsorption-desorption isotherms of hydrothermally treated samples of fly ash and activated fly ash were analyzed. X-ray diffractometry was used to characterize the hydration products and SEM was used to analyze microstructure. The shapes and sizes of the hysteresis loops of isotherms and the pore size distribution data indicated that the pore structures of samples were comprised primarily of wedge-shaped pores with open ends. The surface area obtained when fly ash reacted with Ca(OH)2 under hydrothermal treatment at 100 °C was 33.4 m2/g, while that of untreated fly ash was only 1.3 m2/g. The surface area of fly ash after reaction with CaSO4 · 2H2O was 2.9 m2/g. For fly ash reacted with Ca(OH)2, the volumes of the pores with radii of 19Aincreased with increasing temperature of thermal treatment. Depending on the temperature, calcium silicate hydrate, calcite and anhydrite formed. Because the pozzolanic reaction produces calcium silicate hydrate with a very large surface area, it controls the pore structures in which fly ash is activated by Ca(OH)2. Therefore, a realistic assessment of the pore structure of activated fly ash is needed to understand those important physical and mechanical properties of concrete.

Epitaxy in the Crystallization of Feldspar Gels and Glasses

The crystallization behavior of stoichiometric feldspar gels and glasses before and after seeding has been compared, and the role of epitaxy in their crystallization upon seeding has been studied. The crystallization products were mainly determined by X-ray diffraction (XRD) analysis and epitaxy upon seeding was examined with a scanning electron microscope (SEM), and energy dispersive spectrometry (EDS). It was found that upon isostructural seeding, Na and K-feldspars crystallized epitaxially from their gels and/or glasses. Little or no seeding effect was found in CaAl{sub 2}Si{sub 2}O{sub 8} and SrAl{sub 2}Si{sub 2}O{sub 8} gels and glasses. Isostructural seeding in BaAl{sub 2}Si{sub 2}O{sub 8} gets increased the formation of Ba-feldspar (monoclinic celsian) epitaxially, whereas without seeding hexacelsian was the crystallization product.