Microfabric collapse of swollen materials such as soils and clays, dried by critical point technique, is considered to seriously affect pore size distribution measurements by Hg intrusion. Nevertheless some interesting aspects of pore space evolution, related to the hydration-dehydration processes, can be elucidated by collapse dynamics. These findings are reported in this work. The following clays have been studied: Na- and Ca- Wyoming bentonite. Ca- Montana Illite and Ca-Zettlitz Kaolinite. Small clods of these materials (0.5-1 cm3) were equilibrated at pF 2.5 and 7 and the wet samples (pF 2.5) were dehydrated by CO2 critical point technique. Pore size distribution was measured by Hg intrusion and, for comparison, by N2 desorption at 77° K in the pore range of overlapping of the two methods (3.7-50 nm pore radius). Micrographs by Backscattered Electron Scanning Images (BESI) were also obtained in order to reveal larger pores (up to10 limy Total samples volume was measured by hydrostatic up thrust method in kerosene. Our results confirm that Hg intrusion forces the microfabric of clay samples to collapse when they are equilibrated at pF 2.5 and dried by critical point technique. The collapse is clearly evidenced by a reduction of total volume and by the measurement of a second Hg intrusion. It occurs to a different extent, depending on the nature of the clays. The collapse is complete, attaining the same microfabric as at pF 7, for Kaolinite and Na-bentonite samples; it is partial, but still strong for Illite and relatively low for Ca-bentonite. For the samples at pF2.5, the N2 desorption method shows that the pores involved in the swelling and collapsing are largely those <50 nm in Kaolinite and Illite. Moreover, the collapse occurs at a mercury pressure lower than that expected for the penetration into pores of this size Therefore, the diagrams of differential pore size distribution are misleading in that they indicate the presence of a class of pores not existing in the samples. In bentonites, as shown by BESI, a larger pore range is involved in microfabric dynamics. The "apparent" Hg pore size distribution agrees satisfactory with data from N2 and BESI analyses. In general, the collapsing curves (volume to Hg pressure) were found to roughly match the swelling curve (volume to swelling pressure) of each clay on hydration. This correspondence found in our experiments, can be helpful for describing the swelling behaviour of clay samples. The different tendencies of the clay samples to collapse can be ascribed to the properties of clay surfaces and particularly to the extent of contact surfaces and to the type of interactions between the single particles.
Dynamics of clay fabric on hydration as revealed by pore space collapse on mercury intrusion
D'Acqui L P;E Sparvoli;
1993
Abstract
Microfabric collapse of swollen materials such as soils and clays, dried by critical point technique, is considered to seriously affect pore size distribution measurements by Hg intrusion. Nevertheless some interesting aspects of pore space evolution, related to the hydration-dehydration processes, can be elucidated by collapse dynamics. These findings are reported in this work. The following clays have been studied: Na- and Ca- Wyoming bentonite. Ca- Montana Illite and Ca-Zettlitz Kaolinite. Small clods of these materials (0.5-1 cm3) were equilibrated at pF 2.5 and 7 and the wet samples (pF 2.5) were dehydrated by CO2 critical point technique. Pore size distribution was measured by Hg intrusion and, for comparison, by N2 desorption at 77° K in the pore range of overlapping of the two methods (3.7-50 nm pore radius). Micrographs by Backscattered Electron Scanning Images (BESI) were also obtained in order to reveal larger pores (up to10 limy Total samples volume was measured by hydrostatic up thrust method in kerosene. Our results confirm that Hg intrusion forces the microfabric of clay samples to collapse when they are equilibrated at pF 2.5 and dried by critical point technique. The collapse is clearly evidenced by a reduction of total volume and by the measurement of a second Hg intrusion. It occurs to a different extent, depending on the nature of the clays. The collapse is complete, attaining the same microfabric as at pF 7, for Kaolinite and Na-bentonite samples; it is partial, but still strong for Illite and relatively low for Ca-bentonite. For the samples at pF2.5, the N2 desorption method shows that the pores involved in the swelling and collapsing are largely those <50 nm in Kaolinite and Illite. Moreover, the collapse occurs at a mercury pressure lower than that expected for the penetration into pores of this size Therefore, the diagrams of differential pore size distribution are misleading in that they indicate the presence of a class of pores not existing in the samples. In bentonites, as shown by BESI, a larger pore range is involved in microfabric dynamics. The "apparent" Hg pore size distribution agrees satisfactory with data from N2 and BESI analyses. In general, the collapsing curves (volume to Hg pressure) were found to roughly match the swelling curve (volume to swelling pressure) of each clay on hydration. This correspondence found in our experiments, can be helpful for describing the swelling behaviour of clay samples. The different tendencies of the clay samples to collapse can be ascribed to the properties of clay surfaces and particularly to the extent of contact surfaces and to the type of interactions between the single particles.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
