BREAKDOWN PROCESS OF AGGREGATES WITH NON-SWELLING AND SWELLING CLAYS AS AFFECTED BY ELECTROLYTE CONCENTRATION AND AIR CONDITION

Abstract

Breakdown of soil aggregates by wetting is caused by air slaking (disaggregation by compression of an entrapped air inside the aggregates), swelling and dispersion of clays contained in the aggregates. Recent studies have attempted to clarify the relative importance of these mechanisms using the aggregates from cultivated fields, however, it appears that diversity and impurity of the cultivated soil have caused inconsistent results. In this study, aggregates made of pure materials (Na-bentonite and Na-kaolinite) were used to clarify the relative importance of air slaking and swelling/dispersion on the aggregate breakdown. To separate the effect of an entrapped air and electrolyte (NaCl) concentration on breakdown process, aggregates were enclosed in glass ampules under vacuum and atmospheric conditions and were subjected to sudden wetting in a series of NaCl solution. Breakdown pattern and time for breakdown were determined by visual observation. The results show that for aggregates of Na-bentonite, breakdown pattern and time for breakdown were strongly dependent on NaCl concentration, but not on air condition. The lower the NaCl concentration was, the more the aggregates dispersed and the longer the time for breakdown was. This dependency was consistent with the findings of the interlayer expansion of Na-montmorillonite in the past study. In contrast, aggregates of Na-kaolinite wetted in vacuum were not broken, whereas aggregates were quickly broken down into small pieces in the presence of the entrapped air irrespective of NaCl concentration. In conclusion, this study clearly demonstrates that the dominant breakdown mechanism of aggregates with swelling clay was dispersion/swelling and that of aggregates with non-swelling clay was air slaking.