Use of a migration technique to study alteration of compacted sand-bentonite mixture in contact with concrete

In this research, a migration technique was applied to accelerate the migration of calcium ions from the pore solution of concrete so as to investigate the alteration of compacted bentonite in contact with the concrete. A 15 V of direct current was used for a composite specimen with 100 mm in diameter and 100 mm in length in which the electrical potential gradient was equal to 167 V/m. This composite consists of compacted bentonite (Na-type) sand mixture and concrete that is adjacent to each other. After a target cumulative electric charge of 13 × 103 C was reached, the specimen was removed for analysis. The results of electron probe micro analyzer (EPMA) showed that the concentration of calcium became higher in the compacted bentonite in the vicinity of the interface with the concrete. This observation was supported by the results of thermogravimetry/differential thermal analysis (TG-DTA) that provided distinct evidences of reduced quantities of Ca(OH)2 in the concrete in the vicinity of the interface with the compacted bentonite. Therefore, calcium ions migrated from the concrete to the compacted bentonite under the given electric field. The results of the X-ray diffraction (XRD) curves indicated possible transformation of the characteristic of the compacted bentonite. However depending on the mix proportions of concrete employed, altered characterization of the compacted bentonite was different. It was found that the swelling capacity of the compacted bentonite in contact with normal concrete was decreased to 64% of its initial swelling capacity as the ratio of calcium content to sodium content (Ca/Na) increased up to 3.7. On the other hand the swelling capacity of the compacted bentonite in contact with fly ash concrete was 85% of its initial swelling capacity with a ratio (Ca/Na) of 2.3. Accordingly the use of the electrical migration technique enables quantitative evaluation of the reduced swelling capacity of the compacted bentonite in accordance with different types of hardened concrete.