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Mineralogical Aspects of Diaphragm Wall Materials for the Encapsulation of Hazardous Waste Sites

Abstract:


A diaphragm wall material containing clay, cement, silica filler and water was evaluated to fulfil following requirements:

1. Longevity in the presence of high leachate concentrations.

2. High density of the suspension to replace the bentonite slurry used during excavation of the trench.

3. Small porosity to ensure a low hydraulic conductivity.

4. High adsorption capacity.

5. Workable mixture over a long period of time. The mixture should not disintegrate before hardening.

6. Sufficient strength under hardened condition to support the surrounding material.

The laboratory program included the investigation of more than 30 different mixtures of clay and cement. After a preliminary screening three mixtures were chosen for further investigations on: a) workability of the mixture, b) mineralogy immidiately after hardening and changes in mineralogy with time, c) changes in pore size distribution with time, d) hydraulic conductivity as a filnction of time, e) difusion coefficients, and f) chemical stability of the hardened mixture. As the result of these tests two mixtures were chosen to constmct a full scale experimental diaphragm wall. The construction of the diaphragm wall showed that one of the two preliminary selected mixtures was less suitable and thus had to be eliminated for further testing. Finally one mixture was selected for an in-situ encapsulation of a large hazardous waste site in Germany by a diaphragm wall up to 55 meter in depth.

The main goal of this mineralogical study was to investigate the changes in mineralogy and fabrics in the material after hardening and after testing hydraulic conductivity by water and leachate. It was demonstrated that silica filler containing mixtures show not only a decrease in average pore size and hydraulic conductivity with time but also a high resistance against chemical attack of the cement matrix. The investigations permitted a good estimation of the long term behaviour of the diaphragm wall material. The clinker minerals alite and belite remained stable whereas portlandite disappeared.

Authors:

Madsen, Fritz and Kahr, Günter and Hermanns Stengele, Rita

Index Terms:

Clay; ClayGroup

Further Information:

Date published: 1996