Developing a Metakaolin Based Material as Cover Layer for Near Surface Waste Disposal Facilities

Key Words: metakaolin, geopolymer, compressive strength, cover layer, erosion

The Israeli national site for near surface radioactive waste disposal facilities is located in the Yamin Plain (YP) in Israel. In the disposal site, waste containers are usually placed in trenches or constructed vaults and when full, backfilled and eventually covered and capped with an impermeable membrane and topsoil. Many studies which focused in YP, showed that erosion and removal of the cap layer overlying the waste, is the main pathway for radioactive dispersion to the biosphere [A. Dody et al., 2017]. Since safety assessment for the waste disposal facilities should be done for hundreds of years, the cap layer should be stable and effective throughout that period.

The cover layer at the YP is composed of sandy soil, which can be stabilized using ordinary Portland cement (OPC). Geopolymers are alkali-activated binders based on alumino-silicate materials such as metakaolin or industrials by-products such as fly ash or slags. It has been found that geopolymers can exhibit high compressive strength and excellent chemical and thermal resistance, therefore offering a promising alternative to OPC [L. Chen et al., 2016, M. Zhang et al., 2013].

In the present study, we intend to develop a new composite alkali-activated material designed to serve as a cover layer, part of an engineered barrier system (EBS) for near surface radioactive disposal facilities. This composite is designed to have extended anti-erosion/abrasion properties and prevent infiltration of water, therefore isolating the waste from the biosphere. The capping layer was designed and prepared by molding of two layers of soil mixture, the upper layer intended to extend the anti-erosion properties and the lower layer to prevent water infiltration.

The materials used in the current study include local clays and sandy soil taken from YP, molded with metakaolin as the geopolymeric binder, activated with sodium silicate and sodium hydroxide mass ratio of 1.3:1 as the activating solution. Six types of activated mixtures were studied differing in their clay/sand/metakaolin content.

The experimental methods of the study were divided into several categories: mechanical properties, physical degradation, chemical and microscopic characterization. It was found that increasing the sand content in the mixtures, created a stronger, more workable material. The best mechanical properties were obtained for the sample prepared with sand/metakaolin mass ratio 1:0.3 and Solids/activator mass ratio of 1:0.33 reaching a compressive strength of 33.4 MPa, similar to the reference mixture. Abrasion tests for this formulation were performed, showing the matrix meets the standard for abrasion resistance.

Two layered samples were cast and the adhesion between these layers was tested using SEM and four-point flexural test, showing a very dense and undisturbed interface that was formed.

Overall, results show that this family of materials have high potential to serve as cap layer for waste disposal facilities.