Interaction processes at the concrete-bentonite interface after 13 years of FEBEX plug operation. Part I: Concrete alteration

Abstract

This paper evaluates the modifications created in the concrete of the FEBEX shotcreted concrete plug after 13 years in the Grimsel Test Site conditions. During this time the concrete interacted with granite groundwater and also with bentonite porewater at the concrete/bentonite contact. Three long cores and 6 small cores from different parts of the concrete plug were evaluated. Mechanical performance was not modified during this time but hydraulic conductivity increased. The main transport mechanisms involved in the alteration of the concrete were groundwater flow from the host rock to the concrete and diffusion at the concrete/bentonite interface. Leaching occurred in the concrete parts near the host rock due to the action of granite water with further portlandite dissolution. The joint action of granite groundwater and bentonite porewater has caused many changes to the concrete matrix which was located at a depth lower than 5 cm from the bentonite-concrete interface. In the first centimetre C-S-H was significantly altered, incorporating elements like Al, S and Mg which change the initial microstructure by loss of compactness. The ettringite content was very high along the length of the concrete plug due to the shotcreting technique which made use of accelerator additives that caused the formation of ettringite. An increase in the ettringite content is also shown near the bentonite barrier. Therefore, sulphate diffused from the bentonite into the concrete, causing the massive formation of new ettringite. Chloride also diffused from the bentonite barrier deeper into the concrete by up to 4–5 cm from where the formation of Friedel's salt was detected.