Ageing of high and low pH concretes after short and long periods in contact with groundwaters

Trabajo presentado al Second Workshop of the HORIZON 2020 CEBAMA Project, celebrado en Espoo (Finlandia), del 16 al 18 de mayo de 2017.
During the second year of the Cebama Project, the work performed by CSIC was focused in two directions: 1) Study of the long-term interaction of concrete with groundwaters: i) samples of the FEBEX shotcreted concrete plug, after 13 years in the Grimsel Test Site conditions under the interaction of the granite and bentonite porewaters, at the concrete / bentonite contact and ii) concrete of the HB6 cell, after 10 years operation with the interaction of clayey and bentonite porewater, and 2) short-term interaction, under percolation tests, of aged or new high-pH (hpH) and low-pH (lpH) concretes. The general objective is to analyse the performance of different concrete types in real and simulated deep geological repository conditions. In the long term studies, the interaction of the concrete of the Febex concrete with the bentonite porewater has caused additional ettringite formation due to the sulphate penetration from the saturated bentonite. This ettringite formation has been evidenced from the concrete / bentonite Interfacial Transition Zone (ITZ) towards 1 cm inside the concrete. Alterations of the C-S-H enriched in Al and portlandite dissolution was also observed. Additionally, Cl diffusion from bentonite up to 5 cm inside the concrete was evident with Friedel salt formation. The pore structure of the concrete was affected with increase of micropores. In HB6 concrete the clayey and bentonite waters lead also to new ettringite formation, pH decrease and portlandite dissolution. In the short-term percolation tests a decrease in the pH of the percolated liquid was observed from hpH concrete with a sharp decay to that of Grimsel granite water pH. While the pH of percolated liquid remains unchanged with the lpH concrete. The first results of the short-term interaction between concrete and granite groundwater showed the formation of more ettringite in FEBEX hpH as consequence of the particular concrete composition that favours the movement of sulphates and Al to form new ettringite. Although the pore pH decreases below portlandite stability the bulk concrete still remains alkaline. On the contrary in Äspö lpH no new solid phases have been detected and alkalies are not leached. Longer and deeper studies are in way.
The research leading to these results has received funding from the European Union's Horizon 2020 Reasearch and Training Programme of the European Atomic Energy Community (EURATOM) (H2020-NFRP-2014/2015) under grant agreement n° 662147 (CEBAMA).