Low pH cements based on CAC for underground repositories of HLW: long-term hydration and resistance against ground water aggression

Abstract

Cementitious materials play an important role in the structural stability and integrity of a purpose built repository for the geological disposal of high level waste (HLW). However, the expected generation of an alkaline plume from the concrete by the ingress of groundwater would have detrimental effects on the intended use of a bentonite buffer. To limit this risk, low-pH cementitious materialss are being developed to have a target pH < 11 corresponding to the upper stability limit of bentonite. The research on low-pH cementitious materials is being addressed from different approaches depending on the type of cement used, emphasising the use of Ordinary Portland Cements (OPC based) and Calcium Aluminates Cements (CAC based), in both cases with high mineral admixtures contents with high silica percentage in their composition. It is well known that the high mineral admixtures contents used in the low-pH binders significantly modify the microstructure and the pore fluid of these cementitious materials. Therefore, once the short-term hydration of these special cements is known, this paper deals with the modifications generated in the chemical composition of the pore solutions of low-pH cement pastes based on CAC and in their solid phases after 2 years of hydration. The results show a high stability of the solid phases formed in the shortterm (90 days). This paper also deals with the resistance of these low-pH cementitious materials to long term ground water aggression using two types of aggressive agents: deionised water and ground water from the real site of Äspö (Sweden). Low-pH concretes based on CAC plus silica fume have been evaluated. This evaluation implies the study of the modifications made in the microstructure of lowpH concretes due to their exposure to a constant water flow and in the chemical composition of the leaching fluid.