Andra, the agency in charge of the radioactive waste management in France, is responsible for the design and the conception of the future deep geological disposal facility purposed to host among others the intermediate level long lived wastes (ILW-LL). The design proposed envisaged the sealing of the disposal ILW-LL wastes’ cells by a swelling clay and two low-pH concrete plugs. Those Low-pH cementitious materials are obtained by massive incorporation of siliceous bearing materials in usual cements, which aimed to lower the materials’ alkalinity (pH). The recourse to commonly used cementitious materials for the sealing would lead to an inhibition of the swelling properties of the clays due to the cements’ high alkalinity. Recent studies have highlighted for low-pH materials an increased permeability, cracking and pore size distribution upon carbonation.
IRSN as a French nuclear safety expert raised the question (through this study) of the durability of Low-pH materials in the framework of the carbonation. The carbonation is defined in cementitious materials as the reaction of calcium (Ca) from Ca-bearing phases with the dissolved CO2 in the pore structure, initially present in the atmosphere. A model material approach was adopted in order to unveil the chemical changes and to allow mass balances after carbonation.
The results obtained evidenced the role of the calcium content and the aluminum content. Materials with high Ca content evidenced the slower carbonation kinetic. In presence of aluminium and high calcium content TAH and pentahedral Al species were formed, their presence and reactivity correlated with a decreased rate of C-A-S-H degradation compared to the one induced by high ca content. The microstructural evolutions and the changes in volumes and the shrinkage were quantified, the coupling between the chemistry and the transport properties was demonstrated comparing the model materials and the industrials low-pH material.