Study of CEM I and low pH cement pastes leaching in multi-ionic underground water
ABSTRACT: Within the framework of radioactive waste geological disposal, concretes must be adapted to the chemical conditions. Ordinary Portland cement is characterized by a high pH which could produce an alkaline plume in the repository near-field. To avoid this issue, low-pH cements were designed. This study compares the behavior of this low-pH with a CEM I cement paste, submitted to an underground solution combining various chemical attacks. The cement pastes were immerged in the solution. pH and pCO2 were maintained constant during the experiment. After 1 month, the CEM I paste presented a 50 μm depth altered zone. Partial portlandite leaching, deep ettringite precipitation and exogenous calcite crust formation were the phenomena observed and reproduced by modeling with the ALLIANCES reactive transport code. The low-pH paste showed a degraded thickness ten times higher than the CEM I paste: C-S-H decalcification, calcite precipitation and ettringite dissolution. These transformations generated a porosity coarsening.