Geochemical behaviour of a bentonite barrier in the laboratory after up to 8years of heating and hydration

Abstract

Abstract The conditions of the bentonite in an engineered barrier for high-level radioactive waste disposal were simulated in a series of tests. Cylindrical cells with an inner length of 60 cm and a diameter of 7 cm were constructed. Inside the cells, 6 blocks of FEBEX bentonite compacted to a dry density of 1.65 g/cm 3 were stacked, resulting in a total length similar to the thickness of the clay barrier in a repository, as per the Spanish Reference Concept. The material was heated at 100 °C on the bottom surface and hydrated through the top surface with granitic water. Tests were performed for durations of 6, 12, 24 and 92 months. At the end of the heating and hydration (HH) treatment, exhaustive post-mortem analyses were performed to check the mineralogical, geochemical and pore water chemistry variations in the bentonite, the objective being to identify the bentonite–water interaction hydrogeochemical processes that occur over time. DRX, SEM and FTIR analyses did not show any evidence of smectite alteration. However, the chemical composition of the pore water evolved with time as a function of hydration of the bentonite, which was affected in turn by the temperature and by the geochemical processes in the bentonite–water system. The pore waters were initially of the Na Mg Cl type with an ionic strength of 0.2 M, and evolved to 0.03 M Na Cl SO 4 type water close to the hydration source, and to Na (Mg) Cl type water further away, with ionic strengths ranging from 0.3 to 0.9 M, depending on the particular location and the experiment run time. Dilution and evaporation of the pore water due to hydration and heating at the top and bottom of the bentonite, respectively, were the main processes controlling the conservative species (such as Cl − ), which moved through the bentonite by advective–diffusive transport. The non-conservative species, such as carbonates, sulfates and cations, were involved in dissolution-precipitation processes and exchange reactions. The pH of the pore water was controlled by surface complexation reactions at edge sites and by carbonates. In the hydrated zones, there was dissolution of sulfates and carbonates, affecting the distribution of the exchangeable cations and the pH of the pore water. The CaX 2 content increased at interlayer sites, and CEC values tended to increase, probably due to pH changes towards more alkaline values. In the heating zones, water evaporation resulted in a concentration of the pore water and in the precipitation of anhydrite, mainly in the stages before the hydration of the warmest zones. Evaporation and CO 2 degassing due to temperature led to the precipitation of calcite, decreasing the pH of the pore water, which probably caused the CEC values to decrease. There was also an increase in the content of MgX 2 and KX at the interlayer sites close to the heater zone, decreasing the NaX and the CaX 2 content with time.