Characterization and sorption properties of low pH cements

Characterization of three low pH cement pastes, including the description of their sorption properties for tritiated water (HTO), 36Cl- and 129I- is described in this work. SEM-EDX and NMR analyses show that after 90 days of hydration, the main hydrated phases are C-S-H and C-A-S-H gels with a Ca:Si ratio between 0.8 - 1.0 and a Al:Si ratio of 0.05. TG-DSC and XRD indicate the presence of calcite in the mixtures where limestone filler has been used. Additional techniques were used to identify minor hydrated solid phases like ettringite (i.e., XRD and solid state NMR). Porosity and pore size distribution was characterized by MIP observing that the size of the pores in the hydrated cement phases varies from the micro to the nanoscale. Uptake studies of HTO, 36Cl- and 129Ifrom batch sorption experiments indicate very weak sorption (Kd < 0.40 ± 0.13 L/kg) for the 3 selected radionuclides. The uptake process of 36Cl- and 129I- is probably associated with surface processes in the C-S-H and C-A-S-H phases with competition for sorption sites, between them. In the case of HTO, isotopic exchange with the interlayer water of the C-S-H and the C-A-S-H seems to be the main uptake process.