Caesium sorption by hydrated cement as a function of degradation state: experiments and modelling

Area of Science:

• Geochemistry
• Materials Science
• Environmental Science

Background:

• Cementitious materials are crucial for radioactive waste repositories.
• Accurate Cesium (Cs) sorption data (K(d)) are essential for performance assessments.
• Existing models require refinement for diverse cementitious matrices.

Purpose of the Study:

• To determine reliable K(d) values for Cs on various hydrated cement paste (HCP) and mortar samples.
• To simplify and validate a surface complexation-diffuse layer model for Cs sorption.
• To understand the factors influencing Cs sorption in cementitious systems.

Main Methods:

• Experimental determination of Cs sorption on hydrated cement paste (HCP), mortars, and mineral phases.
• Simplification and parameter re-assessment of a surface complexation-diffuse layer model.
• Comparison of experimental data with model predictions across various conditions.

Main Results:

• CSH minerals are identified as the primary Cs sorbing phase in HCP.
• K(d) values are significantly influenced by pH and dissolved Na, K, and Ca concentrations.
• The simplified model accurately predicts Cs sorption across a wide range of conditions (K(d) from 10^-4 to 3.2 m^3/kg).

Conclusions:

• The developed model effectively quantifies Cs sorption variability in cementitious materials.
• Na and K concentrations are key factors explaining scatter in Cs sorption data for fresh HCP.
• Degraded HCP offers more predictable Cs sorption behavior than fresh HCP.