Long-term concrete performance

Cement and concretes are used for many purposes in the disposal of radioactive wastes: for incapsulation/solidification of the waste, as backfill and construction material for low- and intermediate level short-lived radioactive waste (LILW-SL) in near-surface disposal facilities.

Especially for LILW-SL, the cementitious engineered barriers (CEB) have an important safety function: to provide

• to provide limited water flow through the disposal facility;
• to provide strong sorption of radionuclides.

The driving force of chemical concrete degradation is the difference in chemical potential of components between the different solid phases, the aqueous and the gaseous phase. In other words, it is the thermodynamic disequilibrium that leads to typical detrimental chemical reactions as chloride ingress, carbonation, decalcification and sulphate attack.

This disequilibrium originates from environmental loading processes such as moisture, solute and heat transport. One of the most important factors maintaining this disequilibrium is the physical transport of chemical components. Both the absorption of fresh water into the cementitious material, the migration of water and mass within the porous microstructure of the material and the desorption from the cementitious material enhance the chemical degradation rate of the CEB. Quantification of the chemical cement degradation rate is only possible when mineral dissolution and precipitation processes are coupled with flow and diffusion in the cementitious porous medium.

Another factor indirectly contributing to the chemical degradation of concrete is the presence of (micro)cracks by their effect on the transport properties. Also, in a strongly coupled process description, moisture status and chemical degradation reactions in turn may influence the mechanical damage of concrete. Assessment of concrete durability thus requires a coupled description of migration of water and mass, geochemical reactions, and physical-mechanical damage processes. Developing such multi-process model requires process information deduced from well-controlled experiments.

The objective of the concrete durability assessment research is to develop an integrated model able to simulate the long-term durability of cementitious based materials used in radioactive waste disposal facilities. The research considers:

• estimation of the concrete degradation due to chemical and physical-mechanical stresses, which is essential information for assessing concrete durability;
• estimation of the water and flow properties during variably saturated flow conditions in homogeneously and inhomogeneously (cracked) degraded concrete.

Over long time scales, the optimal properties of concrete for safety will deteriorate as a result of chemical and physical-mechanical stresses. For assessing the safety of radioactive waste disposal facilities, knowledge on the long-term behaviour of cementitious engineered barriers (CEB) under disposal conditions (with interactions between CEB, soil and atmospheric conditions for near-surface disposal facilities) plays a crucial role. SCK•CEN contributes to developing models for assessment of concrete durability.

PHREEQC-2 Version OF CEMDATA07.02

The recently developed database for cement hydrates for ordinary Portland cement systems CEMDATA07.2 developed at EMPA (see CEMDATA07 website for more information) together with the auxiliary information from the Nagra/PSI-Thermodynamic Data Base was converted to PHREEQC-format for temperatures 0-50 °C by Jacques 2009.

PHREEQC-2 cement database is based on CEMDATA07.2 released on 14.08.2008
PHREEQC-2 cement database (26 kB)

References

Database conversion and benchmarking

D. Jacques, 2009. Benchmarking of the cement model and detrimental chemical reactions including temperature dependent parameters.
NIROND-TR 2008-30 E (9 MB)

Applications

D. Jacques, L. Wang, E. Martens, and D. Mallants, 2010. Modelling chemical degradation of concrete during leaching with rain and soil water types. Cement and Concrete Research, 40:1306-1313. doi:10.1016/j.cemconres.2010.02.008

E. Martens, D. Jacques, T. Van Gerven, L. Wang, and D. Mallants, 2010. Geochemical modeling of leaching of Ca, Mg, Al, and Pb from cementitious waste forms. Cement and Concrete Research, 40:1298-1305. doi:10.1016/j.cemconres.2010.01.007

Responsible: Jacques Diederik