CRACK2. Modelling calcium carbonate deposition from bicarbonate solutions in cracks in concrete

The numerical CRACK2 model simulates precipitation of calcite from calcium bicarbo-nate solution (e.g. groundwater) passing through cracks in concrete or other cementitious materials. A summary of experimental work is followed by a detailed description of the model. Hydroxyl ions are transported by diffusion in pore systems in columns of cementitious materials. The hydroxyl is precipitating calcite from a flow of bicarbonate solution in a crack connecting the ends of a row of such columns. The cementitious material is simulated as calcium hydroxide mixed with inert material but with sodium hydroxide dissolved in the pore solution. Diffusive migration of cesium as radioactive isotope is also considered. Electrical interaction of the migrating ions is taken into account. Example calculations demonstrate effects of parameter variations on distribution of precipitated calcite in the crack and on the composition of the outflowing solution, which can be compared directly with experimental results. Leaching behavior of sodium can be used to tune the model to experimental observations. The calcite is mostly precipitated on top of the original crack surface and may under certain circumstances fill the crack. The produced thin layers of low porosity calcite act as a diffusion barrier limiting contact between cement and solution. Pore closure mechanisms in such layers are discussed. Implications for safety assessment of radioactive waste disposal are shortly mentioned. The model is also relevant for conventional uses of concrete. Acknowledgements: Part of the CRACK2 model development and most of the supporting experimental work were carried out under the EU Research Programs 1988-2000 on Management and Disposal of Radioactive Waste. ISBN 87-550-2612-5 Internet 87-550-2613-3 ISSN 0106-2840 Print: Pitney Bowes Management Services Denmark A/S, 2003