Calcium hydroxide (CH), along with C-S-H, are the end products of the reaction of alite and belite with water. The abundance of CH in the hydrated cement paste varies with the degree of hydration of the cement, and can reach approximately 26 % of the total volume of a mature paste. Contrary to the C-S-H gel that is an ill-crystallized phase, CH is present predominantly in the form of well-defined crystals in the hydrated cement paste. The size of these crystals tends to vary significantly from one to approximately 100 microns in diameter 1, 2.
The leaching of calcium may be a matter of concern for the durability of concrete3. In some cases, CH dissolution and the decalcification of C-S-H may increase the porosity of the surface layers of concrete, and detrimentally affect the resistance of the material to deicer salt scaling and ion penetration4. In other instances, the leaching of calcium may also affect the core of the material and have a negative influence on the engineering properties of concrete structures3. For instance, CH dissolution and C-S-H decalcification have been found to have a detrimental influence on the mechanical and transport properties of hydrated cement systems5-9.
Over the years, several studies have clearly demonstrated that the investigation of calcium leaching mechanisms by laboratory experiments is often difficult and generally time-consuming8, 9. Furthermore, since both phenomena readily affect the pore structure of the material, the kinetics of CH dissolution and C-S-H decalcification quickly become non-linear, and a reliable prediction of the evolution of the concrete properties upon leaching can hardly be made on the sole basis of experimental results.
This paper presents the main results of a study of the influence of CH dissolution on the mechanisms of ion transport in hydrated cement systems. The effects of CH dissolution on the pore structure and transport properties of various hydrated cement paste mixtures were investigated using the NIST CEMHYD3D cement hydration and microstructure development model 10, 11. These results were then implemented in another numerical model called STADIUM12, 13. This model can be used to predict the transport of ions and water in reactive porous materials (such as concrete). The degradation characteristics of neat cement paste mixtures immersed for 3 months in deionized water served as a basis for the validation of the model.