Durability of Cementitious Materials in Long-Term Contact with Water
Doctoral thesis, 2015
Nuclear electricity is considered to be an alternative energy production solution for the power industry in many countries. To ensure the sustainability of this energy solution, the disposal of the produced waste is one of the biggest issues facing nuclear electricity industries. Deep geological disposal of waste with multi-layered engineered barriers has been shown to be one of the safest solutions. However, degradation induced in barrier material by long-term contact with water during the required operational life time of the repository should be accounted for in safety assessments. Cementitious materials are considered to be one of the most efficient alternative barrier materials, providing high pH buffering capacity, good mechanical properties and low diffusivity. The major degradation scenario to consider for these barriers is the dissolution of calcium-containing phases and the eventual leaching of calcium. Decalcification occurs due to the low concentration of calcium ions in the groundwater that comes in long-term contact with the barriers. To facilitate long-term durability predictions, acceleration methods that enhance the calcium leaching process from cementitious materials are needed. However, experimental studies of the natural leaching process under long-term degradation are hampered by the tedious and complicated process of manufacturing large enough decalcified specimens with a composition and pore structure that corresponds to that of concrete leached under natural leaching conditions. In this study, a new acceleration test method for cementitious specimens of flexible size is developed. The electrochemical migration method facilitating both the dissolution and transport of calcium ions provides a higher acceleration rate than other available methods. With application of a current density of 125-130 A/m2 for 53 days a depletion depth of 75 mm is obtained. The dissolution front, comparable to a natural leaching process, corresponds to the complete leaching of Portlandite, with a certain degree of phase changes in calcium silicate hydrates. The changes in the pore structure, adsorption, ionic diffusion, mechanical strength, elastic modulus, permeability and frost resistance of Ca-depleted concrete, mortar and paste specimens are demonstrated. The results indicate that a considerable increase in pore volume and specific surface area can be expected due to the complete leaching of the Portlandite. This coincides with up to 70% decrease in mechanical strength, more than 40% decrease in elastic modulus and a significant increase in the adsorption capacity and ionic diffusion rates of the leached specimens.