Abstract: A backfill for an engineered barrier used to contain radioactive waste has a predetermined amount of clayic material and a predetermined amount of a reinforcement material with hydrophobic surface characteristics. The reinforcement material may include hydrophobic compounds selected from group consisting of organic polymers or inorganic materials on which a layer of hydrophobic compounds is formed. The hydrophobic reinforcement material results in the backfill maintaining a very low water permeability while providing high mechanical strength and other properties suitable for use in a repository of radioactive waste.