Waste sludges produced from the recycling of concretes contain a large amount of aggregate powders and pre-reacted cements so that they have no more cementing properties. However, since they contain unreacted Ca(OH)(2), pozzolanics such as blast slag or meta kaoline can activate the waste concrete, and they can be reused as low-grade cement or pozzolanics. When meta kaoline was added with some other pozzolanic materials such as gypsum or blast slags, the density was increased so that the compressive strength, especially early stage of curing time, was enhanced. When meta kaolin was added, more densified C-S-H gel networks were formed at aggregates, and ettringites were produced between the C-S-H gel networks.