Crushed concrete is a mixture of different types of crushed rock, such as limestone, sandstone, and granite. Limestone is a sedimentary rock that is formed from the skeletal fragments of ancient marine organisms. Limestone is a soft rock that is used in a variety of ways, such as to create mortar and concrete. Limestone is often crushed into a powder to be used in concrete because it is an inexpensive, common rock. Limestone is used in concrete because it can be heated and mixed with water to create mortar. Limestone also reacts with cement to create calcium silicate hydrate, which is an excellent binding agent.