Recycled concrete materials are aggregate matter that would otherwise be thrown away to occupy landfills and other waste disposal sites. But they can be reclaimed and used in concrete instead of sand or gravel. An example is the fly ash byproduct produced by coal use; this can be used to make cheap concrete materials.
Recycled concrete is very similar to normal concrete. It looks the same, acts the same and tends to have similar durability and strength. This makes it an ideal substance to use for retaining walls, since inexpensive recycled materials can often cost less than using traditional concrete while providing the same benefit.
Waste concrete pieces are concrete chunks that have been thrown away but not broken down into parts. The aggregate is not reclaimed, but stays in the original concrete formation. These chunks can be taken and piled up for walls, used for filler material, or positioned as stepping stones in landscaping projects.
The primary benefit to entire waste concrete pieces is affordability. They are either very cheap or free. And while they can be stacked and mortared together to form a wall, the wall should not be used to hold up much weight. Concrete pieces do not have the strength-bearing capacity necessary for a retaining wall project like this and should generally be used as a decorative addition.