Backyard patio - updated

Home from the picking up highway trash, most of the snow was melted. Needed to make some progress on the backyard and clear up the two piles of grass and dirt that were still left from leveling the patio area. So built retaining walls to hold the dirt and terrace the yard.
Can you tell what the walls are made from?

The jack-hammered concrete side walk.

Still need to fill the cracks with the polymeric sand that is supposed to hold it all together. The directions say it has to be above 40 degrees and very very dry. That rules out last weekend. And makes we wonder if there is going to be time before winter really sets in.