Hello all, I’m working on a gardening system for my 3rd year product design studio. I’d like to cast this system out of concrete and I’m looking through my options. Weight has the potential of being an issue so I’m looking for something a bit lighter than regular concrete. My professor pointed me in the direction of “Syndecrete” which is a concrete containing recycled glass, wood scraps, vinyl LPs etc. and weighs half what normal concrete does. I’m having trouble finding a way to purchase bags of their powder instead of designs that they have created using the material. Does anyone have a suggestion to an alternative?
I’d try experimenting with a few different options from Sakcrete (avail. at Home Depot and other home centers), and create your own “aggregate” with the above mentioned materials. Maybe a mortar product will give you a lighter weight base. They also have a light-weight concrete mixes with stone/sand aggregate already included.
As Chris suggested, make your own.
When I was in school I made “garden products” (planters, pots, etc.) that I sold at a local nursery. They were made of a material know as “hypertufa”; they were either hand fashioned, or made in simple plywood molds. I made the hypertufa in an electric cement mixer using both sphagnum moss and chipped wood bark as fillers.
The finished loses weight as the organic materials decompose. I don’t see any reason materials like expanded styrofoam beads, crushed cinders (available from a “redi-mix” company), or any other available material couldn’t be substituted.
See: Hypertufa - Wikipedia (and a ton of other sites)
Aggregates are generally Sphagnum (peat moss), sand, and perlite or vermiculite.[1] Hypertufa made with the classic proportions for mortar (1 part cement: 3 parts aggregate) has a composition of
3 parts Type I Portland cement
4 parts Sphagnum
5 parts perliteor 3 parts cement: 9 parts aggregate[2]
To increase structural strength and longevity, polymer fibers, liquid acrylic,[2] and fiberglass[3] may be incorporated into the mixture, along with various grades of sand, pebbles, and crushed rock which add to the final object’s overall strength and stone-like appearance though they increase its weight. Powdered concrete dyes (in small amounts) also tint the hypertufa to resemble natural rock.[2]
BTW, for what it’s worth, it’s important to know that cement is one thing. And concrete is another.
Cement is a very fine powder made of limestone, calcium, silicon, other minerals, and gypsum. When water is added to cement, it triggers a chemical process that allows it to harden. It’s basically a glue.
Concrete is a composite material made of water, aggregate (crushed stone, rock, and sand, etc.) and cement.
With a father in the concrete products industry, that’s an argument I’ve had to make my entire life. Thanks for putting that out there, Lew.
Too many civil engineers in my life for me to have escaped an understanding of that definition; next door neighbor, two nephews, my old college roommate (not to mention my current line of work). 
WJF… google is y0ur friend.
Using “light weight concrete fillers” > light weight concrete fillers - Google Search
Also see: US Department of the Interior Bureau of Reclamation / CONCRETE MANUAL