Raised boxes and cement are both major interests of mine for more than 20 years.
Flyash is available in types C and F. F is higher purity. Both are waste products from coal furnaces. Adding flyash to the concrete mix reduces the amount of cement by about a third.
Adding flyash also increases the strength and workability of the concrete. I do not know what type flyash is used by concrete block mfgs. I do know the processor of the limestone to make the Portland cement also makes the concrete block. These mfg plants are all around the country.
Concrete block, in my opinion, makes a good garden box. I made one ferrocement garden box 3ft x 13 ft by 20in deep. This is the middle of the 3rd season. I don't till, or add "processed" chemicals of any kind. It is doing great.
If you are really sensitive to certain chemicals, I suggest you call the manufacturer. Block is cheaper at the source, and you at least can ask questions of someone who may know.
Concrete block with a paver on top is very comfortable to sit upon while tending the garden.
I am in the process of making a strawberry planter using cardboard boxes, stucco mesh, and cement.
First I had an artist friend help arrange the boxes. I glued them into that shape. I painted the cardboard with a good primer. Then I covered all the surfaces with the mesh. I used rubber cement to attach the mesh. I added about (3) 1/8" layers of the cement mix. Without dirt or the little solarwater fountain it weighs about 100 pounds, stands about 5 ft tall x 18in x 18in.
Another related thought regards separating the concrete block from the garden soil. Consider an old billboard vinyl. They are cheap, UV protected, and large enough to cover a lot. I have used 1 making 3 raised garden boxes. I used salvage welded wire electrical trays (U shaped - 6in x 1 ft x 6in 10ft long). I wrapped the billboard around the trays and put a 1x6 on top for a seat. They are doing great this year (their first). I think it would work well for a liner between the concrete block and the garden soil.
You might consider taking tree prunings and lining the inside of the boxes instead of plastic.
This one time, at band camp, I had relations with a tiny ad.