Called Menards and he said they were concrete.........but who knows for sure?

The two terms "cinder" and "concrete" have no clear cut differences since there are continuous variances between the two extremes. They are both "cement" blocks. Concrete at one extreme end has no ash component, but in reality that's not always so. Cinder block at the other end has high ash and cinders, both related waste products from industrial burnings. Both are made with cement made from impure mined materials like limestone, shale, sand, and iron. Some have plastics, pumice, perlite, vermiculite, coal. Other than incidental impurities from many possible sources, neither should be nastier or be safer than the other, but if I HAD to pick one as the least likely to be funky, I'd choose a common cheapest possible cement block.

Leaching is over-expected.

For some there is little to no difference between "cinder" or "concrete" blocks, but in reality the concrete blocks should have only cement and sand and stone aggregates in the mix while "Cinder" blocks can have coal cinders and even coal ash in the aggregates well as perlite or vermiculite and if common in the area maybe volcanic ash.. "Cinder" blocks are sometimes referred to as "lightweight" blocks for that reason.

The major leaching problem with either would be lime from the cement. If the blocks are to be used to raise the stock tank and the blocks are not to be in contact with the soil that the crops will be raised in what might leach from those blocks would not be of concern.

kimmq is kimmsr