Well, while I don't have any anecdotal evidence either way, I am a QC Manager at a concrete company, and knowing how the cement powder is made, I personally wouldn't put it in my tank.
Most cements are made from a combination of mined clays that are high in silicates, crushed limestone, plus a few other ingredients..... While set/hardened concrete is typically inert for most applications, my fear would be the slow leeching silicates feeding an extended diatom bloom, and the presence of the limestone rich material acting as a buffer to raise the pH.
Someone else may be able to chime in with some actual experience contrary to this....
