Is there evidence showing antacids are worthless as calcium supplements?

PROBABLY: The main reason might be that the amount of calcium in antacids is not enough to meet the required 1000 - 1200 mg/day recommended for bone health. As well as the need to take multiple antacid pills to achive this dose.

Not necessarily: I have understood that it is perfectly ok to use Tums for instance as a calcium supplement-- it comes in different strengths, including a 750 and 1000 mg strength ( of calcium carbonate). What i had read was that your body cannot deal with more than about 500mg at a time anyway so i'm not sure the strength actually matters. It's important to take the calcium with meals several times daily.