Your information is not quite correct. Yes, strontium is heavier than calcium but not heavier than bone. No, strontium will not replace bone; strontium will be used to grow bone scaffolding just like calcium. Yes strontium is heavier than calcium and DXA scans that are gauged to measure calcium in bone will show about 10% denser than if only calcium was present, because strontium is heavier.

Bone is made from mainly collagen and calcium phosphate, a mineral that adds strength and hardens the framework. In your bones, calcium-sensing receptors are found on osteoblasts (cells that build new bone). Strontium kick-starts osteoblasts, but not as much as calcium does.

According to one source though, see citation below, a new version of calcium-sensing receptor has been identified in osteoblasts. And this sensor seems to be activated only by strontium. In other words, your body recognizes strontium and it uses it to build bone just as it uses calcium, to add strength to the bone matrix. It has been known for awhile that strontium activates calcium sensing receptors. If this new research is borne out, about a separate receptor for strontium, then that will be a big break thru in strontium use for osteoporosis.

We already know strontium occurs naturally in water and many foods; we know that the body recognizes it and uses it similarly to calcium in bone. If strontium has its own receptor then those who have said that in the past the body was subjected to strontium regularly and in much larger amounts than now will gain a valid talking point in its use for osteoporosis. But even if this new receptor is not borne out in further study, it does not change the fact that the known calcium receptors know and use strontium to make the bone stronger. Yes, they use calcium first, but they also use strontium.
https://www.algaecal.com/algaecal-ingredients/strontium/strontium-benefits/
Indeed, recent 3d visualizations from bone biopsies of strontium users show plate like healthy bone with good mineralization. See photo inserted here with a link to the research study
https://asbmr.onlinelibrary.wiley.com/doi/full/10.1359/jbmr.071012