If they're going to build an inert projectile -- which is what a JDAM without an explosive warhead is, basically a really huge, really smart bullet rather than a bomb -- I'm surprised they couldn't find something more dense than portland cement. Jacketed lead or iron, or even something exotic like DU (depleted uranium, not that other dense substance with the same initials). For that matter, something like a Sabot round strapped to the front of a JDAM would be precise enough for a pilot to ask, "which nostril?"
Is it possible that the cement was chosen because the resulting mass and CG are comparable to those for which the guidance systems were designed?