If replacement is not an option, about the only thing you can do is to paint the tiles. Although most any clear
sealer (possibly even including your floor sealer) could be used, you probably want to use white paint to maintain the look of the tiles. Again, you want to use a washable-type paint, which tends to have a harder surface, probably in a semi-gloss rather than flat since that also tends to provide a better seal. Obviously, this is going to ruin the sound-absorbing qualities of the tiles and will need to be done outside the data center, which means removing and replacing every piece. I'm assuming we're talking here about an operational data center, so that process alone will cause a lot of dirt and flake-off and will require substantial equipment protection. This is why I suggested looking into replacement rather than sealing. It is also one of the reasons we generally prefer that there be no dropped ceiling in a data center unless it is necessary for either aesthetic reasons (the data center is also a "show place") or the volume of the room must be reduced to minimize the cost of an inert gas fire suppression system.
As to the concrete slab sealer, its main purpose is to prevent concrete dust flake-off that can get into the air stream and equipment. Water sealing, we certainly hope, is not what it's ever going to need to do. You can usually see if any areas of the slab have been missed by shining a good light across the surface, particularly if two coats have been applied (which should always be the case). After drying, the slab should be completely damp-mopped at least twice and inspected. As to actually testing, however, the only way we can think of -- other than visual observation, which is the norm -- is to spray water over the floor after it has been mopped and dried and watch for anywhere water appears to be absorbed instead of beading. (This would be done with something like a garden sprayer, not a hose. You're trying to spray, not flood.) We've never done this, but if you are that concerned about the quality of the job, it might be something to try.
In our opinion, however, there's a bigger consideration -- namely compatibility of the sealer with the mastic used to attach the raised floor pedestals. Be sure to verify with your raised floor installer that the sealer to be used is appropriate for the mastic he will use. Some of these products react chemically, which causes the floor to come loose as well as the slab seal to be broken.
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