From the reseach I have done it is hard to tell exactly what the composition of Hardblok really is. However if it is a true cement it will probably outlast the block itself.
Once it sets nothing will come loose, the iron filler will be locked into the cement matrix much like the pigment in paint when it drys.
Unlike paint cement sets by a chemical reaction making it very stable and long lived. The main concern is whether the manufacturer had got the co-efficient of expansion right to match that of the block
regards
David
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