We have a type of rock on which we are looking for damage mechanics which have been caused by magnesium salts in situations of extreme drying (less than 29% RH).
The mechanism of damage will depend on the specific type of salt, i.e., it may vary for different magnesium salts.
If you subjected your material to wet-dry cycles, magnesium sulfate for example could have given rise to massive contact-induced damage. It is because of this high destructive power that magnesium sulfate is used in crystallization tests since always (Thury 1828). Today, it is used, for example, in the ASTM standard test method for soundness of aggregates (ASTM 2005).
However, I don’t think this kind of mechanism (contact-induced damage) is representative of what happens in the field (see for example https://www.researchgate.net/publication/260539644_Alteration_kinetics_of_natural_stones_due_to_sodium_sulphate_crystallization_can_reality_match_experimental_simulations)
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Reference to damage by magnesium salts in field conditions is made, for example, by Arnold (1981) and even, although in a more general way, by Schaffer (1932).
Some magnesium salts are highly hygroscopic, eg,, magnesium chloride. Its relative equilibrium humidity at 20ºC is around 33%, but since you’ve dried the material at 29% there should be no problem in this case.
You can find the complete references I’ve mentioned in the list at the end of https://www.researchgate.net/publication/260405382_Salt_crystallization_in_plastered_or_rendered_walls?ev=prf_pub
Article Alteration kinetics of natural stones due to sodium sulfate ...
Thesis Salt crystallization in plastered or rendered walls
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