Metal nail reaction in wall filler

![enter image description here](https://i.stack.imgur.com/gI5kb.jpg)


Please help explain what is happening to this nail. It has been in this wall filler for a good few months. 


The substance being excreted was rising up forming these stalagmites, with a liquid forming on them.


Removing the nail, there were also excretions at the part that was sunken.




It is such a beautiful picture, perhaps good for a chemistry book cover. It is a similar phenomenon as what happens in a so-called chemical garden. Water seems to be creeping up to the nail head, and helping in oxidation of iron salts to insoluble hydroxides. Finally the gelatinous membranes act like semi-permeable membranc in the osmosis like experiment. [![Garden](https://i.stack.imgur.com/ZY0x5.jpg)](https://i.stack.imgur.com/ZY0x5.jpg)


The figure shows several salt crystals soaked in a slightly alkaline medium.


Ref: From Chemical Gardens to Chemobrionics,Chemical Reviews, 2015, 115(16)
DOI: 10.1021/acs.chemrev.5b00014


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