The matter of calcium chloride: CaCl is much more reactive than NaCl. Hence the the warning label. Its not explosive, but admixtures that would have neutral reactions in the presence of NaCl may not in the presence of CaCl. Probably the most common reaction would be precipitates.
Waterproofing: There are quite a few paints that would work quite well. The only problem being the cost. The other issue is papercrete to a certain extent needs to breathe. Sealing it up completely might lead to issues. __._,_.___
