[Ferro List] more Tom's crud testings

[Keith B]

Tom

I just got back onto the Internet after being down since early July, 
when accumulating computer problems led to massive hardware failure. 
Email should have been held on the server, starting 2005, but Comcast is 
quirky and seems buggy re. "stuck mail". (Efforts to retrieve around 
25,000 emails broke Evolution in Linux, Comodo Antivirus/Thunderbird in 
XP, and corrupted and lost data in Win 2K/Thunderbird.) I hope to piece 
more together with time, but email data is missing in various degrees 
from Spring 2006 on, probably earlier. I'm now getting all mail (but 
this is 3rd try at posting to list), saw your Crud thread, and tried to 
follow it back but can't find what construction you eventually used.

Regarding passification by weathering, what I found in the literature 
re. FC seemed to stem from problems with bright galvanizing, resulting 
in warnings to hold new stock until the surface had dulled naturally, 
else to chromate it. Such a thin coat may significantly differ from 
weathering which has proceeded to incipient or actual "white rust" of 
the zinc. The difference might be oxidation proceeding to hydrolysis or 
carbonation, penetration through the zinc, or simple mechanical 
thickness permitting water entrainment, film cracking or flaking.

It's known, and I think was noted, that natural passivation leaves a 
soft film, a chromate film being far more tenacious and stronger. Use of 
sharp sand and/or heavy working of the mortar might have been a major 
factor in displacing a natural coating.

During my literature searches, I never saw mention of a perceptible zone 
of attacked or inhibited mortar as remote from the wire as you describe. 
I'm a bit at a loss to understand that, though it might not be out of 
line with OPC and hydrogen from raw, fresh zinc. One possibility, 
perhaps, might be a white rust layer being ground up mixed into the 
mortar near the wires by abrasive action of sand and working of the 
mortar. I don't see what the chemistry would have been though, to so 
drastically affect OPC cure.

I can send you a small amount of a dichromate if you want to test that 
as a dip. You might also test galvanized mesh aged only to first 
dullness, and try "bad" mesh after an asphalt emulsion dip.

For something like a tank, where you may worry about rust but actually 
need little or no bond if your reinforcement is hoops, an asphalt 
emulsion dip might work. I didn't see much (in what I have of the 
thread) on suggestions for salvaging the situation. If the mortar is so 
rotted that it's easy to remove (a shop vac might help), and if a test 
with asphalt emulsion works, then you may be able to work your way round 
the tank 1/4 to 1/8 or so at a time, removing crud, treating the wire 
and replastering against a curved or flat backer board. If you had 
perfect or very good penetration before, perhaps use the same mortar, 
else consider an all fines mix. You might also benefit from pretreatment 
of the asphalted wire.


kb