[Ferro List] More On tom's crud Testing (Keith)

[Keith B]

Hi Chris

There seem to be several potential or actual problems.  The intent is to 
end up with steel wire reinforcement which is durable in a sound and 
durable OPC mortar matrix.  It's probably desirable that the 
reinforcement be strongly bonded to the matrix, but both because it's a 
mesh and for reasons related to ligament behavior and as considered for 
fiber reinforcement, that might be questioned. 

Bearing the above in mind, the galvanizing is there to protect the steel 
from rusting.  It's assumed that it will passively isolate the steel 
from the matrix environment and, if/when penetrated, protect it 
sacrificially by galvanic action. It should not compromise the matrix 
chemistry.  Some loss of bond strength is expected and viewed as 
unfortunate.  It's known that bright zinc reacts rapidly with aqueous 
calcium hydroxide, but assumed that action stops due to a self formed 
barrier and/or change from slurry to a solid state environment.

In the real situation, the wire is vigorously flexed, probably impacted 
and certainly exposed to abrasion as mortar is forced and vibrated both 
past and against it.  The mortar may not be a simple OPC mix, and that's 
not well defined anyway regarding  major let alone trace components.  
Its cure  is protracted to a notional  solid state,  but has known solid 
state slow reactions in late cure which may or may not preserve a high 
pH which is protective of steel - but a steel/zinc sandwich?...?  Then, 
of course, the OPC dries porous, very porous if workability has driven 
the w/c ratio up, porous and structurally impaired if drying is too 
fast.  Late porosity allows water and atmospheric gases to act, plus 
access by electrolytes, and any developed "crud zone" a damp water 
channel adjacent the metal.

Considering all the above, one is inclined to question the wisdom of 
galvanized versus plain steel, but if galvanized is to be used, it's 
clear that the zinc must have protection at least through first cure.  A 
natural thin oxide/hydroxide coating may suffice, and one may maneuver 
to limit the problems from its fragility.  An all fines mix with rounded 
particles would help, as might laying up the mesh and leaving it 
unmolested for enough time for a displaced film to reform before 
mudding, and certainly designing reinforcement to minimize need for 
vigor to flood it with matrix.  Martin Iorns technique is notably 
gentle, for instance, and fewer layers and/or larger holes would help 
for chicken wire or hardware cloth.   Other than that, one's looking for 
either a stronger and more tenacious thin barrier, or a thicker but 
flexible one.

A hard barrier pretty much has to be thin to conform to flexure of the 
wire and avoid the flaking likely with thicker coats.  Ideally, it would 
bond with the zinc and rapidly bond with curing OPC to both maximize 
long term bond and rapidly recruit an incremental resistant coating to 
further defend it.  As it happens, a chromate coating does exactly that 
and may be unique in doing so.  Further, chromate ions adjacent zinc or 
iron will preferentially plate out on their surfaces to molecular but 
chemically protective thicknesses.  There are other thin coatings, like 
silica deposited from silicates, which offer chemical protection, but 
they aren't easy to apply nor potentially self healing. 

A "thick" coating could offer passive protection from gases, 
electrolytes and other chemical action, and though soft enough to 
conform to wire flexure, and actually offer mechanical protection during 
emplacement.  The downstream trade-off though, would likely be poor or 
no bond to the matrix.  That's what you get with vinyl or epoxy coated 
steel.  (How long does it take for a vinyl coated cyclone fence to 
rust?)  There is perhaps one interesting candidate though.  That's 
asphalt deposited from an emulsion.  Further, it's probably possible to 
spike such an emulsion with chromate; one could certainly asphalt coat a 
chromated wire; and, one could probably dope asphalt with solids which 
bond with OPC.  As prepared in emulsions, the asphalt is a viscous 
liquid becoming an amorphous solid.  Leave it months to decades, and it 
slowly crystallizes to a hard, bitumen solid - still clinging 
tenaciously in air and watertight bond to its substrate.  Deposited from 
emulsion, it's a water displacer, and it bonds strongly to dry rust.  
Doping with chromate or prior chromating should result in strong bonding 
to zinc.

kb


Christopher Glasspool wrote:
> Keith or anyone having an idea on this:
> Seems to me that if galvanized material could be passivated by exposure to the elements then some other less innocuous material might be used to passivate the zinc other than adding  chromium trioide to the mix design . Do you think wetting the galvy mesh with an acid or alkaloid might pre-passivate in a more efficent and speedy manner? Lime water maybe? Thanks, chris
>
>