[Ferro List] strandslip_galv_JL-00-MARCH-APRIL-9

[Keith B]

Thanks Greg for finding postable text. The last reference was 
interesting, but the paper is only by purchase from NTIS. The abstract 
does give relevant and disturbing information though:

"The nature and constitution of ferro-cement is reviewed with special 
reference to the structure of cement paste. The microstructure of low 
water to- cement ratio pastes is presented in schematic illustrations to 
show that a thin paste layer can be used to protect steel reinforcing 
material from a marine environment. A galvanic cell between the plain 
steel reinforcing bar and the galvanized steel mesh ordinarily used in 
ferro-cement is identified. This galvanic cell gives off hydrogen gas at 
the plain steel reinforcing bar which leads to poor bonding. Possible 
solutions to this problem are presented and one, the use of chromium 
trioxide (CrO3) in the mix water, is shown to solve the problem most 
effectively. This leads to improved mechanical properties as well as a 
sounder barrier to corrosion of the reinforcement."

This is definite in relating hydrogen evolution to UNCOATED steel in 
contact with Galvanized material. That says hydrogen is NOT involved in 
the "crud" by the galvanizing. It also associates hydrogen evolution 
with poor bonding of the UNCOATED metal. Without reading the paper, 
there's nothing to say whether the harm caused is chemical or simply 
mechanical.

Some searching in the patent literature suggests that zinc compounds can 
act as a retarder or activator/accelerator. In US4455171, an "hydration 
retarder" gives indefinite retardation with a reactivator being needed 
to restart cure. Zinc carbonate was one listed. DE10113978, teaches a 
mortar formulation with " e) from 0.02 to 4.0% by weight of one or more 
compounds from the group consisting of zinc oxide, zinc hydroxide, and 
zinc hydroxide carbonate," so zinc compounds certainly affect the 
chemistry of cure in low concentrations, specifically those found in 
"natural" passivation. Study continues...

kb

greghall at elands.com wrote:
> 3.   Christensen, K. A., and Williamson, R. B., “Solving  the Galvanic
> Cell Problem  in Ferrocement,” Report No. UC-SESM 71-14, Department of
> Civil Engineering, University of California, Berkeley, CA, July 1971, p.
> 58.
>
> Greg
> -
>