Cement wash coating for steel protection

[MikeJohns]

Does anyone know what to use for mixing a good protective alkaline cement wash for coating the insides of ships steel water tanks?

I've come across it many times in situ working well but never specified it. Some older vessels are coated inside extensively with a cement wash and are sweet and bright under the coating after many decades.

Could probably use a similar recipe to the ferro boatbuilding mix.

[DGreenwood]

I saw a fellow do the inside of his steel water tanks this way. They stayed like new and the water was clear and sweet. As I recall he used straight Portland (cement only, no aggragate) mixed to a thick paint consistency. I can't really remember him putting in any other ingredients???
I was impressed by the job it did.

[D'ARTOIS]

It is as DGreenwood says, save for an addition of ................I forgot, something to do with Sodium. I have the recepe somewhere, will look it up later in the day.
Come back on the issue later.

[MikeJohns]

D'Artois
I would have expected something like Silica Ash. The problem in seawater environments is to stop the chloride ions permeating the cement. The aditives do this.
I would be intersted to see what you come up with. This is "OLD Lore" now and seems to be mostly forgotten. Modern equivalents are available at a cost with fancy brand names but they don't tell you what they use. An old Iron sailing ship sat sunken in Tasmania for decades, when she was raised the plating was in good order where she had been cement washed. The cement was still in place and adhering well.

[D'ARTOIS]

Today I started to look in the attic where I store my files, but I couldn't find my notes that are in the manual I wrote lightyears ago for amateur builders.
I come back on it.

[D'ARTOIS]

Mike, I don't find the notes. But I remember - and that is easy to check for you - that it is the same stuff they use in Ferrocement.
The books where I have all those old recipe from is in my house in France.

[MikeJohns]

Thanks anyway Dartois I'll do a bit of experimentation.

[Nels Tomlinson]

D'Artois, were you thinking of sodium silicate? I have no idea if that's right; I'm just trying to jar your memory.

Nels

[phylus]

Hi, very interesting thread you've got going here.
Any progress with the missing secret ingrediant/additive for cement mix?
thanks
phil

[D'ARTOIS]

It is not secret, just missing for the moment. It is my other residence, in France,
so a bit far away for the moment.....

[DGreenwood]

D'artois
I think I might have just remembered...see if this jogs your memory. Calcium Chloride additive. I think it was 2% to accelerate the cure and act as a plasticizer. But I am unsure if I have the right chemical?

[SamSam]

Originally Posted by DGreenwood

I saw a fellow do the inside of his steel water tanks this way. They stayed like new and the water was clear and sweet. As I recall he used straight Portland (cement only, no aggragate) mixed to a thick paint consistency. I can't really remember him putting in any other ingredients???
I was impressed by the job it did.

How do you apply the stuff? There is a latex based additive that is used in stucco that makes it stick better to old work, available at Home Depot, perhaps that would work. Was this concoction used only in water tanks, or over the whole interior?

[MikeJohns]

I am still a long way from having a firm recipe that I could specify for a mix-your-own cement wash. Maybe when Dartois gets back to his summer residence!


Some material I have from the pertland cement association may be interesting:

Cement wash protects steel from corrosion through its highly alkaline nature. The high pH environment in the cement (usually greater than 12.5) causes a passive and noncorroding protective oxide film to form on steel. However, the presence of chloride ions from seawater can penetrate the film. Once the chloride corrosion threshold (about 0.15% water-soluble chloride by mass of cement) is reached, an electric cell is formed on the steel and the electrochemical process of corrosion begins. Once it starts, the rate of steel corrosion is influenced by the concrete's electrical resistivity, moisture content, and the rate at which oxygen migrates through the concrete to the steel. Chloride ions alone can also penetrate the passive film on the steel; they combine with iron ions to form a soluble iron chloride complex that carries the iron into the cement coating where it can later oxidise expanding and cracking the cement coating.

The resistance of cement based morter to chloride is good; however, it can be increased by using a low water-cementing materials ratio, and supplementary cementing materials, such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing chloride ingress include the use of corrosion inhibiting admixtures commercially available from a variety of manufacturers.

Another approach is surface treatment to stop or reduce chloride ion penetration at the cement surface. Silanes, siloxanes, methacrylates, epoxies, and other materials are used as surface treatments.

Reference :
Portland Cement Association;
Kerkhoff, Guide to Protective Treatments, 2001, 36 pages.



And a commercial addmix from Tapcrete:

TAPECRETE MARINE COATING
It is a three component water-based coating containing a liquid blend, portland cement and fine silica sand all supplied in requisite proportions. Bonds well with mild steel and galvanised steel. Provides corrosion resisting coatings to steel structures in sea or fresh water, salt spray on environment, suitable for bridge decks, ship docks, ballast tank for ships, steel water mains, pipings and other above grade and subgrade tanks. Correct surface preparation is important for the performance of coating. The surface to be treated should be properly cleaned to remove all deleterious materials. After mixing the component materials in paddle type mixer at site, the resulting creamy slurry of thick paint consistency can be applied with roller, brooms, brushes or air operated spray system to build up the desired thickness. Air cure for about 3 days for normal case and water cure for about 3 days when the structure treated is subjected to continuous water exposure. One cubic meter of Marine coating required the following
Marine Coating 446 kg
Portland Cement 858 kg
Fine silica sand 858 kg
For a 2mm thickness of coating the requirement will be about 4 kg of the three component Marine Coating/ per sq. mtr.The generally applicable thickness ranges from 0.8 to 1.0 mm. The corresponding requirement can be worked out from the 2 mm thickness value.

Of course Tapcrete don't divulge the admix content. The ferro boat building literature talks about Pozolan and I am not sure what that is either.

[D'ARTOIS]

I believe indeed it was Calcium Chloride; however I am not sure about it. I am stuck but again, t could be calcium chloride.

[DGreenwood]

Interesting ...
From the article above it seems that silica is the answer. It looks like chlorides may not be such a good thing??
I would love to find out the method as I also have an immediate use for it.