Oyster shell in concrete or tufa

tufaenoughSeptember 22, 2005

Has anyone used it?

I'm buying a bag today, it is sold as chicken grit.

I'm wondering if it has any bad chemical reactions with cement.

I would like to add it to my growing list of aggregates.

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Just a sidenote on this subject: When I googled oyster shells and concrete, I ran into this site. Oyster shells have been used as aggregate to create this Pervious Concrete. Would be great to have a sidwalk that did not puddle, but what would freezing do to it?

"Pervious concrete
As an item of interest, Windham noted that the sidewalk by the parkÂs education center is made with "pervious" concrete. It allows rainwater to flow through the concrete and into the ground. Normal concrete sidewalks are smooth and hard, and water runs across them and causes puddles, or may run on into drainage ditches and then into the marsh. By going into the ground immediately, the soil helps filter out oils and other chemicals so the water carries less pollutants into the wetlands. "ItÂs an environmentally-friendly surface," Windham said.
The Education Center opened in April 2002, and the sidewalks are still in great shape.
"ItÂs more expensive, but not that much more expensive," Windham said. "I think for the preservation value you get, the benefit outweighs the cost."
HeÂs seen people and businesses use pervious concrete as an apron around a parking lot. The water runs off the regular asphalt and the pervious concrete catches it.
Windham noted that Southeast Redi-Mix from Charlotte contacted him about this concrete. That company is getting into using it for its customers, he said." In South Carolina- Education Center at Huntington Beach State Park.

Here is a link that might be useful: Pervious concrete

    Bookmark   September 22, 2005 at 6:22PM
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sounds neat. We always used oysters to fill potholes where i grew up. If you can find a way to crush them you could make it at little to no cost if you have a oyster house nearby.

    Bookmark   September 22, 2005 at 6:32PM
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As a matter of fact, I do, and the owner is a friend of mine. I guess I could put the shells between two sheets of plywood and drive over them for a few days!! Or just scoop them off his lane to the oyster house. They are crushed and could be sifted to get the fine stuff out. They put them on lanes around here all the time. I put my clam shells in the potholes after we go clamming and have a feast!! Isn't it funny how traditions keep on going?

    Bookmark   September 22, 2005 at 7:09PM
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Sounds like it would be nice, but don't oysters come from salt water?

I would hate to think that the salt was distributed throughout the shell's layers, just waiting to decimate the cement.

And would the word pervious also mean porous? Seems like shell would be made of a brittle calcium base. Maybe great for walkways ( and potholes!) but to add strength to a tufa project?

    Bookmark   September 22, 2005 at 7:24PM
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kobold(Vancouver BC)

Without going too deep into biochemistry, compare two important calcium sources, the oyster shell and the limestone, both made of Calcium carbonate.

Oyster (clam, scallop) shell: marine organisms, molluses secret it to build up their shell. Biological origin, made of mostly of Calcium carbonate.Doesn't contain salt from the water. Can dissolve in acid.

Limestone : sedimentary rock by origin, can contain impurities, such clay, iron oxide, etc.. this can give different colors, weathered surfaces and in specific circumstances to form the tufa(rock).Can dissolve in acid.

IMO you can look at the shells as a cleaner form of limestone, rock or sand. As BB and brighteyes mentioned, they fill up potholes with it, like with rocks. Very safe to use in concrete. Alkali.


    Bookmark   September 23, 2005 at 3:39AM
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Someone out there knows more about this than I, but didn't they use it in building? They mixed oyster shells with sand and water and made like a mud, built walls, burned the walls and the heat chemical reaction made a very sturdy concrete like wall. Didn't they call it "Tabby"? I'm talking colonial times.

    Bookmark   September 23, 2005 at 6:56AM
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kobold(Vancouver BC)

for centuries, before the use of the cement, masons used lime mortar to build houses, castles, etc.
To start, heat limestone(high), the product, white powder called quick lime. Mixed with water formed the lime putty.This was used for making the lime putty mortar, white wash walls, pour over corpses in mass graves, etc..

Maybe the oyster shell was used to make the quick lime or for aggregate in the mortar. Interesring!

    Bookmark   September 23, 2005 at 11:55AM
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I'm picking up some oyster shell today. I was told it is grey in color which is kind of disapointing.

    Bookmark   September 23, 2005 at 11:56AM
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gottatufa(z5 ny)

I only have one thing to say......TABBY

Here is a link that might be useful: nothin' new here :)

    Bookmark   September 23, 2005 at 5:53PM
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Tabby was & is generally regarded as "a cement of last resort". While the calcium content makes oyster shells an excellent candidate for manufacturing lime plaster, removing the sea salt from the outside of the shells is an almost impossible task which makes it a very poor choice for use in concrete. Salt results in concrete that is basically self-destructing with it's life expectancy inversely proportional to it's salt content. Can be fine for short-term applications, but even with the tremendous efforts that were made to clean the salt from the shells used in historical Tabby, these structures became quite fragile and unusable in fairly short order. It is important to note that builders only employed oyster shells for lack of "proper" local materials.

I understand there are some modern, high-tech efforts afoot to try and make this renewable resource usable in concrete, but haven't heard of any results yet. Would be a real boon to third-world and remote coastal countries if it can be made to work, but 'til then, I'm going to avoid it.

    Bookmark   September 24, 2005 at 10:31AM
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Hi Tango
Does this apply to shells sold as grit?
I asked my supplier about this and was told there was zero salt content in the shells sold as grit.

    Bookmark   September 24, 2005 at 10:53AM
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All I know about "Grit" is that chikens seem to love the stuff. As for "dealer supplied info", I am generally inclined to be suspect regarding their actual knowledge (consider the quality of the information you get from the "experts" at Home Depot). All of my information indicates that salt is quite difficult to thoroughly remove.

It might behoove us all to investigate a howngrown test for salt content. Surely there must be a relatively simple way to detect something that basic. I'm going to look into it, but meanwhile, is anyone out there scientifically inclined???

    Bookmark   September 24, 2005 at 11:50AM
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Hi Tango
I'm suspicious of their salt free claims as well.
I recall a simple electrical test. I think saline conducts electricity better than water alone.
So I guess water could be tested then pour in the shells and test a few days later.
Sounds like a trip to Radio Shack is in order but not for me. I have enough hobbies. I can't afford any more.:)

    Bookmark   September 24, 2005 at 11:56AM
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Ditto the hobbies...it's a lot easier to simply stick with "known quantities" and thereby avoid the potential problem.

    Bookmark   September 24, 2005 at 1:00PM
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Hi Tango
True enough, but considering all my concrete work is of the exposed aggregate variety I'm always on the lookout for anything that will give me a new finish.
I'm going to soak my Oyster shell grit for a couple weeks before giving it a try.

I have a question about rust.
I would like to make a pot with blooming rust. Rust that just appears on the surface over time.
I'm thinking iron filings mixed into the surface concrete should give me this effect. What do you think?
I would also love to have copper oxidize to that great green in a similar fashion.
I find it very difficult to oxidize copper. Do you know any way to speed up this process?

    Bookmark   September 24, 2005 at 1:27PM
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kobold(Vancouver BC)

BB to bring Tabby to our attention, we can learn in this forum a lot.

tango88 to add more information.

I googled Tabby and learned more. Buildings made with Tabby, many are still there altough some needs repair. After 2-300 years! They still use oyster shells in concrete after cleaning the shells from the salt.

To give interesting texture to our work, IMO, it would give some terrific result to use the oyster shell. Maybe it won't last for a few hundred years!Do you care?


    Bookmark   September 24, 2005 at 3:22PM
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tufaenough --- Adding iron filings to the surface does create rust "bloom". It can be applied to a finished surface, added into a surface finishing mix or into the full mix. The latter is generally not recommended since it can have structural consequences, but I have seen it used that way for casting smaller items. The coolest idea I've seen involved spraying it onto a damp, fresh surface using a cheap-o sand blasting set up (if you have a compressor, the blasting gun set-up is less than $20 bucks at HD). The sculptor just "sprayed" a coating of steel filings all over the surface and they embedded. He even had some areas masked off with pieces of lite, stiff plastic. Great effect. I have also heard of, but not actually seen, copper being used the same way.

No need to accelerate the steel, as it will start oxidizing immediately. Especially on the surface where it has free access to oxygen. As for copper, there are dozens of exotic techniques and formlae for creating patinas. Personally, I just use a Muriatic acid solution on sculpture that has copper I want patinated. At first it brightens & cleans the copper (it is routinely used for that purpose). Don't rinse it. Just let it sit for a day or two and it will produce a nice "verde" (green) finish. Sometimes it takes several applications...just let it dry in between. Of course, the acid will also attack the 'Tufa or concrete, so I would think starting with a very weak solution would be highly advisable. You can always add more...BUT...



Never, never, ever pour water into acid!!! It can instantly boil and create a steam explosion!!! ...in your face!!! "Muriatic" acid is Hydochloric acid in solution (already cut with water), but is still very nasty stuff. I don't recommend anyone working with it without doing some serious homework first.

Salt can also be used as an accellerant, but you don't want it penetrating cementious material so it has to be applied with care.

You could also try using the oyster shells strickly as a surface finish mix. A light spray of sugar water will retard the set on the surface for an exposed aggregate effect. That way you wouldn't be compromising the structural integrity. Just a thought.

As for "Tabby"...every historical reference I've read regarding it's construction use emphasized how critical it was to remove as much of the salt as humanly possible. And while there are indeed numerous architectural elements still standing that were made from it...thanks to the effects of the salt they could NOT get out...they haven't been usable for most of those years...and a great many more failed very early on. It's interesting stuff, but to my limited knowledge, no one has quite yet cracked the code on making it a viable construction material. Work out the details...and you could become a billionaire. Really.

    Bookmark   September 24, 2005 at 5:47PM
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Thanks for the tips Tango.
I have a cool sand blasting gun but I'm waiting for a bigger compressor that arrives next week to operate it properly. My little compressor only gives me a few seconds of air at a time.
I'm going to grind up some iron and copper and try spraying both.

To date I have avoided using Muriatic acid. I have a quart bottle that is very old but never opened. I'm sticking with vinegar for now.:)

    Bookmark   September 24, 2005 at 7:01PM
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You could crumble steel wool (even use an old grater) and mix it with cement, make a sloppy mix and paint all over the surface. The steel wool will rust very quickly.

In the past, if I wanted to age concrete very quickly, I made hot tea (just with teabags) and crumbled steel wool into it. Let it sit overnight, it goes very black, paint over fresh concrete surface, wipe off the high spots. It will soak in and lasts pretty well.

You can buy very fine copper powder from fibre-glass & resin suppliers. I'm experimenting with it at the moment, mixing it with cement and painting on tufa surface.

    Bookmark   September 27, 2005 at 9:02AM
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wannadanc(z8 WA)

RD - this is your second great contribution to my life today - thank you so much! Keep on talking and sharing - I love it!!!!!!

Thank you....


    Bookmark   September 27, 2005 at 9:13AM
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Hi Running Dog
Steel wool does rust quickly but it's certainly nasty stuff to handle. Millions of rusty needles ready to infect me.:)

I prefer machine shop sweepings. I screen them to size but they aren't consistant. I'm trying to develop a method to get the rust to bloom from under the surface without blowing it up. Ive thought of winding long thin strands into my work
I like your silicone method. In my reno days I always used soap to smooth silicone with my fingers. Squishing the entire tube into a soapy basin sounds cool to me.

    Bookmark   September 27, 2005 at 11:24AM
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Gardening gloves!

    Bookmark   September 27, 2005 at 12:56PM
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Hi Dog
BTW it's great to see someone from Ireland in the group.
Old School Tufa!
I use gloves and have every kind going but most of my projects using these types of finishes require a very fine touch and steel wool goes right through even my toughest doctor type gloves.
Most of this work is fingertips only. I'm having good success with cutting the fingers off gloves and rolling them onto my fingers like condoms.
Laugh all you like but I have little green nitrile finger condoms all over the place.:)
My plams and knuckles are very thankful.
What I need is some sort of liquid latex that I could dip my fingers in then when finished just peal it off.

    Bookmark   September 27, 2005 at 1:11PM
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If working on something small you could maybe mash up the steel wool in something like a pestle and mortar, sift out the really fine stuff from the bottom of the bowl to mix into creamy cement/sand mix, and apply to surface of tufa object using a small paint-brush. Then sand back after ten hours or so, or when the cement has set a bit. That way you could get as fine as you like without having to use your fingers.

    Bookmark   September 27, 2005 at 2:30PM
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BTW tufaenough - I wonder if 'our' steel wool is the same as 'your' steel wool? This is the kind of stuff I'm thinking of...

    Bookmark   September 27, 2005 at 2:34PM
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Hi Dog
Thanks for the advice.
As to steel wool I use basically the same stuff although 0 is the finest I have.
Maybe 0000 will be less poky.:)
I think a cheap Mortar pestle might be a good investment for my shop. I have a fine one, Jamie Oliver style, in my kitchen.

    Bookmark   September 27, 2005 at 2:50PM
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club_53(z5 MA)

If you can stand one more comment about this subject:

I do not know anything about using oyster shells, but was curious and asked a friend that's a construction engineer who has been putting up buildings, etc. with concrete for the last 40 years or so. This is what he says about combining oyster shells and portland cement:

"Oyster shells in concrete; salt not the issue; shells almost pure calcium carbonate; affects the chemical make-up of the portland cement. Stay away from."

So, that's what he says.

    Bookmark   October 3, 2005 at 3:46PM
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dgaroutte(z8 WestWA)

Just a comment about grit. I have chickens and grit and oyster shell are differnt. Grit is very small rock for the chickens gizzard(so they can grind up their food) and oyster shell is ground up oyster shell used so their egg shells don't get thin. I'd just check to make sure they aren't selling you grit as oyster shell.

    Bookmark   October 8, 2005 at 11:00PM
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kobold(Vancouver BC)

Club 53

Your friend can make fantastic concrete work, but he erred about the chemistry. The oyster shell(Calcium carbonate)is relatively very stabil compound, reacts only with acid, doesn't affects any of the chemical reaction during concrete making or the chemical make-up of the portland cement. Portland cement contains lime (manufactured from calcium carbonate), a very active, corrosive compound, not the inactive calcium carbonate. The oyster shell in the concrete is a form of aggregate, the surface salt affects the longevity of the concrete.

    Bookmark   October 9, 2005 at 4:22AM
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club_53(z5 MA)

Well, I will just have jaunt on down to Maryland and make him wear a salty oyster shell necklace for leading us astray.

    Bookmark   October 9, 2005 at 11:04AM
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Hi Club
That is what's so great about this hobby.
It seems that even after centuries there are almost as many unknowns about concrete as there are facts.
Even seasoned pro's have odd ideas about concrete.
I amazed at how many experienced people in the business of selling concrete products believe things that are simply not true.

    Bookmark   October 9, 2005 at 11:11AM
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bznmygarden(Z5 IL)

Hi folks ~

My local pet store has an entire section of cement bird perches, ladders, toys, etc., all of which contain oyster shells. The perches and rungs on the ladders help to keep the bird's beaks and claws trimmed, and the calcium from the shells is a supplement for the bird's diet. They use regular old food coloring to color the mix, so it wouldn't work to use the perches outside. They want a pretty penny for these, too!

My cockatoo has one big perch that he came with, and it's his fave. It's lasted for several years, so I don't think that the shells cause the mix to degrade. I plan on trying to make some, for all of my other featherkids. Just have to explore if there's a certain mix required to be bird-safe!

- Lisa

    Bookmark   October 10, 2005 at 10:22AM
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I've been soaking my Oyster shell for 3 days now and the water tastes fresh no hint of salt.
These 'flakes of shell' are less that 2/3 the weight of sand by volume and have that pearly luster. They just might look terrific as a surface treatment, especially sealed with a gloss sealer.
By volume the Osyter shell doesn't cost much more than sand although there are a lot of very fine materials so I will be washing the stuff until the water runs clear before using. I'm not sure what the fines will do to the curing concrete.

    Bookmark   October 14, 2005 at 3:12PM
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Maybe tasting the water wasn't such a good idea.
I drained the water off and left the shells sitting in a stainless steel bowl for a couple days. Well today when I was getting ready to use them they smelled very GAMMY.
I assume there is still very tiny fragments of meat attached and in the damp conditions they started to decompose. From now on I will drain my washed shells pronto.
Just finished a Crete square little bowl using Perlite and the shells as my aggregate. The shell fragments are all less than 1/4 of an inch. Looks kinda cool, I just added a pinch of blue pigment to the white portland. I can hardly wait to see what a stiff brushing will do to remove the Portland haze. I'm hoping that pearly glisten of the shells is visible. It will be interesting how it feels to the touch.

Very hard on gloves BTW.

    Bookmark   October 19, 2005 at 3:47PM
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you should take the various rinses from your oyter shell-washing and pour it on your plants. I wouldn't put it all in one place, but many plants enjoy a calcium boost. At the very least you can pour it on your compost for the same reason.


    Bookmark   October 20, 2005 at 10:10AM
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That's a great idea.
The water is vey muddy the first few rinses and would be great for the garden. I always run my egg shells through my blender and dump those on the garden.
Two feet of snow slows me down a bit but I get most stuff to the compost.:)

    Bookmark   October 20, 2005 at 11:52AM
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kobold(Vancouver BC)

Make sure that you don't put any of that around rhododendrons and azaleas!!

    Bookmark   October 22, 2005 at 6:14AM
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