ARAGOCRETE RESEARCH BY TRACY GRAY
MOORHEAD SENIOR HIGH'S REEF FARMING PROJECT part 1
Several weeks ago I sent an e-mail to LeRoy asking for his input on several issues and commented on my experiences with "artificial" live rock. LeRoy felt that it might be worth while to have me share these experiments with other readers of GARF's newsletter. In my e-mail I had made comments regarding "aragocrete" on several different subjects so I'll arrange this correspondence as follows:
General Chemistry and Terminology
Portland Cement Chemistry
1. Some of the very first pieces I made.
Not a lot of surface area or easily accessible interior space.
These are rather flat broad pieces intended to be interlocked as in photo #2.
|2. Here the pieces are shown interlocked. A third flat piece on there rocks makes a good elevated shelf. The overall structure takes up a fair amount of space. I liked the idea for the shapes but the rock itself could be greatly improved.|
General Chemistry and Terminology
Portland Cement - A mixture of calcium and aluminum silicates that will hydrate upon addition of water to form crystalline materials with high compressive strength (when properly cured). The major components of Portland cement types one and two are:
Tricalcium silicate - 3CaO.Si2O3 - about 55%
Dicalcium Silicate - 2CaOSi2O3 - about 25%
Tricalcium aluminate - 3CaOAl2O3- about 10%
Tretracalciumaluminoferrite - CaO.Al2O3.Fe2O3 *
Other inorganics - about 2%
* Taken from a cement manufacturer's web site but the name given suggests there should be a four in front of the CaO in this formula. I believe type three to be a slight modification of the above compositions.
3. These are pieces now in my aquaruim they take up little room on the sand surface and provide flat elevated space for placing corals.
|The first two constituents are what actually give the cement its strength and primary properties. The tricalcium aluminate does not
contribute greatly to concrete's final strength, but is necessary in the manufacturing process. The tetracalcium aluminoferrite is the
material that gives cement its grayish color.
The white Portland cements are manufactured with tighter control on this impurity. The majority of the "other inorganics" is gypsum. Gypsum is added to suppress the hydration reaction of the tricalcium aluminate which if not suppressed will occur instantly during mixing. This is what cement people call false set.
Calcium Hydroxide - Ca(OH)2 or Kalkwasser to many hobbyists. As pointed out by GARF, this material may be obtained at your local supermarket as pickling lime. Also only slightly soluble, but can increase pH greatly as the Hydroxyl group adds directly to the (OH) concentration in aquarium water.
4. This photo contrasts the first "wet" pour forms to one made by sprinkling a "dry" mix into a form.
|Sodium Carbonate- Na2CO3, sometimes called washing soda. Most readily obtained by purchasing Buffex from your salt water aquarium
store, but much less expensively obtained by calcining sodium bicarbonate. To calcine sodium bicarbonate, simply spread in cookie sheet
in your oven and bake for about 1 hour at 400 deg F.
Sodium carbonate increases pH in water very quickly and is highly soluble. The carbonate ion is a major portion of the buffering system in salt water. Quick addition of carbonate to sea water can precipitate calcium carbonate where the carbonate concentration is locally high during the addition.
Perlite - This is a gardening soil additive used to prevent soil compaction which helps drainage and aeration. This product is made by high temperature roasting of the naturally occurring volcanic mineral whose main constituents are oxygen, silica, and aluminum. Perlite is essentially a glass and does not appear to absorb water even long submersion periods and seems work well as an additive to aragocrete mixes.
Vermiculite - Pretty much the same use as Perlite but based on a mineral similar to mica. Its major constituents are oxygen, silica, aluminum, and magnesium. Vermiculite is also very light weight when roasted but seems to absorb water and break apart in cement mixes. I see no advantage to using vermiculite in aragocrete and it might add some undesirable elements. Aggregate - General term which refers to materials being bound together in a concrete mix.
|Portland Cement Chemistry|
I found volumes of references on Portland cement covering everything from its basic components to 100's of patents for special additives for hardening accelerators, hardening decelerators, strength enhancers, and strength during freezing enhancers. There were more sections on x-ray diffraction studies to identify the actual crystalline forms present after hydration and kinetic data for various formulations, and more I can't even remember. Suffice it to say Portland cement has been extensively studied and seems still not to be completely understood. What is important here is that it will hydrate to form a high compressive strength material capable of bonding many aggregates securely together. There are a few properties however, which are worth specifically listing.
5. These slabs were originally intended to be background pieces but use up more space than one might think. However, water runs through the pieces like cheese cloth. When broken up, these pieces make a strong, high porosity base rock.
There are in fact three different reactions occurring when Portland cement cures. The last two are the di and tricalcium silicate hydrations and the first one is the tricalcium aluminate. Most formulations add gypsum to suppress the first hydration reaction to prevent "false set".
The hydration reactions that occur produce calcium hydroxide and the wet cement has a very high pH. The rate of calcium hydroxide production follows the hydration reactions. That is, it will be very high initially and very slow afterwards. The calcium hydroxide is produced throughout the mix and so much of it will take some time to diffuse into the water from the interior portions of the piece.
Cured Portland cement usually will have rather poor shear stress strength characteristics. That is to say it is very difficult to crush by direct compression, but is usually easy to break into pieces by holding one end firm and applying force on the other end perpendicular to the axis of the form. Very much like fiberglass resin, one can add fibrous materials into the aggregate to add shear strength to the piece.
5b. Tripod #1 in my tank. Only just now am I getting coralline algae to cover this piece.
When I first found the GARF website, I was most pleased to find there might be some way I could have live rock without having to feel like I was mugged on the way home from the aquarium shop. I also visualized my patio in pieces and placed in my aquarium. Though dry concrete absorbs water, this didn't seem like a high surface area material which would harbor many bacteria, or have much (usable) penetration of water into its matrix.
I therefore set out to continue on LeRoy's quest for a good material to add to the mix which would make it lighter and have more surface area. I wanted to use something fairly readily available and also rather inexpensive.
So here I'll list a few ideas which did not work and add a few ideas I've not tested which might have promise.
Sodium sulfate - Same idea as above but it dissolves too quickly. (However, large marble sized chunks might work.) Not too hard to get and highly soluble.
Rock Candy- Again, same idea as above. The sugar actually reacts with the cement and quickly forms a very thick sticky mix almost able to support its own weight. With fine sugar one could probably make a clay like material. However, subsequent curing is extremely retarded, possibly even stopped entirely.
Pasta - I know now that some people have had success using pasta but I didn't like it because I found it took forever to remove from the rock, even on the outside. I felt that pasta would stay on the interior of the rock for many months and I didn't want a continuous leaking of starch into the water. (Maybe Jake might comment on this?)
Sodium Bicarbonate - This idea was really reaching. I wanted to put the baking soda in the mix and bake the piece immediately after forming hoping to get the same effect one gets when making biscuits. Baking soda also dissolves in the water which is not so bad, but it reacts almost instantly with the cement. It creates the false set condition and also seems to ultimately slow the final setting rate. I can't say if it hurts the final strength after several days curing.
Sodium carbonate - Large chunks might work, but the carbonate will react with the calcium hydroxide and form calcium carbonate (as does the baking soda). It is likely this calcium carbonate will not be strongly incorporated into the matrix of the cement. Sodium carbonate is also a strong set accelerator.
Ice - The idea was to have the ice melt away and leave holes behind. The ice cooled the mix and severely retarded the set time. After the ice melted, the water added to the cement and made a sloppy mix that settled into pancakes with almost no interior surface area.
6. The two based stalagmite shelf.
|Sometime around this point I read Karen Holt's letter describing a good and easy method to make a high porosity material. Her reasons for
wanting to do this were well stated and echoed my own very closely. Even with the description printed, I didn't precisely picture what she
was doing with the small blobs of mix being dropped onto one another, but I got the basic idea. And besides,"dry" mixed cement has loser
structure than "wet" mixes. I made several pieces using this idea, but sprinkled the material into its form instead of dropping blobs.
This "sprinkling" of material is something like holding wet sand in one's hand and working the fingers so that the material is very loosely spread across the designated area. I was pleased with the result, but I wanted to make the pieces still lighter in weight. I had experimented with Vermiculite already and was not happy with it, so I tried Perlite as an additive to the mix. I used as much as 50% Perlite in the aggregate, making up the difference with aragonite sand and crushed oyster shells. This made extremely lightweight forms with good porosity. Except for being stubborn, and somewhat worried about what Perlite would do long term in salt water, I would have stopped experimenting here. (P.S. I am no longer concerned about Perlite adding silica to the water, but these pieces do seem to be colonized by the brown algae more readily.) I also wanted something a bit more natural looking with even higher surface area. I went back to the ice and tried to find a way around the limitations mentioned above.
Shredded Plastic 35%
Fine Aragonite Sand 20%
Crushed Oyster Shells or Coarse Aragonite 10%
Portland Cement 27%
Ice volume ice/1 volume aggregate
(All percentages are by weight as volume percentages vary by aggregate type. Large deviations will definitely be possible)
The amount of ice and its size are very important. If you use too much, it tends to make the mix get too wet when it melts and the desired porosity will not be achieved. Too much ice will also make the pieces very weak. The piece will be stronger if you increase the amount of cement mix compared to aggregate and ice, but I don't think it looks quite as good and also seems to take longer to colonize. If the ice is too small, it will not leave holes behind when it melts. If it is too big, there will locations of extreme weakness in your rock. For me, a range from say large BB's to marble size works well. I strongly suggest making very small test quantities until you find a procedure and formula that works for you. I followed this plan while trying to develop the technique. It really cuts down on waste and unwanted rocks. I probably formed 30 to 40 different mixes in 12 oz Styrofoam cups with small holes punched in them for drainage. Actually, if you want small fist size rocks this works well, but I'd try to find something with a little less regular shape for aesthetic reasons.
Pre-crush the ice and leave in freezer
Pre-mix Portland cement, NaCl, and Na2CO3 and put in freezer
Pre-mix the aggregate ingredients in your cement mixing bucket and put in freezer
(I have always kept every thing cooled before mixing to minimize melting during mixing but this may not be necessary. After all, I then add water to make the mix)
Add cement mixture to aggregate and mix while still dry
Add ice to mixture and begin mixing
Add water SLOWLY and mix between additions until you have a somewhat "dry" uniform paste/slurry.
(This step is the hardest to get right and it's easy to over shoot, as the water you add also melts ice and you get more water than you expect.)
Form pieces in a bed of aragonite sand using shapes you find desirable and sprinkle with oyster shells Do not bury the freshly made rock as the weight will reduce the effect of the ice
7. Tripod #2, the newest addition. Its not visible in the photo but coralline algae is starting to grow nicely on this piece.
Cure for at least two days (three is better) before moving the piece. These pieces do not approach their final strength very quickly. I have broken quite a few by losing patience. I also do not acid wash right away when making rock this way as the acid interferes with the curing of the rock and reduces its strength. But you can put it into water after three days or so. I acid wash, if at all, about a week after its initial mixing. I don't acid wash for pH reasons, but instead for aesthetic reasons. To me, the pieces look better when the excess cement is cleaned from the form.
Acid washing may also serve to open up some of the otherwise clogged pores in the cement structure. I have never had a high pH problem with or without acid washing, but I let the rock soak in water for about a month before using it. I also have never filled an entire aquarium with aragocrete all at one time. (For those who are having trouble with high pH, you might try letting the piece dry completely and then acid washing. The idea would be to use surface tension to pull acid into the structure of the cement and neutralize the caustic values without waiting for diffusion of calcium hydroxide to the rock's surface.)
8. A shelf and colum on 2 pedastal supports. The blue discosoma are on top of the self.
Well, for what its worth, that's the method. This method is by no means optimized. It is likely that baking soda can be directly substituted
for the sodium carbonate or that the same set acceleration can be achieved by using salt or calcium chloride only. I have not used the type
three Portland cement as it is hard to find around here unless you want to buy about a ton. I have used both the white and type 2 Portland
cements and both work fine. Also I should mention that the "dry" mix and sprinkling technique is really much easier than the ice method,
so this is what I do to make pieces I plan to use as base rock. If done right, it really does pass water through its structure like a sieve. If
anybody improves on the ice technique I would like to hear about it. I am certain improvements are possible.
Finally, guess I'll cover some miscellaneous thoughts and experiences.
I have tried a number of different shapes. These shapes include flat puzzle shaped pieces that interlock and are easy to stand upright in the aquarium, circles, starfish shaped pieces, platforms on legs, caves molded around paper mache, slabs molded in kitty litter pans, and whatever one gets when the piece breaks. The flat puzzle shaped pieces seemed like a good idea, but they must be pretty large before they are strong enough. The size can then be a bit limiting on how they are used. Caves are always nice. The platforms are nice as they leave a lot of sand uncovered, but the tripod legs don't look real. I sent GARF some pictures of a few of these tries so readers can judge for themselves.
I have found the highly porous pieces made a la Karen Holt or using ice to be rather weak. This is not unexpected but does limit the
design of ones rock. I have made (only) one piece where I added many short pieces of thin fishing line to add strength. It seems to have
worked as the piece did not break, but I didn't care to repeat the effort of cutting many feet of fishing line into
9. A close up of one of the newer pieces showing the cave like openiness within the form. The blue specs are on a different piece; an experiment I will not repeat.
This last idea is something I have not tried, as I'm not sure what fairly freshly mixed cement might add to the water. However, since cement creates calcium hydroxide when it cures (and raises pH as some have found), it seems like the periodic addition of small pieces of "aragocrete" to one's sump might be a useful and easy way to maintain pH and calcium levels. How much and how often is obviously critical.
10. The work in progress. Note the large green Bowlus plasticus in the upper right.
More next time - LeRoy
REEF PROPAGATION RESEARCH
GEOTHERMAL RESEARCH AND EDUCATION
| Moorhead Senior High's Micro-Reef
Left to right: Jesse Palczewski, Kristi Alverson, Ashley Tessier, and Jeremy Buth, all posing with our small reef.
A full photograph of our reef with the new acropora cuttings just glued. At the top and center can be seen 4 of the tiny fragments.
Reef Ecosystem is Like Art
This aquarium has several different species of coral. The colors range from dull to bright pinks, purples and greens. The tank is illuminated by two fluorescent lights and a blue light so the colors show up very bright. The various shapes of coral make a nice composition.
I like having this in the room, because I can learn a little about the ecosystem of the reefs just by watching the tank in my spare time. It's a nice thing to add to the classroom if you're living in the midwest.
Aquarium Adds Serene Beauty
My name is Nathan, and I am a student at Moorhead Senior High
School. I am in Mr. Raymond's Introduction to Art class. His classroom
is one of the neatest rooms that I go to during my school day. He has a
really cool fish tank.
When I first went to his class, I picked the seat closest to the fish tank. I really enjoy looking at it. If you are having a stressful day, all you have to do is look at the beautiful salt water tank. It is so calm and serene that it can help you relax. It helps me.
The salt water tank adds so much color and vibrancy to the room. Yet at the same time, it offers life and energy to the room. Mr. Raymond has it set up to look like a real underwater ocean scene. He does a good job of putting a variety of different corals in it. All in all, I do not think that Mr. Raymond could have picked a more beautiful centerpiece to help brighten the room.
A Full Tank
Jeremy surveying the tank before adding cuttings.
|I'm Jeremy Buth, a freshman Drawing and Design student at Moorhead Senior High. I'm writing about Mr. Raymond's aquarium. Three Yellow tailed Blue Damsels swim amongst coral and rock. Red legged and blue legged hermit crabs walk on the crushed coral bottom and on the live rock. There are also snails.|
|The corals are colorful and add excitement to the aquarium. Sinularia finger leathers reach their fingers out around themselves. Bright green star polyps bloom creating a grassy look, and green centered button polyps spread their spiky heads. Candy cane coral gives the aquarium a psychedelic look with their intense green centers and raised, reddish brown color.||
Jeremy holding the bag of acropora cuttings.
Jeremy placing the next frag in reef aquarium
Holding a fragment on a rock while the glue sets.
The center of the tank showing the new cuttings at the top.
The right side of our little reef.
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