Geopolymer shrinkage results

[etownandrew]

This past Friday I mixed up a batch of geopolymer and put it into a long rectangular mold. I hadn't intended to test the shrinkage but I was surprised by how much it shrank in two days. Note that I left it covered in plastic overnight. The mold cavity is 27 1/16" long and the clay is now 26 7/16" long. So far it has shrunk 2.3%. I'm glad that I put 3% rock wool fibers into the mix by weight because it has not yet cracked. Here is the mix I used. Note that I did not use any sand in the mix because I was trying to obtain the most insulating result. I had mixed it to a "peanut butter" like consistency. I was intending to try a castable mix and so used the 20% binder but then got nervous about it and did not add that much water.

117.528 oz. Clay wet
(reference)
93.456 oz. clay dry

3.6 oz fiber

97.056 oz total aggregate

20.00% Desired % binder

17.47 oz Sweet PDZ 3:1

5.82 oz lye

120.35 Total oz



The sweet PDZ and lye were allowed to sit mixed with a little water for several hours before use.
 (I don't know how to upload pictures. I don't know what I am doing wrong.)
app.box.com/s/h954bi7blvd2mvksqxg70rp294azksfp

My main concern is that I had thought to mold this around a wood core form. So I'm afraid that the clay will crack as it shrinks. I can see several options to try and avoid problems with cracking.
1) Add sand or another aggregate that will reduce shrinkage but also reduce the insulating effect.
2) Make the core out of wood boards and soak the core for a day or two before use so hopefully it will shrink with the clay. Avoid plywood because I don't think it would expand and contract as much as plain wood boards.
3) Make the core out of something soft that will "give" as the clay contracts. Possibilities are cardboard or foam insulation boards. My concern with these is that they may be too weak to hold shape as I put a couple hundred pounds of clay all around them.

[smartliketruck]

As clay dries over a form it shrinks not just outside in but also inside out. So a tube would have a larger inside diameter and smaller outside diameter when dried, so long as it is not sticking to the form. Packing tape works well to cover a wooden or cardboard form, kitchen cling wrap also works if you can wrap it completely several times then heat shrink it with a hair dryer or heat gun.

[pigbuttons]

ETA, thanks for the thread, I look forward to your results.

Shrinkage can always present a challenge.
1)You don't mention the specific kind of clay used, but some clays shrink more than others.
2) The stresses set up in this kind of moulding/casting is why most often you will see this type of product split longitudinally down the middle and made in two pieces then bonded together.
3) Cardboard may work, but may also fail if it gets too wet; foam insulation board is surprisingly difficult to compress by the small loads that shrinking clay would afford.
4) Others don't agree with me, but I like "concrete aggregate vermiculite" as an insulative bulking aggregate. Most often people use agricultural vermiculite and the two are worlds apart.

[Deleted]

Nothing prevents one from using much larger amounts of fibers.
My small coated core sample was made with 20% mineral fiber.
One may use even more for making a laminating putty.

[branja]

Just a thought, you don't actually need "peanut butter" (forgive me I never seen its viscosity) , ram mass, hand shaped "modeling clay" high viscosity sometimes works also.

[coastalrocketeer]

Thank you for the excellent info on your mix and experiment.
I am looking forward to hearing how your sample performs after it is fully cured!

One question: Are you seeing much physical evidence of dissolution of the sweet PDZ granules at that lye concentration? Big chunks becoming smaller?

Since you and I are the only ones using that particular Clinoptilolite zeolite product at the moment, me with the power and you with the granular, I'm looking for your experience with that.

Grains not dissolving all the way apparently does not prohibit success at Geopolymerization and can be due to natural inclusions of less easily dissolved, or non-dissolvable minerals according to Karl.

[coastalrocketeer]

Jan 15, 2018 8:02:21 GMT -8 branja said:

Just a thought, you don't actually need "peanut butter" (forgive me I never seen its viscosity) , ram mass, hand shaped "modeling clay" high viscosity sometimes works also.

Ok... I finally learned what ram mass consistency would be. Thank you Branja!

[coastalrocketeer]

Jan 15, 2018 8:02:21 GMT -8 branja said:

Just a thought, you don't actually need "peanut butter" (forgive me I never seen its viscosity) , ram mass, hand shaped "modeling clay" high viscosity sometimes works also.

Peanut butter at room consistency is able to be stirred with effort, but holds it's shape and does not have any "slump" when peaks and ridges are formed... Thicker then hummus, for sure... Can't really think of another good, relatively universally known "equivalent viscosity" example from daily life. The thickest automotive/industrial greases I've used are about the same consistency, but that's not a very good universal example as it is characterized based only on what I've worked with.

Here it's so universally consumed and sold, that I hadn't even considered that it would be unknown to others around the world, but it makes sense... Invented in the US, to develop broader use for peanuts as a crop, long after everyone else developed their eating and farming habits.

[etownandrew]

Jan 15, 2018 11:25:41 GMT -8 coastalrocketeer said:

Thank you for the excellent info on your mix and experiment.
I am looking forward to hearing how your sample performs after it is fully cured!

One question: Are you seeing much physical evidence of dissolution of the sweet PDZ granules at that lye concentration? Big chunks becoming smaller?

Since you and I are the only ones using that particular Clinoptilolite zeolite product at the moment, me with the power and you with the granular, I'm looking for your experience with that.

Grains not dissolving all the way apparently does not prohibit success at Geopolymerization and can be due to natural inclusions of less easily dissolved, or non-dissolvable minerals according to Karl.

I had some interesting initial results with the sweet PDZ/lye mix. My long slender mold I treated differently. I left it covered in plastic cling wrap overnight. Then the next morning I placed it in a warm spot next to my wood stove. Then later in the day when I could see moisture collecting under the cling wrap I pulled it off so it could start drying. I have not seen any efflorescence. However, I shaped some of the leftover material into pucks but did not cover them or put them in a warm place. They have developed efflorescence over the entire surface. So I am having thoughts that maybe I should try to somehow warm the overall core mold after I cast clay around it. I'm imagining an insulated box with some light bulbs inside it. One study called "Effect of Curing Profile on Kaolin-based Geopolymers" indicated that a 3 day curing at 60C was ideal based on their test results.

The sweet PDZ granules did not seem to dissolve.

[etownandrew]

Jan 15, 2018 6:15:53 GMT -8 pigbuttons said:

ETA, thanks for the thread, I look forward to your results.

Shrinkage can always present a challenge.
1)You don't mention the specific kind of clay used, but some clays shrink more than others.
2) The stresses set up in this kind of moulding/casting is why most often you will see this type of product split longitudinally down the middle and made in two pieces then bonded together.
3) Cardboard may work, but may also fail if it gets too wet; foam insulation board is surprisingly difficult to compress by the small loads that shrinking clay would afford.
4) Others don't agree with me, but I like "concrete aggregate vermiculite" as an insulative bulking aggregate. Most often people use agricultural vermiculite and the two are worlds apart.

You didn't mention pearlite as an option. What do you think about pearlite? I know that Karl feels it produces a weak fragile outcome.

[etownandrew]

Jan 14, 2018 14:43:42 GMT -8 smartliketruck said:

As clay dries over a form it shrinks not just outside in but also inside out. So a tube would have a larger inside diameter and smaller outside diameter when dried, so long as it is not sticking to the form. Packing tape works well to cover a wooden or cardboard form, kitchen cling wrap also works if you can wrap it completely several times then heat shrink it with a hair dryer or heat gun.

I had not considered that factor and it would help. It sounds like you have done this and it worked fine. I think though that it would not help enough without taking other steps. For example, if we had 1" thick clay walls and the sides of the core were 10" on a side. The 1" walls would shrink back maybe 3% to the midpoint of the 1" thickness. However, the 10" long clay side would also shrink the same % and it would end up shrinking about 10x more than the 1" thick. (I'm speaking in approximates. But I think close).

So I can see that I want to do more shrinkage experiments with minimal water.
1) Check to see if more fiber will reduce the shrinkage.
2) Start adding fillers. I might try a 1:1 clay to sand and check the shrinkage.
3) Try a clay and pearlite mix and check shrinkage. Also, check the durability.

Other options that were suggested by others above and that have occurred to me.
A) Instead of molding on the outside instead build up the clay on the inside. That way it can shrink to the inside freely. Unfortunately, I can see some practical difficulties with this idea.
B) Come up with some sort of a two or three part mold that does not restrain the clay from shrinking and then put the parts together.
C) Of course, there is always the build it out of insulating ceramic boards option.

[Deleted]

Maybe each of you could send me a pound or so Sweet PDZ as a letter.
Send me a PM for my address.

[pigbuttons]

I don't particularly like pearlite for several reasons.
1) It is rare in "concrete aggregate" form. I haven't looked for it in several years, but the last time I bought it I had to travel 300 miles to get it.
2) As with vermiculite, the agricultural stuff is NOT ok.
3) It produces very fine dust that is extremely irritating to eyes and lungs. Think microscopic glass shards; vermiculite is smooth sided.
4) The big one! Its melting point is too low for a rocket stove burn chamber. This is probably why it leads to brittle geopolymer in our applications.

[smartliketruck]

As you are finding, it is a balancing act between reactivity of mixture, moisture levels, grog/aggregate and time or energy applied to curing.

I can't speak specifically to the zeolite mixture you are playing with as I haven't found zeolite available locally.

The amount of shrinkage you are experiencing with the current mixture can be reduced by preventing moisture loss till more polymerization has occurred.
Till you have the moisture loss under control don't add additional heat, the uncoated wood is probably wicking away a lot more moisture than you'd expect and the additional heat accelerating that. This will of course will require more time and patience, less moisture to start with will likely produce satisfactory results sooner.

[coastalrocketeer]

Jan 15, 2018 21:29:21 GMT -8 smartliketruck said:

I can't speak specifically to the zeolite mixture you are playing with as I haven't found zeolite available locally.

Thanks for all the excellently clear advice and analysis SLT!

What we are using is SweetPDZ, sold as a "horse stall freshener" so feed and farm supply stores are the ones that would be most likely to have it. Not likely available in NYC, SF or Detroit... But if you have any farming area nearby it might be available at one of those places. In my rural area the most local feed store did not carry it, but the next closest one 22 miles away did. I bought 3 40lb bags for $60. Quite a bit more than clays, but my nearest pottery supplier is 4 hours/120 miles round trip. So it will be a bit until I have time and money at the same time to go buy a few hundred pounds of rock powders, clay, and other aggregate and additive materials to play with.

Unless you are in the US, or maybe Canada, all of my advice on locating it is probably moot.




I have the "powder" (mostly coarse powder, with some random large grains as wide as 3mm, as I think they use a blower system to remove the finest particles and sell them to other markets, to reduce inhalation hazard from the sprinkling application this product's label recommends.)

Andrew has the "granules" version, which is significantly coarser.