I have mixed caustic burnt magnesium oxide with the swelling kind of bentonite in 1/1 ratio and added 5% citric acid. To the mixture I have added just enough water fo form a thick paste.
Layers thicker than 5mm take very long to dry and tend to crack. Adding some fine sand could improve the crack resistance. Once dried it is quite hard and wear resistant, but requires firing to get water resistant. It has very good insulating properties.
Likely a smaller amount of caustic burnt magnesium oxide will suffice and even pure bentonite may work.
I will have to make some more tests.
If it sticks well to mineral surfaces it would be a very simple way to protect softer materials against abrasion eg. vermiculite board, very easy to repair as well.
Oxalic acid is said to be a much stronger organic acid, but very unhealthy.
As expected, based on my knowledge, it works with the magnesium oxide containing swelling kind of bentonite, without additional magnesium oxide.
Unfortunately it does not stick well to surfaces. With high amounts of sand it could be used to make insulating tiles, which require firing. Obviously a stronger kind of acid eg. phosphoric acid would be needed to make it water resistant without firing and more sticky.
Anyway, as I know acid treatment increases chemical reactivity of bentonite and increases the inner surface as well. The likelyhood was high that organic acids can make it reactive enough for the usual way of geopolymerisation without calcination at high temperatures. Thus I have decided to give it a try.
The swelling kind of bentonite needs many times it's own weigt of water to make a slurry with about 5% organic acids, which makes either powdered waterglass or and extremely concentrated DIY waterglass a requirement. Simply giving the acid to a mixture with waterglass will not work as the acid would react with the much more reactive waterglass and not with the bentonite.
As expected it works. The extremely high water content of the mixture requires very careful curing and drying. Although I do not like to wait I can be very patient.
Our cellar is three-quarters of its height in the earth, making in even in a hot summer to a cool place. There I have placed the mixture to slowly cure many days. Today I have moved it to our balcony to continue sealed curing at a warmer place.
I am curious how strong it will become, but have to wait some time more.
Post by firewatcher on Aug 29, 2016 7:52:19 GMT -8
I have just finished casting a puck and a ring using bentonite/waterglass/perlite chunks to see if i can come up with something a little more light weight and maybe more insulative for the heat riser. In a couple of days I'm hoping to "fire" the both of them and see what happens. The only means of temperature curing that i really have is by "camp fire" so this is what i mean by "firing".
Judging by the greenish color your bentonite seems to be of the magnesium containing swelling kind. If you preaktivate it with some kind of acid the result will be stronger and much more insulating than with perlite chunks, even it it contains high amounts of slag sand. Even vinegar should work.
Post by firewatcher on Aug 29, 2016 8:23:25 GMT -8
I'll have to give it a shot then...any recommended ratios for the amount of vinegar to use...a starting point? you mentioned in your recent post 5% perhaps? I don't want to assume either...is it 5% by weight as always?
Yes, it is 5% by weigt of the bentonite. You will have to look up how much acetic acid is in common vinegar. Vinegar essence has defined concentrations of acetic acid. Preactivate it a day or so before.
Desiccants require very large inner surfaces. The very large inner surface of the layered particles is responsible for the swelling property of magnesium containing bentonite. Likely it has undergone some heat treatment to improve its adsorbing property.
As I really do not like to wait, to check other things and to keep me bussy I have made another mixture, which would give faster results.
The new mixture contains 45g bentonite, 5g talc, 5 g citric acid and 300g slag sand, which gives a slag/bentonite ratio of ~ 6.7/1. As I this time did not add powdered waterglass to compensate for the water required for preactivation the resulting solid waterglass concentration was only about 200g/liter.
The mixture got very hard with a dry density of only 1.3g/cm³, which indicates a porosity close to 50%. Contrary to popular belief more sand doesn't always mean a higher density, but sometimes the contrary.
It did not get water resistant. But wait: Do not jump to false conclusions. It is not the fault of the bentonite. Like with Davidovits LTGS bricks such extremely small amounts of reactive material need more heat energy for a propperly working chemistry. As with the LTGS bricks about 450°C should do the job.
The first mixture containing 100g bentonite, 5 g citric acid, 300g slag sand and powdered waterglass for compensation became water insoluble. Small pieces of the still not completely dry mixture lie in water since many hours, without any sign indicating that they will ever fall apart.
Geopolymers are like a dream come true for stove builder, with properties which has no other material. They can be made with many different materials, many of them quite cheap or even free. It is possible to make geopolymers completely from materials occurring in nature. One can do it without to buy any chemicals.
Post by firewatcher on Sept 2, 2016 7:24:09 GMT -8
Karl, you've done it again...got me wanting to make more test pucks...with the new mixes that you've come up with.
Can you provide your process for making this mix as far as preactivation of the bentonite and amount of waterglass that you added to the mix?
I may be able to get my hands on all the bentonite clay that i can handle for free...citric acid is fairly cheap and i can get that from the local home brew shop (the only reason i know that is because i make wine periodically and have seen it there).
The only remaining hang up may be how to cure the mixture at 450 C. Not sure yet how i would do that.
In the mean while though, I would definately like to try this mix out.
Post by firewatcher on Sept 2, 2016 8:54:32 GMT -8
Looking on line, my only possibility for temperature curing (camp fire), should be sufficient for getting up to 450 C. From a couple of sites, it was indicated that an average temp would be around 600 C (around a 3 hour fire of modest size).
The first mixture with 100g bentonite, which took so long to cure and dry got much more water resistant. Small pieces of it did not fall apart in water. It got very hard and pretty wear resistant at a dry density of only 1.24, indicating slightly more than 50% porosity. If you knock it rings a bright sound.
With respect to the second sample with an extremely hight slag/bentonite ratio of 1.67/1, in a stove it would hardly ever encounter any water. If used as a riser in a barel the hot gases at the outside have very likely still more than the 450°C.
The process is simple. First mixing bentonite and acid with water to a thick slurry and let it rest. At best this is prepared a day or so before. Then mixing the preactivated bentonite slurry with aggregate. 10% lye by dry weight of the bentonite and waterglass, by either adding powdered waterglass or use waterglass with a very high solid content. Small amounts of lime or cement can be added at the very end as accelerator.
Post by firewatcher on Sept 2, 2016 9:49:37 GMT -8
Maybe I'm missing it in reading your last post...sometimes tapatalk is goofy when i read posts on my phone and it doesn't show the entire post.
Two questions: 1. How much powdered waterglass should be used in the mix? 2. How much solids should be used in a "concentrated" liquid waterglass that you mentioned? ...and how much of this concentration should be used in the mix? (I still have some 1000g/2L concentration waterglass left and, in addition, some 1000g/1.5L concentration liquid waterglass)
Post by firewatcher on Sept 2, 2016 11:06:58 GMT -8
So it sounds like solid waterglass is easiest to use in this case for measurement purposes. I'm still not sure that I'm clear on waterglass concentration though, so perhaps a mathmatical scenario would help me the most.
For example (completely making up numbers here for the sake of an easy mathmatical example)...
If i used 1L of water to make the bentonite slurry, i would use 500g of powder waterglass? What I'm getting at asking is what is considered a "high concentration" for the waterglass.
You mentioned in the past that the 500g solids (sodium hydroxide + silica gel combined) to 2L water ratio was enough to turn even dirt into a refractory. I just wanted to confirm whether this was concentrated enough or if higher concentrations were necessary.
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