[marty63]

Hello, I'm new to the site and I would like to know if this is the right board for asking questions about geopolymer?

[Borek]

Fire away, we will think what to do with the question later.

[marty63]

Ok. I should just explain that I am a sculptor trying to make geopolymers so these calculations could be all wrong! Most probably I have made a complete mess of things.

According to the literature a molar ratio of SiO2 to K2O of 1.83 is sufficient to ensure room temperature reactivity, but weak enough to be classed as irritant rather than caustic. The calculation below is for Potsil rather than the Kremer potassium silicate. K-silicate 1.83 is the hopeful result. From the soapy feel I get on my fingers when I handle the wet casts in geopolymer that I have made so far, I can only think that I've massively overdone the KOH added to the Potassium silicate! It works great and sets at room temperature but is it in the right ballpark even as a calculation. There is no point making sculptures that dissolve human flesh on contact!

Potassium silicate conversion to K-silicate right version?

Potsil content:
                                K2O           SiO2
wt% /100g                10               20
weight ratio               1        :        2
molar weight          94.2mr       60.08mr
wt%/mr                  0.106n   :    0.333n
molar ratio                 1        :      3.14


Kremer Potassium Silicate content:
                                K2O           SiO2
wt%/100g                  8               20.8
weight ratio               1        :      2.60
molar weight          94.2mr       60.08mr
wt%/mr                 0.0849n  :   0.3462n
molar ratio                 1        :     4.08

Density (20OC)     1.25 - 1.26g/cm3
Viscosity (20OC)   31 mPa*s


K-silicate content:
                                 K2O             SiO2
wt%/100g                 17.8               20
weight ratio                 1        :       1.12
molar weight           94.2mr        60.08mr
wt%/mr                    0.1803   :     0.333
molar ratio:                 1         :      1.83

n=m/mr

         Potsil                 K-silicate
      SiO2 :  K2O           SiO2  :  K2O
n    3.14  :  1               1.83  :  1
mr  60.1  :  94.2          60.1  :  94.2


SiO2 = 3.14n*60.1mr = 188.7g
K2O = 1n*94.2mr       = 94.2g
188.7g + 94.2g        = 282.9g

Mass Potsil = 188.7:94.2
Total mass  = 282.9


    Potsil       K-silicate
    SiO2         SiO2
n  3.14          1.83

Difference
n  3.14-1.83 = 1.31mols
m 1.31*60.1 = 78.7g


           SiO2      +     2KOH       =       K2SIO3 + H2O (i.e. formula for potassium silicate balanced
m       188.7g              87g                                          with ingredients SiO2 + KOH on right.)
mr      60.1                  56.1                                  Si       O        K       H
n        3.1                   1.55                 Left side      1       2         1       1
                                                          Right side    1       4         2       2
used  1.31n             2.62n
used  78.7g           126.786g            By adding a 2 in front of the KOH on the left hand side we                                                               
                                                         we balance the equation.

Potsil    :   KOH
282.9g :   126.786g
100g      :   44.81g

Adding 44.81g KOH to 100g Potsil = K-sil 1.83.

Is this way too much KOH? It seems like an awful lot when I'm adding it!

[marty63]

I should maybe add that the starting solution is 30-40% w/w according to the bottle. So the amount of H2O isn't that clear! Perhaps I'm missing something obvious like not adjusting for the H2O content? It took me days to get this far and I feel like I've lost the wood for the trees!

[Borek]

It is a little bit messy - as you have not told what you are trying to do.

If I understand you correctly, you are trying to convert one mixture* into another by adding KOH?

What is the starting mixture, what is the target?

*geopolymer, whatever, looks like we are dealing with just a simple stoichiometry or even just a mass balance here.

[marty63]

Maybe my workings out aren't too clear. I feared as much! The starting mixture is a potassium silicate solution containing 10% by weight K2O to 20-24 parts by weight SiO2 in an aqueous solution - 'potsil' in the workings out above. I need to raise the molar ratio to 1.83 SiO2 to K2O - 'k-silicate' in the workings out above. The starting mixture has as far as I can understand it a molar ratio of 3.14 SiO2:K2O. According to the literature k-silicate can be made by adding KOH to the starting mixture - potsil.

I worked out that for every 100g of the starting mixture I need to add 44.81g of KOH. I really would like to know if my attempt above is the right way to go about it and if my result is anywhere near being right. I am just trying to get something that works and is not too hazardous to use - given basic precautions like goggles and nitrile gloves etc.

The procedure is normally carried out in a laboratory by the suppliers of the potassium silicate and so there are not many clues in the literature of how much KOH is added. The molar ratio of 1.83 is all that is given.

[marty63]

The actual recipe I am making is as follows, in case its of any relevance:

30g GGBS (blast furnace slag), 125g Fly-ash (low Ca), 15g H2O, and 30g K-sil 1.83 (or is it 1.83?).

This makes a geopolymer that sets at room temperature overnight. But id the K-silicate 1.83 prepared with too much KOH? That is the question.

[mjc123]

There is no point making sculptures that dissolve human flesh on contact!

Sounds like a good horror movie to me!

[marty63]

Well that's what I'm trying to avoid! But yes, I am working on the evil cackle!

[Arkcon]

I have to admit, even as I read your explanations, the concept is a bit hard to follow.  Let me try to break it down:

You start off with a word I've never heard of: geopolymer.  And Wikipedia isn't much help.  I think I have the gist, following Wikipedia and your postings:  geopolymer is a type of concrete, made of silicates and other earthy compounds, instead of the carbonates used to make cement since antiquity.  And they have specialized properties, thermal properties for example.  Cool.  You're making artwork out of cement.  Lets use that definition from now on, since you're not fireproofing a house, or building a medical implant.

You have a recipe, that gives explicit proportions, but lacking the correct raw material, you made some yourself following another recipe.  Before we dive in right at the calculations, we have to remember:  either recipe could be wrong.  So this may be a futile exercise.

You've quoted some recipes from manufacturers.  And you're wondering if there's too much KOH.  Here we come smack against a problem of chemistry:  not everything exists as a compound.  Large, inorganic compound may exist as complex coordination complexes.  What I mean to say is, maybe potassium silicate doesn't exist as a compound, but instead, a mixture of silicon dioxide and KOH and we just call it potassium silicate.  In which case, we may need more KOH to keep the resulting mixture stable.  The manufacturers know this, and so may some materials chemists, but this isn't the first topic in Undergraduate courses.

Of course the best chemical stability may prevent other reactions from happening, for example, cause a failure to set.  Did that happen?  'Cause that would be an easier question for a materials chemist or cement making company to answer.

I still don't know the dilemma.  You seem to be worried about the KOH in the mixture making the artwork, a set cement, unsafe to touch.  That's kinda out there.  For example, typical cement contains CaO, lime, which will likewise burn skin.  But it sets over time into a solid with long-range organization, internalizing some of the CaO, and I guess, washing off some of the surface.  We don't usually worry about touching other set cements.

[marty63]

Information on geopolymers can be found here:

Geopolymer.org

http://www.geopolymer.org/science/introduction

Or you can google 'Davidovits' as he is the chemist most associated with the development of them. They are distinct from cements in that they are formed of long chains of molecules giving an amorphous, nano-particle, rather than crystalline structure. Think of a plastic but in place of carbon you have silica. I might not have it quite right but this is the basic principle to the best of my knowledge.

They are generally formed by some combination of alumina and silica in the presence of alkaline solution although phosphate based geopolymers can also be made. they are basically a room, or low temperature, setting ceramic. Sorry if I didn't introduce what they were from the start but I didn't know that they were that obscure.

The recipe I gave does work fine, as do many of the others I have made, but the worry is that too much KOH remains unreacted and therefore makes the thing caustic to the touch when wet. This is after the thing has set hard into a sculpture. I know that the starting solution is a kind of water glass of which you can have lithium, sodium or potassium. All of these will work to make the polymers.

I am quite happy to work with trial and error as far as finding things that work but obviously one error that I would like to avoid is seeing my hand dissolve in front of my eyes! The main thing is, then, to get this initial solution right - i.e. to know how to work out how much KOH to add to the starting solution of potassium silicate. I also know that the molar ratio of the basic solution ought to be in the region of 1.83 SiO2:K2O.

The question can therefore be confined to this:

'How much KOH do I need to add to a potassium based water glass solution of a given molar ratio - in this case 3.14 - to change the alkaline content to 1.83 SiO2:K2O.'

If I can find the answer to this question then I know that I have a solution that is safe to use, and a product that is safe to handle. As I say, practical experience already indicates that I may have too much KOH according to my workings out above since it does definitely remove a few layers of skin when handling, and washing off, the resulting casts. I have worked with cement for many years and never had this effect from the lime in the cement!

Possibly in my naiveté I am asking either a ridiculously easy question or a ridiculously difficult one, either way I don't know the answer as to whether my calculations are correct.

The potassium hydroxide flakes I can get are 90% purity.

Any help would be greatly appreciated!

[marty63]

Oh! And by the way Davidovits thinks that many of the 'cements' of antiquity are in fact Geopolymers and not cements. He even thinks that some of the pyramids were made of geopolymer stone cast in situ. He cast up some 'pyramid limestone' geopolymer and sent it to laboratories for analysis - without telling them that it was cast - and they saw nothing unusual about this stone he had sent in for analysis.

[DrCMS]

I should maybe add that the starting solution is 30-40% w/w according to the bottle.

Perhaps I'm missing something obvious like not adjusting for the H2O content?

Yes that needs to be done but I think your other calculations are wrong.

If your composition data is correct 100g of Potsil contains 10g of K₂O and 20g of SiO₂ and 100g of the solution you want contains 17.8g K₂O and 20g of SiO₂.

So if you added 7.8g of K₂O to 100g of Potsil it would be just about what you want (not exactly because it would be 17.8g K₂O and 20g of SiO₂ in 107.8g but I'd guess that would be close enough). 

As you're adding KOH instead of K₂O you'd need 9.3g ((7.8/94.2)*2*56.1) and as you've only got ~90% KOH I think you'd be safe to use 10g of your KOH per 100g of Potsil to get pretty close to what you want.

[marty63]

So - I have a kind of 'penny dropping' moment here - I don't need to think about anything other than molecular weight? I simply work out the weight of K2O needed, and translate this into the equivalent weight of KOH? I don't need to know what happens to the KOH in order that it becomes K2O, and take account of anything being used up/ejected in any reaction? I just find out how much K2O is needed and then translate this molecular weight into the equivalent of KOH?

Have I got the gist or completely misunderstood?

[Borek]

I don't need to think about anything other than molecular weight? I simply work out the weight of K2O needed, and translate this into the equivalent weight of KOH?

Yes, although you contradicted yourself in a funny way. "Translating K₂O into equivalent weight of KOH" means using more than just a molecular weight, so both phrases you wrote can't be right at the same time