(Na,K)-PS

Sodium or potassium Poly-Sialate

My experience with this high aluminum to silicon system has been very naive. Somehow, some things have worked, and I like them. But I have many (many) jars of neglected goo.

My very first geopolymer experiments were with a potassium-grog mix. Grog is kaolin (and sometimes other clays) burnt at significantly higher a than is ideal for chemical reactivity. I dry mixed the potassium hydroxide and grog and then added water while stirring to get a thick slop. It got very hot from the dissolving alkali while I stirred. Somehow it did cure, even though the kaolin was over calcined and poly-sailates are not known to cure well at ambient temperature. The hot solution and the hot hot Texas summer sun was somehow enough. Perhaps also potassium poly-sialates are stronger than sodium, or cure more quickly or at lower temperatures. Trying things like this now they are failures, and rightly so. But this first success encouraged me through the many failures that followed.

Some samples of unknown history from that period have been strong but very light, scratched by fingernail, and extremely dry tasting. Like plaster of paris, I lick it and the dryness sticks to my tongue. This is probably some zeolite configuration rather than the more closed sodalite structure. I like the quality of it, but I don’t imagine it ever being strong in anything other than thick pieces.

My Na-PSs all too often crumble to pieces. Or appear to be cured but are not much stronger than simple dry clay. I did grow some interesting rocks once though. In an attempt to repeat my first experiment, I mixed dry sodium hydroxide and MK750, and then added water. This was frighteningly exothermic, much more so than just adding water to sodium hydroxide, or to NaOH and raw kaolin. I dashed in extra water to try to absorb some of the generated heat. I set the jar of the now over watered experiment down. I came back a couple hours later and found a few pink pumice looking pebbles, the size of a grain of rice, had formed. 24 hours later, there was one piece of pumice the size of a raspberry. (Simultaneously, I did this experiment with raw kaolin, and only 1/2 grain of rice sized pebbles formed, also pumice like. The raw kaolin did not get as hot as the MK750. A third sample in this experiment used MK750 and diatomaceous earth. It turned to goo which did not precipitate rocks.)

Trying this technique again with grog, and comparing sodium to potassium would be informative. Perhaps when it is summer again.

I have not done much with (Na,K)-PS for a couple reasons. One is that they require elevated temperatures to cure. In the literature, they are often cured above 100C under enough pressure to keep the water from boiling (this can be done with a hot press and does not require an autoclave, but still,that is equipment I dont have). The other reason is that they are not as strong as PSS (poly sialate-siloxo and poly sialate (di)siloxo) which use alkali silicate solutions, unless higher temperatures and pressures are used. Thus they tend to be failures more often for me. They are more suited for industrial processing. At some point I may come back to them, but there is no reason I can see to exclude soluable silica from the mix.

The stoichiometric ratio of sodium hydroxide to MK750 is: 80g pure NaOH to 222g MK750. Adding water to this will be scary, so it is best to dissolve the alkali first.