_ Sodium Hydroxide /strong>
Potassium Hydroxide /strong>
Sodium Silicate /strong>
Potassium Silicate _
Alkali refers to sodium or potassium, though lithium and some other rare elements are also in the alkali family. Geopolymerization requires a pH 14 solution, so alkali hydroxide is used. If some alkali salt is to be used, the solution should still be have high pH and the acid in the salt had better be something that is structurally integrated in the final polymer. Silicic acid (hydrated silica) is the obvious choice. phosphoric or boric acid are potential other candidates, but there has been very little exploration of these option published. Phosphorus is often incorporated into zeolites (SAPO-n), but the only author I read mentioned only aluminum phosphate, not sodium phosphate. In solution, isn’t it all just a bunch of free ions anyway? More research is needed.
Silica is de-polymerized by alkali ions. The two can be fused in a furnace to yield a water soluble silicate glass. Alternatively, amorphous silica (powdered glass, diatomaceous earth, opal) can be dissolved in a strong boiling caustic solution. For geopolymers, highly depolymerized silica is desired, which is achieved through a high alkali to silica ratio. I have used a commercial sodium silicate solution (from a ceramics supply shop) of known molar ratio, which I add sodium hydroxide to in order to achieve the degree of (de)polymerization I want.
I use two modified sodium silicate solutions. One theoretically consists of dimers and trimers (2 and 3 unit long polymers) and will cure at room temperature but prefers elevated curing conditions. The other is monomers and dimers, and cures more quickly at room temperature. See the discussion on Na-PSS for details.
Sodium is much cheaper and easier to find than potassium. Soap and bio-desil makers both use sodium or potassium hydroxide, and I have found sodium hydroxide extremely cheaply by asking people what their source is, and then asking that source what their source is. I pay $21 for a 50 lb bag. Most people are paying $42-$60 for 50 lbs. From that same source, potassium CARBONATE is four times that price, indicating that potassium hydroxide would be over $100 per 50 lb if my source even had it.