Calcium Geopolymers

A mixture of quicklime and commercial sodium silicate sets almost instantly. The reason for this is the precipitation of calcium (poly?) silicate. Free sodium hydroxide is also formed. A mature sodium geopolymer resin cosists of sodium silicate, sodium aluminate and sodium poly-aluminosilicates. Therefore, quicklime would also cause a setting of this resin through the formation of calcium silicates, calcium aluminates and calcium poly-aluminosilicates.

According the Breck (”Zeolite Molecular Sieves”), in the calcium-alumina-silica system: “although hydrous aluminosilicate gels have been employed as reactants, significant crystalization of a zeolite phase does not occur below 225C. Most successful experiments were conducted in the temperature range of 300C-400C” at autogenous pressure in an autoclave. (Breck also reports that no Magnesium zeolite phases have been synthesized under any conditions, though he does not discuss the sodium-magnesium system). In my own experience, lime and Mk750 alone was not significantly stronger than lime with an inert aggregate.

The addition of sodium however allows for ion exchange and precipitation routes which do result in zeolite phases. This has been my experience, without quantitative data. In my understanding, to a stoiciometric Na-PS or Na-PSS precondensate resin, one would add calcium hydroxide and enough Mk750 to absorb the free lye released by the ion exchange of calcium for sodium. The weight ratio of Ca(OH)2:NaOH:(MK750 or activated kaolin) would be 74:80:(444 or 516).

Rather than trying the above ratio, I made a Na-PS paste and mixed it 10% by volume into a lime paste. The mixture set very quickly, within 15 minutes, and though I had mixed it as I would mix an hydraulic portland cement mixture, the Ca-geopolymer quickly sequestered the free water and was thirsty for more. This is the behavior of an hydraulic cement, which neither lime nor Na-PS are on their own. The resultant block was much harder (more difficult to scratch) than lime mortar, and set when Na-PS alone would not. It was decently strong, requiring solid effort to break off marble sized pimples.

This water starved specimin was the only Ca Geopolymer I made, being more interested in the Na-PSS. I have not experimented with optimizing this system and I do not know if my thinking behind the proportions or mechanisms is accurate. This should have been clear evidence for me to be more careful in using wollastonite and silica flour interchangeably, though I did not heed it.