This recipe from the repp.org listserv was forwarded to me by LeissKG, one of our readers. Low cost, readily available materials vary from place to place so it’s good to have numerous recipes to choose from.
Shawn Hicks wrote : George Monbiot in his book Heat talks about geopolymeric cements being made from readily available materials (including wastes) and being produced at a much lower temperature than other cements. My local concrete folks have never heard of them. Does anyone have any experience with these ?
Chris Green replied : Until recently I have a history of being highly skeptical of the claims made by the people who are promoting this stuff. However, Prasad Boudreaux, a member of the Organic Architecture Yahoo Group who lives and works in Oregon, has been experimenting with geopolymer cements, so my skepticism is being tempered a bit.
Prasad was kind enough to send along a recipe he worked out for geopolymer cement to the Organic Architecture list last month. I haven’t tried this yet, but here is an edited copy of his recipe post for those who want to try making some.
Safety First : When using lye, be sure to wear eye protection and gloves.
For anyone who might be interested, here is a starting point for a simple geopolymer recipe. In Joy, Prasad
Geopolymer cement notes.
The making of alkaline solution :
*12 hr before mixing, slowly ! dissolve 320gm sodium hydroxide (pure lye, as in a drain cleaner) into a liter of water. This should be stirred in slowly, with care, wearing gloves and goggles as it is very caustic. This mix will generate some heat while dissolving.
*After the lye solution is fully dissolved (12hr) mix one part lye solution with 2 1/2 parts sodium silicate. (available at pottery supplies)
* Basic recipe #8
4 1/2 parts metakaolin [Ed. : heat treated kaolin]
1/2 part lime (type-S)
8 parts aggregate (sand mix)
alkaline solution as needed (about 1/3 the amount of metakaolin and ash, by weight)
*Mix all the dry ingredients together then stir in just enough alkaline solution to make a stiff mix. Keep the liquid content as low as possible. Cure like concrete, warm and moist.
*Class C fly ash can replace the metakaolin and lime, if its type F fly ash replace only the metakaolin.
Geopolymer concrete negates the need for Portland cement as a binder. Instead, materials such as fly ash are activated by alkaline liquids (most often at temperatures below 150 degrees Celsius) to create the cement. Hence concrete can be produced without the need for large quantities of fuel, making it far more energy efficient, and avoids much of the environmental pollutants associated with traditional Portland cement production.