Geopolymers and Roman Concrete are two ancient technologies that have been rediscovered in recent times. Roman Concrete is something that has lasted for over two thousand years ever since the time when Jesus Christ walked the earth. Geopolymers have been around even longer since it is claimed the Ancient Egyptians used the technology to cast the stones used to build the Pyramids. Both civilizations had the materials and the skills to use either technology.
These technologies share some common materials, and practices, but widely differ in the materials they use. Roman Cement and Concrete are actually derived from the ancient practice of whitewashing rammed earth structures to protect them from the rain. The hydrated lime in the whitewash combined with the materials found in the rammed earth wall. The Egyptian invention depended on their use of Natron, sodium carbonate to preserve their dead.
Roman Concrete structures have lasted for over 2,000 years whereas modern concrete structures are falling apart in as little as 50 years. What is the difference between the two concretes, and why has the Roman concrete lasted so long? Chemically their compositions are quite similar yet they are so different in quality. Once again we have to ask why?
To make cement the Romans combined two simple ingredients; hydrated lime that was made by burning lime leaving behind calcium oxide or quicklime that has been slacked in water. To this they added volcanic ash of which they had in plenty. Red ash came from near Rome, and a more yellowish/gray ash came from near Naples.
If you mix there two ingredients together with additional aggregate you form concrete. The Romans didn’t have cement mixers like we do today, but they knew how to mix their concrete in mortar tubs.
The concrete was mixed using very little water so it was delivered to the jobsite as a non-slumping concrete that was rammed in place using several different tools to assure that all the spaces in the concrete were filled in, and the concrete was compacted. This technique left no open spaces in their concrete that allowed it to be penetrated by water or salt.
Geopolymers are another matter and here they deviate from Roman Cement in their choice of materials. The active ingredient in Geopolymers was sodium silicate that can be made from natron by heating it to drive off the carbon dioxide leaving behind sodium oxide that if it is added to water forms a solution of sodium hydroxide and gives off a lot of heat. By adding quartz sand to this solution the sand is dissolved by the sodium hydroxide leaving a solution of sodium silicate.
If you add finely ground alumino-silicate minerals like those found in granite you wind up with a superior material with cement like characteristics. You can use volcanic ash as well to form Geopolymers. If you fill the Geopolymer with a suitable aggregate filler it will set like regular concrete.
Both of these materials use a special material that is called pozzelanic that is an alumino-silicate that has been affected by high heat. One example of this is volcanic ash, but other pozzelanic materials are found in finely ground glass or ceramics including bricks.