Geopolymers — PART I, The Introduction
by Mallory A. Westbrook
What are Geopoylmers?
Although geopolymers have been around for centuries, they have been gaining traction in the construction industry over the last 30 or so years. So what exactly are geopolymers?
Geopolymers are a construction material that do not require the parent materials that make up the calcium-silicate-hydrate gel (found in Portland Cement Concrete, PCC) but utilize silica and alumina to achieve superior strength. The modern pioneer of geopolymers, Joseph Davidovits, defines geopolymers as “amorphous to semi-crystalline three-dimensional silico-aluminate materials.”
Geopolymers can provide comparable performance to traditional cementitious binders with significantly reduced greenhouse effects. By utilizing varying waste materials, such as fly ash, geopolymers can exhibit a wide variety of properties including high compressive strength, low shrinkage, fast or slow setting, acid resistance, fire resistance and low thermal
conductivity.
How are Geopolymers Made?
Geopolymers are usually synthesized using an aluminosilicate raw material and an activating solution which is mainly composed of alkalis of sodium or potassium and water glass (FHWA 2010).
Fly ash is the most used waste material in geopolymers as it is estimated that around 780 million tons are produced annually and its great workability. Fly ash is composed mainly of amorphous silica and alumina with a favorable shape and size that improve the workability and make this material suitable for geopolymer production.
Recently, different studies have evaluated utilizing additional waste materials mixed with fly ash to create geopolymers. These materials include but are not limited to (Davidovitts 2003):
- Granulated blast furnace slag has been utilized to create fire resistant geopolymers. It has been found that the addition of blast furnace slag accelerates setting time and enhances compressive and flexural strength.
- Lateritic earth clay is being used to create geopolymer bricks for consturction. Compared to a traditional brick fired at 1000°C in a kiln,
the lateric earth clay brick needs about eight times less energy for an equivalent strength and is significantly less expensive to produce. These bricks achieve the compressive strength required to build walls and consume less energy than a traditional brick.
The choice of the materials to produce geopolymers depends on availability and the alumina and silicate content.
So how do we know the ideal material to produce a geopolymer that achieves the desired featrues and benefits? The second part of this series will explore the balance between alumina and silicate in geopolymerization.
