Geopolymers

Geopolymers

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Geopolymer is a term covering a class of synthetic aluminosilicate
Aluminosilicate
Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals....

 materials with potential use in a number of areas, essentially as a replacement for Portland cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 and for advanced high-tech composites, ceramic applications or as a form of cast stone
Cast stone
Cast stone is defined as “a refined architectural concrete building unit manufactured to simulate natural cut stone, used in unit masonry applications”. In the UK and Europe cast stone is defined as “any material manufactured with aggregate and cementitious binder, intended to resemble in...

. The name Geopolymer was first applied to these materials by Joseph Davidovits
Joseph Davidovits
Joseph Davidovits is a French materials scientist who has posited that the blocks of the Great Pyramid are not carved stone but mostly a form of limestone concrete...

 in the 1970s, although similar materials had been developed in the former Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 since the 1950s, originally under the name "soil cements". However, this name never found widespread usage in the English language, as it is more often applied
Soil cement
Soil cement is a construction material, a mix of pulverized natural soil with small amount of portland cement and water, usually processed in a tumble, compacted to high density...

 to the description of soils which are consolidated with a small amount of Portland cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 to enhance strength and stability. Geopolymer cements are an example of the broader class of alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...

-activated binders, which also includes alkali-activated metallurgical slags
Ground granulated blast furnace slag
Ground-granulated blast-furnace slag is obtained by quenching molten iron slag from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder.-Applications:GGBS is used to make durable concrete structures in combination with ordinary...

 and other related materials.

Research


Much of the drive behind research carried out in academic institutions involves the development of geopolymer cements as a potential large-scale replacement for concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 produced from Portland cement. This is due to geopolymers’ alleged lower carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 production emissions, greater chemical and thermal resistance and better mechanical properties at both ambient and extreme conditions. On the other side, industry has implemented geopolymer binders in advanced high-tech composites and ceramics for heat- and fire-resistant applications, up to 1200 °C. There is some debate as to whether geopolymer cement has lower emissions compared to Portland cement. Calcination of limestone in production of Portland cement is responsible for emissions (one ton of cement produced releases one ton of ), while some processes of formation of lyme also release . Mainly it is the ratio of reduction that is under debate, and it is process-dependent.

Production


Geopolymer binders and geopolymer cements are generally formed by reaction of an aluminosilicate powder with an alkaline silicate solution at roughly ambient conditions. Metakaolin
Metakaolin
Metakaolin is a dehydroxylated form of the clay mineral kaolinite.Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.-Forming...

 is a commonly used starting material for laboratory synthesis of geopolymers, and is generated by thermal activation of kaolinite
Kaolinite
Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...

 clay. Geopolymer cements can also be made from sources of pozzolanic
Pozzolana
Pozzolana, also known as pozzolanic ash , is a fine, sandy volcanic ash. Pozzolanic ash was first discovered and dug in Italy, at Pozzuoli. It was later discovered at a number of other sites as well...

 materials, such as lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 or fly ash
Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...

 from coal. Most studies on geopolymer cements have been carried out using natural or industrial waste sources of metakaolin and other aluminosilicates. Industrial and high-tech applications rely on more expensive and sophisticated siliceous raw materials.

Theory


The majority of the Earth’s crust is made up of Si-Al compounds. Davidovits proposed in 1978 that a single aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

-containing compound, most likely geological in origin, could react in a polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 process with an alkaline solution. The binders created were termed "geopolymers" but, now, the majority of aluminosilicate sources are by-products from organic combustion, such as fly ash from coal burning. These inorganic polymer
Inorganic polymer
Inorganic polymers are polymers with a skeletal structure that does not include carbon atoms. Polymers containing inorganic and organic components are named hybrid polymers. One of the best known examples is polydimethylsiloxane, otherwise known commonly as silicone rubber...

s have a chemical composition somewhat similar to zeolitic materials but exist as amorphous solids, rather than having a crystalline microstructure. Some have alleged that ancient "Roman cement" is a geopolymer cement, but in reality this material is chemically unlike alkali activated geopolymer cements because it is made using lime and forms calcium-silicate-hydrates, making it much closer to Portland cement from a chemical standpoint.

Structure


The chemical reaction that takes place to form geopolymers follows a multi-step process:
  1. Dissolution of Si and Al atoms from the source material due to hydroxide ions in solution,
  2. Reorientation of precursor ions in solution, and
  3. Setting via polycondensation reactions into an inorganic "polymer" (actually a crystalline-like lattice).


The inorganic polymer network is in general a highly-coordinated 3-dimensional aluminosilicate gel
Gel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...

, with the negative charges on tetrahedral Al(III) sites charge-balanced by alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

 cations.

History


Davidovits has proposed that some of the major pyramids
Egyptian pyramids
The Egyptian pyramids are ancient pyramid-shaped masonry structures located in Egypt.There are 138 pyramids discovered in Egypt as of 2008. Most were built as tombs for the country's Pharaohs and their consorts during the Old and Middle Kingdom periods.The earliest known Egyptian pyramids are found...

, rather than being blocks of solid limestone hauled into position, are composed of geopolymers, cast in their final positions in the structure. He also considers that Roman cement
Roman cement
For the architectural material actually used by the ancient Romans, see Roman concrete."Roman cement" is a substance developed by James Parker in the 1780s, and finally patented in 1796...

 and the small artifacts, previously thought to be stone, of the Tiahuanaco
Tiwanaku
Tiwanaku, is an important Pre-Columbian archaeological site in western Bolivia, South America. Tiwanaku is recognized by Andean scholars as one of the most important precursors to the Inca Empire, flourishing as the ritual and administrative capital of a major state power for approximately five...

civilisation were made using knowledge of geopolymer techniques. However, because Roman cement forms calcium-silicate-hydrates, and requires calcined limestone as a reactant/precursor, it is more similar to Portland cement than alkali-activated "geopolymer cements" such as Pyrament cement of LoneStar.

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