Geopolymer Concrete
Smita Singh, Associate Professor, Amrita School of Engineering, Bangalore
Dr.Aswath M U, Professor, BIT, Bangalore
Dr. R.V.Ranganath, Principal, BMSIT, Bangalore
Geopolymer concrete has emerged as a feasible alternative to ordinary Portland cement concrete. The term "geopolymer" was first introduced by Davidovits in 1978. "Geopolymer" describes a family of mineral binders that have a polymeric silicon-oxygen-aluminium framework structure, similar to that found in zeolites. These binders could be produced by a polymeric reaction of alkaline liquids with the silicon and the aluminium in source materials of geological origin or by-product materials such as fly ash, metakaolin, GGBS, red mud and rice husk ash. It has been found that the ancient buildings like the Egyptian Pyramids and the Roman Coliseum were built from concrete which resembles the geopolymer concrete of today. In the recent years there has been a dramatic improvement in understanding the reaction mechanism and property development of geopolymer concrete and it can be used in many construction applications.
Chemical composition and polymerization of geopolymer cement
The polymerisation process involves a substantially fast chemical reaction under alkaline condition on Si-Al minerals, which results in a three dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds, as follows:
Mn [-(SiO2) z-AlO2] n . wH2O
Where: M = the alkaline element or cat-ion such as potassium, sodium or calcium; the symbol – indicates the presence of a bond, n is the degree of polycondensation or polymerisation; z is 1, 2, 3, or higher.
The schematic formation of geopolymer material can be shown as described by Equations (A) and (B), fig 1.
Fig 1: Schematic Diagram of Geopolymerisation
To date, the exact mechanism of setting and hardening of the geopolymer material is not clear.
The chemical reaction may comprise the following steps:
- Dissolution of Si and Al atoms from the source material through the action of hydroxide ions.
- Transportation or orientation or condensation of precursor ions into monomers.
- Setting or polycondensation / polymerisation of monomers into polymeric structures.
However, these three steps can overlap with each other and occur almost simultaneously, thus making it difficult to isolate and examine each of them separately.
Fig 2: Geopolymeric Structure Showing Al-O-Si Bonds
A geopolymer can take one of the three basic forms, Fig.3.
- Poly (sialate), which has [-Si-O-Al-O-] as the repeating unit.
- Poly (sialate-siloxo), which has [-Si-O-Al-O-Si-O-] as the repeating unit.
- Poly (sialate-disiloxo), which has [-Si-O-Al-O-Si-O-Si-O-] as the repeating unit. Sialate is an abbreviation of silicon-oxo-aluminate.
Fig 3 Three basic forms of geopolymer
Advantages
Use of geopolymer concrete offers many advantages over Portland cement concrete. Apart from the alkalies, the base material used in the synthesis of geopolymer concrete is mostly an industrial waste which would ultimately be dumped, causing hazard to the environment. Some hazardous minerals like uranium, radium, arsenic and lead are present in the industrial wastes. Geopolymer provides a solid matrix for binding such hazardous minerals. The pollutants get locked into the three dimensional geopolymeric-zeolitic framework. Also, the CO2 emission is lesser than that of the conventional Portland cement. The production of 1 tone of Geopolymeric cement generates 0.180 tonnes of CO2, from combustion carbon-fuel, compared with 1tonnes of CO2 for Portland cement [1]. Hence, geopolymer concrete is rightly called a "green concrete".
Depending on the raw material used, geopolymer concrete either exhibits a higher strength or a comparable strength to the conventional concrete. Also, the rate of gain of strength is faster, which in turn reduces the construction time. Fly ash based geopolymer concrete cured at 60oC for 24 hrs exhibited a strength of 55-60oC [2]. Other advantages of using geopolymer concrete are [3]:
- Long-term durability.
- Unique high temperature properties.
- Fire resistant: Geopolymer can withstand 10000C to 12000C without losing its functionality
But the main advantage of geopolymerisation over OPC technology is its ability to consume high volume industrial wastes and transform them into high performance concrete.
Factors Affecting Geopolymerisation
Fig 4 Polymeric Structures from Polymerization of Monomers
(i) Curing temperature
Pozzolanic reactions are accelerated by temperature increase. Viscosity of the geopolymer mix in wet state indirectly indicates the geopolymer's compressive strength. With increase in temperature from 45oC to 65oC, the viscosity of the mix increases 5 times and with increase in temperature from 65oC 85oC, the viscosity increases 10 times. Temperature is especially important during 2- 4 hours of curing.
(ii) Curing Time
Fig 5: Cross-Section of a Geopolymer Concrete Pavement
Increase in curing time increases the degree of polymerisation and consequently the strength. However, increase in strength for curing periods beyond 48 hours is not very significant. The initial increase in temperature favours the dissolution of reactive species. On curing at elevated temperature for a longer period of time, the granular structure of the geopolymer mix is broken. This results in decrease of strength.
(iii) Silicate and hydroxide ratio
Fig 6: Geopolymer Concrete Retaining Wall for a Private Residence
Increase in the M2O/SiO2 ratio (M=Na/K/metallic ions) increases the concentration of alkali or decrease in the added silicate. Excess of sodium silicate hinders the water evaporation and structure formation. Hence the increase in M2O/SiO2 ratio increases the compressive strength. Use of K+ is beneficial over Na+. K+ is more basic and larger in size, hence it allows higher rate of solubilized polymeric ionisation and dissolution resulting in denser polycondensation reaction.
(iv) Alkali Concentration
Fig 7: Water Tank Made of Geopolymer Concrete
Solubility of aluminosilicate increases with the increase in hydroxide concentration. Hence, increase in the alkali concentration increases the compressive strength up to an optimum limit. Higher alkalinity of the hydration water slows down the rate of hydration due to the presence of excess OH-1 ions in the system. It is reported that addition of 5% NaOH tends to increase the strength of the binder at early age (less than 7 days) but the strength decreases at later age. This may be due to the excessive NaOH that resulted in undesirable morpholology and non-uniformity of hydration products in the pastes, thereby reducing the binder strength.
(v) Silicate and Aluminate ratio
Strength increases as SiO2/Al2O3 ratio increases. Geopolymers with SiO2/Al2O3 ratio in the range of 3.16-3.46 have better strength. Long precuring before heat treatment narrows this range in the alumina-silicate gel. Precuring is the rest period after thermal curing and before testing.
(vi) Aluminium and Silica source
Geopolymer containing clays (kaolin and metakaolin) were found to be the strongest under compressive strength testing while fly ash lacked compressive strength alone.
(vii) Age of Concrete:
Geopolymer gains about 70% of its strength in the first 3-4 hours of curing. Strength of concrete does not vary significantly with age of concrete when cured for 24 hours.
Strength of Geopolymer Concrete using different base materials
[5] Properties of geopolymer concrete vary with the constituent materials. Kaolin or metakaolin based geopolymer binders give the maximum strength, but fly ash is predominantly used for the synthesis of geopolymer as it is an industrial waste.
Strength of fly ash geopolymer concrete cured at 60oC for 24 hours is 55 Mpa in steam curing and 65 Mpa in dry curing. The molarity of the alkali used is 10M. Test cylinders casted from the same mix exhibited mean 7th day compressive strength of 35 Mpa and 28th day strength of 47 Mpa. The heat-cured specimens did not have increase of strength with age.
Red mud has also been utilized in the synthesis of geopolymer concrete. The strength of the red mud geopolymer concrete is very low. Hence red mud is mostly used along with fly ash, GGBS or metakaolin for the synthesis of geopolymer concrete. It is reported that the strength of fly ash geopolymer concrete increases specially at ambient temperature if GGBS is incorporated to it. Incorporation of rice husk ash [6] results in higher geopolymeric matrix quality with denser gel. Denser gel structure causes greater connectivity in the load distribution hence the load carrying capacity of the concrete also increases.
Field Applications
Fig 8: Precast Geoplymer Concrete Bridge Deck
The characteristic of geopolymer concrete to cure faster at high temperatures has enabled them to be used efficiently in the precast industry. It is widely used in many structural applications worldwide.
(i)Pavements
Fig 5 shows a pavement, 900 metre long and 5.5 metres wide using geopolymer concrete of Grades 25 Mpa and 40 Mpa. Geopolymer has also been used in the construction of footpaths.
(ii)Retaining Wall
Precast panels of size 6 meters long and 2.4 meters wide were used for construction of retaining wall for a private residence, Fig 6. The 40 MPa panels were cured under ambient temperature and designed to support earth pressure of 3 meters.
(iii)Water Tank
Fig 7 shows water tank constructed with 32 Mpa geopolymer concrete using 10 mm down size coarse aggregates.
(iv)Precast Bridge Decks
One of the earliest applications of geopolymer concrete was the Murraie Plant site bridge in Australia. It is a composite bridge structure where the bridge deck was casted from 40 Mpa geopolymer concrete, fig 8.
(v)Precast Beams
Precast beams of strength 40 Mpa were used to design the Global Change institute in Queensland, Australia. These beams were not only used as structural floor elements but were also a key architectural feature of the building with their arched curved soffit, fig 9
Limitations
Fig 9: Precast Geopolymer Concrete Beam of a Multi-Storeyed Building
The advent of geopolymer concrete marks a new era in the construction field. This innovative material is not only faster but also utilizes wastes materials. There are certain limitations to the adoption of this technology. Firstly the synthesis of geopolymer concrete requires some standard laboratory conditions which are difficult to replicate in the field. Use in precast construction industry is a more feasible option. Secondly, the cost of producing it depends on the nearness to the material source location. Lastly the technology is yet to be developed in terms of codal provisions and standard methods. Much work has been done using fly ash. Still there is a lot scope for research using other alumina-silicate materials.
Conclusion
- Despite a few limitations, the geopolymer concrete is a greener and cost effective replacement of the conventional Portland cement. The limitation in its use is mostly due to the gap between the scientific researches and the actual application of these researches in the construction world. This limitation can be overcome by systematic and thorough researches.
- Geopolymer concrete is comparable or better than the Portland cement with respect to mechanical properties
- It possesses excellent properties in both acidic and alkaline environments.
- Geopolymer concrete has high fire resistance and low thermal conductivity.
References
- Prof. Dr. Joseph Davidovits (October 28-29, 2002), Geopolymer 2002 Conference, Melbourne, Australia Geopolymer Institute
- Wang, J.W., and Cheng, T.W.(2003). "Production geopolymer materials by coal fly ash." Proceedings of the 7th International Symposium on East Asian Resources Recycling Technology, Tainan, Taiwan.
- Divya Khale and Rubina Chaudhary, Mechanism of geopolymerization and factors influencing its development: a review, Journal of Material Science, Jan 2007
- James Aldred and John Day (29-31 August 2002), "Is geopolymer concrete a suitable alternative to traditional concrete", 37th conference on our world in concrete & structures, Singapore.
- B. Vijaya Rangan, Djwantoro Hardjito, Steenie E. Wallah, and Dody M.J. Sumajouw,"Studies on fly ash-based geopolymer concrete, Geopolymer: green chemistry and sustainable development solutions", Faculty of Engineering and Computing, Curtin University of Technology, Perth, Australia.
- Andri Kusbiantoro, Muhd Fadhil Nuruddin, Nasir Shaiq, Sobia Anwar Qaz, "The effect of microwave incinerated rice husk ash on the compressive and bond strength of ?y ash based geopolymer concrete", construction and building materials, 36(2012) 695-703.