Dr. T.Ch.Madhavi, Professor and Head, Rameshwaran P.M, Asst Prof (Sr Grade), Department of Civil Engineering, SRM Institute of Science and Technology, Ramapuram

Geopolymer Concrete - The Eco Friendly Alternate to Concrete

What is Geopolymer Concrete?
The name geopolymer was given by “Joseph Davidovits” in 1978. Geopolymer concrete (GPC) is an eco friendly product which uses industrial waste by-products such as fly ash (waste from thermal power plants) and ground granulated Blast Furnace Slag (waste from Iron production) as complete replacement for cement in concrete. As a result of this geopolymer concrete reduces CO2 emissions by 80%. Geopolymer is gaining importance and acceptance as it ensures sustainability. Fly ash and GGBS are rich sources of silcon and aluminium which are polymerized by alkali activating solution to form molecular chains and networks to create hardened binder.

Why Geopolymer concrete?
Ordinary Portland Cement, results from the calcination of limestone (calcium carbonate) at very high temperatures of about 1450-1500°C, and silico-aluminous material as given in the equation below.

5CaCO3 + 2SiO2 —> (3CaO,SiO2) + (2CaO,SiO2) + 5CO2

which means that, production of every 1 metric tonne of cement generates 1 metric tonne of CO2.With increasing consumption of cement, the world’s atmosphere gets destroyed. Cement industry is one of the worst source of atmospheric pollution than any other industry. Thus the necessity for an alternative material arose and soon Geopolymer emerged as an alternative material which is eco friendly with reduced carbon dioxide emissions.

What are the Advantages of Geopolymer Concrete?
The main benefit of geopolymeric cement is the reductions in carbon dioxide emission since the chemical process emits zero carbon dioxide, and the fuel much less, resulting in reduction of carbon dioxide emissions by 80% to 90%. The other benefits are shown in fig 1.

Advantages of geopolymer concreteFig 1 - Advantages of geopolymer concrete

Applications of Geopolymer Concrete
Lot of research has been carried out on in the recent past on Geopolymer Concrete as an alternative to conventional cement concrete. Geopolymer concrete has high potential due to its enhanced durability, chemical and thermal resistance to heat and early age strength. Some of the applications are
  • Pavements
  • Sewer piper
  • Marine members
Recently world’s first building with geopolymer concrete has been constructed at, the University of Queensland’s Global Change Institute (GCI). It is a four storey high building. 33% panels used in the GCI’s floor plates are made of GPC.

Advantages of geopolymer concreteFig 2 - University of Queensland’s Global Change Institute (GCI)

The Brisbane West Wellcamp Airport is a significant milestone in civil engineering. It is the world’s largest geopolymer concrete project and was built with about 40,000 m3 (100,000 tonnes) of geopolymer concrete, making it the largest application in the world and saved 6,600 tonnes of carbon emissions in the construction of airport.

The geopolymer concrete is developed by the company Wagners, Heavy duty geopolymer concrete, 435 mm thick, is used for the turning node, apron and taxiway aircraft pavements, which welcomes a heavy 747 cargo for regular air traffic between Toowoomba-Wellcamp BWWA airport and Hong Kong.

Geopolymer

Composition of Geopolymer Concrete
Production of Geoploymer concrete requires great care and proper composition of materials. The formulation of the GPC mixtures requires systematic numerous investigations. Geopolymer concrete is produced using Flyash, GGBS, Fine aggregate and coarse aggregate and Alkaline activator Solution. The two main constituents of geopolymer are 1) The source materials which are rich in Silicon (Si) and Aluminium (Al) such as Fly Ash, Rice husk, Silica Fume, GGBS etc and 2) the alkaline liquids. Strength of GPC is affected by the curing time and temperature. The same sand and coarse aggregate as used in normal concrete can be used in the production of GPC.

Alkaline Activator Solution
Catalytic liquid is used as alkaline activator solution. It is combination of solutions of Sodium or potassium silicates and hydroxides or a mixture of those which are soluble in water. The role of this alkaline solution is to activate Fly and GGBS. The highly alkaline solution should be handled carefully considering the safety of the user.

Mix design
Development of mix design for geopolymer concrete is difficult compared to OPC concrete because of the various parameter involved. The factors that effect the mix design of GPC are
  • Ratio of liquid to solid ratio
  • SiO2/Al2O3
  • Na2O/Al2O3 ratio for flyash
  • Water content
  • Curing temperature
  • Molarity of NaOH
Increase in the ratio of Na2SiO3/ NaOH by mass results in higher compressive strength. Workability can be improved by adding superplasticizer upto 4% of fly ash content by mass. The concentration of NaOH solution can be from 16 to 8 Molar. Higher concentration of NaOH gives higher strength (Wallah et al, 2005).

Production of Geopolymer Concrete
Production of Geopolymer concreteFig 5 - Production of Geopolymer concrete
The geopolymer paste is produced by the reaction between the alkali activator with the Al2O3 and SiO2 of fly ash. The performance of geopolymer concrete depends on the coupling of the above two main constituents because of the chemistry that occurs between the two. For example, Sodium Silicate solution gives better activation to GGBS than other activators while Sodium Hydroxide gives better activation of fly ash (Fareed et al., 2011). Experimental investigations showed that high early age strength can be obtained with mass ratio of SiO2 / Na2O = 0.75 of Sodium Silicate solution. (Bakharev et al., 1999a,b). The ingredients of geopolymer concrete can be mixed in mixers used for conventional concretes such as pan mixer, drum mixer, etc. a conceptual diagram is given in fig 5 to depict the production of geopolymer concrete.

Geopolymerisation
The reaction between alkali sources and precursors is called as geopolymerisation. The geopolymerisation can be explained as follows (Duxson et al. 2007)
  • Now, the aluminates and silicates react together to form an alumino silicate gel which is initially formed as an aluminium rich gel since the aluminum is more reactive and dissolves faster than silicon. At a later stage as more silicon dissolves, the gel structures gets reorginsed to form zeolite gel which is more stable than the previous gel since Si –O bonds are more stronger than Al-O bonds.
  • This reorganization process continues and results in crystallized zeolite formation. Thus, the gel forms into a solid mass similar to the hydration of OPC.
Conceptual model of geopolymerisationFig 6 - Conceptual model of geopolymerisation

Workability
The workability of GPC depends on the ratio of Na2SiO3 to NaOH and concentration of NaOH. Bleeding is lower in GPC than OPC concrete. Workability can be improved by adding Napthalene based superplasticizer from 2 to 4%. For flyash the workability increased with decreased molarity of NaoH. NaOH as alkaline activator alone, without sodium hydroxide, can significantly reduce the slump value of the geopolymer concrete.

Setting time
The quantity of Na in Na2SiO3 has significant affect on the setting time of GPC. The setting time of GPC can be reduced by increasing the content of GGBS. Use of GGBS with fly ash has significant affect on the setting time.

Mechanical Properties of Geopolymer Concrete
Geopolymer concrete has similar and sometimes superior properties than cement concrete. The mechanical strength of geopolymer concrete is affected by the nature of alkali activator Concentration of the solution and the curing temperature. Heat curing of GPC improves the geopolymerisation and subsequently its mechanical properties. For slag based GPC, heat curing accelerates strength gain at early ages. But, at later age the strength is lower than the specimens cured at ambient temperature. This is because the fast reaction, localization of reaction product occurs near slag grains which form barriers for further reactions resulting in slow strength at later ages. Hence heat curing is not essential for slag based GPC. Higher molarity of NaOH results in higher compressive strength at early age because of the increase in the geopolymerisation reaction. The mechanical properties are listed in the table below.

Table 1 Mechanical properties of GPC compared to OPC concrete
S.No Property GPC
1 24 hr compressive strength 25 to 35 MPa
2 28 days compressive strength Upto 70 MPa
3 Rate of gain in strength Faster than normal concrete
4 Modulus of Elasticity Marginally lower than normal concrete
5 Porosity Low
6 Chloride Penetration Low or Very low as per ASTM 1202C standard
7 Drying Shrinkage Low
8 Heat of Hydration Low
9 Fire resistance High
10 Acid Resistance High
11 Geopolymer concrete beams At service loads behaves similar to cement concrete beams
12 Geopolymer concrete columns Exhibit failure modes and Crack patterns similar to cement concrete columns.

Micro structure of Geopolymer concrete
Geopolymer concrete has denser microstructure than normal cement concrete. The C-A-S-H matrix chains in GPC are longer than the C-S-H chains in OPC concrete. Fig 7 (a) shows the SEM image of flyash which shows different sizes of spherical vitreous particles. Fig 7b) shows the micro structure of flyash activated with alkaline solution and 7(c) shows the microstructure of fly ash activated with sodium silicate solution.

Conceptual model of geopolymerisationFig 7 - SEM Images of Fly ash based GPC (Keane et al., 2006; Mehta and Monteiro, 2006;van Deventer et al., 2015).

Conceptual model of geopolymerisationFig 8 - Pore size distribution of GPC and OPC at 28 days age (Aligizaki, 2006).
The microstructure analysis of pore size distribution using mercury porosimetry and gas adsorption techniques revealed that slag based GPC showed more mesopores, with pore size less than 50nm compared to normal concrete. GPC thus showed lower porosity than normal concrete. A graph showing comparison of pore size distribution in GPP and OPC paste is shown in Fig 8 below. Most of the pores in OPC paste range from 10 to 100 nm. But, most of the pores in GP paste are less than 20 nm. (Collins and Sanjayan, 2000; Garboczi, 1990; H€akkinen, 1993a).

Concluding Remarks
Geopolymer concrete exhibits good strength and durability properties than OPC concrete. Geopolymer concrete is a potential material for future because it is not only eco friendly but also possess good strength and durability properties. Application of geopolymer concrete in precast elements has high potential and needs to be explored. Though it is accepted that geopolymer concrete is a powerful alternative material and as a sustainable concrete, its application to structural members has not yet gained wide acceptance because of lack of proper structural design standards and codes. Development of standard code for geopolymner concrete is the need of the day. Further research is required on long term behaviour and durability of geopolymer concrete.

References
  1. Amer Hassan, Mohammed Arif , M. Shariq, Use of geopolymer concrete for a cleaner and sustainable environment-A review of mechanical properties and microstructure, Journal of Cleaner Production, 223 (2019) 704-728.
  2. Aligizaki, K.K., 2006. Pore Structure of Cement-Based Materials. Taylor & Franics, New York.
  3. Bakharev, T., Sanjayan, J.G., Cheng, Y.-B., 1999a. Alkali activation of Australian slag. cements. Cement Concrete Research 29, 113-120.
  4. Collins, F., Sanjayan, J.G., 2000. Effect of pore size distribution on drying shrinking of alkali-activated slag concrete. Cement Concr. Res. 30, 1401-1406.
  5. Duxson, P., et al., 2007b. Geopolymer technology: the current state of the art, Journal of Material Science. 42, 2917 - 2933.
  6. Fareed, Fadhil, Nasir, M., 2011. Compressive strength and workability characteristics of low-calcium fly ash-based self-compacting geopolymer concrete. International Journal of Civil, Environ. Struct. Constr. Archit. Eng. 5, 64-70.
  7. Mehta, P.K., Monteiro, P.J.M., 2006. Concrete : Microstructure, Properties, and Materials. McGraw-Hill, New York.
  8. Wallah, S.E., Hardjito, D.S.D.M.J., R.B.V.,2005. Performance of fly ash-based geopolymer concrete under sulphate and acid exposure’. Geopolymer Proc 153-156.

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