Geopolymer Concrete: Leading the World Towards a Sustainable Future

Concrete is the most widely used construction material after water. It requires large amount of OPC as binder material, but production of OPC involves huge energy consumption, destruction of natural resources and emission of large quantities of green house and pollutant gases like CO2 and NOx. In order to significantly reduce CO2 emissions (which are major contributor to global warming and climate change) by cement industry, we need an eco-binder which can partially or fully replace OPC in concrete. Geopolymer technology has been proved to be promising one in this context. This paper describes test results obtained on large number of Geopolymer concrete units by various researchers around the world and illustrates methods adopted for preparation, mixing, curing of ecoconcrete, mechanical properties of GPC, and other useful properties like shrinkage, creep, fire and chemical resistance. Basic Properties of Geopolymer Concrete and OPC concrete based on test results are being compared. Economic benefits, recent developments and applications of geopolymer concrete are also discussed.


INTRODUCTION
The global problems associated with us in today's world like environmental pollution, global warming (and hence climate change) are threats to sustainable future of this planet. Global warming is caused by emissions of green house gases like methane, carbon-dioxide (CO2) to the atmosphere. Contribution of CO2 is appreciable accounting for about 65% of global warming. Concrete is most widely used construction material after water, and conventionally it is produced using OPC as primary binder material. So with increasing demand and usage of concrete, production of OPC also demands an increase. Moreover in manufacturing of cement CO2 is released. It has been estimated that 1 ton of OPC production emits about 1 ton of CO2 and world cement production generates 2.8 billion ton man-made greenhouse gases annually. Efforts are being made to replace OPC partially or fully as binder material in concrete so as to reduce carbon emissions into atmosphere. An achievement in this regard is the development of Geopolymer technology which utilizes wastes from various industries like fly ash, blast furnace slag, rice husk ash, silica fume etc. with alkaline medium to replace cement in concrete. This novel eco-friendly technology using geopolymer as eco-binder is considered to be promising in reducing CO2 emissions caused by cement industries.

GEOPOLYMERS
The term "Geopolymer" was coined by a French materials scientist named Prof. Joseph Davidovits in 1978. He proposed that an alkaline liquid could be used to react with Silicon (Si) and Aluminium (Al) present in a source material of geological origin or by-products such as fly ash, blast furnace slag, and rice husk ash to produce binders. These are essentially inorganic alumino-silicate polymers synthesized from a (fast) chemical process called "Polymerization." That is why Davidovits called them geopolymers.

CONSTITIUENTS OF GEOPOLYMER
There are two main constituents of geopolymer namely, the source material and the alkaline liquid. Source material should be rich in Si and Al. These could be natural material like kaolinite or alternatively by-products such as fly ash, blast furnace slag, rice husk ash etc. Several minerals and industrial wastes have been investigated in past as source materials. Metakaolin (Davidovits 1999 The alkaline liquids are from soluble alkali metals generally sodium (Na) and potassium (K). The alkaline liquids are formed from the combinations of sodium hydroxide (NaOH) and sodium silicate or potassium hydroxide (KOH) and potassium silicate. Most commonly used alkaline activator is combination of NaOH and Na2SO3. Generally NaOH is available in the market in pellets or flakes form with 96% to 98% purity. Solution of NaOH is formed by dissolving these pellets in water to obtain a solution of particular molarity. It is strongly recommended that the sodium hydroxide solution must be prepared 24 hours prior to use and also if it exceeds 36 hours it terminate to semi solid state (R Anuradha et al. 2011), So the prepared solution should be used within this time. Sodium silicate is available in gel form in market. It is also diluted with water and then mixed with NaOH to form alkaline activator liquid.  (Satpute et al. 2012). Mane and Jadhav (2012) observed that even when exposed to high temperature of 500 0 C geopolymer specimen show less reduction (29%) in the capacity than that for OPC (36%). In general GPC has good fire resistance compared to OPC when exposed to more than 800 0 C ( Zhao and Sanjayan 2011).

Chemical Resistance
Durability of concrete structures is very important property which affects service life of structures. Penetration of aggressive substances may damage concrete and corrode reinforcement inside it. GPC has been tested under many aggressive environments, and has proved to have excellent resistance against chemicals like Sulphates, chlorides, and acids. GPC can be used to build structures exposed to marine conditions (Reddy et al. 2011). Wallah and Rangan (2006) studied the effect of immersing low calcium fly ash GPC in 5% sodium sulphate solution for different time durations up to 1 year and the sulfate resistance was evaluated based on visual appearance, change in length, change in mass, and change in compressive strength. On visual appearance it was observed that there was no sign of surface erosion, cracking or spalling of specimen, change in length was extremely small and less than 0.015%, and there was a slight increase (1.5%) in the mass of specimens due to the absorption of the exposed liquid after one year. Sanni and Khadiranaiker (2012) showed that GPC lost only 15% of its compressive strength on an average compared with 25% for OPC. Acid resistance of fly ash-based geopolymer concrete has been studied by soaking concrete in various concentrations of sulfuric acid solution up to one year, and by evaluating the behaviour in terms of visual appearance, change in mass and change in compressive strength after exposure (Wallah and Rangan 2006). It was seen that specimens exposed to sulfuric acid undergoes erosion of the surface. The damage to the surface of the specimens increased as the concentration of the acid solution increased. The compressive strength decreased about 20% after one year exposure, concentration and time of exposure influenced it . By exposing to 5% sulfuric acid and hydrochloric acid, Davidovits (1994) reported that geopolymeric cements remained stable in acidic environment with mass loss in the range of 5-8%, compared to 30 to 60% mass loss of calcium-aluminate cement.

BOND STRENGTH
Even though GPC has higher tensile strength compared with OPC, its structural performance still depends on the bonding between concrete and steel bars. Bonding strength between the reinforcement and surrounding concrete is an essential factor to examine the structural performance of the material. D B Raijiwal and H S Patil (2011) concluded that In Pull Out test, GPC increases over controlled concrete by 1.5 times. GPC shows higher bond strength to the reinforcement because of its higher tensile strength (Sarker2010; Sarker 2011).

ECONOMIC BENEFITS OF USING GPC
Use of fly ash, slag, rice husk ash, and GGBS which are by-products of industries enhances economic benefits of GPC.  This makes GPC cheaper than Portland cement in terms of the materials cost. After allowing for the price of alkaline liquids needed to the make the geopolymer concrete it is 10 -25% cheaper than that of portland cement concrete.  Nearly 1 ton fly ash is utilized for 2.5 m 3 of GPC thus cutting the world's carbon.  1 ton fly ash or GGBS earns one carbon-credit and hence earn monetary benefits through carbon-credit trade.  Hectares of land which would otherwise be required for dumping of industrial wastes can be saved now. In addition to the lower price of the production of GPC, its superior properties in shrinkage, creep, resistance to fire and chemical yield in excellent durability and long lifetime for the structure. As a result, fewer damages and less rehabilitation costs will be incurred, which is beneficial for the economic growth of a country.

DEVELOPMENTS AND APPLICATIONS OF GEOPOLYMER CONCRETE IN RECENT SCENARIO
Geopolymer concrete has potential for use in civil engineering applications. High-early strength gain is a characteristic of geopolymer concrete when dry-heat or steam cured, although ambient temperature curing is possible for geopolymer concrete. It has been used to produce precast railway sleepers, sewer pipes, and other prestressed concrete building components. Recently, geopolymer concrete has been tried in the production of precast box culverts with successful production in a commercial precast yard with steam curing(Gurley J T and Johnson 2005). The products included sewer pipes, railway sleepers, floor beams and wall panels. Reinforced geopolymer concrete sewer pipes with diameters in the range from 375 mm to 1800 mm have been manufactured using the facilities currently available to make similar pipes using Portland cement concrete (N A Lloyd and B V Rangan 2010). HySSIL is a light weight precast geopolymer concrete product. HySSIL (High Strength Structural Insulated Light weight) products has developed a range of cellular Geopolymer precast panels which are half the weight of conventional concrete precast panels, with similar durability and strength and up to five times more insulative than conventional concrete. Geopolymer concrete bricks produced on an industrial scale are found to meet the minimum compressive strength requirement with low water absorption (Dr. S Ramchandra Murthy 2014). Another class of geopolymer concrete is fibre reinforced geopolymer units which are gaining attraction due to their high tension capacities.
GPC is becoming popular in Marine structures construction also due to its high resistance against chemical attacks and Due to low permeability values it is being used in Waste containments and mining waste encapsulations.

CONCLUSION
Geopolymer concrete offers environmental protection by means of up cycling low-calcium fly ash and blast furnace slag, waste/by-products from the industries, into a high value construction material needed for infrastructure developments. The document presented brief details of GPC, its properties, relevant comparisons with conventional concrete, economic benefits to the society, and its applications. Following conclusions can be arrived at about GPC 1. Geopolymer concrete has many superior properties compared with its counterpart OPC concrete and GPC is an environmentally friendly sustainable construction material which is becoming increasingly popular. 2. The reduced CO2 emissions of Geo-polymer concrete make it a good alternative to Ordinary Portland Concrete. 3. Geo-polymer concrete shows significant potential to be a material for the future because it is not only environmentally friendly but also possesses excellent mechanical properties. 4. It is possible to utilize various waste products from different industries ( FA, GGBS, Red mud, Copper ash, RHA etc. ) through geo-polymer technology for the development of eco-friendly construction material. 5. Recommendations on use of geo-polymer concrete technology in practical applications such as precast concrete products and waste encapsulation need to be developed.