WO2011135584A2

WO2011135584A2 - Geopolymer concrete

Info

Publication number: WO2011135584A2
Application number: PCT/IN2011/000274
Authority: WO
Inventors: Arati Varma; Mohammed Qayyumi
Original assignee: Aditya Birla Science And Technology Company Limited
Priority date: 2010-04-29
Filing date: 2011-04-26
Publication date: 2011-11-03
Also published as: WO2011135584A3

Abstract

Disclosed is a geopolymer-based concrete composition comprising fly ash, an alkaline activator having Na₂O:SiO₂ molar ratio in the range of 0.68-1.13, calcium hydroxide, and water, along with inert aggregates and ordinary Portland cement. The composition provides concrete having high thermal stability, high acid resistance, quick setting, cost effective, and can be cured at room temperature. The composition is easy-to-make and reduces CO₂ emissions by up to 70%.

Description

GEOPOLYMER CONCRETE

FIELD OF INVENTION

The present invention relates to a concrete composition.

Particularly, the present invention relates to a geopolymer concrete composition.

DEFINITIONS OF TERMS USED IN THE SPECIFICATION

The term "fly ash" used in the specification means the material that is pozzolanic in nature of class F which contains less than 20 % lime (CaO), being extracted from the flue gases of power stations firing harder, older anthracite and bituminous coal via the use of electrostatic precipitators.

The term "OPC grade 53" used in the specification means the material obtained by inter grinding high grade clinker with gypsum having a minimum compressive strength of 53 MPa at 28 days.

The term "curing" used in the specification means initiating a hydration process at a favorable temperature for a prefixed duration, thereby setting a chemical reaction between the concrete mixture and water to cause the concrete to harden. In this case, the concrete mixture is kept in a dry chamber for a prefixed duration. BACKGROUND

Architectural and construction applications such as roads, bridges, commercial and residential buildings and water pipelines require a significant amount of cement- based concrete. Ordinary Portland Cements (OPC), primarily comprising limestone, clay, cement rock, and iron ore, are hydraulic cements that chemically react to harden with addition of water, have been extensively used in the past; where, to produce the cement the ingredients are blended together and cured at a temperature of about 1400 - 1700 °C and finally ground. The Portland cements are high cost and susceptible under increased fatigue stresses, acid rain, and/or road salts. Further, concrete made from Portland cement takes time to harden. To overcome these limitations due to Portland cement, geopolymer concretes have been recently developed which exhibit greater heat, fire, and acid resistance, and harden rapidly.

Where, one ton production of OPC liberates 0.7 to 0.9 tons of C0₂ due to the chemical reactions of limestone, thermal energy and electrical energy, in comparison, the geopolymers release much lesser C0₂. Geopolymers mainly comprise silicon and aluminum atoms bonded via oxygen into a polymer network, and are prepared by dissolution and poly-condensation reactions between an aluminosilicate binder and an alkaline silicate solution. The application of geopolymers depends largely on their chemical structure and the atomic ratio of silicon to aluminum, where, recently they are preferred in cementing systems due rapid setting and hardening, superior hardness and chemical stability. Further, with the increasing demand for reducing the greenhouse gas (GHG) emissions, where C0₂ emissions from cement industry in India contribute to about 6 - 8 % of the total C0₂ emissions, the geopolymers are therefore becoming highly preferable over OPC. Geopolymers use an aluminosilicate source from activated clay, industrial waste such as coal fly ash, with or without a metallurgical slag. These aluminosilicates on reaction with the alkaline silicate solution give zeolite like polymers. Geopolymers are durable, environmental friendly and mechanical properties are superior to OPC.

Several attempts have been made in the past to provide geopolymer concrete compositions. Some of these disclosures are listed in the prior art below.

US7794537 discloses a geopolymeric composition which is formed as a suspension comprising an aluminosilicate source, a metal silicate, an alkali activator, and a carrier fluid. The composition further comprises a retarder and an accelerator which are active between 20 - 120 °C. The composition is applicable for oilfield cementing applications.

US7141 1 12 discloses a cementitious material comprising a geopolymer and stainless steel slag, at least one activator component and optionally containing Portland cement.

EP 1801084 and WO2008048617 disclose the use of geopolymeric chemistry to prepare cement and concrete compositions. These formulations utilize fly ash or amorphous silica as aluminosilicates source. A drawback of these inventions is that the cement and concretes require curing at temperatures in the range of 40 - 90 °C to obtain the desired strength. Also, geopolymers cured at 40 °C require further addition of milled blast furnace slag. This limits the application of the cement and concretes thereof. US4509985, US20060272551 , and WO2005049522 disclose geopolymer compositions which require addition of ground blast furnace slag along with fly ash for room temperature curing. Also, the amount of alkali used in these formulations is substantive.

WO2005049522 and US20070125272 disclose geopolymers using fly ash with OPC and ground slag for achieving room temperature curing.

WO2007109862 describes a dry mix cement composition using fly ash, portland cement (OPC), a polymeric aid and Ca(OH)₂. The polymeric aid used in the formulation is prepared by activation of aluminosilicate and NaOH fused at 600°C, followed by grinding to particle size essentially less than 25 microns. This high temperature activation requires an additional processing step and ample energy which limits its commercial application.

FR2904307 discloses a geopolymer having the molar ratio of Si0₂ : M₂0 between 1.28 - 0.78. The geopolymer is cured at room temperature. This composition uses slag, which limits that application of the composition.

Cement and Concrete Research vol. 38 (2008) pp. 554 - 564 discloses geopolymers that are cured at 40 °C using metakaolinite and different calcium silicate sources. Preparing metakaolin involves calcination of kaolin at about 700 °C and subsequent grinding to a very fine size. This consumes ample energy and also limits the ease of applicability of this formulation.

Journal of Hazardous Materials vol. 167 (2009) pp. 82 - 88 discloses a geopolymer paste prepared using fly ash and Ca(OH)₂ cured at room temperature. The strength of the geopolymer paste is about 23 MPa after curing for 7 days at 20 °C. The geopolymer strength is significantly lower than standard OPC mortar.

Typically, ground blast furnace slag is used in the known geopolymer compositions. The drawbacks of ground blast furnace slag include: the blast furnace slag requires processing prior to use and must be finely ground to 5 - 25 μ, this process step requires specialized machinery and high energy input; the slag is not readily available in the Indian sub-continent as the steel industry is comparatively small, and also the cement industry utilizes the slag to produce blended cements (portland slag cements) incorporating between 35 - 70 % by wt. of the slag. It is observed in the prior art that the geopolymer compositions either uses slag or require curing at elevated temperatures to gain the mechanical strength. The present invention, therefore, aims at overcoming the afore-listed drawbacks by providing a geopolymer composition which uses easily available raw materials, is cost-effective, and environmental friendly.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a geopolymer concrete composition having high thermal stability and high acid resistance properties.

Another object of the present invention is to provide a geopolymer concrete composition which is cost-effective and easy-to-make.

Still another object of the present invention is to provide a geopolymer concrete composition which is environmental and user friendly. Yet another object of the present invention is to provide a slag- free geopolymer concrete composition which has mechanical properties similar to or better than OPC-based concrete.

One more object of the present invention is to provide a geopolymer concrete composition which is cured at room temperature.

Yet one more object of the present invention is to provide a geopolymer concrete composition whose raw materials require no pretreatment/preprocessing.

Still one more object of the present invention is to provide a geopolymer concrete composition which can be made using the conventional concrete equipments.

SUMMARY OF THE INVENTION

In accordance with the present invention, is disclosed a composition for a geopolymer-based concrete comprising: at least one inert aggregate in a proportion in the range of 49 - 65 wt%, fly ash in a proportion in the range of 19 - 35 wt%, an alkaline activator having Na₂0:Si0₂ molar ratio in the range of 0.68 - 1.13 in a proportion in the range of 3 - 6.5 wt%, calcium hydroxide in a proportion in the range of 0.5 - 1.4 wt%, and water in a proportion in the range of 7 - 12 wt%, and ordinary portland cement in a proportion in the range of 0.5 - 5 wt%.

Typically, in accordance with the present invention, the amount of said aggregate in said composition is in the range of 53 - 65 wt%. Preferably, in accordance with the present invention, the amount of said fly ash in said composition is in the range of 21 - 35 wt%.

Typically, in accordance with the present invention, the amount of said alkaline activator in said composition is in the range of 4 - 4.5 wt%.

Preferably, in accordance with the present invention, the amount of said ordinary portland cement in said composition is in the range of 1.2 - 5 wt%, preferably in the range of 1.2 - 3.2 wt%.

Typically, in accordance with the present invention, the amount of calcium hydroxide in said composition is in the range of 0.7 - 1.4 wt%.

In accordance with the present invention, said inert aggregate is selected from the group of aggregates consisting of sand, gravel, crushed stone, metal and recycled concrete.

Typically, in accordance with the present invention, said fly ash is F-type fly ash.

Preferably, in accordance with the present invention, said alkaline activator is an alkaline earth metal silicate.

More preferably, in accordance with the present invention, said alkaline activator is sodium silicate.

Typically, in accordance with the present invention, said ordinary portland cement is OPC grade 53. Preferably, in accordance with the present invention, purity of calcium hydroxide is greater than 97 %.

Typically, in accordance with the present invention, particle size of said aggregate is in the range of 0.09 - 20 mm.

In accordance with the present invention, is disclosed a process for manufacturing a geopolymer-based concrete, said process comprising the steps of: combining at least one inert aggregate, fly ash, an alkaline activator having Na₂0:Si0₂ molar ratio in the range of 0.68 - 1.13, calcium hydroxide, and water, and ordinary Portland cement, to obtain a reactive mixture; and curing the reactive mixture at a temperature in the range of 20 - 40 °C for a period in the range of 24 hours - 28 days, to obtain the geopolymer-based concrete.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The invention will now be described with the help of the accompanying drawings, in which,

Figure 1 illustrates a graph showing the % weight loss in concrete over a period of time on exposure to 30 % HCL, comparing standard OPC-based concrete and the geopolymer-based concrete composition of the present invention; and

Figure 2 illustrates a graph showing the strength of concrete over a range of elevated temperatures, comparing standard OPC-based concrete and the geopolymer-based concrete composition of the present invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention envisages a composition for a geopolymer-based concrete. The composition of the present invention comprises fly ash, an alkaline activator, calcium hydroxide, and water, along with inert aggregates, and ordinary portland cement. The ingredients once mixed are cured at room temperature to impart strength to the concrete composition. The composition of the present invention is environment friendly and lowers the green house gas emissions by reducing the C0₂ emissions by 40 - 70 %. The composition of the present invention is cost- effective as the prime ingredients fly ash and inert aggregates are easily available at a low cost; and none of the ingredients require any pretreatment or processing. Further, the composition can be made using conventional cement manufacturing implements, therefore no added capital expenses.

The composition for the geopolymer-based concrete comprises at least one inert aggregate, typically selected from sand, gravel, crushed stone, metal and recycled concrete, having particle size in the range of 0.09 - 20 mm. The amount of the inert aggregate in the composition is in the range of 49 - 65 wt%, more preferably in the range of 53 - 65 wt%. The inert aggregate used in the concrete composition of the present invention may be sand having particle size between 0.09 - 2mm in case of mortar formulations or a mixture of sand and larger aggregates including metal having particle size between 0.6 - 20 mm in case of concrete. Further, the composition for the geopolymer-based concrete comprises fly ash, typically nonclassified F-type fly ash having a composition as stated in TABLE 1. The fly ash used in the composition is generally obtained from power plants where it is generated as a waste in large quantities. The composition of the present invention comprises 19 - 35 wt%, more preferably 21—29 wt%, of fly ash.

TABLE 1 : Fly ash composition in accordance with the present invention

The composition of the present invention further comprises the alkaline activator, typically an alkaline earth metal silicate, having Na₂0:Si0₂ molar ratio in the range of 0.68 - 1.13. Sodium silicate is preferred as the alkaline activator as it is cheap and easily available. The amount of the alkaline activator in the composition of the present invention is in the range of 3 - 6.5 wt%, more preferably in the range of 4 - 4.5 wt%. The alkaline activator is typically in the form of a solution which can be obtained directly from a manufacturer with the specified Na₂0:Si0₂ molar ratio or can be prepared by mixing the two components, viz., a commercial grade alkaline sodium silicate solution containing 14.7 wt% of Na₂0 and 29.4 wt% of Si0₂ with a total solid content of 44.1 % in water; and a 8 M solution of NaOH prepared by mixing commercial grade of NaOH flakes with calculated amount of water. Another important ingredient of the composition of the present invention is commercial grade calcium hydroxide Ca(OH)₂ having purity greater than 97 %. Calcium hydroxide is present in the composition in the range of 0.5 - 1.4 wt%, more preferably in the range of 0.7 - 1.4 wt%. Water in added to the composition to be present in the range of 7 - 12 wt%. Ordinary portland cement, typically OPC grade 53, is added to the composition in the range of 0.5 - 5 wt %, preferably in the range of 1.2 - 5 wt%, and more preferably in the range of 1.2 - 3.2 wt%, to impart further strength and hardness. The process for manufacturing the composition of the present invention comprises the steps of combining the at least one inert aggregate with fly ash, the alkaline activator, calcium hydroxide and water, and ordinary portland cement, to obtain a reactive mixture. The reactive mixture is cured at room temperature for at least 24 hours to obtain the geopolymer-based concrete composition of the present invention. In accordance with the present invention, the geopolymer-based concrete composition is cured at a temperature in the range of 20 - 34 °C, more preferably, at a temperature greater than 28 °C. These are the daily average temperatures in the Indian subcontinent and other tropical countries. Temperatures higher than these are found to accelerate the strength development; however, not much difference is expected in the final 28-day strength of the concrete. The geopolymer-based concrete was found to achieve the 28-day strength within 24 hours on curing at temperatures ranging from 50 - 70 °C at normal pressure. This provides rapid strength development and can be advantageously used for several industrial and commercial applications.

The aluminosilicates from the flyash in presence of the alkaline activator and silicate precursor undergo dissolution and polycondensation reactions to form a geopolymer binder. This geopolymer binder provides desired mechanical properties to the concrete. Simultaneously, ordinary portland cement reacts with the water to form calcium silicate hydrate (CSH) gel due to hydration of the tricalcium silicate (C₃S) and dicalcium silicate (C₂S) present in portland cement. This hydration reaction is enhanced by the high alkalinity provided by the alkaline activator. Also, due to the high pH and presence of Ca(OH)₂, the fly-ash reacts as per the pozzolonic reactions to form calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gel. The CSH and CAH gels undergo further geopolymerization reactions and form aluminum silicate hydrates stabilized by Na or Ca ions. The combined geopolymer and CSH structures provide the required hardening and early-age strength even by curing at room temperatures and thus enhanced properties to the geopolymer composition.

The geopolymer-based concrete composition of the present invention provides compressive strength development similar to ordinary portland cement based concrete at all ages viz. 3-day, 7-day, 28-day and long term strengths. Referring to FIGURE 1, therein is illustrated a graph showing the % weight loss in concrete over a period of time on exposure to 30 % HCL, comparing standard OPC-based concrete and the geopolymer-based concrete composition of the present invention. From the graph it can be estimated that the weight loss in the geopolymer-based concrete of the present invention is negligible in comparison with a standard OPC concrete which shows a significant % weight loss. The composition of the present invention further provides a large flexibility in terms of altering the parameters to obtain a desirable strength development particularly based on the application, thereby providing convenient and wider applicability of the composition. The composition is environmental friendly as it liberates 40 - 70 % less C0₂ compared to ordinary portland cement (OPC) concrete (including C0₂ emissions from all ingredients and activators). The direct C0₂ emissions during manufacturing the concrete by mixing the ingredients is similar for both geopolymer and OPC concrete as the equipments used are the same. The indirect C0₂ emissions caused during manufacturing the raw materials used in the concrete contribute to the largest amount of C0₂ emission in OPC concrete. The basis for calculation of the total C0₂ emission is based on summation of individual contribution from all the ingredients used in its preparation. It was found that the geopolymer-based concrete composition of the present invention emits 40 - 70 % less C0₂ compared to conventional OPC concrete. Also, the geopolymer-based concrete is eco- friendly because it comprises 19 - 35 wt% fly ash which is a waste product from power plants causing disposal problems. The composition of the present invention is user friendly as the alkaline activator is used in the form of a solution making it non-corrosive, and further the concrete in its wet form is non- corrosive and safe for human contact.

The geopolymer-based concrete composition exhibits mechanical properties similar to standard OPC concrete. The geopolymer-based concrete of the present invention achieves strength equivalent to the standard OPC concrete at 3 -days, 7- days & 28-days at room temperatures and without the use of slag in any form and less than 5 % ordinary portland cement. However, the composition of the present invention has several added benefits over the conventional OPC concrete in terms of enhanced performance in acidic environment and under elevated temperatures (Refer FIGURE 1 & FIGURE 2). The geopolymer-based concrete shows a high acid resistance when exposed to concentrated hydrochloric and sulfuric acid (Refer FIGURE 1). The physical shape and dimension is retained for longer exposure compared to OPC concrete. Also, the strength retention of the geopolymer-based concrete is higher than standard OPC concrete in acidic environment.

Referring to FIGURE 2, therein is illustrated a graph showing the strength of concrete over a range of elevated temperatures, comparing standard OPC-based concrete and the geopolymer-based concrete composition of the present invention. The resistance of the geopolymer-based concrete composition to high temperature is also significantly higher than standard OPC concrete. The OPC concrete normally looses its mechanical strength when exposed to temperatures higher than 500 °C, however the geopolymer-based concrete retained shape and strength even after exposure to 800 °C for 3 hours. Thus, high temperature and acid resistant properties of the geopolymer-based concrete are significantly superior to the standard OPC concrete.

Further, the geopolymer-based concrete composition of the present invention is easy-to-use and does not require providing any preprocessing or pretreatment to the raw materials prior to use. The raw materials used like flyash, OPC, alkaline activator solution, Ca(OH)2, aggregates are all standard commercial grade and do not require any processing/pretreatment. The ingredients including water have no special quality requirements and commercial grade materials are used. Further advantages of the composition of the present invention is quick setting, high utilization of fly ash which is an industrial waste, room temperature curing & hardening, high early-age strength, and low cost. This composition can be used to make concrete for applications requiring acid resistance, and temperature resistance, and pre-cast products for load bearing and non-load bearing applications.

TEST RESULTS

The invention will now be described with respect to the following examples which do not limit the scope and ambit of the invention in anyway and only exemplify the invention.

EXAMPLE 1:

Sample 1 was of standard OPC-based concrete, prepared as per IS 4031 , and samples 2 - 13 were of the geopolymer-based concrete composition of the present invention using fine aggregates/sand. The fly ash used in samples 2 - 13 was a non-classified F-type fly ash from a coal power plant in Maharashtra, India. The composition of this fly ash is illustrated in TABLE 2. The aggregates used were sands as per Indian standards (IS 1727: 1968). The three sands, viz., Grade I (1 mm - 2 mm), Grade II (0.5 mm - 1 mm) and Grade III (0.09 mm - 0.5 mm) were used in equal proportions in all the 13 samples.

TABLE 2: Fly ash composition for samples 2 - 13.

For samples 2 - 13, the aggregates, fly ash, OPC and Ca(OH)₂ were measured and put in a mortar mixer. These ingredients were dry mixed for 3 minutes at a low speed. The alkaline activator solution was then added to the dry mixture in the mortar mixer for mixing or the mixture was manually mixed for 5 minutes. The mixture was poured into cubes of 70.5 mm x 70.5 mm. The cubes were then vibrated in a vibrating machine for 3 minutes and allowed to set at room temperature. The cubes were then placed in a dry chamber at a temperature of 30 °C +/- 2 °C. The cubes were placed in the dry chamber for 24 hours and then removed. The concrete cubes were then de-moulded and placed exposed to room conditions (27 +/- 3 °C). The cubes were tested for compression strength using compression testing machine at 3-days, 7-days and 28-days. The average strength of the cubes is reported where more than two cubes were tested. The setting time of the mortar was measured using standard VICAT apparatus. The wet concrete after mixing was placed in the vicat moulds and its needles were used for finding the initial and final setting times. The composition of the 13 samples is illustrated in TABLE 3. The corresponding setting time, compressive strength at 3- days, 7-days and 28-days and other relevant properties are reported in TABLE 5. The cost of the composition and C0₂ emissions are calculated by summation of individual contribution from all the raw materials used for each of the composition. The individual cost and CO? emissions by the raw materials are reported in TABLE 4. The calculation of C0₂ emission and cost is intended for comparison with standard OPC concrete and not to be used as absolute numbers.

TABLE 3: Composition of concrete for samples 1 - 13.

TABLE 4: Cost estimate and C0₂ emission by raw materials

TABLE 5: Properties of the concrete for samples 1 -13.

It is observed from TABLE 4 & TABLE 5 that the strength development and the cost of the geopolymer-based concrete composition are comparable with the standard OPC concrete. The C0₂ emissions for all the geopolymer-based concrete compositions (sample 2 - 13) are reduced by 37 to 66 % compared to the standard OPC concrete (sample 1).

EXAMPLE 2:

The geopolymer-based concrete composition was further checked for acid resistance, as illustrated in TABLE 6 & FIGURE 1. The study demonstrates the compressive strength of the geopolymer-based concrete composition and the standard OPC concrete exposed for a duration of 56 days to 30 % HC1 and 21 days to 10 % H₂S0₄. It is observed that the strength retention of the geopolymer-based concrete composition is higher than the standard OPC concrete when exposed to acidic conditions.

TABLE 6: Mechanical properties of the geopolymer-based concrete composition and the standard OPC concrete, under acidic conditions.

EXAMPLE 3:

The geopolymer-based concrete composition was further checked for thermal stability, as illustrated in TABLE 7 & FIGURE 2. The study demonstrates the compressive strength of the geopolymer-based concrete composition and the standard OPC concrete when exposed to a temperature of up to 800 °C. The strength of the OPC concrete is drastically reduced on exposure to a temperature of 550 °C, whereas the geopolymer-based concrete composition of the present invention illustrated good strength retention even at 800 °C.

TABLE 7: Mechanical properties of the geopolymer-based concrete composition and the standard OPC concrete, under elevated temperatures.

TECHNICAL ADVANTAGES

A geopolymer-based concrete composition comprising fly ash, an alkaline activator having Na₂0:Si0₂ molar ratio in the range of 0.68 - 1.13, calcium hydroxide, and water, along with inert aggregates, and comprising ordinary Portland cement; as described in the present invention has several technical advantages including but not limited to the realization of:

• the composition of the present invention provides concrete having high thermal stability, from FIGURE 2 it can be ascertained that where the strength of the OPC concrete is drastically reduced on exposure to a temperature of 550 °C, the composition of the present invention demonstrated good strength retention even at 800 °C;

• the composition of the present invention provides concrete having high acid resistance, from FIGURE 1 it can be ascertained that the strength retention of the composition of the present invention is higher than the standard OPC concrete when exposed to acidic conditions;

• the composition of the present invention reduces C0₂ emissions by up to 70

%;

• the composition of the present invention uses 19 - 35 wt% fly ash which is an industrial waste, therefore, environmental friendly;

• the composition of the present invention is easy-to-use, quick setting, cost- effective and non-corrosive;

• the composition of the present invention does not use slag in any form and does not require providing any pretreatment to the raw materials; and

• the composition of the present invention is cured at room temperature thus conserving energy.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary. In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

CLAIMS:

1. A composition for a geopolymer-based concrete comprising: at least one inert aggregate in a proportion in the range of 49 - 65 wt%, fly ash in a proportion in the range of 19 - 35 wt%, an alkaline activator having Na₂0:Si0₂ molar ratio in the range of 0.68 - 1.13 in a proportion in the range of 3 - 6.5 wt%, calcium hydroxide in a proportion in the range of 0.5 - 1.4 wt%, and water in a proportion in the range of 7 - 12 wt%, and ordinary portland cement in a proportion in the range of 0.5 - 5 wt%.

2. The composition as claimed in claim 1 , wherein the amount of said aggregate in said composition is in the range of 53 - 65 wt%.

3. The composition as claimed in claim 1 , wherein the amount of said fly ash in said composition is in the range of 21 - 35 wt%.

4. The composition as claimed in claim 1 , wherein the amount of said alkaline activator in said composition is in the range of 4 - 4.5 wt%.

5. The composition as claimed in claim 1 , wherein the amount of said ordinary portland cement in said composition is in the range of 1.2 - 5 wt%, preferably in the range of 1.2 - 3.2 wt%.

6. The composition as claimed in claim 1 , wherein the amount of calcium hydroxide in said composition is in the range of 0.7 - 1.4 wt%.

7. The composition as claimed in claim 1 , wherein said inert aggregate is selected from the group of aggregates consisting of sand, gravel, crushed stone, metal and recycled concrete.

8. The composition as claimed in claim 1 , wherein said fly ash is F-type fly ash.

9. The composition as claimed in claim 1 , wherein said alkaline activator is an alkaline earth metal silicate.

10. The composition as claimed in claim 1 , wherein said alkaline activator is sodium silicate.

1 1. The composition as claimed in claim 1 , wherein said ordinary portland cement is OPC grade 53.

12. The composition as claimed in claim 1 , wherein purity of calcium hydroxide is greater than 97 %.

13. The composition as claimed in claim 1 , wherein particle size of said aggregate is in the range of 0.09 - 20 mm.

14. A process for manufacturing a geopolymer-based concrete, said process comprising the steps of: combining at least one inert aggregate, fly ash, an alkaline activator having Na₂0:Si0₂ molar ratio in the range of 0.68 - 1.13, calcium hydroxide, and water, and ordinary portland cement, to obtain a reactive mixture; and curing the reactive mixture at a temperature in the range of 20 - 40 °C for a period in the range of 24 hours - 28 days, to obtain the geopolymer-based concrete.