**Abstract**

Geopolymer concrete (GPC) has significant potential as a more sustainable alternative for ordinary Portland cement concrete. GPC had been introduced to reduce carbon footprints and thereby safeguarding environment. This emerging eco friendly construction product finds majority of its application in precast and prefabricated structures due to the special curing conditions required. Sustained research efforts are being taken to make the product suitable for in situ applications. The developed technology will certainly address the issues of huge energy consumption as well reduce water use which is becoming scarce nowadays. Ground Granulated Blast Furnace Slag (GGBS) a by-product of iron industries in combination with fly ash has proved to give enhanced strength, durability as well reduced setting time. This study investigates the effect of GGBS as partial replacement of fly ash in the manufacture of GPC. Cube and cylindrical specimens were cast and subjected to ambient curing as well to alternate wetting-drying cycles. The 28 day compressive strength, split tensile strength, flexural strength and density of GPC specimens were found. The study revealed increase in compressive strength, split tensile strength, density as well flexural strength up to 40 percent replacement of fly ash by GGBS.

**Keywords:** geopolymer concrete, ambient curing, Flyash, GGBS, Setting time

## **1. Introduction**

It is well known that concrete is one of the most widely used reliable and effective construction material all over the World [1, 2]. Rapid urban growth development leads to the usage of concrete in construction industry increase day by day which further increases the demand of ordinary Portland cement. In order to meet the huge demand, the production of ordinary Portland cement (OPC) increases every year. During the production of OPC, an enormous amount of greenhouse gas such as carbon dioxide (CO2) will be emitted giving rise to global warming issues [3]. As such invention of alternative binding materials evolved. Geopolymer concrete proved itself to address the issues by reduced carbon footprints. Further, the enormous energy required in the production of OPC could be avoided leading to energy conservation.

Geopolymer had proved further superior to OPC in terms of acid resistance, sulphate resistance, withstanding heat [4], fire and possessing corrosion resistance. Geopolymerization involves a heterogeneous chemical reaction between silicon and aluminium in a source of geological origin or industrial by products such as flyash and GGBS (binder) with high alkaline solution of sodium hydroxide and sodium

silicate resulting in the form of three-dimensional amorphous to semi crystalline polymeric and ring structure comprising Si-O-Al and Si-O-Si bonds.

There exist some limitations in practical usage of geopolymer concrete in construction industry. Usage of alkaline solution in geopolymer concrete contributes to cost aspect. Further, requirement of specialized curing namely heat curing or steam curing makes it difficult for in situ applications.

Due to the heavy need of electricity, numerous thermal power stations have been installed throughout the country which gives rise to fly ash generation. Flyash is a heterogeneous by-product material produced in the combustion process of coal used in power stations. Fly ash particles are almost spherical in shape which allows them to flow and blend freely in mixtures. This characteristic makes fly ash a desirable binder for concrete. Further, control of high thermal gradients, pore refinement, depletion of cement alkalis, resistance to chloride [5] and sulphate penetration and continued micro structural development through a long-term hydration and pozzolanic reaction contributes to added durability aspects. Also, the magnitude of reinforcement steel production is also enormous so as to meet the present day needs of multistoreyed structures. The by product of iron manufacturing by heating iron ore, lime stone and coke at very high temperature of 1500 degree celcisus is GGBS.

GGBS posseses good mobility characteristics arising from consistent fineness, unique particle shape and from lower relative density. Also, workability gets improved due to the smoother surface texture and glassy surface of GGBS. Also, efflorescence and staining of concrete shall be prevented by the use of GGBS. It is evident that due to its superior performance, it replaces Sulphate Resisting Portland Cement (SRPC) and useful against severe chloride attack in reinforced concrete in marine environment.

In this work, an attempt was made to study the performance of GGBS admixed geopolymer concrete under ambient curing condition. The optimum percentage replacement of fly ash by GGBS had been determined by conducting compressive strength and split tensile strength tests. The results obtained would help resolve the issues addressed above.

## **2. Experimental procedure**

#### **2.1 Materials**

In this study, Class F fly ash with specific gravity 2.3 and GGBS having specific gravity 2.1 was used. River sand was used as fine aggregate as per standards [6].

Coarse aggregate of different sizes 7 mm, 14 mm and 20 mm was graded using sieve analyser with specific gravity 2.717, 2.81, 2.76 has determined using pycnometer. The water absorption was found as 0.26%, 0.15%, 0.42% respectively while the aggregates are immersed in water for 24 hours. Sodium hydroxide (NaOH) having 14 M molarity with 97–98% purity in the form of pellets was used [7]. Sodium silicate available in the form of gel was used. Both chemicals have brought from the local supplier (Thirumala Nadar and Sons, Madurai, India-625001).

#### **2.2 Concrete mix proportions and specimen preparation**

The geopolymer concrete comprises binder, fine aggregate, coarse aggregate and alkaline solution. The materials were quantified before mixing using standard guidelines as per Rangan and Hardjito method [8]. Sodium hydroxide was mixed with water and then sodium silicate was added to the diluted sodium hydroxide to make alkaline solution. The alkaline solution was prepared one day before concrete mix to facilitate polymerization process.

#### *Geopolymer Concrete under Ambient Curing DOI: http://dx.doi.org/10.5772/intechopen.97541*

The percentage replacement of fly ash by GGBS was varied from 0–50%. Including the control mix, six mix proportions were used for both ambient curing and for wetting-drying cycles. Due to the reason that quick setting developed [9], percentage replacement was not done above 50%.

It is a usual procedure to first mix the binder materials, fine aggregate and coarse aggregate using mixer machine [10] for about three to four minutes. Then the prepared alkaline liquid should be poured over the dry mix concrete and mixing to be continued for about four to five minutes to initiate geopolymerisation process.

After getting an intimate mix, they were cast using moulds. The cast specimens were cured under ambient curing for 28 days while another set of specimens were subjected to wetting for 2 days in curing tank and drying for 2 days in ambient condition. These wetting and drying cycles were continued upto 28 days. The cast cube and cylinder specimens subjected to ambient curing are shown in **Figures 1** and **2** respectively.

#### **Figure 1.** *Cube specimens under ambient curing.*

**Figure 2.** *Cylinder specimens under ambient curing.*

Prisms of size 500 mm x 100 mm x100mm were cast to study the flexural behaviour of GPC using fly ash and GGBS combination. Prisms exposed to ambient curing for 28 days are shown in **Figure 3**.

The nomenclature of GPC specimens is presented in **Table 1**.
