Study on Effect of Superplasticizer on GGBS Blended Geopolymer Concrete

Geopolymer concrete is an alternate to conventional concrete with reduced carbon emission, embodied energy and global warming potential and transforming waste product into an useful material. Geopolymer concrete is produced by mixing highly alkaline activator solution with alumino silicate source materials. Geopolymer concrete is highly viscous or sticky in nature, to overcome this drawback, an attempt has been made to study the effect of naphthalene based superplasticizer on geopolymer concrete blended with GGBS. Fly ash was replaced by GGBS at 20% increment levels, keeping a constant superplasticizer dosage at 3% by weight of binding material. Workability and strength characteristics were compared with those of geopolymer concrete blended with GGBS without superplasticizer. Up to certain replacement level of fly ash with GGBS, results in decrease of strength parameters with increase in workability, further replacement results in increase of strength with reduced workability with the addition of superplasticizer.


General
Concrete is the widely used material in the world after water. Ordinary Portland cement has been used traditionally as a binding material for preparation of concrete. The world-wide consumption of concrete is believed to rise exponentially for the infrastructural development taking place in China and India.
One tone of carbon dioxide is estimated to be released to the atmosphere when one ton of ordinary Portland cement is manufactured; also the emission by cement manufacturing process contributes 7% www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 2, No. 2, 2019 46 Published by SCHOLINK INC. to the global carbon dioxide emission. It is important to find an alternate binder which has less carbon footprint than cement. Geopolymer concrete is one such alternate binding material, wherein the cement is totally replaced by industrial byproducts such as fly ash, GGBS etc., with rich content of aluminium and silicon. Davidovits (1988;1994) proposed that an alkaline liquid could be used to react with the Silicon (Si) and Aluminum (Al) in a source material of geological origin or in by product materials such as fly ash and GGBS to produce binders. Because the chemical reaction that takes place in this case is a polymerization process, he coined the term geopolymer to represent these binders. The final properties of geopolymer concrete is influenced by large number of factors like type of curing, water content, alkali concentration, solids content, silicate and aluminates ratio, pH and others.
Geopolymer concrete is highly viscous or sticky in nature, which reduces the workability. To increase the workability, extra water or superplasticizer will be added. Therefore, the present work aims to study the effect of adding superplasticizer on workability and strength characteristics of GGBS blended geopolymer concrete.

Constituents of Geopolymer Concrete
Geopolymer concrete can be manufactured by using the low-calcium fly ash obtained from coal-burning power stations. Most of the fly ash available globally is low-calcium fly ash formed as a by-product of burning anthracite or bituminous coal. Although coal burning power plants are considered to be environmentally unfriendly, the extent of power generated by these plants is increasing due to the huge reserves of good quality coal available worldwide and the low cost of power produced from these sources. The energy returned-to-energy invested ratio of coal burning power plants is high, and second only to the hydro-power generation plants. (Lloyd, 2009) The main constituents of the geopolymer concrete are source materials and alkaline liquids. The source materials should be rich in silicon (si) and aluminum (Al), these could be natural materials such as kaolin, clays etc. Alternatively, by-product materials such as fly ash, silica fume, slag, rice husk ash, red mud etc. could be used as source materials. The choice of the source materials for making geopolymers depends on factors such as availability, cost, type of application, specific demand of the end users. The alkaline liquids are from soluble alkali metals that are usually sodium or potassium based.

Literature Review
Nuruddin et al. (2011) have studied the effect of superplasticizer (SP) dosage and molarity of NaOH solution on workability and compressive strength of Self-Compacting Geopolymer Concrete. The workability properties such as filling ability, passing ability and resistance to segregation were assessed using slump flow, T-50, V-funnel, L-Box and J-ring test methods. It was found that the essential workability requirements for self compactability were satisfied. Results show that low superplasticizer content 3, 4 and 5% had poor filling and passing ability and the workability results were not satisfied the limits of self compacting concrete. As the concentration of NaOH solution increased from 8M to 12M, the compressive strength of geopolymer concrete has increased. They have concluded that 6% of   superplasticizers caused higher instantaneous slump and larger spread than polycarboxylate based type.

Materials
The alumino silicate material used in this study is a combination of fly ash and ground granulated blast furnace slag (GGBS). Fly ash is procured from Bellary thermal power plant, Kudithini. GGBS is obtained from JSW, Bellary. Sodium silicate and Sodium hydroxide are procured from Shree Chem, Bangalore and locally available coarse and fine aggregates are used.

Mix Proportions
As there are no codal provisions for the mix design of geopolymer concrete, the geopolymer concrete mix was prepared as per the procedure given by Patankar et al. The alkaline liquid to fly ash ratio is kept as 0.35. The ratio of sodium hydroxide to sodium silicate is taken as 1.00 Concentration of sodium hydroxide solution is maintained as 13M. Mix design is carried out for M30 grade of geopolymer concrete. Quantity of materials required is shown in Table 2.

Casting and Curing
Fine aggregate, coarse aggregate, fly ash are mixed in dry condition for 3-4 minutes and then the alkaline solution, which is a combination of sodium hydroxide solution and sodium silicate solution with superplasticizer dosage of 3% by weight of binding material is added to the dry mix. Figure 1 shows the casting of specimens. Specimens are demoulded after 24 hrs of casting and kept in ambient condition for curing as shown in Figure 2.

Results and Discussions
The specimens are designated as FA100 for 100% fly ash and FGn for fly ash and GGBS blends at n% of fly ash. Fly ash is replaced at an increment of 20% by weight with GGBS. Workability and strength characteristics are studied for GGBS blended geopolymer concrete with and without SP.

Figure 3. Slump for GGBS Blended Geopolymer Concrete with and without Superplasticizer
Workability in terms of slump is shown in Figure 3. It is observed that the slump for geopolymer concrete with superplasticizer is more when compared with geopolymer concrete without superplasticizer, for all replacements of fly ash with GGBS. Slump for geopolymer concrete without superplasticizer, reduces with increase in GGBS level but for geopolymer concrete with superplasticizer, slump increases up to 40% replacement of fly ash with GGBS and then it decreases which may be due to the presence of calcium in GGBS.
Strength properties such as compressive strength, split tensile strength and flexural strength are conducted for GGBS blended geopolymer concrete with and without superplasticizer and the results are shown in Table 3.

Conclusions
In this study the effect of naphthalene based superplasticizer on GGBS blended geopolymer concrete has been made. Based on the experimental results, following conclusions are drawn.
• Workability increases with the addition of superplasticizer for all replacement levels of GGBS. Maximum of 40% increase in workability is observed for 40% replacement of fly ash with GGBS.
• Strength properties are decreased by 20% upto 40% replacement level when superplasticizer is added.
• Strength properties such as compressive strength reduces by 10%, split tensile strength reduces by 20% and flexural strength reduces by 20%, when fly ash is replaced upto 40% with GGBS with the addition of superplasticizer.
• Strength properties are increased by 100 to 450% with the addition of superplasticizer beyond 40% replacement level of fly ash with GGBS.