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Study on the compressive strength of fly ash based geo polymer concrete

Introduction of the alternative materials for complete replacement of cement in ordinary concrete will play an important role to control greenhouse gas and its effect. The 100% replacement of binder with fly ash (in integration with potassium hydroxide (koh) and potassium silicate (k2sio3) solutions) in concrete gives a significant alternative to conventional cement concrete. This paper focuses on the effect of alkaline solutions koh and k2sio3 on strength properties of fly ash based geo polymer concrete (fgpc); compared the strength at different molarities of alkaline activator koh at different curing temperature. Fly ash based geo polymer concrete was produced from low calcium fly ash, triggered by addition of koh and k2sio3 solution and by assimilation of superplasticizer for suitable workability. The molarities of potassium hydroxide as 8m, 10m and 12m molarities were used at various curing temperatures such as 60°c, 70 °c and 80°c. Results showed that for given proportion to get maximum compressive strength the optimum molarity of alkaline solution is 12m and optimum curing temperature is 70 °c.


Introduction
Concrete is one of the essential materials for the infrastructure development due to its versatile applications and global use of it is second only to water. To fulfil this huge demand of the concrete, the cement production is also increasing day-by-day. Carbon dioxide (CO 2 ) emissions from the cement producing industries are generating up to 5% of worldwide man-made emissions of it. Hence, to reduce the environmental impact due to cement manufacturing, innovation of the alternative materials has now become the need of the hour. Flyash based Geopolymer Concrete (FGPC) is one of the alternative to the conventional Ordinary Portland Cement (OPC) concrete where the pozzolanic material like fly ash is used as a binder instead of cement [1,2,3,4,5]. Use of the fly ash as a geopolymer binder can save up to 80% of CO 2 emission. The fly ash is composed of fine particles and is a by-product of the coal fired electric and steam producing plants. Fly ash based GPC is ideally used for building and repairing infrastructure. FGPC has high early strength, also their time of setting can be controlled and they remain integral for a very long time. Due to the characteristics the FGPC is used for pre-casting units [7,8,9,10].

Objectives
The objectives are to study the effect of alkaline solution of KOH and K 2 SiO 3 on the Strength properties of a FGPC, to compare the strength of the FGPC at different molarities of alkaline activator (KOH) and to compare strength of FGPC at different curing temperatures.

Research Significance
From the most of the research papers it is found that KOH and K 2 SiO 3 alkali were used as an activator by a little number of investigators. No one has provided the optimum molarity and optimum curing temperature for KOH activator to get maximum compressive strength of geopolymer concrete.

Materials
The Coarse aggregate and Fine Aggregate used for the FGPC mixes were satisfied the requirements given in IS: 2386 Part 3. The specific gravity values of fine aggregate and coarse aggregate used is 2.57 and 2.66, respectively. For the concrete mixes, Class F fly ash from Ennore thermal power plant was used. The Combinations of KOH and K 2 SiO 3 were used to activate the binder material. The alkaline solutions were made 24 hours prior to use [6]. Three various concentrations of KOH like 8 M, 10M and 12M were used. The workability of FGPC was enhanced with the use of Sulphonated Naphthalene based superplasticizer. The Chemical Composition of Fly ash is given in Table 1. The properties of Potassium Silicate and Compositions of Potassium Hydroxide are given in Table 2 and Table 3, respectively.

Mix Design
There are no standardized procedures for the mix design for FGPC. With the help of previous research work, different trials were made. Three trial mixes with different molarities of Potassium Hydroxide such as 8M, 10M, and 12M were used. Also, the ratio of potassium silicate to potassium hydroxide as 1.50 was fixed. The 100 mm cube specimens were cast with trail mixes given in Table 4 and specimens were kept in the oven for 24 hours at varying curing temperatures of 60˚C, 70 ˚C and 80 ˚C. After hot air curing, the specimens were kept in ambient air. The ratio between alkaline solution and Fly ash is given in Table 5.

Results and Discussions
All the cubes were tested to find compressive strength by using the Compression Testing Machine (CTM). The rate of loading was 2.5kN/s during the test. The Compressive strength of concrete cubes were tested at 7 and 21 days. The results of the findings of the current work is absolutely matching with the earlier investigators. At the temperature of 70˚C for the period of two hours, the specimens were showing higher compressive strength for the molarity of 12M of alkaline solution of KOH. The optimum usage of superplasticizer was 1.5% by weight of binder. The failure pattern of the cubes is shown in Figure 1.