Performance Of Geopolymer Concrete Under Various Curing Conditions

This paper presents the study done on development of strength for various grades of geopolymer concrete with various types of curing conditions (ambient, steam and oven curing). The grades choosen for the investigation were M-40, M-50 and M-60, the mixes were designed for molarity of 12 M. The test specimens were 150x150x150 mm cubes, 100x200 mm cylinders, for hot air oven and steam curing the specimens were cured at 60°C. The alkaline solution used for experimental study is a combination of sodium silicate and sodium hydroxide solution with the ratio of 2.5. Out of these three curing conditions heat curing gave better results. Since it utilizes the industrial wastes such as fly ash for producing the binding system in concrete, it can be considered as eco-friendly materials. Performance Of Geopolymer Concrete Under Various Curing Conditions Introduction Portland cement is widely used in concrete industry since many decades ago, however it releases green house gases, i.e. carbon dioxide (CO2), into the atmosphere during its manufacture [1]. Geopolymer technology is one of the new technologies attempted to reduce the use of Portland cement in concrete. Geopolymers are amorphous to semi-crystalline three-dimensional alumino-silicate polymers similar to zeolites [2]. Geopolymers are environmental friendly materials which do not emit green house gases during polymerisation process. Besides, they need only moderate energy to produce. Geopolymers are made from source materials with silicon (Si) and Aluminium (Al) content, thus they can be made using fly ash, waste-product of coalfired power station, as the source materials [3]. The geopolymer technology was first introduced by Davidovits in 1978. His work considerably shows that the adoption of the geopolymer technology could reduce the co2, Na2siO3, kg/m3 emission caused due to cement industries. Davidovits proposed that an alkaline liquid could be used to react with aluminosilicate in a source material of geological origin or in by-product materials such as fly ash to make a binder [4]. Fly ash is the most common source material for making geopolymers. Normally, good high-strength geopolymers can be made from class F fly ash [5]. Geopolymer is synthesized by mixing aluminosilicate-reactive material with strong alkaline solutions, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium silicate or potassium silicate. The mixture can be cured at room temperature or temperature cured [6]. The Alkaline activating solution is important for dissolving of Si and Al atoms to form geopolymer precursors and finally alumino-silicate material. The most commonly used alkaline activators are NaOH and KOH. Experimental Investigations Materials: The following materials have been used in the experimental study [9] a) Fly Ash (Class F) collected form Raichur Thermal power plant having specific gravity 2.00. b) Fine aggregate: Sand confirming to Zone–III of IS:383-1970 [13] having c) Fly Ash (Class F) collected form Raichur Thermal power plant having specific gravity 2.00. d) Fine aggregate: Sand confirming to Zone–III of IS:383-1970 [13] having specific gravity 2.51 and fineness modulus of 2.70. e) Coarse aggregate: Crushed granite metal confirming to IS:383-1970 [13] having specific gravity 2.70 and fineness modulus of 5.90. f) Water : Clean Potable water for mixing g) Alkaline liquids: Specific gravity of i) Sodium Hydroxide (NaOH) = 1.16 ii) Sodium Silicate (Na2SiO3) = 1.57 Tests were conducted on specimen of standard size as per IS: 516-1959 and IS:5816-1999 [15 and 16]. Details of tests conducted and specimens used are given in Table 1. Table 1: Details of specimen used and tests conducted Type of test conducted Size of specimen No. of specimen cast for different grades Compressive strength 150x150x150mm 5 Split tensile strength 100x200mm 5 Mix design of geopolymer concrete In the design of geopolymer concrete mix, coarse and fine aggregates together were taken as 77% of entire mixture by mass. This value is similar to that used in OPC concrete in which it will be in the range of 75 to 80% of the entire mixture by mass. Fine aggregate was taken as 30% of the total aggregates. The density of geopolymer concrete is taken similar to that of OPC as 2400 kg/m3 [7]. The details of mix design and its proportions for different grades of GPC are given in Table 2. Mixing, Casting, Compaction and Curing of Geopolymer Concrete GPC can be manufactured by adopting the conventional techniques used in the manufacture of Portland cement concrete. In the laboratory, the fly ash and the aggregates were first mixed together dry on pan for about three minutes. The liquid component of the mixture is then added to the dry materials and the mixing continued usually for another four minutes [Fig.1]. In preparation of NaOH solution, NaOH pellets were dissolved in one litre of water in a volumetric flask for concentration of NaOH (12M). Alkaline activator with the combination of NaOH and Na2SiO3 was prepared just before the mixing with fly ash. The ratio of alkaline liquid to fly ash by mass varies with the grade of concrete [8]. The alkaline liquid (Na2SiO3 / NaOH) used in the current study was 2.5 for all the mixes. The fly ash and alkaline activator were mixed together in the mixer until homogeneous paste was obtained. This mixing process can be handled within 5 minutes for each mixture with different molarity of NaOH. Fresh fly ash based geopolymer concrete was usually cohesive [Fig. 2]. The workability of the fresh concrete was measured by means of conventional slump test [Fig. 3]. For easy working of fresh GPC mixes, superplasticizer Conplast SP430 was used. After casting the specimens, they were kept in rest period for two days and then they were demoulded. The demoulded specimens were kept accordingly to the various curing conditions namely ambient room temperature, steam curing and hot air oven curing both maintained at 60°C for 24 hours [Fig. 4 to 6]. Results and Discussions Workability Fresh GPC mixes were found to be highly viscous and cohesive with medium to high slump. The workability of the geopolymer concrete decreases with increase in the grade of the concrete as presented in Fig. 7, this is because of the decrease in the ratio of water to geopolymer solids. Hence we can say that as the grade