STRENGTH AND THERMAL STABILITY OF FLY ASH-BASED GEOPOLYMER MORTAR

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1 A.05 STRENGTH AND THERMAL STABILITY OF FLY ASH-BASED GEOPOLYMER MORTAR Djwantoro Hardjito- Senior Lecturer; M.Z. Tsen- Student Curtin University of Technology, Malaysia. ABSTRACT: This paper presents the engineering properties of geopolymer mortar manufactured from class F (low calcium) fly ash from Kuching, Sarawak, Malaysia, with potassium-based alkaline reactor. Tests were carried out on 50x50x50 mm cube geopolymer mortar specimens. The results revealed that as the concentration of KOH increased, the compressive strength of geopolymer mortar increases. The range of potassium silicate to KOH solution ratio by mass to produce high strength geopolymer mortar is in between This study also found that geopolymer posses superior thermal stability at least up to 800 C. The setting time of geopolymer was found dependent on the alkaline concentration. KEYWORDS: Fly ash, Geopolymer Mortar, Setting Time, Strength, Thermal Stability. 1. INTRODUCTION The manufacture of ordinary Portland cement (OPC) releases large amount of carbon dioxide (CO 2 ) to the atmosphere that significantly contributes to greenhouse gas emissions. One ton of carbon dioxide gas is released into the atmosphere for every ton of OPC produced. Currently, the world annual OPC production is about 1.6 billion tons or about 7% of the global loading of carbon dioxide into the atmosphere [1-3]. By the year of 2010, the global cement consumption is expected to reach 2 billion tons, meaning that approximately 2 billion tons of CO 2 will be released into the atmosphere. Production of one ton OPC requires approximately 1.5 tons of limestone, and considerable amount of both fossil fuel and electrical energy [4]. Therefore there is a need to find alternative type of binders to produce more environmentally friendly concrete. A promising alternative is to target reduction in CO 2 emissions from cement manufacture through the substitution with fly ash (a coal combustion waste product). The use of fly ash may reduce the total energy demand for producing concrete, lower the emissions of greenhouse gasses into the atmosphere from the concrete industry, and recycle the fly ash that otherwise only disposed in landfill. In this view, the use of fly ash can make valuable contribution to the reduction of environmental impact from concrete industry. In 1978, Joseph Davidovits developed a binder called geopolymer as a result of polymerization process of silicon and aluminium from the source materials with alkaline solutions. The source materials can be of geological origin or by-product materials, such as fly ash and slag. Geopolymer binder is a low CO 2 cementitious material compared to OPC because it does not rely on the calcinations of limestone that generates CO 2.[5]. Most of the previous studies on the engineering properties of fly ash-based geopolymer concrete or mortar utilized sodium-based alkaline reactor. On the other hand, only limited information available on the properties of geopolymer mortar produced using potassium-based alkaline 144

2 reactor. The objective of this research was to study the engineering properties and mix design of fly ash-based geopolymer mortar with potassium-based alkaline solution as the reactor. 2. MATERIALS AND EXPERIMENTAL DETAILS 2.1. Materials Fly ash, known also as pulverized-fuel ash, is precipitated electrostatically or mechanically from exhaust gases of coal-fired power stations. In this study, low-calcium fly ash (ASTM C a Class F) from Sejingkat Power Plant in Kuching, Sarawak, Malaysia, was used as the source material. The fly ash particles are spherical, and grey in color. The chemical composition and the loss on ignition (LOI) of fly ash, as determined by XRF, are given in Table 1. The local fine aggregate from Miri, Sarawak, Malaysia, with fineness modulus of 1.48, was used. Table 1: Chemical Compositions of Fly Ash as Determined by XRF Oxides SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO TiO 2 K 2 O Na 2 O SO 3 LOI Percent A combination of potassium hydroxide (KOH) and potassium silicate solution was used as the alkaline reactors in this research. Distilled water was used to dissolve KOH pellets to prevent any effect of unknown contaminants. The mass of the KOH dissolved in distilled water varies according to the required concentration of solution. The range of KOH concentration used in this research was in between 6 to 14 M Details of the tests Mix Design, Manufacture and Curing of Mortar Specimens Mixture No Table 2: Mixture Composition (kg/m 3 ) Sand Fly Ash Potassium Silicate Solution 145 Potassium Hydroxide Solution Mass Molarity All geopolymer mortar specimens were made with sand to fly ash ratio by mass of 2.75, whereby the sand was in saturated surface dry condition. The concentration of potassium hydroxide (KOH) solution varied from 6 to 14M. The ratios (by mass) of potassium silicate to potassium hydroxide solution were varied in the range of 0.4 to 2.5. The fly ash and the fine aggregates were dry mixed together for 2 minutes using Hobart mixer at gear 1, followed by the addition of

3 activator solution containing potassium hydroxide and potassium silicate to the mixture, and mixed for another 10 minutes. The mixing was carried out in a room temperature of approximately C. There are nine mixtures were made to study the influence of various parameters on the compressive strength of fly ash-based geopolymer mortar manufactured using potassium-based alkaline reactor. The details of these mixtures are presented in Table 2. The fresh geopolymer mortar was cast in the 50x50x50 mm cube moulds. The specimens were compacted in two layers. Each layer was tamped 25 times with a rod. This was followed by an additional vibration of 2 minutes using vibrating table to remove air voids. All the specimens were put in the oven without delay time for curing at elevated temperature of 60 C for 24 hours. After curing at elevated temperature, the specimens were demoulded and cured at undisturbed room temperature until the day of testing Testing The compressive strength tests were performed in accordance to ASTM C109. The specimens were tested for 1, 7, 28 and 70-day compressive strength. The specimens were subjected to a compressive force at the rate of 1800 N/sec until it failed. Each of the test data points plotted in various Figures corresponds to the mean value of the compressive strengths of three test cubes in a series. The standard deviations are plotted on the test data points as the error bar. The geopolymer setting time measurements were conducted by using Vicat needle apparatus in accordance to ASTM C191 with some modification. The needle used was 1 mm diameter. Firstly, the amount of fly ash, potassium hydroxide and potassium silicate required were weighted. In this case, fine aggregate was excluded from the mixture proportion. Then the fly ash and activator solution were placed in the mixer bowl and the Hobart mixer was started at gear 1 for 10 minutes. The geopolymer paste was place into the 40mm height, 80 mm diameter conical mould in two layers. Each layer was tamped 25 times with a rod. The specimen was then placed into the oven for curing at elevated temperature at 60 C. At every interval of 15 minutes, the specimen was taken out and placed on the Vicat apparatus to measure the initial and final setting time. To investigate the thermal stability of geopolymer mortar or strength under high temperature, the specimens were exposed to fire in furnace with temperature in the range of 400 C to 800 C for three hours. The compressive strength results were then compared to those of mortar specimens with no fire exposure. 3. RESULTS AND DISCUSSION 3.1. Concentration of potassium hydroxide solution (KOH) Mixtures 1 to 5 were prepared to study the effect of molarity or concentration of potassium hydroxide solution on the compressive strength of geopolymer mortar. From Figure 1, it is observed that alkaline concentration is proportionate to compressive strength of geopolymer mortar. It is clear that the concentration of KOH has an effect on the compressive strength of geopolymer mortar. This might be due to the acceleration in the geopolymerisation process with the increase of the KOH concentration or molarity. Similar results had also been found elsewhere [6]. However, these results are in disagreement with the study conducted by Alonso and Palomo [7] using metakaolin and calcium hydroxide. The study indicated that when activator concentration increased above 10M, a delay in the formation of alkaline polymer happened and hence resulting in decrease in flexural strength. 146

4 Compressive Strength (MPa) KOH Concentration (M) 1 day 7 days 28 days Figure 1: The Influence of KOH Concentration on the Compressive Strength This disagreement might be due to the differences in the type of source material and proportion of the mixtures. Alonso and Palomo [7] used a highly pure metakaolin (57% SiO 2, 41.5% Al 2 O 3 ) produced by subjecting the raw kaolin to thermal treatment at 750 C for 24 hours. 3.2.Ratio of potassium silicate to potassium hydroxide solution by mass Figure 2: The Influence of Potassium Silicate to Potassium Hydroxide Solution by Mass on the Compressive Strength The results indicated that with K 2 SiO 3 /KOH ratio in the range within 0.8 to 1.5, geopolymer mortar achieved highest compressive strength. This is rather unexpected as the previous studies on low calcium fly ash-based geopolymer concrete reacted by sodium-based alkaline solution indicated that the higher the silicate to hydroxide solution ratio resulted in higher compressive strength [8, 9]. The compressive strength results of Geopolymer mortar with different ratio of K 2 SiO 3 /KOH solution by mass is shown in Figure 2. In this series, the concentration of potassium hydroxide solution was kept constant at 8M for all the mixtures. All mixture was cured at 60 C for 24 hours. 147

5 3.3.Thermal stability of geopolymer mortar at high temperature Mixtures 4 were prepared to study the thermal stability of geopolymer mortar, especially on the compressive strength. Geopolymer mortar specimens were sent to furnace at the age of 7 th day for three hours under 400 o C, 600 o C and 800 o C respectively. From Figure 3, it is observed that geopolymer mortar specimens exposed to 400 C yielded the highest compressive strength of 81 MPa. With the increase of high temperature exposure up to 800 o C, Figure 3 shows steady loss in strength with an increase in temperature; however it remains higher than those of control mixture. These facts confirm the excellent fire resistance of geopolymer mortar, as also reported by Cheng and Chiu [6]. When exposed to temperature of 400 o C for three hours, the compressive strength doubled than the one of control mixture. This indicates that the geopolymerisation process continues when geopolymer mortar exposed to high temperature, at least up to 400 o C, resulted in very high strength gain. From then on, with the increase in temperature, the compressive strength starts to decrease Control Temperature ( o C) 3.4. Setting time Figure 3: Compressive Strength of Geopolymer Mortar at High Temperature Mixture 1, 3 and 5 were used to investigate the setting time of Geopolymer paste at curing temperature of 60 C. The difference of all these 3 mixtures is the concentration of alkaline solution. For Mixture 1 the concentration of KOH solution is 14M, while for Mixture 3 and 5 are 10M and 6M respectively. The alkaline solution-to-fly ash ratio by mass was held constant at 0.3. Fine aggregates were removed from these mixtures. 148

6 Depth from Surface (mm) M KOH 10M KOH 14M KOH Setting Time (min) Initial Final Figure 4: The effect of KOH molarity on setting time at 60 C Figure 4 shows that the geopolymer paste setting time is faster when KOH concentration increased. This is rather unexpected compared to study conducted by Cheng & Chiu [6]. The study indicated that when the alkaline reactor concentration increased, a longer setting time was required. However, this might explain the increase of compressive strength when the KOH molarity increased, as shown in Figure 1. The strength gain resulted from the increase in the KOH molarity might be due to the faster geopolymerisation process, as indicated by faster setting times. 4. CONCLUSIONS From the experimental results reported in this study, the following conclusions are drawn. a) Higher concentration (in terms of molarity) of potassium hydroxide solution produces higher compressive strength of fly ash-based geopolymer mortar. b) Ratio of potassium silicate-to-potassium hydroxide by mass in the range between produced highest compressive strength geopolymer mortar. c) Geopolymer mortar posses excellent fire resistance at least up to 800 C exposure for three hours. d) Setting times of geopolymer mortar reduced with the increase in the molarity of KOH solution. REFERENCES 1. Mehta, P.K., Reducing the Environmental Impact of Concrete. ACI Concrete International, (10): p Malhotra, V.M., Making Concrete "Greener" With Fly Ash. ACI Concrete International, (5): p Malhotra, V.M., Introduction: Sustainable Development and Concrete Technology. ACI Concrete International, (7): p Greer, W.L., Johnson, M. D., Morton, E.L., Raught, E.C., Steuch, H.E. and Trusty Jr., C.B.,, Portland Cement, in Air Pollution Engineering Manual, Anthony J. Buonicore and Waynte T. Davis (eds.) , New York: Van Nostrand Reinhold. 149

7 5. Davidovits, J. Properties of Geopolymer Cements. in First International Conference on Alkaline Cements and Concretes Kiev, Ukraine, 1994: SRIBM, Kiev State Technical University. 6. Cheng, T.W. and J.P. Chiu, Fire-resistant Geopolymer Produced by Granulated Blast Furnace Slag. Minerals Engineering, (3): p Alonso, S. and A. Palomo, Alkaline Activation of Metakaolin and Calcium Hydroxide Mixtures: Influence of Temperature, Activator Concentration and Solids Ratio. Material Letters, (1-2): p Hardjito, D., et al., On The Development of Fly Ash-Based Geopolymer Concrete. ACI Materials Journal, (6). 9. Hardjito, D. and B. Rangan, Development and Properties of Low Calcium Fly Ash-based Geopolymer Concrete, Research Report GC , Curtin University of Technology: Perth, Australia. 150