Brown Coal Fly Ash Geopolymer Concrete

Transcription

1 Brown Coal Fly Ash Geopolymer Concrete Rahmat Dirgantara 1,2, David W. Law 1 and Thomas K. Molyneaux 1 1 RMIT University, Melbourne, Australia 2 STT Harapan College, Medan, Indonesia Abstract: The production of Portland Cement (PC) as the main binder in concrete raises environmental concern over its CO 2 emission. Other concerns arise over the use of burning coal as a primary energy source, as it releases Fly Ash (FA) by-product of which some becomes environmental waste. ASTM classifies FA as class F produced from anthracite and bituminous or class C produced from lignite and sub-bituminous coals. Victoria FA is produced from lignite known as Brown Coal (BC), but due to the chemical composition cannot categorize as class C. Recent studies have shown the viability of using 100% class F FA as the binder to create alkali activated concrete or geopolymer concrete. The activation process of geopolymer varies notably from PC concrete, due to the activation of the aluminosilicate by high concentration alkali. Thus, if the content of the aluminosilicate in the BC FA is sufficient it may possible to produce BC FA geopolymer concrete. The aim of this study is asses the viability of producing BC FA geopolymer with a compressive strength comparable to class F geopolymer and PC concrete. The key steps to reach the aims are to determine the chemical composition of BC FA, and note how its composition could lead to the design a geopolymer mix. This paper reports on a study of Loy Yang BC FA to produce geopolymer mortar and concrete. The experiment was initiated with mortar specimen followed by concrete specimens. Experiments using Loy Yang BC FA gave a strength of 56MPa for mortar and 59MPa for concrete which is comparable to class F geopolymer and OPC specimens. Results indicate that Loy Yang BC FA geopolymer concrete can produce strengths acceptable for use in the construction industry. Keywords: brown coal, fly ash, geopolymer, mortar, concrete 1. Introduction The production of Portland Cement (PC) as the main binder material in concrete raises a number of environmental concerns regarding the energy consumption and the emission of CO 2 (1, 2). Other concerns have also highlighted the use of coal as a primary energy source in the world. The use of coal releases Fly Ash (FA) as a by-product, some of which becomes environmental waste (3). ASTM classifies FA as class F produced from anthracite and bituminous or class C produced from lignite and sub-bituminous coals. Victoria FA is produced from lignite, known as Brown Coal (BC) (4), but because of the chemical composition cannot categorize as class C. The use of FA as a replacement material for OPC is well established. However, geopolymer concrete is a new kind of material that uses a different chemistry to OPC, and does not need to contain OPC to work (5). At present class F (low calcium) FA so far is the most commonly used in the synthesis of this alkaliactivated binder (6). Activation of FA involves using a highly alkaline solution which will then form an inorganic binder through a polymerization process (7) e.g. dissolution, speciation equilibrium, gelation, reorganization, polymerization and hardening. The activation process for geopolymer concretes is due to the activation of the aluminosilicate by high concentration alkali rather than the activation of the FA by the Ca(OH) 2 produced by the hydration of the OP cement (8). Therefore, if the composition of the aluminosilicate in the BC FA is sufficient it may be feasible to use them to produce geopolymer concrete. Some previous research has been undertaken using class C FA (also referred to as high Ca FA) to explore the feasibility of using it as a binder in geopolymer concrete (9, 10). According to the Concrete Institute of Australia (CIA), Australian BC FA is less preferred as a potential alkali reactive binder as it tends to show little geopolymer reaction and more likely to be classified as non-reactive fillers (4). However, preliminary research on geopolymer mortar using Loy Yang BC FA as 100% replacement of OPC demonstrated compressive strength comparable to those obtained from OPC mortar (11). The potential demonstrated for the geopolymer mortar could also be applied to produce geoplolymer concrete. This potential use of BC FA could result in utilization of a widespread industrial by-product from coal burning power stations, which presently are dumped into the environment resulting in the saving of natural resources and energy.

2 This paper reports on a research project undertaken to investigate the possible use of Loy Yang BC FA to produce geopolymer concrete. 2. Materials 2.1 Victoria-La Trobe Valley Brown Coal Fly Ash The FA used in this study came from Loy Yang Power Stations in the La Trobe Valley, Victoria, Australia. The La Trobe Valley contains large deposits of low-rank brown coal (lignite) also known as BC. There are 3 major power stations in La Trobe Valley Victoria: Loy Yang, Yallourn and Hazelwood. ASTM C defines FA into 2 classes, class F and class C. Both have pozzolanic properties, and in addition to this, class C also has some cementitious properties and the total calcium (CaO) content is typically higher than class F. ASTM C also differentiates the FA based on the minimum percentage of silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ) and iron oxide (Fe 2 O 3 ), and the maximum sulfur trioxide (SO 3 ) content. The minimum combined content of silicon dioxide, aluminum oxide and iron oxide is 70% and 50% for class F and class C respectively. The maximum sulfur trioxide (SO 3 ) content is 5.0% for both classes of FA. Based on the type of coal used and previously published data the La Trobe Valley FA was classified as class C FA (13). However analysis of the chemical composition of the material supplied for this study could not categorized into those two classes of ATM C This is due to the high SO 3 content and the percentage of SiO 2, Al 2 O 3 and Fe 2 O 3 in the Yallourn and Hazelwood FA (Table 1). As such, the term Brown Coal Fly Ash will be used for all materials. A significant variation in the chemical composition was observed despite the same source and type of burning coal (14). Table 1 shows chemical composition of La Trobe Valley BC FA previously reported and the current materials supplied. The SiO 2, Al 2 O 3 and Fe 2 O 3 contents are significantly different between Yallourn, Hazelwood, and Loy Yang. The CaO content of Loy Yang BC FA is significantly lower compared to typically found in class C FA. Table 1. Chemical composition of brown coal fly ash materials. Oxide % Brown Coal Fly Ash La Trobe Valley Class C Fly Ash* * (13) 2.2 Alkali activators Loy Yang Loy Yang Yallourn Hazelwood SiO Al 2O Fe 2O CaO MgO K 2O Na 2O TiO P 2O SO Cl <0.1 < Cl 2O 0.44 Mn 2O 0.10 LOI A sodium silicate solution with composition of 14.7% Na 2 O, 29.4% SiO 2 and 55.90% water, and a 15 M sodium hydroxide solution with composition of 37.5% NaOH and 62.5% water were used as the alkali activator. Sodium hydroxide solution (15 M NaOH) was used together with grade D sodium silicate (Na 2 SiO 3 ) with a 2.00 ratio liquid sodium silicate (Na 2 O = 14.7% and SiO 2 = 29.4%) of 1.52 g/cc density.

3 3. Mix design, Casting and Testing 3.1 Mix proportion of specimen The geopolymerization reaction depends on the chemical properties of the FA, the availability of soluble silicates and aluminates, and the concentration of added sodium hydroxide (5, 15). There are several factors need to be investigated to be able to produced BC FA geopolymer concrete: the significant variation of the chemical composition of BC FA from different sources (14), the liquid to solid ratio of the mixture (15, 16) and the alkali modulus of activator (11, 17). The mortar specimens were produced using BC FA with different mix designs (11, 18). For all specimens, the FA content was designed to have minimum 10wt% of the mass of the mixture. The content of the FA in the mixture was evaluated by mass ratio of the FA to total weight of mixture (wt% FA). Sand (fine aggregate) used for the mixture was uncrushed sand in oven dried condition. The liquid mass (water content) was determined taking into account all of the water in the mixed alkali activators (water quantity of sodium silicate and sodium hydroxide solutions) (5). The solid mass was determined by accounting for all solid materials of the binder and aggregate including the solid content of the sodium silicate and sodium hydroxide solutions (16). In order to obtain a workable geopolymer mortar molding, a liquid to solid ratio (water content) of 10-15% was maintained. The dosage and modulus of activator have significant influence on the properties of the mortar (19). The dosage of activator is defined as the ratio of the Na 2 O content of the alkaline activator to the mass of the binder, whereas the activator modulus is the mass ratio of the SiO 2 to the Na 2 O in the alkaline activator Loy Yang BC FA Geopolymer Mortar The alkali modulus of activator was varied based on the composition of the Loy Yang FA supplied and those contents from sodium silicate and sodium hydroxide solutions. The alkali modulus of activator was investigated over a range from 2.75 to The content of alkali activator was calculated including silicon dioxide (SiO 2 ) and sodium oxide (Na 2 O) content from the BC FA. Table 2 is the mix proportion of mortar specimens from a previous study to get the optimum strength of the BC geopolymer mortar (11, 18), and detail of mixtures ratios. Table 2. Loy Yang BC FA mortar mix proportion and mass ratio. Mixture Mass (kg) Mass ratio Fly ash Sand Na 2SiO 3 NaOH SiO 2/Na 2O Liquid/Solid Fly ash wt% Mix LY Mix LY Mix LY Mix LY Mix LY Mix LY Mix LY Loy Yang BC FA Geopolymer Concrete Initial Mix Proportion BC FA GC mix is initial mix proportion for Loy Yang BC FA geopolymer concrete based on the the best mixture of the Loy Yang BC FA geopolymer mortar. Using the mix LY3 fine aggregate (sand) proportion was substituted by a combination of aggregates of 43% sand, 38% 10mm aggregate and 19% 7mm aggregate (Table 3). Those percentages of combined aggregates were adopted from previous research at RMIT with same source of the aggregates (20). The fine aggregate was uncrushed sand and the coarse aggregates were crushed basalt aggregate. The aggregate moisture condition of BC GC Mix1 was saturated surface dry (SSD), while other mix BC GC Mix2 used oven dried aggregates same as for the sand (fine aggregate) used in previous mortar specimens.

4 Table 3. Loy Yang BC FA concrete initial mix proportion. Mixture Mass (kg) Fly ash Aggregates Na 2SiO 3 NaOH Sand 7-mm 10-mm BC FA GC Mix Specimen moulding and testing The mortar specimen mixing was performed using a 5 liter Hobart mixer, while the concrete specimen mixing was performed using 25-litre mixer. Directly after mixing the mortar was placed in 50 x 50 x 50 mm cubic molds and the concrete used 100 x 100 x 100 mm cubic molds. The specimens were compacted with two-layer placing and tamping, and placed on a vibration table. The specimens were left for 24 hours at room temperature. The specimens were then covered with oven bag to prevent moisture loss and put in the oven heated at 120 C for 10 hours. The specimens were left to cool at room temperature before being de-molded. The specimens were kept at room temperature prior to testing. Testing will be undertaken to investigate the compressive strength. Compressive strength measurements of mortar and concrete were performed on Universal Testing Machine and MTS machine accordance with BS EN Three cubes were tested for each data point. 4. Compressive strength testing result Compressive strength result of Loy Yang BC FA geopolymer mortars are given in Table 4. The strengths obtained range from 13.44MPa, Mix LY4 at 7 days to a maximum of 56.81MPa, Mix LY3 at 7 days. The results show little variation of strength with time which would be expected due to the elevated curing temperatures applied. Similar results have been observed by other authors and is attributed to the geopolymer materials requiring heat curing to achieve activation of the binder (21). Table 4. Loy Yang BC FA mortar compressive strength result. Mixture Compressive Strength (MPa) 7days 28days Mean SD Mean SD Mix LY Mix LY Mix LY Mix LY Mix LY Mix LY Mix LY Table 5. Loy Yang BC FA geopolymer concrete compressive strength result. Mixture BC FA GC Mix1 BC FA GC Mix2 Compressive Strength (MPa) Mean SD Mean SD Mean SD 3days 7days 28days days 56days 91days The BC FA geopolymer concrete Mix 1 gave compressive strength comparable to Loy Yang BC FA mortar with same mix proportion. Compressive strength of initial Mix 1 Loy Yang BC FA geopolymer concrete are 45.98MPa, 43.60MPa and 40.95MPa for 3, 7 and 28 days testing respectively. Compressive strength of oven dried aggregate Mix 2 Loy Yang BC FA geopolymer concretes are 59.59MPa, 52.08MPa and 60.38MPa for 7, 56 and 91 days of testing (Table 5).

5 5. Discussion Figure 1 is the data from the previous studies of alkali modulus mass ratio versus compressive strength (11, 18). The results show that this maximum strength is confined to a small range with values of AM 3.19 and 3.30 having compressive strengths 27.17MPa and 32.72MPa at 7 days compared to AM 3.11 for 56.81MPa of Mix LY3. Those results show the influence of the alkali modulus (SiO 2 /Na 2 O) on the strength of the mortars. The CIA (5) highlighted the SiO 2 /Na 2 O mass ratio as one of the key parameter on the mix design to optimized the concrete strength. The polymerization process in geopolymer binder/gel is due to the aluminosilicate precursor being activated in a concentrated alkali hydroxide solution, and the compressive strength of the geopolymer mortar was contingent of the geopolymer gel, concentration of the alkaline compound, water content, curing temperature (5, 17), and bonding between geopolymer binder and aggregate (22). Figure 1. Compressive strength (7days) vs alkali modulus SiO 2 /Na 2 O. The results for the Loy Yang BC FA geopolymer concrete mix are given in Figure 2. The BC FA GC Mix1 gave compressive strengths of approximately 45MPa, with a slight reduction observed from 3 days to 28 days. The BC FA GC Mix2 gave compressive strengths of between 55-60Mpa, with similar strengths at 7 and 91 days, with a small reduction at 56 days. The variations are attributed to the variability in the mix. The results obtained are comparable with those observed for Class F FA geopolymers and also are greater than those specified for OP concretes for reinforced concrete structures categorized as exposure class A or B, AS Figure 2. Compressive strength results of Loy Yang BC FA geopolymer concrete. The difference in the results is attributed to different moisture content in the aggregates. Those results reflected the liquid to solid ratio of the mixture and shows narrow range of liquid to solid ratio (5, 15). Geopolymer mortars with lower liquid to solid ratio are normally stronger than higher ratio (5, 11, 17). The 7 days strength of BC FA GC Mix1 gave lower strength 45.98MPa compare to mortar results 56.81MPa. However, BC FA GC Mix2 gave comparable strength 59.59MPa to mortar result 56.81MPa. The Mix 2 had the aggregates with the same level of moisture, dry sand and aggregate, compared with the Mix 1 which had dry sand but SSD aggregates. These results indicate that good

6 correlation is obtained between the mortar strengths and concrete strength if the aggregates are in the same condition. Again the variation can be attributed to the change in the liquid to solid ratio due to the SDD aggregates. This would indicate that the moisture level of the aggregates should be taken into account when designing the concrete mix. It is interesting to note the comparable result between mortar and concrete specimens, as the only different between those two were fine aggregate(sand) being substituted with combination of fine and coarse aggregates. As likely as concretes, the basic ingredient of the geopolymer concrete are aggregate skeleton (stone and sand) and geopolymer binder/gel (a cement paste in OPC concrete), and to minimize gross void space in the compacted skeleton the skeleton particle size distribution should be optimized (5). These grading optimization could lead the possibility to produce BC FA geopolymer concrete with a compressive strength more than 60MPa, indicating that it may feasible to use for structural purposes. 6. Conclusion The maximum strengths of geopolymer concrete obtained from Loy Yang BC FA are comparable to those obtained from previous research on class F and class C FA. The strengths are also comparable to those obtained from OPC specimens and are indicative that Loy Yang BC FA geopolymer concrete can produce compressive strengths acceptable for use in the construction industry. A dependable geopolymer product could be produced despite an inherent variation of fly ash content as corrections of any deficiencies or changes in ash properties are able to be made in mix design (5). These optimization corrections could be made in a geopolymer mortar mix design rather than in a geopolymer concrete mix design, as this research also showed comparable compressive strength result of mortar and concrete geopolymer from same source of BC FA. Further investigation is required to determine the mechanical properties and durability characteristics of geopolymer concrete for use in the construction industry. 7. Acknowledgement Chemical analysis and imaging are performed using the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the RMIT Microscopy & Microanalysis Facility, at RMIT University. The authors would like to thank AGL Loy Yang Power Station for providing the Brown Coal Fly Ash and PQ Australia Pty Ltd for providing the Sodium Silicate. 8. References 1. Davidovits, J., "Global Warming Impact on the Cement and Aggregates Industries", World Resource Review, 1994, 6(2), pp Berry, M., Cross, D., et al., "Changing the Environment: An Alternative "Green" Concrete Produced without Portland Cement", in 2009 World of Coal Ash (WOCA) Conference, 2009, Lexington, KY, USA. 3. Naik, T.R., Singh, S.S., "Fly Ash Generation and Utilization - An Overview", Geoscience Australia, ABARE, "Australian Energy Resource Assessment", 2010, Canberra. 5. CIA, "Recommended Practice Geopolymer Concrete", 2011, Sydney, Concrete Institute of Australia. 6. Guo, X., Shi, H., et al., "Alkali-activated complex binders from class C fly ash and Cacontaining admixtures", Journal of Hazardous Materials, 2010, 173(1 3), pp Barbosa, V.F.F., MacKenzie, K.J.D., et al., "Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers", International Journal of Inorganic Materials, 2000, 2(4), pp Neville, A., M., "Properties of Concrete", 3rd ed, 1981, London, Pitman Publishing Limited. 9. Guo, X., Shi, H., et al., "Performance and Mechanism of Alkali-Activated Complex Binders of High-Ca Fly Ash and other Ca-Bearing Materials", in 2009 World Of Coal Ash (WOCA) Conference, 2009, Lexington, KY, USA.

7 10. Chindaprasirt, P., Chareerat, T., et al., "High-Strength Geopolymer Using Fine High-Calcium Fly Ash", Journal of Materials in Civil Engineering, 2011, 23(3), pp Dirgantara, R., Law, D., et al. "Brown coal fly ash geopolymer mortar", in ACMSM 22, 2013, Sydney, CRC Press. 12. ASTM, "ASTM C618-12", in Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, 2012, ASTM International, West Conshohocken, US. 13. Macphee, D.E., Black, C.J., et al., "Cements Incorporating Brown Coal Fly Ash from The Latrobe Valley Region of Victoria, Australia", Cement and Concrete Research, 1993, 23(3), pp French, D., Smitham, J., "Fly Ash Characteristics and Feed Coal Properties", 2007, Cooperative Research Centre for Coal in Sustainable Development, Pullenvale, Qld 4069, Australia. 15. Fansuri, H., Prasetyoko, D., et al., "The effect of sodium silicate and sodium hydroxide on the strength of aggregates made from coal fly ash using the geopolymerisation method", Asia- Pacific Journal of Chemical Engineering, 2012, 7(1), pp Lloyd, N.A., Rangan, B.V. "Geopolymer Concrete with Fly Ash", in Second International Conference on Sustainable Construction Materials and Technologies, 2010, Universita Politecnica delle Marche, Ancona, Italy. 17. Li, X., Ma, X., et al., "Mechanical Properties and Microstructure of Class C Fly Ash-Based Geopolymer Paste and Mortar", Materials, 2013, 6(4), pp Law, D.W., Molyneaux, T.M., et al. "The Use Brown Coal Fly Ash To Make Geopolymer Concrete", in ACCTA 2013, 2013, Johannesburg. 19. Wang, S.-D., Scrivener, K.L., et al., "Factors Affecting The Strength of Alkali-activated Slag", Cement and Concrete Research, 1994, 24(6), pp Adam, A., "Strength and durability properties of alkali activated slag and fly ash-based geopolymer concrete", in School of Civil, Environmental and Chemical Engineering, 2009, RMIT University, Melbourne. 21. Sindhunata, "A Conceptual Model of Geopolymerisation", 2006, The University of Melbourne, Melbourne. 22. Temuujin, J., van Riessen, A., et al., "Preparation and characterisation of fly ash based geopolymer mortars", Construction and Building Materials, 2010, 24(10), pp