Properties of Brown Coal Fly Ash Geopolymer Mortars

Transcription

1 Properties of Brown Coal Fly Ash Geopolymer Mortars David W. Law 1, Thomas K. Molyneaux 1 and Rahmat Dirgantara 1 1 RMIT University, Melbourne, Australia Abstract: The use of industrial by-products to totally substitute Ordinary Portland cement (OPC) has become a major research area of late due to the environmental impact caused the production of CO 2 in the manufacture of cement. The use of industrial by-products as partial replacements for OPC is well established, with Class F Fly Ash and Ground Granulated Blastfurnace Slag being the most widely used. Recent research has also shown that concrete with similar strengths to both OP and blended cements can be achieved using Class F Fly Ash as a 100% replacement material with a suitable alkali activator. This material is known as geoploymer concrete. However, much of the Fly Ash produced in Australia is Brown Coal Fly Ash (Class C), which cannot be used as a replacement material for OPC due to the chemical composition adversely affecting the properties of the concrete produced. The reaction to form geopolymer concrete is different to that when the Fly Ash is used as a partial replacement material. As such the possibility exists to use Brown Coal Fly Ash to make geopolymer concrete. This paper reports on a research project investigating the development of Victoria Brown Coal Fly Ash for geopolymer concrete and the properties of the material produced. Keywords: geopolymer, brown coal, fly ash, sustainability 1. Introduction The production of Portland cement 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). FA as an industrial by-product contains silicate materials that have been used as an alternative binder material to OPC. The FA produced can be categorised as either class F or class C. Class F FA is produced from burning anthracite and bituminous coals, while class C FA is produced from lignite and sub-bituminous low-rank brown coal also known as Brown Coal (BC). The use of FA as a replacement material for OPC is well established, class F FA so far is the most commonly used in the synthesis of this alkali-activated binder (4). Activation of FA involves using a highly alkaline solution which will then form an inorganic binder through a polymerization process (5) e.g. dissolution, speciation equilibrium, gelation, reorganization, polymerization and hardening. The formation of [M z (AlO 2 ) x (SiO 2 ) y nmoh mh 2 O] gel has been identified as a dominant step in formation of an amorphous structure of geopolymer with the OH ion acting as a reaction catalyst during the activation process (6), while Davidovits has categorised the geopolymer structure as being based on the ratio of Si/Al (7). This initial development of geopolymer was undertaken using class F FA, as a higher proportion of silica (SiO 2 ) and or the sum of silica (SiO 2 ), alumina (Al 2 O 3 ) and iron (Fe 2 O 3 ) is needed to ensure that sufficient potential reactive constituent is present in FA. Much of the FA produced worldwide is class C FA from lignite and sub-bituminous (BC) and Australia has 25% of the world s BC total reserves (8), which cannot be used as a replacement material for OPC, due to the chemical composition adversely affecting the properties of the concrete produced. However, 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. Therefore if the composition of the aluminosilicate in the class C FA is sufficient it may be feasible to use them to produce geopolymer concrete (9). 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 (10, 11). Preliminary research using Loy Yang BC FA as 100% replacement of OPC demonstrated compressive strength comparable to those obtained from OPC mortar (12). The potential demonstrated for Loy Yang BC FA could also be applied to other BC FA produced in the La Trobe Valley Victoria which is a by-product from same source (sub-bituminous and lignite coals). 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 La Trobe Valley, Victoria BC FA to produce geopolymer mortars, and the optimization of the mix design. 2. Materials 2.1 Victoria-La Trobe Valley Brown Coal Fly Ash The FA used in this study came from Power Stations in the La Trobe Valley, Victoria, Australia. The La Trobe Valley contains large deposits of low-rank brown (lignite and sub-bituminous) coal. The FA supplied from 3 major Power Stations in Victoria: Loy Yang, Yallourn and Hazelwood. ASTM C defines FA into 2 classes, class F and class C. Class F is produced from burning anthracite and bituminous coals, while class C is produced from lignite and sub-bituminous coals. 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 (14). 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 samples (Table 1). As such, the term Brown Coal Fly Ash will be used instead of class C FA for all materials. A significant variation in the chemical composition was observed despite the same source and type of burning coal (15). Table 1 shows chemical composition of La Trobe Valley BCFA 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 Yallourn and Hazelwood BC FA (as usually found with class C FA) are significantly higher than Loy Yang BC FA, while the SiO 2, Al 2 O 3 and Fe 2 O 3 are significantly lower than Loy Yang. Table 1. Chemical composition of brown coal fly ash materials. Oxide % Brown Coal Fly Ash La Trobe Valley Class C Fly Ash* Loy Yang Yallourn Hazelwood Loy Yang Yallourn Hazelwood SiO Al 2 O Fe 2 O CaO MgO K 2 O Na 2 O TiO P 2 O SO Cl <0.1 < Cl 2 O Mn 2 O LOI * (14) 2.2 Alkali activators A grade D sodium silicate (Na 2 SiO 3 ) solution with composition of 14.7% Na 2 O, 29.4% SiO 2 and 55.90% water (a 2.00 ratio liquid sodium silicate of 1.52 g/cc density), and a 15 M sodium hydroxide (NaOH) solution with composition of 37.5% NaOH and 62.5% water were used as the alkali activator.

3 3. Mix proportions, Casting and Testing 3.1 Mix design 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 (16). 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 (15), the liquid to solid ratio of the mixture (16, 17) and the alkali modulus of activator (12, 18). The mortar and concrete specimens were produced using BC FA with different mix designs based on previous research at RMIT (12, 19). 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). 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). 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 (17). In order to obtain a workable geopolymer mortar 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 (20). 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. As the content SiO 2 of BC FA Yallourn and Hazelwood are significantly different compare to Loy Yang, the initial mix of Yallourn and Hazelwood were designed based on the ratio of SiO 2 +Al 2 O 3 +Fe 2 O 3 to the Na 2 O Loy Yang BC FA Mortar Pilot Mix Proportion The alkali modulus of activator of the mix was varied based on the composition silicon dioxide and sodium oxide content 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. Loy Yang BC FA specimen mix proportion. Mixture Mass (kg) Fly ash Sand Na 2 SiO 3 NaOH LY LY LY LY LY LY LY Table 3. Loy Yang BC FA specimen mass ratio. Mixture Mass ratio SiO 2 /Na 2 O SiO 2 +Al 2 O 3 +Fe 2 O 3 Liquid/Solid Fly ash wt% / Na 2 O LY LY LY LY LY LY LY

4 Table 2 is the mix proportion of mortar specimens. Mix LY1-LY5 were from a previous pilot study to get the optimum strength of the BC geopolymer mortar (12). Mixtures LY6 and LY7 are additional mix to investigate the alkali modulus gap of the pilot study. Detail of mixtures ratios are shown in Table Yallourn and Hazelwood Mortar Mix Proportion The initial mix proportions for Yallourn and Hazelwood BC FA were calculated based on the best mix from the Loy Yang geopolymer mortar trials, LY3. Since the SiO 2 content of Yallourn and Hazelwood BC FA are significantly lower compared to Loy Yang BC FA, the alkali activator ratio of these mixes will differ from the Loy Yang geopolymer mortar. Taking into account these differences in chemical composition, the initial mix was designed based on ratio of the sum of silica (SiO 2 ), alumina (Al 2 O 3 ) and iron (Fe 2 O 3 ) to Na 2 O instead of ratio of SiO 2 /Na 2 O. These gave a ratio of SiO 2 +Al 2 O 3 +Fe 2 O 3 to Na 2 O ranging from 3.5 to 4.3. As some of the initial mixes did not set, four addition mixes were prepared using the same proportions of the Loy Yang BC FA, as in LY 3, 5, 6 and 7. The mix proportion and specimen mass ratio are shown in Table 4. Table 4. Hazelwood and Yallourn BC FA specimen mix proportion and mass ratio. Mixture Mass (kg) Mass ratio Fly ash Sand Na 2 SiO 3 NaOH SiO 2 /Na 2 O SiO 2 +Al 2 O 3 +Fe 2 O 3 /Na 2 O H H H H H Y Y Y Y Y Specimen casting and testing The mortar specimen mixing was performed using a 5 liter Hobart mixer. Directly after mixing the mortar was placed in 50 x 50 x 50 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 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 demolded and kept at room temperature prior to testing. Testing was undertaken to investigate the compressive strength. Compressive strength measurements of mortars and concrete were performed on a Universal Testing Machine, in accordance with BS EN Specimens were tested at 7 and 28 days. 4. Compressive strength testing result Compressive strength result of Loy Yang BC FA geopolymer mortars are given in Table 6. The strengths obtained ranged from 13.44MPa, LY4 at 7 days to a maximum of 56.81MPa, LY3 at 7 days. The additional Mixes LY6 and LY7 gave 7 days strength of 27.17MPa and 32.72MPa and the 28 days strength of 26.44MPa and 28.77MPa respectively. The results show little variation of strength with time which is as 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). However, compressive strength of Hazelwood and Yallourn BC FA geopolymer mortars shown in Table 7 did not show comparable result with those obtained from Loy Yang BC FA geopolymer mortars. The strengths obtained are less than 10MPa, with mixes H2 and H4 not setting, even following heat curing.

5 Table 6. Loy Yang BC FA mortar compressive strength result. Mixture Compressive Strength (MPa) 7days 28days Mean SD Mean SD LY LY LY LY LY LY LY Table 7. Hazelwood and Yallourn BC FA mortar compressive strength result. 5. Discussion Mixture Compressive Strength (MPa) 7days Mean SD H H2 Not set N/A H H4 Not set N/A H Y Y Y Y Y Figure 1 shows the influence of the alkali modulus (SiO 2 /Na 2 O) on the strength of the mortar for the Loy Yang specimens. The Figure 1 (a) is the data from the previous study of alkali modulus mass ratio versus compressive strength (12). To further investigate the peak compressive strength mixes LY6 and LY5 were designed with Alkali Modulus (AM) ratio of 3.19 and 3.03, Figure 1 (b). The new data confirms that a maximum compressive strength is obtained by mix LY3, with an AM of The results show that this maximum strength is confined to a small range with values of AM 3.19 and 3.03, having compressive strengths 27.17MPa and 32.72MPa at 7 days compared to 56.81MPa for mix LY3. At 28 days these values are 26.44MPa and 28.77MPa for mixes LY6 and LY7, and 43.16MPa for LY3. Figure 1. Compressive strength (7days) vs alkali modulus SiO 2 /Na 2 O geopolymer mortars.

6 Figure 2 shows the ratio of the sum of silica, alumina and iron (SiO 2 +Al 2 O 3 +Fe 2 O 3 ) to Na 2 O. The shape of the graph being similar to that of the alkali modulus SiO 2 /Na 2 O graph. Both graphs show narrow ranges of the ratios that give higher strength. The ratio for the SiO 2 +Al 2 O 3 + Fe 2 O 3 to Na 2 O of 3.96 provided the best result. Figure 2. Compressive strength (7days) vs SiO 2 +Al 2 O 3 +Fe 2 O 3 to Na 2 O ratio. 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 (18), and bonding between geopolymer binder and aggregate (22). Figure 1 and 2 show the influence of the alkali modulus and aluminosilicate on the strength of the mortars. It is interesting to note the small range of mix compositions for which compressive strengths above 40 MPa can be achieved. This emphasizes the need for careful consideration of the chemical composition of the FA material when designing a specific mix. In particular the careful adjustment of the activator content, the AM and Na 2 O content. The results do show that by selecting these parameters correctly mortars with a compressive strength of 60 MPa can be achieved indicating that it may be feasible to produce a geopolymer concrete from BC FA that could give compressive strengths acceptable for structural purposes. Although the Yallourn and Hazelwood BC FA geopolymer mortar specimens were designed based on the optimum Loy Yang specimens, the initial results of both Yallourn and Hazelwood BC FA gave significantly lower compressive strengths than the Loy Yang specimens. Both mixes gave compressive strength less than 10MPa. Indeed a number of mixes, H2 and H4 did not set at all, Table 7. The low strengths of Yallourn and Hazelwood BC FA obtained is attributed to the content of aluminosilicate (SiO 2 +Al 2 O 3 ) being significantly lower (8.72% Yallourn, 5.14% Hazelwood compared to 64.81% Loy Yang). Although the alkali modulus ratio and the sum alumina, silica and iron to sodium oxide are the important factors in the reaction process, the main factor in the geopolymer reaction is the aluminosilicate contents of the mixture (18). For Yallourn and Hazelwood BC FA appeared that there is not sufficient aluminosilicate content available to react with the alkali activator to produce a geopolymer mortar with a compressive strength significantly above 10Mpa based on the mix designs employed. 6. Conclusion The maximum strengths of geopolymer mortar 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 mortar can produce compressive strengths acceptable for use in the construction industry. Further investigation is required to determine the mechanical properties and durability characteristic of the geopolymer concrete for use in the construction industry. For Yallourn and Hazelwood BC FA, future investigation is needed to examine the composition of the aluminosilicate and alkali activator that will create polymerization process to provide geopolymer binder, despite the fact that both BC FA has low content of aluminosilicate. As previous study stated: despite class C FA s lower content of silica and alumina, high strength could also be obtained when other factors affecting compressive strength remained in an optimistic level (18).

7 7. Acknowledgement Chemical analysis and imaging are performed using the facility, 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, Energy Australia Yallourn Power Station and GDF SUEZ Australian Energy Hazelwood 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", 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 Fernández-Jiménez, A., Palomo, A., "Composition and microstructure of alkali activated fly ash binder: Effect of the activator", Cement and Concrete Research, 2005, 35(10), pp Davidovits, J., "Geopolymer chemistry and sustainable Development. The Poly(sialate) terminology : a very useful and simple model for the promotion and understanding of greenchemistry.", in Geopolymers, Green Chemistry and Sustainable Development Solutions, Davidovits, J., Editor, 2005, Institut Géopolymère, Saint-Quentin, France, pp Geoscience Australia, ABARE, "Australian Energy Resource Assessment", 2010, Canberra. 9. Neville, A., M., "Properties of Concrete", 3rd ed, 1981, London, Pitman Publishing Limited. 10. 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. 11. 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. 13. 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. 14. 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. 16. 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. 18. 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

8 19. Law, D.W., Molyneaux, T.M., et al. "The Use Brown Coal Fly Ash To Make Geopolymer Concrete", in ACCTA 2013, 2013, Johannesburg. 20. Wang, S.-D., Scrivener, K.L., et al., "Factors Affecting The Strength of Alkali-activated Slag", Cement and Concrete Research, 1994, 24(6), pp 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