The Use of Fly Ash to Reduce the Environmental Impact of Concrete

Transcription

1 The Use of Fly Ash to Reduce the Environmental Impact of Concrete Djwantoro Hardjito and Ng Tyam Pui School of Engineering and Science, Curtin University of Technology, Sarawak Campus CDT 250, Miri 98009, Sarawak, Malaysia ABSTRACT The production of cement is increasing about 3% annually. As one of the most popular and major construction materials worldwide, the Portland cement production is expected to rise from 1.7 billion tons in 2000 to 2 billion tons in As the production of one ton of Portland cement liberates about one ton of CO 2 to the atmosphere, currently the contribution of Portland cement production worldwide to the greenhouse gas emission is about 1.35 billion tons annually or about 7% of the total greenhouse gas emissions to the earth s atmosphere. Aside from that, cement is among the most energy-intensive construction materials as well. Due to the environmental concern, the production of ordinary Portland cement attracts critics. One strategy to make concrete greener construction material is to utilize fly ash, a by-product material from burning coal at power plants, either as partial or total substitute for Portland cement in concrete. This attempt results in twofold benefits, i.e. to provide a solution with regard to the concern on the carbon dioxide emission from Portland cement production, and to provide way to effectively use fly ash. Fly ash is available abundantly worldwide. Its availability is increasing and yet its utilization to date is still very low. Without proper plan, the management of fly ash may incur cost, and potentially harm the natural environment as well. Two noted achievement in this attempt are the development of high volume fly ash (HVFA) concrete which replaces more than 50% of Portland cement by fly ash, and the development of fly ash-based geopolymer concrete which totally replaces the use of Portland cement in concrete. This paper reports the results from initial study on HVFA concrete utilising up to 50-70% by mass of fly ash from Kuching, Sarawak. This paper also elaborates the current status of the development of fly ash-based geopolymer concrete. Key words: Cement, Concrete, Fly Ash, and Environmental Concern 1. INTRODUCTION Environmental issues related to carbon dioxide (CO 2 ) emissions from the production of Portland cement require serious attention from the concrete industry. Partial or total replacement of Portland cement in concrete using materials, such as fly ash and rice husk ash, should be implemented in increasing quantities. As one of the most popular and major construction materials worldwide, the Portland cement production is expected to rise from 1.7 billion tons in 2000 to 2 billion tons in 2010, whereby the major increase will take place in China and India [1]. Portland cement production raises environmental concern, not only due to highly energy intensive, but also due to high amount of carbon dioxide released to the atmosphere. Production of one ton Portland cement will release one ton of carbon dioxide into atmosphere [2]. On the other hand, fly ash, the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gases from the combustion zone to the particle removal system [3], is available abundantly. Worldwide, the estimated production of coal ash in 2004 was in the order of 990 million tons, with China, India and US as the main producers. Only less than 15 percent of this fly ash was utilized in concrete, while the rest was just mainly disposed in landfills [4].

2 The utilization of fly ash and other byproduct materials, such as granulated blast furnace slag, rice husk ash, as a substitute or partially substitute of Portland cement in concrete has been studied intensively and extensively. Partial substitution of Portland cement by 20-30% fly ash has become a common practice. A more substantial development in this area were the development of high volume fly ash (HVFA) concrete with 50-65% Portland cement replacement by using fly ash, and the development of fly ash-based geopolymer concrete whereby the use of Portland cement is totally replaced by the geopolymer matrix using fly ash as the source material. This paper reports the results of study on the fly ash utilisation to reduce the use of Portland cement in concrete, particularly the development of high volume fly ash (HVFA) and geopolymer concrete using locally available fly ash. 2. OBJECTIVES To present the result of initial study on high volume fly ash (HVFA) concrete using fly ash from Kuching, Sarawak, Malaysia To elaborate the current status of study on fly ash-based geopolymer concrete 3. HIGH VOLUME FLY ASH CONCRETE High volume fly ash (HVFA) concrete is defined as concrete with more than 50% by mass of total cementitious material replaced by fly ash. The fly ash used is usually low calcium (class F) fly ash. Fly ash in itself does not have cementitious properties, however, with the presence of water, the silicon dioxide (SiO 2 ) content in fly ash will react with calcium hydroxide (CaOH 2 ) from the hydration process to form the calcium silicate hydrate (C-S-H), which is the strong cement gel. This process is well known as the pozzolanic reaction. Partially replacing Portland cement with fly ash will enhance the engineering properties of concrete, i.e. increase the strength, reduce the permeability, lower the amount of Ca(OH) 2 in concrete, and thus improve its durability. HVFA concrete has been implemented successfully in various construction projects, such as in buildings, foundations and piers in US [5] and road pavement in India [6]. In this initial study, low calcium (class F) fly ash from Kuching, Sarawak, was investigated its possibility to be used in producing HVFA concrete. The chemical composition of the fly ash as determined by X-Ray Fluorescence (XRF) analysis is shown in Table 1. The specific surface area of the fly ash was 1.51 m 2 /cc, with 80% of the particles size of the fly ash smaller than 41 m. Table 1: Composition of fly ash as determined by XRF (mass %) Oxides % by mass SiO Al 2 O Fe 2 O CaO 2.40 Na 2 O 0.30 K 2 O 3.14 TiO MgO 1.95 P 2 O SO MnO 0.18 LOI *) Method Four concrete mixtures have been prepared. All of the mixtures have the same water, coarse and fine aggregate contents, i.e kg/m 3, 1200 kg/m 3 and 650 kg/m 3 respectively. The water to cementitious ratio was kept constant at To improve the workability of the fresh concrete, superplasticiser commonly available in the market was used in the amount of 7 kg/m 3. The total cementitious content was 370 kg/m 3. The difference among the mixtures was the amount of the partial replacement of Portland cement by fly ash, as shown in Table 2. The workability of the fresh HVFA concrete was determined by measuring its slump. Concrete specimens were then cast

3 Compressive strength (MPa) ISESEE 2006 International Symposium & Exhibition on Sustainable Energy & Environment, in 100x200 mm cylinder steel moulds for compressive strength test. One day after casting, concrete specimens were immersed in the water at room temperature for curing, until the day of testing. Compressive tests were conducted on the age of 7, 14, 28 and 90 days. Table 2: Mixture Proportion of Cementitious Materials Mixture No Fly Ash % Fly Ash Portland cement kg/m % % % % High volume fly ash mortar was prepared with the same mixture proportion as concrete, only without the coarse aggregate. The aim of making HVFA mortar was to investigate the setting time of the mixtures. Preparation of specimens and procedure of testing were conducted in accordance to the current Standards. 3.2 The Slump of Fresh Concrete Slump of the fresh concrete can be seen in Table 3. Table 3: Slump of the Fresh Concrete % Fly Ash Slump (mm) 50% 95 60% % 221 0% 90 The workability of fresh concrete, as measured in term of the slump value, was significantly improved as the amount of fly ash increased. This most probably is due to the spherical shape of fly ash particles, which reduces the internal friction between concrete particles. Improvement in workability is desirable in construction practice. Since the workability is improved without increasing the water content, this results in less segregation of the concrete mixture. Moreover, improvement in workability enables lowering the water to cementitious ratio to achieve better concrete performance. 3.2 Compressive Strength at Various Ages The compressive strength of concrete at various ages can be seen in Figure 1. Only on 7 day, concrete without using any fly ash has slightly higher compressive strength compared to concrete specimens from the other mixtures. After that, especially after 14 days, HVFA concrete showed higher strength development results in higher compressive strength. The reason for this most probably due to the slower pozzolanic reaction compared to the hydration reaction of Portland cement % 60% 70% Ref Age (days) Figure 1: Strength Development of Concrete Specimens up to 90 days HVFA concrete showed significantly higher compressive strength compared to the reference concrete without any fly ash, especially in the later age. 3.3 Initial and Final Setting Time of Mortar The setting times of mortar were measured using the Vicat apparatus with different penetrating attachments. The initial and final setting times of fresh mortar are shown in Table 4. The results indicated that the higher the fly ash content, the longer the initial and final setting times. The results also showed that the incorporation of high volumes of fly ash has more significant impact on the final

4 setting time of high volume fly ash mortars. For reference mortar, the interval between initial and final setting times was only one hour, while in the case of high volume fly ash mortars, this interval increased to more than two hours. Table 4: Initial and Final Setting Time of Fresh Mortar Fly Ash Initial Final hour:minute 50% 4:55 7:10 60% 5:40 7:50 70% 6:10 8:15 0% 3:50 4:50 This again most probably attributed to the slow pozzolanic reaction between the SiO 2 in fly ash with Ca(OH) 2 from the hydration process of Portland cement to form the calcium silicate hydrate (C-S-H). 4. FLY ASH-BASED GEOPOLYMER CONCRETE Another noted achievement in the use of fly ash in concrete was the development of geopolymer concrete. In this case, no Portland cement is used, as it is totally replaced by the geopolymer paste. Any material rich in silicon and aluminium oxide can be used as source material for geopolymer concrete. In this case, aluminium and silicon oxides in the source material will chemically react with the alkaline solutions in a polymerisation process, to form inorganic polymeric binder called geopolymer. Geopolymer technology has been coined by Davidovits et al [7, 8], mainly using calcined kaolin as the source material. Since then, many other possible materials have been investigated its potential to be the source material for geopolymer [9]. Fly ash was found to be one of suitable source materias for geopolymer. The development of fly ash-based geopolymer concrete has gained significant advancement. It is possible to manufacture fly ash-based geopolymer concrete using the common technology to produce normal Portland cement concrete [10]. It shows excellent short term and long term properties, as well as its durability in the sulphate environment [11-13]. The application of fly ash-based geopolymer concrete into structural members has been investigated, i.e. as columns, beams, railway sleepers, walls as well as sewerage pipes [14-17]. It was reported that reinforced geopolymer concrete structures behave in similar manner as those of normal Portland cement concrete. Research to further investigate the behaviour and properties of this newly construction material is on going in many different parts of the world. Currently, at Curtin University of Technology, Sarawak Campus, a research on fly ash-based geopolymer concrete is conducted. The fly ash used is the locally available low calcium (class F) fly ash from a power plant in Kuching, Sarawak. The main objective of the study is to develop a database on the engineering short-term properties of fly ash-based geopolymer concrete for developing the mix design tools of the concrete. An initial report has been published confirming the potential of fly ash from Kuching, Sarawak, to be used as the source material for geopolymer concrete [18]. 5. CONCLUSIONS Fly ash, a by-product from power plants, has shown its potential to reduce the environmental impact of concrete, by partly or totally replace the use of Portland cement in concrete. From the results of initial study on high volume fly ash (HVFA) concrete using locally available fly ash from a power plant in Kuching, Sarawak, it can be concluded that: Fly ash from Kuching, Sarawak, has potential to be used to produce HVFA concrete. The use of fly ash of 50-70% by mass of the total cementitious material showed an excellent compressive strength, especially at the age of 28 days onward. The initial and final setting time or cement mortar increased with the increase in the amount of fly ash incorporated in the mixture. This is attributed to the slow pozzolanic reaction.

5 The developments of fly ash-based geopolymer concrete also shows its promising potential. Thus, fly ash may play an important role to make concrete greener construction material. REFERENCES 1. Malhotra, V.M. Role of Supplementary Cementing Materials and Superplasticisers in Reducing Greenhouse Gas Emissions. in ICFRC Chennai, India: Allied Publishers Private Ltd. 2. Mehta, P.K., Durability - Critical Issues for the Future. ACI Concrete International, (7): p ACI Committee 232, Use of Fly Ash in Concrete. 2004, American Concrete Institute: Farmington Hills, Michigan, USA. p Malhotra, V.M., Reducing CO 2 Emissions, The Role of Fly Ash and other Supplementary Cementitious Materials. Concrete International, (09): p Mehta, P.K. and D. Manmohan, Sustainable High-Performance Concrete Structures. Concrete International, (7): p Desai, J.P. Construction and Performance of High-Volume Fly Ash Concrete Roads in India. in Eighth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete Las Vegas, USA: American Concrete Institute. 7. Davidovits, J. Chemistry of Geopolymeric Systems, Terminology. in Geopolymer '99 International Conference France. 8. Davidovits, J. Green-Chemistry and Sustainable Development Granted and False Ideas About Geopolymer Concrete. in GGC 2005 International Workshop Perth, Australia. 9. van Jaarsveld, J.G.S., J.S.J. van Deventer, and G.C. Lukey, The Characterisation of Source Materials in Fly Ash-based Geopolymers. Materials Letters, (7): p Hardjito, D., S.E. Wallah, D.M.J. Sumajouw, and B.V. Rangan, On The Development of Fly Ash-Based Geopolymer Concrete. ACI Materials Journal, (6): p Hardjito, D., S.E. Wallah, D.M.J. Sumajouw, and B.V. Rangan, Fly Ash- Based Geopolymer Concrete. Australian Journal of Structural Engineering, (1): p Wallah, S.E., D. Hardjito, D.M.J. Sumajouw, and B.V. Rangan. Creep and Drying Shrinkage Behaviour of Fly Ash-Based Geopolymer Concrete. in Concrete ' Melbourne, Australia: Concrete Institute of Australia. 13. Wallah, S.E., D. Hardjito, D.M.J. Sumajouw, and B.V. Rangan, Performance of Fly Ash-Based Geopolymer Concrete under Sulfate and Acid Exposure, in Geopolymers, Green Chemistry and Sustainable Development Solutions, J. Davidovits, Editor. 2005, Geopolymer Institute: Saint Quentin, France. p Sumajouw, D.M.J., D. Hardjito, S.E. Wallah, and B.V. Rangan, Fly Ash- Based Geopolymer Concrete: An Application for Structural Members, in Geopolymers, Green Chemistry and Sustainable Development Solutions, J. Davidovits, Editor. 2005, Geopolymer Institute: Saint Quentin, France. p Sumajouw, D.M.J., D. Hardjito, S.E. Wallah, and B.V. Rangan, Fly Ash- Based Geopolymer Concrete: Study of Slender Reinforced Columns. Journal of Material Science, Accepted for publication. 16. Palomo, A., A. Fernandez-Jimenez, C. Lopez-Hombrados, and J.L. Lleyda. Precast Elements Made of Alkali- Activated Fly Ash Concrete. in Eighth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete Las Vegas, USA. 17. Gourley, J.T. and G.B. Johnson, Development in Geopolymer Precast Concrete, in Geopolymer, Green Chemistry and Sustainable Development Solutions, J. Davidovits, Editor. 2005, Geopolymer Institute: Saint Quentin, France. p Sarker, P.K. Making Geopolymer Concrete using Sarawak Fly Ash. in ACF Conference Chiang Mai, Thailand.