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ABSTRACT
In 2012, more than 100 million tons of Coal Combustion By-products (CCBs) were generated in the USA out of which more than 50% was disposed of in ponds and landfills as tailing materials that could be hazardous to the environment. The other environmental issue is related to Ordinary Portland Cement (OPC), a raw material for construction industry, which is responsible for 5-8% of the total CO2 emissions in the world. Production of one ton OPC generates one ton of CO2, a greenhouse gas affecting global warming. Therefore, the main objectives of this study were to address these two environmental challenges through finding a new application for CCBs stored in landfills, and replacing OPC by a new binder to reduce CO2 emissions. “Geopolymer” or cement-less concrete is the solution to these challenges.
In this study, a coal fly ash sample from Carolina was used in a chemical process known as geopolymerisation to produce a new binder named geopolymer. The developed geopolymer binder could competently substitute the OPC binder in regular concrete application. An experimental design program was conducted to optimize parameters of the geopolymerisation process affecting the strength of the final cement-less concrete product. Mortar and concrete samples were made to compare the strength of geopolymer with OPC concrete. The results showed that the compressive strength of geopolymer-based products at the 60th day could be more than 6600 psi for mortars and 5700 psi for concrete samples which effectively compete with OPC. This paper presents the results of the experiments and discusses the effectiveness of the new cement-less binder.
INTRODUCTION
In 2012, around 110 million tons of Coal Combustion By-products (CCBs) were generated in the US. However, only about 52 million tons of CCBs were consumed in
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different applications including concrete industry, agriculture, structural fills, road base and sub-base, soil modification, roofing granules, mining application, etc1. The remainder CCBs were disposed of in landfills and ponds which could be hazarding the environment and drinking water resources. This has led to short and long term effects on the communities surrounding these ponds. There have also been several coal ash spills into the rivers which maintain the drinking water for downstream communities. Some main coal ash leakages include the spill in Roane County, Tennessee in December 2008 and in Eden, North Carolina in February 2014.
Ordinary Portland Cement (OPC), which is widely used in construction industry, is responsible for about 7% of the total CO2 emissions in the world2. The main part of Portland cement production is the pyroprocessing system in which the raw materials are transformed into clinkers. During this process, substantial amounts of CO2 is generated and released into air. Depending on the conditions of reactions, production of one ton of OPC releases about 0.85 to 1.35 tons of CO2 into the atmosphere3. In addition, other volatile organic pollutants such as CO are emitted during the pyroprocessing system3. Cement production and construction industries have made significant progress in reducing CO2 emissions since it has been one of the main priorities. Improving the reactions in pyroprocessing system and partially substituting Portland cement by alternative cementious materials including fly ash, bottom ash, and boiler slag are some of the attempts have been made1, 4, 5, 6, 7, 8, 9. There have also been significant development in finding new binders which can completely replace Portland cement binder in the concrete10. “Geopolymer” binder is one of the new binders that have been focused especially in recent years to substitute OPC binder in concrete applications.
“Geopolymer” concept was first introduced by Joseph Davidovits in 1979 when he explained that alkaline metals react with high rich Al-Si materials and produce a three dimensional alumino-silicate complex with a strong bindery network of Al-Si element. The raw materials for making geopolymer binder could be any high Al-Si material including natural minerals such as kaolinite and clays, and wastes such as fly ash, bottom ash, red mud, rice-husk, etc. Equation 1 shows the polymerization process in which Al-Si elements react with alkaline metals and produce a polymer product. Because the raw materials are inorganic, the new product and the reaction were named “Geopolymer” and “Geopolymerization”, respectively11.
Si Al Materials Activators NaOH,Na SiO ,KOH Water → Geopolymer binder Water
(1)
Geopolymer is one of the recently introduced solutions to two above-mentioned environmental issues, i.e. disposal of CCBs in ponds and high CO2 emissions of OPC production. During past years, different raw materials which are rich in Al-Si such as fly ash, bottom ash, and slags, have been used to generate geopolymer binders in order to replace OPC in concrete application in construction industry12. This leads to reduction in OPC consumption and consequently, decline in the CO2 emissions. Other advantages of geopolymers include reduction in waste materials such as fly ash and bottom ash, less water consumption in comparison to OPC, less mining activities and natural minerals utilization, and higher resistance to fire and corrosion.