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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 49 1.0 INTRODUCTION 1.1 Background Concrete is the most widely used construction material in the world, with current consumption estimated at 1 m3 per person per year 1. Ordinary Portland cement (OPC) has traditionally been used as the binder material in concrete, however the embodied energy arising from OPC manufacture is high, with carbon dioxide equivalent (CO2-e) estimates ranging from 0.73 kg to 0.99 kg of CO2 emitted for every kilogram produced 2, 3. Meyer 4 estimates the contribution of the production of OPC is approximately 7% of global, man-made CO2 emissions. e cause of high CO2 emissions during OPC manufacture has been attributed to: (i) calcination of limestone, one of the key ingredients, which leads to formation and release of CO2; and (ii) high energy consumption during production 5. e need for reducing greenhouse gas emissions is becoming the focus of the public, governments and industry. Blended cements, comprising OPC that has been partly substituted by supplementary cementitious materials (SCC), are commonly used for concrete making. SCCs commonly include fly ash, a fine waste residue that is collected from the emissions liberated by coal burning power stations, and ground granulated blast furnace slag, a waste byproduct from steelmaking. Flower and Sanjayan 6 showed that blended cements reduced CO2 emissions within the range of 13-22%, although this estimate can vary depending on local conditions at the source of raw materials, manufacturing premises, climate, energy sources and transportation distances. An alternative cementitious binder to OPC, the "geopolymer", commonly comprising of fly ash plus alkaline activator(s), has been considered by many studies as a replacement for OPC binders. Geopolymers were first described by Davidovits 7 as inorganic materials rich in silicon (Si) and Aluminium (Al) that react with alkaline activators to become cementitious. Alkaline "activators" used for geopolymers are usually a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH), and sodium silicate or potassium silicate 2, 7-12, with NaOH and sodium silicate being the most common due to cost and availability. To achieve comparable strength to OPC concrete, geopolymer concretes require elevated temperature curing between 40-80 °C for at least 6 hours 8, 11. Further background to geopolymers is provided in a recent paper by Duxson et al 8. Recent studies have reported the carbon dioxide emissions of geopolymers as a measure used to compare the emissions generated by OPC binders based upon their global warming potential 13-17. e range of estimates was considerable, ranging from 80% less emissions than OPC 13,17 to 26-45% lower than OPC concrete 14-16. Stengel et al 14 considered CO2-e due to sodium silicate manufacturing based on six European production plants in 1995. Two different types of manufacturing were considered: (i) melting silica sand and soda ash at 1400 °C to form water glass; and (ii) hydrothermal process, where sand is dissolved in sodium hydroxide under high pressure and temperature in an autoclave. e two production methods were concluded to expend significantly different energy, namely 4.58 MJ/kg and Geopolymers: A greener alternative to Portland cement? Louise Turner and Frank Collins, Monash University Fly ash based geopolymers have been hailed as a greener cementitious binder to traditional ordinary Portland cement (OPC) for concrete construction because fly ash, a waste byproduct from coal-fuelled electricity power stations, is utilised. is contrasts with OPC which liberates significant carbon dioxide (CO2 ) during manufacturing due to: (i) calcination of limestone which leads to formation and release of CO2 ; and (ii) high energy consumption of the kiln. However, most published studies on the carbon footprint of geopolymers do not consider the embodied energy within the alkaline activators that are an essential ingredient for the fly ash to become cementitious and also the elevated temperature curing that is necessary to achieve comparable concrete strength to that produced using an OPC binder. is study compares the CO2 footprint generated by concretes comprising geopolymer binders with 100% OPC concrete. e study considered the binder manufacturing, concrete production and construction phases. e findings of the study were unexpected. e CO2 footprint of geopolymer concrete was approximately 9% less than comparable concrete containing 100% OPC binder, much less than predictions by other studies. e key factors that led to the higher than expected emissions for geopolymer concrete included the inclusion of mining, treatment and transport of raw materials for manufacture of alkali activators for geopolymers, and expenditure of significant energy during manufacture of alkali activators, and the need for elevated temperature curing of geopolymer concrete to achieve reasonable strength.