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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 51 and Sanjayan 6). e concrete manufacturing activities shown in Figure 1 also apply to the manufacture of geopolymer concrete, based on fly ash plus alkali activators based on NaOH and sodium silicate, with the addition of elevated temperature curing which is a requirement for strength development of geopolymers. Previous studies have indicated that geopolymer-binded concrete is most effective when cured at an elevated temperature. e points of emission release for the alkali activators are summarised in Figure 2. (ii) In addition to fly ash, geopolymers require alkali activators during concrete manufacture to enable the binder to become cementitious. e two activators most commonly used are sodium hydroxide and sodium silicate. Consultation with manufacturers indicated that sodium hydroxide is commonly produced in Australia through the chlorine-alkali (chlor-alkali) process, which produces both sodium hydroxide and chlorine, as outlined in Figure 2: saltwater/brine, transported, cleaned and prepared before undergoing electrolysis, followed by cooling to result in sodium hydroxide. e major energy use in the process occurs in the electrolytic cell, which has a large electricity requirement despite being extremely efficient (95% according to producers, who use a membrane cell). (iii) Sodium silicate with low weight ratios (less than 2.4) are generally used for geopolymer mixes and are produced through melting silica sand and soda ash or by the hydrothermal method of dissolving silica sand in sodium hydroxide. For production via melting of sand and soda ash, the energy expending processes for sand sourcing include the dredging, washing, drying and classifying, as well as delivery in bulk by a pneumatic tanker. For the raw material soda ash, the energy expending processes include the mining, cleaning and classifying of raw soda ash or more likely, if synthetic soda ash is used, additional energy is expended via the Solvay manufacturing process (ie ammonia reacts with CO2 (from calcined limestone) which is then introduced to brine which reacts to create sodium carbonate). Sand and soda ash are then mixed and melted; a process that expends significant energy due to the high temperatures (approximately 1400 °C) and pressures needed. (iv) Estimates of CO2-equivalent emissions (ie estimates of all greenhouse gas emissions as an equivalent CO2, (CO2-e)), were based on the quantities of energy expended at the point of emission release and aggregated to estimate the carbon footprint raised by 1 m3 of concrete. In this study, CO2-e is used as the unit of measurement, which is adjusted to include the effects of other greenhouse gas (GHG) emissions from the same fuel or process that contribute to global warming effects (eg methane, nitrous oxide, halocarbons, and synthetic gases). CO2-e calculations were made in accordance with the Australian National Greenhouse Accounts (NGA) Factors 20. Energy sources and energy expended for each activity were obtained from audited records (for example, operation hours by particular machinery and receipts of fuel consumed). Calculation of CO2-e was based on the collective contributions of CO2, CH4, NO2 and synthetic gases evolved during each activity, taking into account the energy content of the fuel, the global warming gas types produced, and the respective gas global warming potential (GWP), when the fuel is fully combusted: CO2-e =Q×EC×GWP (1) where, Q = quantity of fuel consumed to undertake a particular activity; Figure 2. CO2 emissions system diagram for production of alkali activators for geopolymers.