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Concrete In Australia : March 2012
40 Concrete in Australia Vol 38 No 1 1.0 INTRODUCTION In an effort to obtain a greater understanding of the practical potential of geopolymer concrete, VicRoads has undertaken a small number of trials over the past two years which include the insitu construction of landscape retaining walls at a bridge site 1, precast footway panels on a bridge and construction of significant lengths of footpath. ese trials form part of a strategy to generate a greater understanding on long term performance, particularly with respect to higher risk structural applications, which includes visual inspection, sampling and testing and monitoring of embedded probes. At this stage VicRoads has gained sufficient confidence with regards to low risk general paving works (ie footpaths, driveways, kerb and channel and other concrete surfacings) and has incorporated geopolymer binder concrete into its general concrete paving specification Section 703 2 as an equivalent product to Portland cement concrete. is paper presents the VicRoads experience with regards to use, testing and ongoing monitoring of geopolymer concrete and further discusses the various parameters incorporated into the VicRoads specification for general paving works with regard to geopolymer concrete. 2.0 GEOPOLYMER CONCRETE Geopolymer concrete consists of the normal components of fine and coarse aggregate, as well as any required admixtures and aluminosilicate based industry byproducts such as fly ash and ground granulated blast furnace slag. ese aluminosilicate based industry byproducts can be activated with a concentrated solution of alkali-based chemicals such as sodium hydroxide and sodium silicate in water to form the geopolymer paste that binds the loose coarse and fine aggregates, and other unreacted materials together. is can take place at temperatures ranging from ambient to as high as 200 °C. Coarse and fine aggregates occupy about 75-80% of the mass of geopolymer concrete, similar to Portland cement concrete. e alkaline activators most commonly used for making geopolymer concrete (due to cost and availability) in either research or commercial work are sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) or combinations of these. e alkaline component dosage rates can be manipulated to achieve the desired strength and various other plastic and hardened properties. Although geopolymer is not meant "to contain" any Portland cement, up to 10% may still be used to enhance the chemical reaction especially under ambient conditions. Geopolymer concrete can be formed in one of two ways, namely: (i) Reactive aluminosilicates (ie fly ash or slag) + alkali component (solid or liquid) at concrete batch plant: as a two-part mix; or (ii) Reactive aluminosilicates + manufactured alkaline component preblended as a dry powder geopolymer "cement": as a one-part mix. Water added to dry materials as per Portland cement. e major difference between cement-based concrete and geopolymer binder based concrete is that cement based concrete is characterised by the formation of calcium silicate hydrates (CHS), whereas geopolymer binder concrete is characterised by an amorphous (non-crystalline) microstructure, where the polymerisation process involves the formation a three-dimensional polymeric chain and ring structure consisting of aluminosilicates (Si-O-Al-O). 3.0 ENVIRONMENTAL ISSUES Cement manufacture is considered to be the fourth largest global carbon emission activity following the oil, gas and coal industries. It is estimated that the cement industry is responsible for between 5% and 10% of all CO2 emissions primarily due to the production of one tonne of Portland cement emitting approximately one tonne of CO2 into the atmosphere, mostly from the process step of high-temperature calcination of limestone (ie limestone (CaCO3) → CaO + CO2 ↑). e cement content of concrete is of the order of 10-15%, the balance consisting of fine and coarse aggregates which have their own emissions contribution from quarry and transport operations. One of the primary advantages of geopolymer concretes over traditional Portland cement concretes is largely associated with the much lower CO2 emissions. is is mainly due to the absence of the high-temperature calcination step in the process of geopolymer synthesis. e activators used in geopolymers do reintroduce some CO2 emissions and the byproduct binders provide a use for an otherwise waste product. Overall, the CO2 saving due to the use of geopolymer concrete can be as much as 40-80% when compared with Portland cement concrete. Fine and coarse aggregates still have their own emissions contribution from quarry and transport operations, similar for both concretes. It should be emphasised that VicRoads has been utilising supplementary cementitious materials (SCMs) (ie fly ash, slag and silica fume) in the construction of its bridges and as specified in its structural concrete specification Section 610 3 for more than 20 years in order to improve the qualities and long term performance of the insitu concretes. is of course has consistently contributed over this period to Reducing the carbon footprint -- The VicRoads experience Fred Andrews-Phaedonos -- VicRoads, Australia