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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 37 produced from chlor-alkali process, where the primary purpose is to produce chlorine. It would be a difficult negotiation to decide what proportion of the carbon emissions from chlor- alkali process should be allocated to NaOH production. If we split in the middle at 50:50, then NaOH activator will have an emission about 0.5 tonne/tonne of NaOH. Typical amounts of sodium hydroxide activator used in geopolymers concrete is about 12 kg per cubic metre of concrete (Zhao & Sanjayan, 2011), resulting in about 6 kg CO2 per cubic metre, as compared to Portland cement concrete emissions of 300 kg of CO2 per cubic metre of concrete. While the activators emissions are not zero, due to the small amount of activators used in geopolymers concrete, large reductions in carbon emissions are possible. Figure 5 shows the number of research publications in refereed journals in this subject since 1990. In the 1990s worldwide interest was negligible (<10 per year), but during the last couple of years the publications are well over 100 per year. It is inevitable that the level of research interest worldwide will translate into commercial realities not so far into the future. Extensive research studies conducted have shown that AAS concretes are suitable for construction purposes. Studies (Collins & Sanjayan, 1998, 2001a) based on the paste and mortar investigation showed that a multi-component activator based on powdered sodium silicate, hydrated lime and calcium sulphate was the most suitable activator based on one day strength and workability. AAS pastes showed better dispersion than OPC pastes. It was also found that partial replacement of slag with ultra-fine slag or ultra-fine fly ash improves workability, whereas condensed silica fume significantly reduces workability (Collins & Sanjayan, 1999a). AAS concrete made with liquid alkali activators, namely NaOH plus Na2CO3 and also liquid sodium silicate showed rapid loss of workability with time (Collins & Sanjayan, 1999b). e one-day strength of AAS concrete was almost identical to OPC concrete for various water/binder ratios. Up to 25 MPa one-day strength was achievable with AAS concrete (Collins & Sanjayan, 1999c). Long-term strength studies, up to 365 days, showed that certain AAS concrete mixes may exhibit limited strength retrogression under certain curing conditions. e behaviour of AAS concrete is associated with continuous microcracking and capillary pore network. A more detailed account of this behaviour is explained in Collins & Sanjayan (2001b). Testing up to 365 days showed that drying shrinkage of AAS concrete is greater than OPC concrete, which is a limitation in certain construction applications. e likely reason for higher drying shrinkage of AAS concretes has been explained by studying the pore size distributions of pastes of AAS and OPC (Collins & Sanjayan, 2000a). Studies focused on reducing the shrinkage of AAS concretes showed that to up to 54% reduction in shrinkage could be achieved by the incorporation of a glycol-based shrinkage reducing chemical admixture into AAS concrete. Further, the examination of the effect of gypsum content on shrinkage and compressive strength of AAS concrete showed 2% SO3 to be the optimum gypsum content. It was found that replacement of normal weight coarse aggregate with saturated porous air-cooled blast furnace slag aggregate into AAS concrete achieved 38% less drying shrinkage at 365 days. is is most likely due to the "internal curing" effect whereby saturated blast furnace slag aggregate releases moisture into the cementitious paste during drying (Collins & Sanjayan, 1999c). e cracking tendency of ASS concrete was also studied by numerical modelling (Collins & Sanjayan, 2000b) and experimental investigation (Collins & Sanjayan, 2000c) and was compared with the OPC equivalent concretes. Concretes developed in the laboratory are not necessarily suitable for construction use, as the problems in construction sites can be of different nature. To investigate these type of problems, a large column was constructed using AAS concrete batched and mixed at a commercial operating concrete plant using a mobile mixer. e investigation (Collins & Sanjayan, 1999d) showed that unlike concretes made with slag-blended cements, AAS concretes do not show significant difference in insitu strength properties and control cylinder properties. Detailed studies on the microstructure of AAS concrete to investigate binder behaviour, including isothermal calorimetry, nuclear magnetic resonance, scanning electron microscopy and x-ray micro-analysis confirm that the AAS concrete is suitable for most types of construction (Bakharev et al, 2001a, 1999a, 1998). Research has also shown that elevated temperature curing of AAS can produce superior strength and durability properties (Bakharev et al, 1999b), making AAS concrete highly Figure 5. Research publications in geopolymer/AAS (Scopus database). Figure 6. OPC and geopolymers in elevated temperatures. 0 40 80 0 200 400 600 800 Temperature (°C) Strength (MPa) Geopolymer paste OPC paste