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Concrete In Australia : September 2013
40 Concrete in Australia Vol 39 No 3 CONFERENCE TECHNICAL PAPER m2/s). is result confirms the conclusion of ASTM C1202 test. e NT Build 443 test results and theoretical curve calculated using the diffusion parameters in Table 8 are presented in Figure 8. 2.5 Water-soluble alkali content of concrete One core from each of the downstream and upstream walls was used for soluble alkali content determination. e results are presented in Table 9. e alkali content of the blended slag cement concrete is similar to that of normal concrete (i.e., concrete containing 400 kg/m3 of cement, with a cement alkali content of 0.6% Na2O equivalent has an alkali content of 2.4 kg/m3). e alkali content determined on the geopolymer concrete was very high, which is in agreement with the results of SEM/ EDX examination (see later). is is because the geopolymer concrete included large amounts of soluble sodium silicate in its formulation. 2.5.1 Implication of high alkali content in geopolymer concrete e presence of additional alkali in the alkali-activated (geopolymer) concrete, compared to that in the blended slag cement, would make it more susceptible to AAR if the aggregate used was reactive. Bakharev et al. (2001) assessed the expansion potential of the alkali activated slag in combination with a non-reactive sand and a reactive coarse aggregate. is was compared with the behaviour of the same aggregate combination when used with OPC at alkali level of 1.25% Na2O equivalent. ey found that the alkali activated slag caused much more expansion than the OPC when combined with the same reactive aggregate. is could have arisen from the reaction of their non-reactive sand at higher alkali content of the alkali-activated slag concrete. Shayan (1994) showed that some sands, which were not reactive at 1.25% Na2O equivalent, could undergo deleterious reaction at higher alkali contents. Table 7. Results of ASTM C1202 test results. Table 8. NT Build 443 test results. Table 9. Water soluble alkali content of concrete. Voltage (V) Initial Current (mA) Total Charge after 6 hours (C) Penetrability 60 31.7 648 Very Low Duration of immersion (day) Initial Chloride Content (%) Chloride content at the boundary, Cs (%) Diffusion Coefficient (m2/s) 35 0.01 1.15 1.58 · 10-13 Figure 8. Chloride profile (data points) obtained using NT Build 443 test method and the theoretical curve based on the diffusion parameters given in Table 6. Results of Puertas, et al. (2009), using the accelerated mortar bar test ASTM C1260, showed that deleterious AAR expansion could occur in alkali activated slag mortar (activator containing 4% Na2O), although the expansion was more severe in the corresponding OPC- based mortar. However, García-Lodeiro et al. (2007) and Kupwade-Patil et al. (2013) showed that mortar bars incorporating alkali-activated fly ash caused far less AAR expansion than mortar bars that contained OPC, which is attributed to the lack of sufficient calcium in the fly ash-based geopolymer system. From the above, it is clear that potentially reactive aggregates can cause deleterious expansion in some geopolymer concretes which contain an adequate supply of calcium. 2.5.2 SEM/EDX Examination All the cores indicated in Tables 1 and 2 were used for detailed examination by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis, to identify their microstructural features, reaction products and their semi- quantitative chemical composition. A brief summary of the observations is presented below. Core ID (concrete) Density Kg/m3 Na2O (kg/m3) K2O (kg/m3) Na2Oeq. (kg/m3) C11/2259-2 (geopolymer) 2204 10.82 0.37 11.06 C11/2259-4 (blended slag cement) 2202 1.95 1.42 2.88