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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 41 2.5.3 Cores taken from geopolymer concrete (C10/ 2036-2, C10/2036-2 and C11/2259-2) Figure 9 shows typical views of the geopolymer concrete pastes and their compositions. e paste is compact and rich in Na, Al and Si, with moderate amounts of Ca. Small amounts of Mg are also present. is composition arises from the use of slag and Na-silicate as the main components of the concrete mix. is composition is very different from the composition of the Portland cement paste, which is dominated by Ca, with much smaller amounts of Si and Al, and only trace amounts of alkali. e high Al content helps in reducing the rate of chloride ion ingress into concrete, as indicated by the low diffusion coefficient (1.58 x 10-13 m2/s) obtained in the earlier study. Figure 10 shows views of gel-like phases which are similar to alkali-aggregate reaction (AAR) products. ese products were found lining a pore, or on a sand grain and in the paste. e gel may have formed due to reaction of sand grains, as the coarse aggregate was non-reactive. It is probably more likely that the observed AAR-looking gel originated from the Na-silicate which was used an the activator for the slag. 2.6 Cores C10/2036-4 and other upstream wall cores Details of features observed in these cores are presented in Figures 11. e hydrated cementitious phases have the same Al-rich composition, and the crystalline phases were also similar. However, these core did not exhibit any AAR-like product, although the mortar phase in some locations appeared to have a similar composition to that of the geopolymer concrete. e upstream walls could have contained much less alkali activator. 3.0 SUMMARY AND CONCLUSIONS e results of this work have shown that the half-cell potentials of steel embedded in geopolymer concrete retaining walls have become more positive since the time of installation in 2009, and are stabilising at around the value of -350mV (CSE). e risk of corrosion appears to be small. SEM/EDX results showed that the concrete in the two cores taken from the downstream wing wall were different, one comprising a blended slag cement concrete (panel closer to the abutment face), and the other (second panel) comprising an alkali-activated (geopolymer) concrete containing large amounts of alkali silica gel materials, similar to AAR-gel. e two cores from the upstream wing wall were both of blended slag cement concrete. e microstructural examination of the geopolymer concrete showed that the paste is very different from the paste of Portland cement concrete, and contains much more Al, Si and Na than the latter. e paste also incorporates Mg derived from the slag component. In concrete samples from the downstream wall hydrous sodium silicate products have been identified which resemble AAR-gel, but can be attributed to the excessive Counts1000 Si O 800 600 Ca Al 400 C 200 Mg Na Cl S Ca 00 2 4 6 8 10 Energy(keV) Counts1000 O Si 800 Ca 600 C Al Mg Na 400 200 Cl S Ca 00 2 4 6 8 10 Energy(keV) Figure 9. Typical SEM views of geopolymer paste and EDX compositions.