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Concrete In Australia : March 2014
Concrete in Australia Vol 40 No 1 47 containing 16 M NaOH compared to that containing 14 M NaOH is due to faster and higher dissolution rate of silicate and aluminate in fly ash . During the geopolymerisation process the silicate and aluminate in fly ash is dissolved by the NaOH and form sodium-alumina-silicate geopolymerisation product which contributed to the strength of the geopolymer matrix. Similar results are also reported by other researchers . 5.2 Effect of molarity of NaOH and amount of Na2SiO3 on water sorptivity The water sorptivity values of different geopolymer concretes and ordinary concrete are shown in Figure 3. It can be seen that the water sorptivity of geopolymer concretes is lower than that of ordinary concrete. Results also show that the water sorptivity of geopolymer concrete decreases with increase in Na2SiO3 contents for a given concentration of NaOH solution. The rate is even higher for higher concentration of NaOH in geopolymer concretes. The significant reduction of sorptivity of geopolymer concretes containing high molar NaOH and high amount of Na2SiO3 solutions can be attributed to the formation of an increasing amount of sodium-aluminasilicate gel in the matrix. The low sorptivity value of geopolymer concrete is also an indication of disconnected fine pore structures in the matrix. Research shows that the water absorption and sorptivity of concrete are significantly reduced in disconnected pore system . The positive effect of low sorptivity is also observed on chloride penetration and corrosion resistance of geopolymer concretes and is discussed in the following sections. 5.3 Effect of molarity of NaOH and amount of Na2SiO3 on chloride penetration The effects of different molarities of NaOH and Na2SiO3 contents on chloride penetration of geopolymer concretes are shown in Figure 4. A linear decreasing trend of chloride penetration in geopolymer concretes can be seen with increasing concentrations of NaOH solution and amount of Na2SiO3 contents. The results are consistent with those of water sorptivity values. As discussed before the low water sorptivity of geopolymer concrete might be attributed to the better chloride penetration resistance. 5.4 Effect of molarity of NaOH and amount of Na2SiO3 on chloride induced corrosion resistance Among many corrosion monitoring and measuring techniques, the half-cell potential measurement method is the most convenient and quickest method of monitoring the corrosion of reinforcing steel in the concrete. However, this method only provides the probability of corrosion activity of steel bar in the concrete. The half-cell potential readings shown in Figures 5 and 6 are based on a copper/copper sulphate reference electrode. The effects of two different molarities of NaOH solution on the corrosion resistance of geopolymer concretes in terms of half-cell potential readings are shown in Figure 5 and compared with that of ordinary concrete. The half-cell potential readings shown in the figures are the average of three specimens. It can be seen that all geopolymer concretes exhibited better corrosion resistance in terms of lower negative potential readings than their counterpart ordinary concrete. Figure: 2 Effects of molarity of NaOH and amount of Na2SiO3 on the compressive strength. Figure: 3 Effects of molarity of NaOH and amount of Na2SiO3 on water sorptivity. Figure: 4 Effects of molarity of NaOH and amount of Na2SiO3 on chloride penetration. 44-49 - Corrosion.indd 47 44-49 - Corrosion.indd 47 3/02/14 12:08 PM 3/02/14 12:08 PM