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deionised waterLy, Vance, Perera, Aly and Olufson

The most leachable species in solutions derived from geopolymers exposed to deionised water are alkalis.

Inorganic polymers formed from naturally occurring aluminosilicates been termed geopolymers by Davidovits.

deionised waterVarious sources of Si and Al, generally in reactive glassy or fine grained phases, are added to concentrated alkaline solutions for dissolution and subsequent polymerisation to take place. Typical aluminosilicate precursors used are fly ash, ground blast furnace slags, and metakaolinite made by heating kaolinite at ~ 750oC for ‘6 24’ h to render it ‘X ray’ amorphous and more reactive. The alkaline solutions are typically a mixture of hydroxide and silicate. The solution dissolves Si and Al ions from the precursor to form Si4 and Al4 -monomers in solution. Neighbouring OH ions molecules condense to form an oxygen bond between metal atoms and release a molecule of water and then polycondense at near ambient temperatures to form a rigid polymer with interstitial unbound water. The geopolymers thus consist of amorphous to ‘semicrystalline’ three dimensional aluminosilicate networks.

Geopolymers physical behaviour is similar to that of Portland cement and they are considered as a possible improvement on cement in respect of compressive strength, resistance to fire, heat and acidity, and as a medium for the encapsulation of radioactive and hazardous waste. Fine particles can be added to the paste to make a mortar, or aggregates can be added to make a concrete, as for Portland cement usage. Besides, for industrial applications, fly use ash as a precursor gives a cost advantage over metakaolinite. Fact, because of the complexity of fly ash in terms of the various distinct crystalline and ‘non crystalline’ phases present, to understand the polymerisation science process, use of the latter the use is preferred. See for example, Van Jaarsveld et al a couple of use different aluminosilicate minerals has also been reported, In the literature the use of different fly ashes has been published extensively. See for example Rahier et al, Although metakaolinite use is limited apart from the early work by Davidovits because most workers are interested in industrial applications, recently for more scientific study metakaolinite has been used on account of its ‘near constant’ composition and high reactivity with alkaline media. Kriven et al. Barbosa et al Several workers have studied physical variation and mechanical properties on composition of fly ashbased geopolymers and similarly on ‘metakaolinitebased’ geopolymers.

Geopolymers widespread use is currently restricted due to lack of long term durability studies, detailed scientific understanding and lack of reproducibility of raw materials. One long understanding aspect term durability of geopolymers is geopolymers integrity under the influence of aqueous leaching. This is of particular interest for geopolymers use exposed to wet environments in applications as sewer pipes, railway sleepers, and outdoor paths for instance. Another application in which the dissolution behaviour has regulatory impact is when the geopolymer is a candidate for immobilisation of radioactive or hazardous waste.

Metakaolinite was prepared by heating kaolinite for 15 h in air at 750oC. Si/Al molar ratio from ‘0’ and the /Al ratio from 8 Samples in which the Si/Al ratio was 2 or less were made by mixing metakaolinite with sodium silicate solution D.a 20 g batch having Si/Al = 2 and Na/Al = 1 was made by adding 96 g metakaolinite to 1304 g sodium silicate solution This was mixed by hand for 5 min to make a thick slurry.

Aldrich Chemical Co.

USA

Click Table to Zoom Na2O. Anyways, H2O. Kaolinite heated to 750oC for 15 h in air to form metakaolinite. The XRD analysis showed an amorphous phase with trace amounts of anatase. Remember, the clay contained ~ 1 mass quartz, TiO2 and Fe2O3 according to the supplier. Of course, fumed silica was Xray amorphous. Actually. Make sure you drop a few comments about it in the comment box. H2O. Furthermore, geopolymers were also made using Class F fly ash supplied by Cement Australia Pty.

Ltd, NSW, Australia and originally from the Gladstone power station in Queensland. The ‘X ray’ fluorescence analysis as given by the supplier is listed in Table 2 and our ‘X ray’ diffraction analysis showed mainly an amorphous phase plus mullite, quartz, hematite and iron silicate. That’s interesting. From scanning electron microscopy it was possible to deduce the approximate compositions as quartz, mullite, hematite on solidification. Eventually, 5 -4 were fabricated by dissolving the silica in alkali solutions as far as possible by heating at 75oC overnight prior to mixing with metakaolinite or fly ash followed by curing, Subsequently six samples, both fly ashbased and ‘metakaolinite based’ geopolymers, Si/Al &gt.

The XRD analyses for Na and ‘K based’ metakaolinite geopolymers showed an amorphous phase in all samples, as evidenced by a diffuse hump in the scattering pattern and peaking at a d spacing at the Na SEM images and Kbased geopolymers showed ~ 1 anatase and quartz and

~ 2 mass of incompletely reacted metakaolinite particles. The SEM images observed here were similar to those already published. As noted by others the major phase was an amorphous phase giving rise to the above mentioned diffuse X ray peak, the fly XRD analyses ash geopolymers showed a similar phase distribution for all samples. However, crystalline phases were mullite, quartz and hematite, that were present in the original fly ash but the iron silicate was not detected because it only constituted ~ 2 or less.

With an initial fast rate followed by a period of ‘steady state’ leaching, the Na+ concentrations determined as a function of exposure time to DIW are plotted in Figure All the samples with Na/Al = 1 disintegrated to fine powders during leaching apart from those with Si/Al = 2 and Si/Al = For the Si/Al = 1 sample, the increase in ion leachate concentration at leaching early stages appeared to follow the accelerated leaching model of Jantzen and Pareizs, after which the leach rate appears to revert to the initial rate.

In converting the ICP OES concentration data to a percentage of Na leached from the samples, the highest value was ~ In summary, the overall agreement between concentration results obtained with the ISE and the ‘ICPOES’ was fair. The geopolymers having Si/Al = 5 and 0 for Na at 20 h showed the best correlation between values obtained with each instrument. Also, the errors in the ISE method were limited, this might be because the concentrations were fairly small. On top of this, it was not possible to quantitatively detect the free after curing silica by XRD because it was amorphous. If the silica was not structure part then there would be fewer structural units in the matrix and hence Na would be in excess to charge balance requirements to the geopolymer network, thus more Na would be in the pore water and would easily leach out, as seen here. Lots of info can be found easily by going on the web. Whenever giving rise to the observed high Si concentrations in the leachates for the Si/Al = 5 and 3 samples in Table Concentration data after 20 h for undiluted leachate by ICPOES for Na and Kbased metakaolinite geopolymers of Na or K = 1 were made and their ICPOES releases in tests carried out in the same way as the ISE experiments are listed in Table It is seen that Al, Si and principal alkali releases for the equivalent compositions made without the silica predigestion in the solution by the elemental concentration, The Si which was not incorporated into the ‘3 dimensional’ geopolymer network would also be expected to be somewhat soluble in the pore water. For complete dissolution the value would correspond to 100 g/ the leach rates require separate calculation.

It was confirmed that the most leachable species in water solutions derived from correctly made geopolymers exposed to deionised water are alkalis.

The most leachresistant compositions of both metakaolinite and fly ash based materials are centred around alkali/Al ~

This work was conducted under the Cooperative auspices Research Centre for Sustainable Resource Processing. We thank Davis, Yee and Wong for technical assistance.

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