Aqueous leachability of metakaolin-based geopolymers with molar ratios of Si/Al = 1.5–4

Abstract

The leachability in water of metakaolin based geopolymers with molar ratios of Na/Al = 1 and Si/Al = 1.5–4.0 has been investigated in order to optimise the composition for the immobilisation of nuclear waste. Formulations with Si/Al of around 2 are the most suitable using the ASTM/PCT leach test method. The variability of the leach results is discussed with reference to the microstructure, compressive strength and the degree of polymerisation of the geopolymers as observed here by solid state nuclear magnetic resonance, XRD and infrared measurements.

Introduction

Inorganic polymers formed from the reaction of naturally occurring aluminosilicates with alkaline solutions have been termed geopolymers (GP) by Davidovits [1]. They have been known for over 60 years and are also described as inorganic cements or polysilates. GPs are basically aluminosilicate inorganic polymers composed of chains with three dimensional repeat structures of (Si–O–Al–O–)_n and (Si–O–Al–Si–O–)_n units. Normally Na⁺ is used to maintain the charge balance required when Al³⁺ replaces Si⁴⁺, but monovalent ions other than Na⁺ may be used. To a rough approximation, the structure can be described as a nanoporous aluminosilicate glass [2], [3].

Sources of Si and Al ions in reactive glassy or amorphous phases are added to concentrated alkaline solutions for dissolution and polymerisation to take place [4]. Typical precursors used are flyash, ground granulated blast furnace slag, and metakaolin made from thermal treatment of kaolin to render it X-ray amorphous and thus more reactive. The concentrated alkali solutions are typically a mixture of NaOH, KOH, Na₂SiO₃ and K₂SiO₃. The mechanism of the reaction with aluminosilicates involves dissolution of Si and Al ions to form Si(OH)₄ and $\text{Al}(\text{OH}{)}_4^{-}$ oligomers in solution. Condensation involves the OH ions on neighbouring molecules interacting to form oxide linkages between the molecules and hence releasing a molecule of water [4]. Further polycondensation at curing temperatures of between 20 °C and 90 °C results in a rigid polymer with unbound interstitial water. Thus GPs consist mainly of porous amorphous and semi crystalline three dimensional aluminosilicate networks.

The rigidity and flexibility of GPs depend very much on the Si/Al molar ratio [4]. If this ratio increases beyond a value of ∼3:1, the GP becomes less rigid and therefore more flexible. If the ratio is below ∼3:1, the three dimensional network structure forms and is suitable as a structural material as well as a structure for incorporating waste ions . If the Si/Al ratio is above 3:1 and as high as 35:1, the resulting two dimensional structure is more suited for use as an adhesive, or as an impregnating resin for forming fibre mat composites [4].

The physical behaviour of GPs is similar to that of Portland cement and they have been considered as an improvement on cement with respect to their compressive strength, resistance to fire, heat and acidity, and as noted above, as a medium for encapsulation of radioactive and hazardous waste [5], [6], [7], [8], [9], [10]. For radioactive waste, Cs and Sr are amongst the most difficult radionuclides to immobilise. Attempts have been made to immobilise Cs and Sr in GPs previously [5], [6], [7], [8], [9], [11], [12] and their leach resistance was studied using the MCC-1 [13] test for qualification of high level nuclear waste. According to that test, leach rates for Na, Cs and Sr were high, in the order of tens of g/m²/day (tens of μm/day) for the first few weeks, decreasing with increasing time [14]. Such behaviour is unsatisfactory for the immobilisation of high level nuclear waste, which is basically liquid waste derived from reprocessing of spent nuclear power plant fuel, or production of plutonium for nuclear weapons.

To assess the suitability of a material for the immobilisation of intermediate level nuclear waste (ILW), there is no universal test. Since the MCC-1 test is primarily for high level nuclear waste, it was decided to test only metakaolin-based GPs using the PCT-B method [15] which uses Environmental Assessment (EA) glass as a reference. This method is applicable to both high level nuclear waste (HLW) and low level nuclear waste (LLW), although the criterion for passing the test does vary depending on the class of the nuclear waste. Intermediate level nuclear waste (ILW) is not recognised as such in the United States, but it is recognised in Europe and by the IAEA. Examples of such wastes are those from radioisotope production having activities of >100 MBq/kg.

For assessing suitability of LLW immobilisation, the ANSI/ANS – 16.1-2003 test [16] is normally used, mainly in Europe. However, some countries, especially Slovakia, which already has a nuclear waste repository, use this test to assess mixed LLW/ILW [17], in addition to testing the surface activity of the drum containing the waste package for acceptability.

The formation and leaching behaviour in water of GPs with compositions having molar ratios of Si/Al = 1.0–3.0 and Na/Al = 0.8–1.0 were reported earlier [5]. These were made using commercial sodium silicate solution or adding fumed silica to metakaolin and mixing with NaOH solution. It was deduced from this study that the fumed silica did not dissolve appreciably, and that high Si samples needed pre-digestion of the fumed silica [18] to minimise leachability. Such samples actually showed better leach resistance than comparable samples made with alkaline commercial silicate solutions and metakaolin, and it was concluded that the optimal leach resistant composition of metakaolin-based geopolymers probably lay at a Si/Al molar ratio substantially in excess of 2; hence the range of Si/Al molar ratios chosen for this study.

So for the present study, a series of metakaolin-based geopolymers with differing molar ratios of Si/Al 1.5–4.0 and Na/Al = 1 were made using fumed silica, sodium hydroxide and metakaolin.