Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers

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Abstract

Inorganic polymers based on alumina and silica polysialate units were synthesised from dehydroxylated aluminosilicate clay (metakaolinite) condensed with sodium silicate in a highly alkaline environment. Reaction of the aluminosilicate with alkali polysilicates yields polymeric Si–O–Al three-dimensional structures with charge-balancing positive ions such as hydrated Na+ in the framework cavities. A statistical study of the effect on the polymerisation process of the molar ratio of the component oxides and the water content of the mixture showed the latter to be a critical parameter. The polymerisation mechanism and structures of the products were investigated using NMR, XRD and FTIR spectroscopy. 29Si liquid-state NMR shows that some compositions do not cure properly because of incomplete reaction of the sodium silicate with the metakaolinite. FTIR confirms that during drying of the incompletely cured samples, Na migrates to the surface where it undergoes atmospheric carbonation. The cured polymers were found to be essentially X-ray amorphous, with bulk densities of 1.3–1.9. During polymerisation the coordination of Al in the metakaolinite reactant (IV, V and VI) changes almost completely to IV in all the polymer compositions. The environment of the Na is unchanged irrespective of the polymer composition. The solid-state 29Si NMR spectra indicate a range of Si–O–Al environments. Typical mechanical properties of the best polymers were: Mohs hardness >7, Vickers hardness about 54, and compressive strength (after drying for 1 h at 65°C) 48.1 MPa.

Introduction

New inorganic materials which could increasingly replace conventional cements, plastics and many mineral-based products hold the key to the reduction of world pollution resulting from the manufacturing and use of the older materials. Such pollutants include CO2 emissions from cement kilns, toxic metal contamination of freshwater resources resulting from mining operations, and catastrophic fires involving common organic plastics.

During the last decade a family of new materials called polysialates has emerged as a possible solution to these problems, since their physical properties make them a viable alternative for many conventional cements and plastics [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The manufacture of polysialates does not create CO2 emissions, they can be made from recycled mineral wastes, and they provide a heat-resistant substitute for flammable organic materials.

Polysialates are inorganic polymers derived from aluminosilicates. They can be synthesised at low temperature and have useful properties such as high early compressive strength, a Mohs hardness of 4–7, and they are stable at temperatures up to 1300–1400°C. By changing the Si/Al ratio, it is possible to produce products with a range of physical and mechanical properties [1], [2], [3], [4], [5]. Polysialates are readily synthesised from natural aluminosilicates such as kaolinite, which is one of the most abundant sources of alumina and silica.

Several fundamental polysialate units have been identified [3]; polymers may be composed of the sialate unit [–Si–O–Al–O–] (designated PS when polymerised), sialate siloxo units [–Si–O–Al–O–Si–O–] (designated PSS when polymerised) or sialate disiloxo units [–Si–O–Al–O–Si–O–Si–O–] (designated PSDS when polymerised). In all the polymerised structures, the Al is four-coordinated, creating a negative charge imbalance which is compensated for by the presence of monovalent cations such as Na+ or K+. Some of the details of polysialate formation and technology are still not fully understood. The purpose of this work is to elucidate aspects of the polymerisation mechanism and structure of products based on polysialate siloxo (PSS) units containing Na+. For this purpose, a number of compositions of various oxide molar ratios were selected statistically. The mixtures were then polymerised and cured at both 65°C and room temperature. A structural study of Na–PSS polymer was made on an anhydrous sample which had been maintained at 65°C until no further mass loss was detected. The synthesis and structure of Na–PSS polymer was studied by 27Al, 29Si and 23Na solid-state nuclear magnetic resonance with magic-angle spinning (MAS NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray powder diffraction (XRD).

Section snippets

Experimental

The polymers were synthesised from metakaolinite, prepared by heating kaolinite from Northeast Brazil at 700°C for 6 h [11]. The chemical compositions of the starting materials are presented in Table 1. The other reactants were sodium silicate (Gessy Lever) (Table 1) and sodium hydroxide pellets (Grade P.A., Vetec).

The particle size distributions, measured using a Sedigraph 5100, were: kaolinite, 41%<2 μm, median ESD 2.59 μm, metakaolinite, 11.9% <2 μm, median ESD 4.24 μm. The densities of the

Polymerisation behaviour of the various sample compositions

Considerable differences were observed in the polymerisation behaviour of the various samples, those with the higher water contents (Na–PSS2, Na–PSS4 and Na–PSS6) requiring almost 24 h at 65°C to cure. Furthermore, the mechanical properties (compressive strength and hardness) of these slow-curing samples were too poor to permit their measurement. Table 3 shows the compressive strengths of the samples which did cure. Sample Na–PSS1 showed the optimum behaviour, attaining a high compressive

Conclusions

Reaction of metakaolinite with sodium silicate in a highly alkaline environment produces in mixtures with an Al:Si ratio of 1:2 an inorganic polymer which cures at room temperature or 65°C to a stable material with a compressive crushing strength of 48.1 MPa, Mohs hardness >7 and Vickers hardness of about 54. Optimum curing and polymer properties are obtained when the Na concentration is sufficient to provide a charge-balancing mechanism for the substitution of tetrahedral Si by Al, but not in

Acknowledgements

We are indebted to R.L. Frost, Queensland University of Technology, for the FTIR spectroscopy and to Centro de Tecnologia Mineral (CETEM) for the chemical analyses. VFFB and CT are grateful to Coordenação de Aperfeiçoamento de Pessoal de Nı́vel Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for financial support. KJDM is indebted to the Royal Society of New Zealand for a James Cook Research Fellowship under which part of this work was carried out.

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