Geopolymers With the Potential for Use as Refractory Castables

7/28/2019 Geopolymers With the Potential for Use as Refractory Castables

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Dan S Perera and Rachael L Trautman

Abstract

A geopolymer was prepared by dissolving metakaolinite in a solution of K2SiO3 and KOH

and curing at 80C for 24 h. It was progressively heated from ambient to 1400C in airand the phase changes were studied by X-ray diffraction analysis, scanning electron

microscopy and energy dispersive X-ray spectroscopy. Only an amorphous geopolymer

phase was observed on heating up to 800C. Kalsilite was the major phase at 1000C

and 1250-1400C. At 1200C leucite was the major phase formed. At 1400C there was

no sign of significant melting. The open porosity of the material was ~ 38% at 1000C,

which is sufficiently porous for it to be used as a heat insulation material for continuous

use at this temperature.

Keywords

Geopolymer, Refractory Castable, Insulating Refractory, Kaolinite, Refractory Coatings

Introduction

Inorganic polymers formed from naturally occurring aluminosilicates have been termed

geopolymers by Davidovits [1]. Various sources of Si and Al, generally in reactive glassy

or fine ground forms, are added to concentrated alkali solutions for dissolution and

subsequent polymerisation to take place. Typical precursors used are fly ash, ground

blast furnace slags, metakaolinite made by heating kaolinite at ~ 750C for 6-24 h, or

other sources of Si and Al. The alkali solutions are typically a mixture of hydroxide (e.g.

NaOH, KOH), or silicate (Na2SiO3, K2SiO3). The solution dissolves Si and Al ions from the

precursor to form a condensation reaction [2]. The OH- ions of neighbouring molecules

condense to form an oxygen bond between metal atoms and release a molecule of

water. Under the application of low heat (20-90C) the material polymerises to form arigid polymer containing interstitial water. The polymers consist of amorphous to semi-

crystalline two or three dimensional aluminosilicate networks, dependent on the Si to Al

ratio [1].

Their physical behaviour is similar to that of Portland cement and they have been

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 hazardous

or low/intermediate level radioactive waste [3-6]. Although they have been used in

several applications their widespread use is restricted due to lack of long term durability

studies, detailed scientific understanding and lack of reproducibility of raw

materials. However, if they are to be used as refractory coatings and as low temperature(1000C) refractories, then the lack of long term durability studies will not be a

hindrance. Use of geopolymers for these applications have been mentioned in the

literature [7].

We have previously heated geopolymers made using Na-alkali up to 1200C and studied

their phase formation and microstructure [8]. In the present work we investigated

briefly a geopolymer which was much more refractory than those studied before, based

on metakaolinite precursor additions. The phase formation and microstructure are

discussed.


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Experimental

A ~ 30 g batch of geopolymer was made, consisting of 29.1 wt% metakaolinite, 4.9 wt%

Ca(OH)2 (Merck, Germany), 11.0 wt% KOH (Sigma Aldrich, Australia), 44.7 wt% Kasil

1552 (PQ Corporation, Australia, composition in wt%: K2O 21; SiO2 32; H2O - 47)

and 10.3 wt% added demineralised water. Metakaolinite was produced by heatingkaolinite (Kingwhite 80, Unimin, Australia) at 750C for 15 h in air. An X-ray diffraction

(XRD) trace showed a broad diffuse peak centred at a d-spacing ~ 0.36 nm indicative of

amorphous material, and a minor amount of quartz. The original clay contained ~ 1 wt%

TiO2 but the presence of a Ti-containing phase was not seen by XRD. The dry mixed

powders were added to this solution and mixed by hand to ensure a smooth viscous

liquid was formed. This was cast in sealed polycarbonate containers and vibrated for 5

min on a vibrating table to remove air bubbles. After holding for 2 h at ambient they

were cured for 24 h at 80C. After 5 d at ambient they were removed from the moulds

and tests were performed after further 2 d. To study the effect of heating on the

microstructure and loss of water and other species, the cured pastes were heated at 500,

800, 1000, 1200, 1300 and 1400C for 3 h in an electric furnace with heating and coolingrates of 5C /min.

The density and porosity of each of the geopolymers were determined according to the

Australian Standard [9] by evacuating under vacuum and introducing water to saturate

the pores. The time of saturation and the immersion in water was kept to less than 15

min to inhibit reaction with water (mainly dissolution of alkali, unpublished work).

All samples were analysed by X-ray diffraction (XRD: Model D500, Siemens, Karlsruhe,

Germany) using CoK radiation on crushed portions of material. Selected samples were

cross sectioned, mounted in epoxy resin and polished to a 0.25 m diamond finish and

examined by scanning electron microscopy (SEM: Model 6400, JEOL, Tokyo, Japan)operated at 15 kV and fitted with an X-ray microanalysis system (EDS: Model: Voyager

IV, Tracor Northern, Middleton, WI, USA).

Results and Discussion

The values of density and porosity are listed along with XRD analyses of the samples in

Table 1. The open porosities of all the geopolymers increase and then decrease with

increase of heat-treatment temperature. The most likely explanation is that the increase

in porosity is due to the removal of water and breaking of silanol bonds at 500C, causing

the opening of pores. The porosity decrease from 800-1400C is attributed to sintering

possibly by assistance from a liquid phase. It is quite feasible to envisage the presenceof a liquid phase at 800C for a system consisting of K2O-CaO-Al2O3-SiO2, when the

lowest eutectic temperature for the K2O-CaO-SiO2 alone is 710C [10].

Table 1. Porosity and XRD analysis of Heated Geopolymers

Temperature 0C Open porosity % XRD analysis

20 29.5 Am (m), Q, Ca8Si5O18

500 58.5 Am (m), Q, Ca8Si5O18

800 50.4 Am (m), Q, Ca8Si5O18

1000 37.8 K (m), Q, G, Ca8Si5O18, L (trace)


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1200 37.7 L (m), K

1250 - distorted K (m), L

1300 30.5 distorted K (m), L (trace)

1350 - distorted K

1400 27.6 distorted K

Key: m=major; Am= amorphous; Q=quartz; G=gehlenite (2CaO.Al2O3.SiO2); K=kalsilite

(K2O.Al2O3.2SiO2); L=leucite (K2O.Al2O3.4SiO2).

The XRD traces of all the geopolymers heated up to 800C showed a broad diffuse hump

centred at d ~0.32 nm characteristic of an amorphous phase (Table 1). Trace amounts

of quartz and the calcium silicate phase, Ca8Si5O18 were also present. At 1000C, kalsilite

was the major phase. Apart from the above crystalline phases, gehlenite was also

observed. The SEM image for the geopolymer heated to 1000C shows (Figure 1) a

calcium silicate phase with Ca to Si ratio of 8:5 and another one close to the gehlenite

composition. The EDS analysis of the matrix indicated the composition was close to that

of kalsilite.

Figure 1. SEM image for the geopolymer heated to 1000C shows a calcium silicate

phase with Ca to Si ratio of 8:5 and another one close to the gehlenite composition.

At 12000C the major phase was leucite and it decreased at 1250C (Table 1). At 1250C

and above kalsilite was the major phase and no leucite was detected at 1350-

1400C. The SEM image (not shown) of the 1400C heated sample confirmed this, but in

addition it showed a trace of calcium aluminium silicate in which the Ca:Al:Si ratio was

2:1:2. The d-spacings of the kalsilite phase above 1250C had shifted indicating the

possible incorporation of another cation such as Ca (also confirmed by EDS). Similar

results have been shown for a metakaolinite/K-alkali system by solid state nuclear

magnetic resonance [7]. Kalsilite has a melting point of ~ 1750C [11] and that of

leucite is 1686C [11], so both are quite refractory. Although the liquid forms at ~750C,

the presence of two refractory phases should be sufficient to make the geopolymersufficiently refractory at 1000C for continuous use at this temperature. Heating the


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geopolymer at 1000C for 5 h did not show any slump and this is an empirical indication

of refractoriness.

The high porosity of the geopolymers should make them suitable for use as thermal

insulators. The pore distribution at 1000C is shown in the secondary SEM image at

1000C (Figure 2). Refractory castables are made by mixing high-alumina cement withchamotte (calcined fireclay). When required water is added and cast to the required

shape. Geopolymers could also be used similarly with chamotte. The geopolymers

produced in this work had no expansion or shrinkage after curing which is also an

advantage.

Figure 2. The pore distribution at 1000C is shown in the secondary SEM image at 1000

A geopolymer made without any aggregate gave a compressive strength of ~ 80 MPa

which is sufficiently high compared to alumino silicate thermal insulators used at ~

1000C (~ 15 MPa at 50% porosity [12]). Thermal insulators are used for lining

structurally supporting refractories or as mortars in such structures. Hence, a high

temperature high strength is not a pre-requisite for their use.

Conclusions

The geopolymers heated up to 1400C did not show any major melting. The presence of

two refractory phases kalsilite and leucite should make them sufficiently refractory at

1000C for its continuous use. High porosity of the geopolymers should make them

suitable for use as thermal insulators.

Acknowledgements

Authors thank Joel Davis for unpublished SEM work and Lou Vance for making valuablesuggestions.

References


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1. Davidovits, Geopolymers Inorganic Polymeric New materials, Journal of Thermal. Analysis, 37 [8](1991) 1633-56.

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(2004), CD, paper no. 4589.

7. V. F. F. Barbosa and K. J. D. MacKenzie, Synthesis and Thermal Behaviour of Potassium SialateGeopolymers, Materials Letters, 57 (2003) 1477-82.

8. D. S Perera, E. R. Vance, D. J. Cassidy, M. G. Blackford, J. V. Hanna, R. L. Trautman and C. L.Nicholson, The effect of Heat on Geopolymers Made Using Fly Ash and Metakaolnite, Ceram. Trans., 165

(2004) 87-94.

9. Australian Standard AS 1774.5-2001, The determination of density, porosity and water absorption,Standards Australia (2001).

10.Phase Diagram for Ceramists, Edited by E. M. Levin, C. R. Robbins and H. F. Mc Murdee, p. 156,American Ceramic Society, Westerville, Ohio, USA, (1964).

11. Ibid. p.157.

12. F. Singer and S. S. Singer, "Industrial Ceramics," pub. Chapman and Hall, London, UK, 1963, pp. 1284-90.

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Geopolymers With the Potential for Use as Refractory Castables