Fabrication of Structural Leucite GlassCeramics from Potassium-Based Geopolymer Precursors

Prévia do material em texto

Fabrication of Structural Leucite Glass–Ceramics from Potassium-Based
Geopolymer Precursors
Ning Xie,*,z,y Jonathan. L. Bell,*,z and Waltraud M. Krivenw,z,**
zDepartment of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois
yHarbin Institute of Technology, Harbin, China
Leucite glass–ceramics were fabricated by cold isostatically
pressing K2O .Al2O3 . 4SiO2 . 11H2O geopolymer powders
into pellets followed by firing at 9501–12001C, every 501C in
air. Leucite formation was observed in specimens heat treated to
�10001C. The relative density, Vickers hardness, fracture
toughness, and biaxial flexural strength of sintered samples
ranged approximately 96%–98%, 767–865 kg/mm2, 0.94–
2.36 MPa .m1/2, and 90–140 MPa, respectively. The toughness
and biaxial flexure strength increased with the firing tempera-
ture, while the density and hardness were relatively constant.
Scanning electron microscopic and transmission electron micro-
scopic analysis revealed that the sintered geopolymer formed
leucite crystals and a compositionally variable glassy phase.
Samples heated to 12001C attained the highest biaxial flexure
strength and toughness. This higher strength is believed to
arise from an optimum in density, leucite content, and crystal
size distribution.
I. Introduction
LEUCITE (K2O �Al2O3 � 4SiO2) glass–ceramics have been man-ufactured for many applications such as dental porcelains,
refractories, and structural ceramic materials. These applica-
tions often utilize leucite because of its large coefficient of ther-
mal expansion and relatively high melting point of 16931C.1
There are various methods for manufacturing leucite and leucite
glass–ceramics including conventional melting of mixed oxides,2
sol–gel,3 and hydrothermal ion exchange of nepheline.4 How-
ever, these methods typically require expensive equipment, long
processing times, and costly precursors.
An alternative method to produce leucite and pollucite
(Cs2O �Al2O3 � 4SiO2) ceramics is through the use of geopoly-
mer precursors.5,6 Geopolymers are a class of amorphous, al-
uminosilicate materials that harden hydrothermally at ambient
temperatures, and can be converted into a ceramic by heating.
Chemically, geopolymers consist of cross-linked units of AlO4
�
and SiO4 tetrahedra, where charge-balancing cations are pro-
vided by alkali metal cations such as Li1, Na1, K1, and Cs1.7
Geopolymers are formed by reacting aluminosilicate minerals
(i.e., such as metakaolin or fly-ash) and silica with highly caustic,
aqueous alkaline (MOH) solutions, where M is an alkali cation.
They can be produced over a wide range of compositions, cen-
tered on the molar ratios ofM2O–Al2O3–xSiO2–yH2O, whereM
is an alkali metal cation (typically Na1 or K1), x5 2–6, and
y5 7.5–13.8,9 Alkali aluminosilicate geopolymers of the compo-
sition M2O �Al2O3 � 4SiO2 � xH2O (M5K, Cs), react to form
high-strength monoliths, which can be used as an structural ce-
ment,10,11 or they can be converted to leucite and pollucite
ceramics by heating.12
The feasibility of forming ceramics from geopolymers has
been demonstrated; however, little work has been performed to
investigate the mechanical properties. Prior work has primarily
focused on the microstructural and physical evolution on heat-
ing of as-cast geopolymers.5,6,13,14 Heating of as-cast, monolithic
bodies typically leads to cracking due to large capillary forces
acting on capillary channels during drying.15 In addition, much
of the studies performed on heated geopolymer systems were
conducted using impure materials such as fly-ash.16–19 Fly-ash is
a byproduct of burning pulverized coal. It has a variable chem-
ical and mineralogical composition, which depends on the min-
ing location and the coal combustion conditions, making it
difficult to obtain a consistent supply of raw material. In addi-
tion, fly-ash powders only partially dissolve in the alkali–silicate
solution. The resultant geopolymer formed is therefore a com-
posite material, which consists of the partially dissolved fly-ash
particles surrounded by an aluminosilicate matrix. On heating,
fly-ash geopolymers are expected to form a variety of crystalline
phases.
The use of metakaolin (Al2O3 � 2SiO2) instead of fly-ash has
been found to produce more homogeneous, highly reacted geo-
polymers.20 Compared with fly-ash, metakaolin has higher pu-
rity and superior reactivity in alkali silicate solution.
Geopolymer precursors based on metakaolin therefore can be
used to form high-purity, leucite-based ceramics. It is also pos-
sible to tailor the thermal expansion of the ceramic by varying
the alkali21,22 (e.g., Cs produces a lower thermal expansion,
while K produces a higher thermal expansion) or silica concen-
tration.23
In this investigation, leucite ceramic disks were fabricated by
grinding K2O �Al2O3 � 4SiO2 � 11H2O geopolymer to a powder,
followed by drying, isostatic pressing, and firing under different
heating conditions in air. Die pressing of dried geopolymer suc-
cessfully prevented cracking on heating, which is commonly ob-
served in as-cast geopolymer due to capillary stresses created
from the evaporation of water.15 The biaxial flexure strength,
hardness, and toughness of the pressed fired disks were deter-
mined by mechanical testing. The link between microstructure
and mechanical properties was investigated using SEM and
TEM analysis.
II. Experimental Procedure
(1) Sample Preparation
Potassium silicate solution (2SiO2 �K2O � 11H2O) was prepared
by dissolving amorphous fumed silica (Cabot M-5, T. H.
Hilson Company, Wheaton, IL) into KOH solution with
the molar ratio of K2O � 11H2O. The silicate solution was
allowed to mix for 24 h. Geopolymer of the composition
K2O �Al2O3 � 4SiO2 � 11H2O (deemed KGP) was then prepared
C. Jantzen—contributing editor
*Member, The American Ceramic Society.
This work was supported by the Air Force Office of Scientific Research (AFOSR),
USAF, under Nanoinitiative grant no. FA9550-06-1-0221, through Dr. Joan Fuller.
wAuthor to whom correspondence should be addressed. e-mail: kriven@illinois.edu
**Fellow, The American Ceramic Society.
Manuscript No. 26337. Received November 6, 2009; approved March 19, 2010.
Journal
J. Am. Ceram. Soc., 93 [9] 2644–2649 (2010)
DOI: 10.1111/j.1551-2916.2010.03794.x
r 2010 The American Ceramic Society
2644
by mixing metakaolin (Metamax High Reactivity Metakaolin,
BASF Corporation, Iselin, NJ) into the potassium silicate using
an IKA overhead mixer (Model RW 20, Wilmington, NC)
equipped with a dispersion blade. The metakaolin used in this
study had an average particle size of 1.2 mm, a specific surface
area of 13 m2/g, and had B97% purity.
After being mixed for 10 min, the solution was placed on a
vibration table to remove entrapped air, cast into a sealed plastic
container, and cured for 48 h at 501C. The cured KGP then was
ground using a mortar and pestle followed by attrition milling
(Union Process 1 liter mill, Akron, OH) for 1 h at 120 rpm using
3-mm-diameter ZrO2 balls and 2-propanol as the milling sus-
pension. Milling containers were loaded with 20 g of geopolymer
powder, 120 mL balls, and a sufficient volume of 2-propanol.
The resultant powders were dried at 1501C for 24 h and sieved
with a�325 standard mesh (i.e.,r45 mmmesh). Sieved powders
were then pressed using a 40-mm-diameter cylindrical stainless-
steel die at 16 MPa for 1 min. The pressed bodies were further
consolidated using cold isostatic pressing (Model CP360, Amer-
ican Isostatic Presses Inc., Columbus, OH) operated at 300 MPa
for a 10-min hold time.
The final green body was sintered in a high-temperature Rad-
atherm furnace (Model HT 05/18, Wetherill Park, NSW, Aus-
tralia) to 9501–12001C at 501C intervals, for a 3-h soak time at a
heating and cooling rate of 21C/min. To reduce sample warping,
all samples were firedin a powder bed of Al2O3 powders. All
samples removed from the furnace were intact, with the excep-
tion of those heated at 9501C.
(2) Sample Characterization
The density of samples was measured using the Archimedes
method. The theoretical density of leucite was assumed to be
2.47 g/cm3 based upon lattice parameters of tetragonal leucite as
determined by Mazzi et al.24 The sintered samples were also char-
acterized using a Rigaku D/Max-b X-ray diffractometer (The
Woodlands, TX). For X-ray diffraction (XRD), a copper target at
45 kV and 20 mA was used, and diffraction scans were performed
from 51 to 751 2y at a rate of 11/min, with a step size of 0.021.
Biaxial flexural strength tests were conducted using an
Instron Multifunctional Mechanical testing machine at room
temperature (Model 4502, Instron Corp., Canton, MA). A min-
imum of five specimens of each group were subjected to biaxial
flexural strength testing in accordance with ASTM Standard
F394-78. Each disk specimen was placed centrally on three
hardened steel balls with the diameter of 3.18 mm, positioned
1201 apart on a support circle with a diameter of 11 mm. Ac-
cording to ASTM Standard F394-78, the expression for biaxial
flexural strength (MPa) is
S ¼ �0:2387PðX � YÞ=d 2 (1)
where S is the maximum center tensile stress (MPa), P is the
total load causing fracture (N), d is the specimen thickness at the
fracture origin (mm), and X and Y are constants that are deter-
mined by the Poisson ratio of the material and the radius of the
specimen, support circle, and ram tip. A value of 0.23 was as-
sumed for the Poisson’s ratio.
Vickers indentation hardness, Knoop indentation hardness,
and fracture toughness were determined using a commercial mi-
crohardness tester (Zwick 3212 microhardness tester, Mark V
Laboratory Inc., East Granby, CT). All the indentation marks
were examined at high magnifications using a Hitachi S-4700
high-resolution scanning electron microscope (SEM, Hitachi
Co., Tokyo, Japan) to ensure accurate measurement. A total
of 10–15 measurements were averaged in order to determine the
final toughness value. A typical SEM micrograph used for
Vickers indentations measurements is shown in Fig. 1. The frac-
ture toughness was calculated from SEM micrographs using
the methodology described by Anstis et al.25 via the following
equation:
KIC ¼ a E
H
� �1
2
P=c3=2
� �
(2)
Fig. 1. Scanning electron microscopic micrographs of indents for measuring (a) Vickers toughness, (b) Knoop hardness, and (c) Vickers hardness.
Fig. 2. X-ray diffraction results for KGP after being heated to 10001C,
10501C, 11001C, 11501C, and 12001C. Samples were heated and cooled
at 21C/min and were soaked at temperature for 3 h.
September 2010 Structural Leucite Glass–Ceramics Converted from K-Geopolymer 2645
where a is a dimensionless material-independent constant that
has an empirical value of 0.01670.004 for Vickers indentation-
induced crack systems, P is the applied indentation load, c is the
indentation crack half-length (mm) (taken as the average of the
two orthogonal radial directions as shown in Fig. 1(a)), and E
and H are the elastic modulus and hardness values, respectively,
based on the Knoop indentation hardness26:
H
E
¼ 1
0:45
1
7:11
� b
a
� �
(3)
where b and a are the short and long axis of the Knoop inden-
tation mark (mm) (as shown in Fig. 1(b)). The Vickers hardness
indentations of the samples were measured by the following
equation27:
HV ¼ 1:8544� P=D2 (4)
where HV is the Vickers hardness (kg/mm
2), P is the applied
load taken (g), and D is the average diagonal length of the in-
dentation mark (mm).
Transmission electron microscopy (TEM) and TEM/EDS
work were carried out at 120 kV on thin, ion-milled samples
using both a JEOL 2100 Cryo TEM (Sheboygan, WI) and a
Philips CM12 TEM (Eindhoven, the Netherlands). The JEOL
2100 TEM was equipped with an EDAX
s
EDS microanalysis
system (Model EDAX-4, EDAX Corp., Mahwah, NJ). Samples
for TEM were prepared by slicing sintered KGP using a Buehler
low-speed diamond saw, which was then heated at 11001C at a
heating rate of 21C/min and a 3-h soak time in the Radatherm
high-temperature furnace. The sliced samples were subsequently
cut into 3-mm-diameter disks using a Gatan ultrasonic disk cut-
ter (Model 601, Warrendale, PA). The 3 mm disks were thinned
to 100 mm using a Buehler Minimet disk polisher (Model Ecom-
net III, Lake Bluff, IL), followed by dimple grinding to 20 mm
with a Gatan dimple grinder (Model 656). Finally, samples were
ion-milled at a low temperature, using a Fischione ion mill
(Model 2, Export, PA).
III. Results and Discussion
(1) X-Ray and Microstructural Analysis
XRD patterns for KGP heated at different temperatures are
shown in Fig. 2. Consistent with an earlier work on as-cast
KGP,5 the crystallization of leucite was obvious in samples heated
to �11001C. In the samples heated to 11001 and 11501C, the
Fig. 3. Scanning electron microscopic fracture surface morphology of the KGP after being heated to (a) 10001C, (b) 10501C, (c) 11001C, (d) 11501C,
and (e) 12001C. Samples were heated and cooled at 21C/min and were soaked at temperature for 3 h.
2646 Journal of the American Ceramic Society—Xie et al. Vol. 93, No. 9
presence of a small peak at 311 2y suggests that a minor amount of
kalsilite formed; however, this peak disappeared in the sample
heated to 12001C. Figure 3 shows the SEM fracture surface mor-
phology of the sintered KGP samples. There were no obvious
microcracks, and the surface developed a glassy texture on heat-
ing. Leucite crystals could not be directly observed on any of the
Fig. 4. Scanning electron microscopic fracture surface morphology of the KGP after being heated to (a) 10001C, (b) 10501C, (c) 11001C, (d) 11501C,
and (e) 12001C and etched in 3 wt% HF at room temperature for 20 s. Samples were heated and cooled at 21C/min and were soaked at temperature
for 3 h.
Fig. 5. Transmission electron microscopic results for sintered KGP showing (a) a bright-field micrograph, (b) a selected area diffraction for region A
along the [001] zone axis of tetragonal leucite, and (c) a selected area diffraction pattern for region B showing a diffuse diffraction ring indicative of a
disordered glassy phase.
September 2010 Structural Leucite Glass–Ceramics Converted from K-Geopolymer 2647
fracture surfaces despite their noticeable presence in X-ray results
for samples heated to �11001C. Similar results were observed for
as-cast KGP heated between 9251 and 11001C.5
After etching the samples in 3 wt% HF acid, the glassy phase
dissolved away and leucite crystals could be observed (Fig. 4).
The heated KGP contained a polydisperse distribution of leucite
crystals, a glassy matrix, and pores left behind from leucite crys-
tals that fell out during etching. All samples contained a large
number of small 1–5-mm-sized leucite crystallites, which in-
creased only slightly in size in samples fired at higher tempera-
tures. This observation is thought to be indicative of a
nucleation-rich environment. It is believed that in samples
heated to 11501 or 12001C (Figs. 4(d) and (e)), the crystals
reached a large enough size to have some level of connectivity,
and were therefore less likely to fall out.
TEM results for KGP after being sintered at 11001C for 3 h
are shown in Fig. 5. In this figure, the darker round areas
(labeled area A) were found to be nanosized leucite crystals,
while the brighter surrounding areas (area B) were glassy phase.
Figures 5(b) and (c) are the diffraction patterns corresponding
to the crystalline phase and glassy phase, respectively. As shown
in Fig. 5(b), the diffraction pattern of area A matched that of
tetragonal leucite along the [001] direction. The diffraction pat-
tern in Fig. 5(c) was a diffuse ring, which is characteristic of an
amorphous material.TEM-EDS results for the glassy area shown in Fig. 5(a) are
given in Table I. A total of six measurements were taken at
different locations of the glassy region. The Si and K content
were found to be more variable than the Al content. The cre-
ation of this glassy phase and chemical variation is believed to
arise from inhomogeneities in the geopolymer, and due to the
ease of forming glasses in the K2O–Al2O3–SiO2 system.
It was determined in a prior work that not all of the meta-
kaolin is dissolved before KGP setting.20 This leads to the pres-
ence of free alkali that persists in the pore water.28,29 Free alkali
has been shown to leach out when metakaolin-based geopoly-
mers were placed in deionized water.30 In the K2O–Al2O3–SiO2
system, there is a substantial amount of glass formation near the
leucite composition field, largely due to the difficulty of crystal-
lizing in the highly viscous glasses produced in this system.31,32
On heating KGP, heterogeneities and free alkali will favor the
formation of compositionally variable, glassy phase(s).
(2) Density and Mechanical Properties
Density values, measured using the Archimedes method, are
listed in Table II. The relative densities of the final materials
were about 96%–98% of the theoretical density of tetragonal
leucite, confirming that the materials were relatively well densi-
fied. This was consistent with the smooth, dense, glassy texture
observed in the fracture surfaces of heated samples using SEM.
KGP has been shown previously to undergo rapid viscous sinte-
ring, surface area reduction, and densification between 9001 and
9501C when heated and cooled at 101C per minute, with no
isothermal soak.5,13 The KGP samples prepared in this study
were heated and cooled at 21C/min with a 3-h isothermal soak,
and were therefore expected to be well densified.
The Vickers hardness, fracture toughness, and biaxial flexural
strength of KGP sintered at different temperatures are listed in
Table III. As shown in this table, the fracture toughness and
biaxial flexure strength increased with the increasing sintering
temperature, except for those samples heated at 9501C, which
cracked into small pieces after being removed from the furnace.
Samples fired at 12001C for 3 h reached a maximum toughness
and biaxial flexure strength of 2.36 MPa �m1/2 and 140 MPa,
respectively. The hardness was relatively constant for all of the
samples tested.
The mechanical properties of the sintered KGP samples com-
pares well to that of leucite-based dental porcelains and leucite
glass–ceramics reported in the literature. For example, Shareef
et al.33 determined that the biaxial flexure strength of a variety of
commercially available dental ceramics ranged from 56 to 137
MPa. Sherrill and O’Brien34 found that the flexure strength of
porcelain specimens, which were prepared by firing commer-
cially available feldspathic dentin powers to 11501C, was ap-
proximately 90 MPa. Cesar et al.35 determined that the fracture
toughness and Vickers hardness of a leucite-based dental por-
celain were 0.82 MPa �m1/2 and 5.69 GPa, respectively. Sheu
et al.36 produced porous leucite specimens of the composition
K2O �Al2O3 � 4SiO2 using the sol–gel route; by sintering above
11001C, flexural strengths ranging from 147 to 175 MPa were
obtained in four-point bend tests. Hashimoto et al.37 fabricated
dense leucite ceramics by sintering pressed leucite powders to
12501C for 1 h; they determined that the bending strength, frac-
ture toughness, and Vickers hardness were 173 MPa, 2.3
MPa �m1/2, and 5.3 GPa, respectively. In the same study, sim-
ilar values reported for dense potash feldspar were 140 MPa, 1.5
MPa �m1/2, and 7.3 GPa, respectively.
Mackert et al.38 recommended a leucite particle diameter of
o4 mm to prevent microcracking due to the phase transforma-
tion observed in leucite-based dental porcelains on cooling. For
the samples we examined, the majority of leucite particles were 5
mm or less. This may also explain why KGP heated to 12001C
had the highest biaxial flexure strength and toughness. The
strongest samples are expected to be the most dense and com-
prised of a large number of small leucite grains. As shown in
Fig. 4(e), the majority of the leucite grains ranged in size from
B1 to 5 mm. Moreover, a larger fraction of leucite grains could
be observed in this sample compared with samples heated at
lower temperatures.
Table I. Chemical Composition (at.%) for the Glassy Phase
of KGP as Determined from TEM-EDS
Chemical elements Average composition Standard deviation
K 21.2 3.2
Al 17.3 1.8
Si 36.2 3.7
O 26.0 4.7
The KGP was heated and cooled at 21C/min, and sintered for 3 h at 11001C.
TEM, transmission electron microscope.
Table II. Density Measurement Results for KGP Sintered at
Different Temperaturesw
Sintering
temperature (1C)
Real
density (g/cm3)z
Relative
density (% TD)
950 — —
1000 2.38 0.96
1050 2.38 0.96
1100 2.41 0.98
1150 2.38 0.96
1200 2.41 0.98
wSamples were heated and cooled at 21C/min, and sintered at temperature for
3 h. zBased on a theoretical density if 2.47 g/cm3.
Table III. Vickers Hardness, Fracture Toughness, and Biaxial
Flexural Strength of KGP Sintered at Different Temperatures
Sintering
temperature (1C)
Vickers hardness
(GPa)
Fracture toughness
(MPa �m1/2)
Biaxial flexural
strength (MPa)
950 — — —
1000 7.58 1.15 87.42721.44
1050 8.02 0.94 107.3711.32
1100 8.49 2.21 114.2477.54
1150 7.84 2.32 119.37710.26
1200 7.52 2.36 139.63717.12
Samples were heated and cooled at 2oC/min, and sintered at temperature
for 3 h.
2648 Journal of the American Ceramic Society—Xie et al. Vol. 93, No. 9
IV. Conclusions
Compared with the conventional methods used to produce
leucite ceramics, the use of geopolymer precursors provides a
simple, low-cost approach to form relatively high-strength, leu-
cite-based glass ceramics. In this study, high-strength leucite
glass ceramics were fabricated by sintering an isostatically
pressed, potassium-based geopolymer. In order to avoid crack-
ing on heating, which is often observed when monolithic geo-
polymers are heated, the precursor geopolymer was ground to a
powder followed by pressing and firing at high temperature.
Firing above 10001C was sufficient to consolidate the sample
and begin leucite crystallization. Heating to 12001C for 3 h re-
sulted in a maximum in density, fracture toughness, and biaxial
flexure strength of 98%, 2.3 MPa �m1/2, and 140 MPa, respec-
tively. The sample hardness, as determined from Vickers inden-
tation, remained relatively constant for all samples fired over the
range of 10001–12001C.
Observations made using SEM and TEM revealed that the
fired potassium geopolymer was comprised of both leucite crys-
tals and a surrounding glassy phase. The typical size of the leucite
crystals ranged from 1 to 5 mm, and increased only slightly with
firing temperature. The toughness and biaxial flexure were more
sensitive to the grain size and leucite concentration, and in-
creased measurably upon firing from 10001 to 12001. It is be-
lieved that the higher firing temperature produced the sufficient
size and quantity of leucite grains needed to reinforce the glassy
matrix with the stronger high-temperature, leucite ceramic phase.
Acknowledgments
The authors acknowledge the use of facilities at the Center forMicroanalysis of
Materials, in the Frederick Seitz Research Laboratory at the University of Illinois
at Urbana-Champaign, which is partially supported by the U.S. Department of
Energy under grant no. DEFG02-91-ER45439. Ning Xie acknowledges Prof.
Liang Zhen and Prof. Wenzhu Shao for their help and support, and also ac-
knowledges the China Scholarship Council (CSC) for awarding a scholarship to
pursue research in the United States as a graduate exchange student.
References
1E. M. Levin, C. R. Robbins, and H. F. McMurdie, Phase Diagramsfor Cera-
mists. The American Ceramic Society, Westerville, OH, 1964, p. 156.
2W. Holand, M. Frank, and V. Rheinberger, ‘‘Surface Crystallization of Leucite
in Glasses,’’ J. Non-Cryst. Solids, 180 [2–3] 292–307 (1995).
3Y. Zhang, J. Q. Wu, P. G. Rao, and M. Lv, ‘‘Low Temperature Synthesis of
High Purity Leucite,’’ Mater. Lett., 60 [23] 2819–23 (2006).
4A. Balandis and I. Sinkyavichene, ‘‘Hydrothermal Synthesis of Leucite and its
Application in Engineering Ceramics,’’ Glass Ceram., 62 [1–2] 49–52 (2005).
5J. L. Bell, P. E. Driemeyer, and W. M. Kriven, ‘‘Formation of Ceramics from
Metakaolin Based Geopolymers: Part II—K-Based Geopolymer,’’ J. Am. Ceram.
Soc., 92 [3] 607–15 (2009).
6J. L. Bell, P. E. Driemeyer, and W. M. Kriven, ‘‘Formation of Ceramics from
Metakaolin Based Geopolymers: Part I—Cs-Based Geopolymer,’’ J. Am. Ceram.
Soc., 92 [1] 1–8 (2009).
7J. Davidovits, ‘‘Geopolymers—Inorganic Polymeric NewMaterials,’’ J. Therm.
Anal., 37 [8] 1633–56 (1991).
8J. Davidovits, ‘‘Mineral Polymers andMethods of Making Them’’; U.S. Patent
4,349,386, September 14, 1982.
9J. Davidovits, ‘‘Early High-Strength Mineral Polymer’’; U.S. Patent 4,509,985,
April 9, 1985.
10P. Duxson, J. L. Provis, G. C. Lukey, S. W.Mallicoat, W.M. Kriven, and J. S.
J. van Deventer, ‘‘Understanding the Relationship Between Geopolymer Compo-
sition, Microstructure and Mechanical Properties,’’ Colloid Surf. A, 269 [1–3]
47–58 (2005).
11M. Rowles and B. O’Connor, ‘‘Chemical Optimisation of the Compressive
Strength of Aluminosilicate Geopolymers Synthesised by Sodium Silicate Activa-
tion of Metakaolinite,’’ J. Mater. Chem., 13 [5] 1161–5 (2003).
12W. M. Kriven, J. L. Bell, S. W. Mallicoat, and M. Gordon, ‘‘Intrinsic
Microstructure and Properties of Metakaolin-Based Geopolymers’’; pp. 71–86 in
International Workshop on Geopolymer Binders, Interdependence of Composition,
Structure and Properties, Edited by W. M. Kriven, Bauhaus-Universita¨t, Weimar,
Germany, 2006.
13P. Duxson, G. C. Lukey, and J. S. J. van Deventer, ‘‘Thermal Evolution of
Metakaolin Geopolymers: Part 1—Physical Evolution,’’ J. Non-Cryst. Solids, 352
[52–54] 5541–55 (2006).
14P. Duxson, G. C. Lukey, and J. S. J. van Deventer, ‘‘The Thermal Evolution
of Metakaolin Geopolymers: Part 2—Phase Stability and Structural Develop-
ment,’’ J. Non-Cryst. Solids, 353 [22–23] 2186–200 (2007).
15M. Gordon, J. L. Bell, and W. M. Kriven, ‘‘Thermal Conversion and Micro-
structural Evaluation of Geopolymers or ‘‘Alkali-Bonded Ceramics’’ (ABCs)’’;
pp. 215–24 in Ceramic Transactions, Vol. 175, Advances in Ceramic Matrix
Composites XI, Edited by N. P. Bansal, J. P. Singh, and W. M. Kriven. The
American Ceramic Society, Westerville, OH, 2005.
16T. Bakharev, ‘‘Geopolymeric Materials Prepared Using Class F Fly Ash and
Elevated Temperature Curing,’’ Cem. Concr. Res., 35 [6] 1224–32 (2005).
17T. Bakharev, ‘‘Thermal Behaviour of Geopolymers Prepared Using Class F
Fly Ash and Elevated Temperature Curing,’’ Cem. Concr. Res., 36 [6] 1134–47
(2006).
18K. Dombrowski, A. Buchwald, and M. Weil, ‘‘The Influence of Calcium
Content on the Structure and Thermal Performance of Fly Ash Based Geopoly-
mers,’’ J. Mater. Sci., 42 [9] 3033–43 (2007).
19D. L. Kong, J. G. Sanjayan, and K. Sagoe-Crentsil, ‘‘Comparative Perfor-
mance of Geopolymers Made with Metakaolin and Fly Ash After Exposure to
Elevated Temperatures,’’ Cem. Concr. Res., 37 [12] 1583–9 (2007).
20W. M. Kriven, J. Bell, and M. Gordon, ‘‘Microstructure and Microchemistry
of Fully-Reacted Geopolymers and Geopolymer Matrix Composites’’; pp. 227–50
inCeramic Transactions, Vol. 153, Advances in CeramicMatrix Composites, Edited
by N. P. Bansal, J. P. Singh, W. M. Kriven, and H. Schneider. The American
Ceramic Society, Westerville, OH, 2003.
21R. L. Bedard, R. W. Broach, and E. M. Flanigen, ‘‘Leucite–Pollucite Glass
Ceramics: A New Family of Refractory Materials with Adjustable Thermal-
Expansion,’’ Mater. Res. Soc. Symp. Proc., 271, 581–7 (1992).
22S. T. Rasmussen, C. L. Groh, and W. J. O’Brien, ‘‘Stress Induced Phase
Transformation of a Cesium Stabilized Leucite Porcelain and Associated Proper-
ties,’’ Dent. Mater., 14 [3] 202–11 (1998).
23J. P. Wiff, Y. Kinemuchi, S. Naito, A. Uozumi, and K. Watari, ‘‘Phase Tran-
sition and Thermal Expansion Coefficient of Leucite Ceramics with Addition of
SiO2,’’ J. Mater. Res., 24 [6] 1989–93 (2009).
24F. Mazzi, E. Galli, and G. Gottardi, ‘‘The Crystal Structure of Tetragonal
Leucite,’’ Am. Mineral., 61 [1–2] 108–15 (1976).
25G. R. Anstis, P. Chantikul, B. R. Lawn, and D. B. Marshall, ‘‘A Critical-
Evaluation of Indentation Techniques for Measuring Fracture-Toughness. 1.
Direct Crack Measurements,’’ J. Am. Ceram. Soc., 64 [9] 533–8 (1981).
26D. B. Marshall, T. Noma, and A. G. Evans, ‘‘A Simple Method for Deter-
mining Elastic-Modulus-to-Hardness Ratios Using Knoop Indentation Measure-
ments,’’ J. Am. Ceram. Soc., 65 [10] C175–6 (1982).
27B. W. Mott,Microindentation Hardness Testing. Butterworths Scientific, Lon-
don, 1966, p. 9.
28P. Duxson, G. C. Lukey, F. Separovic, and J. S. J. van Deventer, ‘‘Effect of
Alkali Cations on Aluminum Incorporation in Geopolymeric Gels,’’ Ind. Eng.
Chem. Res., 44 [4] 832–9 (2005).
29P. Duxson, J. L. Provis, G. C. Lukey, J. S. J. van Deventer, F. Separovic, and
Z. H. Gan, ‘‘K-39 NMR of Free Potassium in Geopolymers,’’ Ind. Eng. Chem.
Res., 45 [26] 9208–10 (2006).
30L. Ly, E. R. Vance, D. S. Perera, Z. Aly, and K. Olufson, ‘‘Leaching of Geo-
polymers in Deionised Water,’’ Adv. Tech. Mater. Mater. Proc., 3 [2] 236–47
(2006).
31W. D. Kingery, H. K. Bowden, and D. R. Uhlmann, Introduction to Ceramics,
2nd edition, Wiley, New York, 1976.
32J. F. Schairer and N. L. Bowen, ‘‘The System K2O–Al2O3–SiO2,’’ Am. J. Sci.,
253, 681–746 (1955).
33M. Y. Shareef, R. Vannoort, P. F. Messer, and V. Piddock, ‘‘The Effect of
Microstructural Features on the Biaxial Flexural Strength of Leucite Reinforced
Glass–Ceramics,’’ J. Mater. Sci.: Mater. Med., 5 [2] 113–8 (1994).
34C. A. Sherrill and W. J. O’Brien, ‘‘Transverse Strength of Aluminous and
Feldspathic Porcelain,’’ J. Dent. Res., 53, 683–90 (1974).
35P. F. Cesar, C. C. Gonzaga, C. Y. Okada, A. L. Molisani, W. Santana, H.
Goldenstein, W. G. Miranda, and H. N. Yoshimura, ‘‘Variables that Affect the
Indentation Fracture Testing (IF) of a Dental Porcelain’’; pp. 663–8 in Advanced
Powder Technology III, Vol. 416-4, Edited by L. Salgado, and F. A. Filho. Trans
Tech Publications (Uetikon-Zurich, Switzerland), SUISSE, INIST-CNRS, 2003.
36T. S. Sheu, W. J. Obrien, S. T. Rasmussen, and T. Y. Tien, ‘‘Mechanical-
Properties and Thermal-Expansion Behavior in Leucite Containing Materials,’’
J. Mater. Sci., 29 [1] 125–8 (1994).
37S. Hashimoto, F. Sato, S. Honda, H. Awaji, and K. Fukuda, ‘‘Fabrication
and Mechanical Properties of Sintered Leucite Body,’’ J. Am. Ceram. Soc. Jpn.,
113 [1319] 488–90 (2005).
38J. R. Mackert, S. W. Twiggs Jr., C. M. Russell, and A. L. Williams, ‘‘Evidence
of a Critical Leucite Particle Size forMicrocracking in Dental Porcelains,’’ J. Dent.
Res., 80 [6] 1574–9 (2001). &
September 2010 Structural Leucite Glass–Ceramics Converted from K-Geopolymer 2649