Characterization of Alkali-Activated Fly-Ash by
Nanoindentation
J. Němeček, V. Šmilauer, and L. Kopecký1
Abstract. Nanoindentation was employed for the characterization of reaction
products, mainly N-A-S-H gel, within alkali-activated fly ash samples. Heat and
ambient-cured samples from ground fly ash were indented in a grid of hundreds of
indents. The intrinsic Young's modulus of N-A-S-H gel was found around the
mean value 17.70 GPa, regardless on the curing procedure. Such finding elucidates intrinsic stiffness of mature N-A-S-H gel with different origin. Partlyactivated slag, slag and fly-ash particles were further distinguished by histogram
deconvolution.
1 Introduction
Alkali-activated fly ash (AAFA) is a new promising material forming stable inorganic
binder. AAFA provides high potential in a partial replacement of ordinary concrete
due to improved durability, acid and fire resistance, low calcium content, low drying
shrinkage, no alkali-silica reaction, good freeze/thaw performance or lower creep induced by mechanical load [11]. The potential utilization of fly ash, as a by-product of
coal power plants, brings attention of several researchers [6, 8, 11, 12].
Chemically, the main reaction product of fly ash is an amorphous aluminosilicate gel (denoted further as N-A-S-H gel) and/or C-S-H gel forming in the
J. Němeček
Czech Technical University in Prague
e-mail: [email protected]
http://mech.fsv.cvut.cz
V. Šmilauer
Czech Technical University in Prague
e-mail: [email protected]
http://mech.fsv.cvut.cz
L. Kopecký
Czech Technical University in Prague
e-mail: [email protected]
http://mech.fsv.cvut.cz
338
J. Němeček et al.
presence of calcium and low alkalinity activator [1]. The chemical composition of
the N-A-S-H gel is similar to crystalline natural zeolitic materials but the microstructure is of amorphous nature. The N-A-S-H gel consists of three-dimensional
structure, built from SiO4 and AlO4 tetrahedra connected by shared O atoms and
forming polymeric chains [0, 0]
M n [−( SiO2 ) z − AlO2 ]n ⋅ wH 2O
(1)
where M stands for sodium, potassium or calcium supplied with alkali activator
and fly ash, n is the degree of polymerization, z quantifies the amount of SiO2
monomer units in the gel, typically within the range from 1 to 3 and w is the
amount of binding water.
Several experimental techniques can be applied to characterize mechanical behavior of individual components of the AAFA composite. Nanoindentation plays
an important role among the experimental techniques working at submicron length
scale. Nanoindentation is based on the direct measurement of the loaddisplacement (P-h) relationship using a very small tip (typically diamond) pressed
into the material. Standard processing of the measured P-h relation is based on the
analytical solution of a contact problem involving an indenter and a semi-infinite
solid body and provides the hardness and Young’s modulus. The Oliver-Pharr [7]
solution assumes perfectly flat surface and isotropic elasto-plastic material. Results from a similar cementititous material can be found in [2, 3].
The objectives of this paper aim at the characterization of intrinsic N-A-S-H
gel properties in the heterogeneous microstructure of AAFA on the scale of micrometers. Ambient and heat-cured samples were prepared from the same composition to explore the differences in the curing procedure.
2 Experimental
2.1 Materials
The raw fly ash (RFA) originates from Chvaletice, Czech Republic, with the
2
-1
Blaine specific surface 210 m kg . The average chemical composition of this RFA
is given in Tab. 1 with SiO2/Al2O3 mass ratio 1.58. RFA was ground in a smallscale ball mill in the quantity of 8 kg for 45 minutes. Activating solution was prepared by dissolution of NaOH in a tap water with the addition of sodium soluble
water glass in the proportions specified in [10]. The cylindrical moulds of 22 mm
in diameter and 40 mm in length were filled, vibrated for 5 minutes and sealed.
o
Curing was performed either at 80 C for 12 hours or at ambient temperature condio
tions at 20 C for 170 days. AAFA remained sealed before cutting and polishing
for nanoindentation.
Characterization of Alkali-Activated Fly-Ash by Nanoindentation
339
Table 1 Average chemical composition of the raw fly ash (main components)
Component SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 As2O3 V2O5 Cr2O3 ZnO C
Weight (%) 51.9 32.8 6.3
2.7 1.1
Rest Total
0.33 2.12 1.89 0.03 0.067 0.29 0.024 0.2 0.5
100
2.2 Methods
Before nanoindentation procedure, samples were polished on a series of emery
papers, polishing cloth and cleaned in an ultrasonic bath. Than, three representative areas from each sample were selected. Nanoindentation was performed as a
series of grids of about 10 x 10 = 100 imprints in each area. The distance between
individual indents varied in order to cover heterogeneity of the sample and was set
in the range between 10 and 50 μm. Nanohardness tester CSM was used for all of
the tests. All together, around 700-800 imprints have been carried out for each
AAFA sample. All experimental nanoindentation measurements were performed
in a load control regime. Trapezoidal loading diagram was prescribed for all tests.
Linear loading 4 mN/s (lasting for 30 s) was followed by the holding period (30 s)
and unloading 4 mN/s (30 s). Maximum load was prescribed 2 mN for all indents.
The applied load led in a maximum penetration depths ranging from 100 nm to
400 nm (average 260 nm) depending on the hardness of the indented material
phase. The effective depth captured by the tip of nanoindenter can be estimated as
four times of the penetration depth. It yields the effective depth around 1 μm for
this particular case.
The environmental scanning electron microscope XL30 ESEM FEI PHILIPS
was employed at gathering pre- and post-indentation images.
3 Results and Discussion
3.1 ESEM
Heat and ambient-cured polished samples were observed by ESEM in back scattered electrons, Fig. 1. The light luminous points are the iron rich particles (Fe-Mn
oxides). The light gray compact spheres are alumina- silica rich glass particles.
Only a small part of porous fly ash particles and slags remain intact by the alkali
activation process. A great portion of the dark gray matter is N-A-S-H gel arising
preferentially from activation of slags and, to a lesser extent, from amorphous silica from spherical fly ash particles.
The grinding process of RFA has a positive impact on the opening of internal
structure of highly porous slag particles. The sickle-like crushed thin shells of
non-activated fly ash particles are observable in the figure. The degree of alkali
activation is estimated by image analysis around 50 %.
340
J. Němeček et al.
Fig. 1 Typical ESEM (back-scattered
electrons) image of heat-cured AAFA
3.2 Nanoindentation
For all indents, elastic moduli were evaluated according to Oliver & Pharr [7]
methodology from experimental P-h curves. Poisson’s ration was assumed 0.2 for
all measurements. Examples of P-h curves belonging to individual material phases
are shown in Fig. 2 in which N-A-S-H phase is the most compliant one while nonactivated fly-ash particle exhibits the stiffest response.
Fig. 2 Typical indentation loaddepth diagrams of distinguished
phases in AAFA
N-A-S-H
2.5
Load [mN]
2
Partly
activated
1.5
Fly ash
1
0.5
0
0
100
200
300
Depth [nm]
Preliminary ESEM observation led to the conclusion that AAFA heterogeneity
occurs not only on a micrometer range but also on the scale of hundreds of μm, far
exceeding the size of fly ash particles. This hypothesis was confirmed experimentally by nanoindentation. Several uniform grids from different AAFA locations
yield different histograms of elastic properties on heat cured samples. From these
measurements containing approximately 100 indents each may be derived that
some areas are rich in a soft N-A-S-H gel while other areas shift toward higher
moduli in the area of less activated fly ash. As opposed, ambient curing seems to
produce homogeneous AAFA on the scale of hundreds of micrometers. The results are averaged through all grids from each AAFA sample.
Characterization of Alkali-Activated Fly-Ash by Nanoindentation
341
Overall results from the measurements (approximately 700 indents for each
sample) are merged and plotted in Figs 3 and 4. The mutual comparison shows
higher frequency of low elastic modulus for ambient cured sample. The explanation lies probably in different reaction kinetics between ambient and heat-cured
sample. Previous microcalorimetry measurement determined the ratio of reaction
kinetics between heat and ambient cured sample as 406 [10], favoring more homogeneous formation of N-A-S-H gel due to ion equilibration over large distances
in an ambient-cured sample.
In order to identify individual phase properties, statistical deconvolution was
applied to both histograms of E modulus. Gaussian distributions were assumed for
the deconvolution. In order to identify several material phases, we suggested to
apply deconvolution of histograms into four phases, namely N-A-S-H phase (well
activated), partly activated phase (higher stiffness), non-activated particles (mainly
slag and high stiffness) and fly-ash particles (the highest stiffness) consisting
dominantly from amorphous SiO2.
Heat-cured samples exhibit two important peaks for the activation products,
Fig. 3. The first peak can be attributed to N-A-S-H gels while the second one to a
partly activated slag. Third and fourth peaks correspond to non-activated particles;
probably slags and fly ash. As opposed, ambient-cured samples in Fig. 4 almost
lack the second peak which points to a better activation with regard to the heatcured sample. Also the third and fourth peaks of non-activated particles are
smaller.
0.25
Normalized frequency
Fig. 3 Deconvolution into
four phases for heat-cured
samples
Experiment
N-A-S-H gel
0.2
Partly activated
0.15
Non-activated particles
0.1
Fly ash particles
0.05
0
0
20
40
60
80
100
Elastic modulus [GPa]
Tables 2 and 3 summarize mean values and standard deviations for individual
components. The N-A-S-H gel phases have almost identical properties for both
heat and ambient-cured samples but frequency of the occurrence in the statistical
set is different. Higher frequency was obtained for ambient-cured samples which
again satisfy the assumption of higher portion of the well activated fly ash. The
elastic properties of minor phases are similar for heat and ambient-cured samples
but again their frequencies are different.
342
0.25
Normalized frequency
Fig. 4 Deconvolution into
four phases for ambient-cured
samples
J. Němeček et al.
Experiment
N-A-S-H gel
0.2
Partly activated
0.15
Non-activated particles
Fly ash particles
0.1
0.05
0
0
20
40
60
80
100
Elastic modulus [GPa]
Table 2 Elastic properties of individual material phases of heat-cured samples
N-A-S-H
Partly activated Non-activated
Fly-ash
Elastic modulus [GPa]
17.65±3.92
31.50±3.37
45.54±5.03
71.49±9.53
Frequency of occurrence [%]
55.3
24.0
13.5
7.2
Table 3 Elastic properties of individual material phases of ambient-cured samples
N-A-S-H
Partly activated Non-activated
Fly-ash
Elastic modulus [GPa]
17.75±3.77
30.50±3.61
46.63±6.45
74.01±10.05
Frequency of occurrence [%]
77.9
10.8
6.8
4.5
4 Conclusions
Nanoindentation was used to characterize dominant phases in the alkali-activated
brown low-calcium fly ash. The main reaction product, N-A-S-H gel, seems to
exhibit an intrinsic Young's modulus irrespective on the curing procedure on the
tested scale of 1 μm. Such finding is important in the view of yet fully unexplained N-A-S-H gel structure [9]. In the parallel comparison with C-S-H gel studies [2, 3], one can speculate about similarly arranged building block with the same
solid fraction in the indentation volume. The nanoindentation technique is therefore indispensable as a tool for the characterization on various length-scales.
Acknowledgments. The presented research has been supported by the Ministry of Education, Youth and Sports of the Czech Republic under grant MSM6840770003 and by the
Czech Science Foundation under projects 103/08/1639 and 103/09/1748.
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