Nanoindentation Study of Resin Impregnated
Sandstone and Early-Age Cement Paste
Specimens
W. Zhu, M.T.J. Fonteyn, J. Hughes, and C. Pearce
*
Abstract. Nanoindentation testing requires well prepared samples with a good
surface finish. Achieving a good surface finish is difficult for heterogeneous materials, particularly those with weak and fragile structures/phases, which are easily
damaged or lost during preparation. The loss of weak structures can be drastically
reduced by impregnating the sample with a resin before cutting and polishing.
This technique is commonly used in SEM microscopy but has not been used for
nanoindentation-testing before. This paper reports an investigation to extract micro-mechanical properties of different phases in resin impregnated sandstone and
1-day hydrated cement samples. The results appeared to show that it is feasible to
use resin impregnated specimens for nanoindentation study of both materials.
1 Introduction
Cement based materials and sandstone are among the most utilized materials, essential to the construction industry and built environment. Studies by microscopic
techniques have revealed that both materials are complex heterogeneous composite materials, with a random microstructure at different length scales, from the
nano to the macro scale. Such complex composites are made even more complicated by the time dependent nature of the cement hydration processes which begin
at the mixing of cement clinker minerals with water and continue for months and
even years. The engineering properties and durability performance of these materials at the macro scale are all significantly affected, if not dominated, by their
structural features and properties at the micro/nano scale where the deterioration
W. Zhu and J. Hughes
School of Engineering and Science, University of the West of Scotland, UK
e-mail: wenzhong.zhu@uws.ac.uk, john.hughes@uws.ac.uk
M.T.J. Fonteyn
Department of Mechanical Engineering, Eindhoven University of Technology, Netherlands
e-mail: M.T.J.Fonteyn@student.tue.nl
C. Pearce
Department of Civil Engineering, University of Glasgow, UK
e-mail: c.pearce@civil.gla.ac.uk
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W. Zhu et al.
and failure process starts. Over the past 10-15 years depth sensing nanoindentation
technique has shown to provide a very useful tool to study properties of
nano/micro-scale features in many different materials, including composites or
multiphase materials [1-5]. Particularly, a statistical or grid/mapping nanoindentation method has recently been applied to determine mechanical properties (e.g.
elastic modulus, hardness) of individual phases in complex materials, including
cement paste, bones and natural rocks, thus providing vital information for understanding of materials behaviour and development of computational models [6-8].
Generally, nanoindentation testing requires well prepared samples with a good
surface finish. However, achieving a good surface finish is difficult for heterogeneous materials, particularly those with weak, porous and fragile structures/phases, which are easily damaged or lost during preparation. Indeed, it has
been reported that there is a limit in obtainable surface roughness for even aged
cement paste due to the porous structure [9]. Roughness of the specimen surface
can also have a significant effect on the nanoindentation test results [10]. Furthermore, it has been impossible to prepare sandstone and 1-day hydrated cement
paste samples using the common preparation steps since almost all the weak
structures and hydrated phases disappear from the prepared surface. Therefore,
the application of nanoindentation to study such materials is hindered by not only
the high roughness of the prepared sample surface but also an unrepresentative
specimen. The loss of weak structures or phases can be drastically reduced by impregnating the sample with a resin before cutting and polishing the surface. This
technique is commonly used in SEM microscopy but has not been used for
nanoindentation testing before. The main objective of this study is to investigate
the feasibility of using resin impregnated samples for nanoindentation, and particularly to extract micro-mechanical properties of different phases in resin impregnated sandstone and 1-day hydrated cement samples.
2 Experimental
The sandstone sample is Giffnock Sandstone, commonly used for buildings in the
Glasgow area. The stone is a well sorted, medium grained, quartz-arenite of Carboniferous age, sub-angular quartz dominating, with minor amounts of mica and
degraded k-feldspars. It contains significant amounts of secondary minerals; 5-10%
of interstitial kaolinite and 10-20% carbonate cement, composed of Fe-Mg bearing
2+
carbonate mineral Ankerite (Ca(Fe ,Mg,Mn)(CO3)2). The cement paste sample was
prepared with a 42.5N Portland cement and a water-to-cement ratio of 0.40. The
fresh cement paste was left in a small mould (10x10x40 mm) covered with a plastic
o
bag at ~20 C for 24 hours before demoulding. After demoulding, the 1-day hydrated sample was placed in bottle of methanol to stop the hydration. Then the
usual sample preparation steps [4] were followed, including resin embedding (but
not impregnating), precision sectioning, grinding, polishing (down to 6 µm) to obtain a disc specimens (φ30x15 mm) for both the sandstone and the cement samples.
These specimens were then resin impregnated using the EPOFIX epoxy resin of
Struers in a vacuum chamber at room temperature. Further grinding and polishing
(down to 1 µm) and ultrasonic cleaning were applied to the impregnated specimens
to produce the final specimens for nanoindentation. Methanol-based liquid was
Nanoindentation Study of Resin Impregnated Sandstone
405
used as a lubricant and for cleaning in all the procedures so as to avoid further cement hydration or possible dissolution of minerals/hydrates.
The methodology and operating principle for the nanoindentation technique
have been reviewed and presented in detail elsewhere [1-2]. Briefly, the test consists of making contact between a sample surface and a diamond indenter of
known geometry, followed by a loading-unloading cycle while continuously recording the load, P, and indentation depth, h. The P-h curve obtained is a fingerprint of the mechanical properties of the test area. Most commonly, the elastic
modulus (E) and hardness (H) of the test area are determined by analysing the initial part of the unloading data according to a model for the elastic contact problem.
For studying multiphase composite materials, a refined statistical indentation
method has been used, which involves testing and statistically analyzing a large
number of indentation points within a representative sample area [7-8].
The nanoindentation apparatus used in this study was Nanoindenter XP fitted
with a Berkovich indenter. In this study, all testing was programmed in such a way
that the loading started when the indenter came into contact with the test surface
and the load maintained for 30 seconds at the pre-specified maximum value before
unloading. In order to provide statistical analysis of the micro-mechanical properties of different phases in the specimen, two randomly selected areas each with a
grid of 12x16 indentation points with indent spacing of 10 µm were used for the
cement paste. For the sandstone sample, a grid of 12x20 indentation points with
indent spacing of 30 µm were used due to its relatively large grain size. The E and
H values were extracted from each test point with the indentation depth range of
100 – 300 nm for the cement specimen by varying the maximum load applied
while for the sandstone specimen a fixed maximum load of 5 mN was used.
3 Results and Discussions
Fig.1 shows typical optical images of the polished surface of the resin impregnated specimens. As shown, good surface finish was successfully achieved with
both specimens. Large resin-filled areas could clearly be seen on the sandstone
surface, while no such resin-filled areas could easily be found on the cement paste
specimen. Nanoindentation trials were carried out on the 1-day hydrated cement
paste specimen with and without the resin impregnation. It was interesting to find
that for the resin impregnated specimen over 50% of the test points had an elastic
modulus value lower than 45 GPa while for the specimen without the impregnation only less than 20% of the test points showed elastic modulus value lower than
45 GPa. As the elastic modulus of cement hydration products is generally lower
than 45 GPa the above results confirmed previous observations that a significant
loss of the weak, hydrates phases occurred during the preparation processes if the
sample was not resin impregnated.
The large number of test results obtained from the grid indentation were statistically analysed to extract mechanical properties of individual phases in the tested
area using a method presented previously [7-8]. Basically, the experimental E and
H values at all test point were statistically analysed to produce a frequency plot.
Then, the best model fit to the experimental results with multi-modal normal distribution curves (or Gaussian distribution) was produced using non-linear least
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W. Zhu et al.
Fig. 1 Optical images of typical polished surfaces (magnification ~100) of the sandstone (a)
and 1-day hydrated cement paste (b) specimens: the arrows showing large resin-filled areas
Fig. 2 Mechanical property frequency plots for test results of the resin impregnated 1-day
hydrated cement specimen, together with model fits for individual hydrate phases
Fig. 3 Mechanical property frequency plots for test results of the resin impregnated sandstone
specimen, together with model fits for individual phases
squares method. These curves are shown in Fig.2 and Fig.3 for the resin impregnated 1-day cement and the sandstone specimens respectively. From each model
fit, the mean values of E and H were extracted, whose association with a specific
hydrate or mineral phase could be ascertained by SEM-BSE and SEM-EDS analysis [8]. The area under each curve also provides an estimate of the volume fraction for the hydrate/mineral phase it associated with. Table 1 presents a summary
of results of the mechanical properties of individual phases and their volumefractions for the 1-day cement and the sandstone specimens. For comparison, previous
results obtained for a 28-day hydrated cement specimen without resin
impregnation are also given. A SEM-BSE image of the indented area for the sandstone is shown in Fig.4. The mineral phases in the sandstone were also identified.
Nanoindentation Study of Resin Impregnated Sandstone
407
Table 1 Properties of different phases in the cement and sandstone specimens
Sample
Properties of phases
1
2
3
4
1
2
3
4
Loose-packed CSH
LD CSH
HD CSH
Ca(OH)2 plus
Sample
Properties of phases
Epoxy resin plus
Ankerite
Quartz A
Quartz B
Resin impregnated 1-day
hydrated cement paste
E, GPa
H, GPa
V%
17.1
24.4
30.6
37.4
E, GPa
6.7
98.9
108.9
119.5
28-day hydrated cement without
resin impregnation
E, GPa
H, GPa
V%
0.63
34.8
18.1
0.99
28.2
24.4
1.27
23.7
31.4
1.54
13.3
37.5
Resin impregnated sandstone
H, GPa
V%
0.40
21.5
5.57
17.0
14.2
28.3
15.7
33.2
-
0.42
0.75
1.05
1.27
5.5
60.2
28.7
5.6
-
-
Fig. 4 SEM-BSE image of the tested area
on sandstone (within the white box):
Area includes quartz (Q), ankerite (A),
kaolinite clay (K) and dark resin-filled
porosity (P)
Results shown in Fig.2-3 and Table 1 appear to suggest that the epoxy resin,
which has an E value of 3 - 4 GPa, could not be detected by the nanoindentation
test in the 1-day cement specimen. This might be due to the small volume of resin
present and pore sizes in the specimen. Generally the phases and properties obtained for the 1-day cement specimen are in good agreement with other published
results for 28-day and 1-year cement samples [6-8]. Particularly, the E and H values obtained for the low and high density calcium silicate hydrates (i.e. LD-CSH
and HD-CSH) were found to be almost the same for the 1-day and the 28-day cement specimens. These appeared to suggest that the mechanical properties of the
CSH phases do not change with the hydration age. For the sandstone specimen,
the epoxy resin phase was detected by nanoindentation. The E and H values of the
quartz phases are in good agreement with the limited published data [3, 8] though
the crystal orientation may have an effect. The results for Ankerite are reasonable.
The kaolinite clay phase seen on SEM was not detected by the nanoindentation,
which might be due to its soft/loose nature and thus merged with the resin peak.
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W. Zhu et al.
4 Conclusions
The study seemed to indicate that it is feasible to use resin impregnated specimens
for nanoindentation study of both sandstone and early age hydrated cement paste
samples. The E and H values of different mineral phases in the sandstone sample
determined by the statistical nanoindentation method appeared to be in good
agreement with the limited published data. The results of the 1-day hydrated cement sample suggested that the mechanical properties of the individual hydrate
phases (e.g. LD-CSH, HD-CSH, etc) did not change with cement hydration.
References
1. Oliver, W.C., Pharr, G.M.: An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater.
Res. 7, 1564–1579 (1992)
2. Ficher-Cripps, A.C.: Nanoindentation. Springer, London (2002)
3. Broz, M.E., Cook, R.F., Whitney, D.L.: Microhardness, toughness, and modulus of
Mohs scale minerals. American Mineralogist 91, 135–142 (2006)
4. Zhu, W., Bartos, P.J.M.: Application of depth-sensing microindentation testing to
study of interfacial transition zone in reinforced concrete. Cem. Concr. Res. 30, 1299–
1304 (2000)
5. Velez, K., Maximilien, S., et al.: Determination by nanoindentation of elastic modulus
and hardness of pure constituents of Portland cement clinker. Cem. Concr. Res. 31,
555–561 (2001)
6. Constantinides, G., Ulm, F.J., Van Vliet, K.: On the use of nanoindentation for cementitious materials. Mater. Struct. 36, 191–196 (2003)
7. Ulm, F., Vandamme, M., Bobko, C., Ortega, J.A.: Statistical indentation techniques
for hydrated nanocomposites: concrete, bones and shale. J. Am. Ceram. Soc. 90(9),
2677–2692 (2007)
8. Zhu, W., Hughes, J., et al.: Micro/Nano-scale mapping of mechanical properties of
cement paste and natural rocks by nanoindentation. Mater. Charact. 11, 1189–1198
(2007)
9. Trtik, P., Dual, J., Muench, B., Holzer, L.: Limitation in obtainable surface roughness
of hardened cement paste: ‘virtual’ topographic experiment based on focussed ion
beam nanotomography datasets. J. Microscopy 232(2), 200–206 (2008)
10. Miller, M., Bobko, C., et al.: Surface roughness criteria for cement paste nanoindentation. Cem. Concr. Res. 38, 467–476 (2008)