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 mate-rials, 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 mi-cro-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, es-sential to the construction industry and built environment. Studies by microscopic techniques have revealed that both materials are complex heterogeneous compos-ite materials, with a random microstructure at different length scales, from the nano to the macro scale. Such complex composites are made even more compli-cated 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 mate-rials 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

404 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 nanoindenta-tion 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 under-standing 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 heteroge-neous materials, particularly those with weak, porous and fragile struc-tures/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]. Further-more, 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 im-pregnating 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 par-ticularly to extract micro-mechanical properties of different phases in resin im-pregnated 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 Car-boniferous 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 carbonate mineral Ankerite (Ca(Fe2+,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 bag at ~20oC for 24 hours before demoulding. After demoulding, the 1-day hy-drated 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 ob-tain 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 ce-ment 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 con-sists of making contact between a sample surface and a diamond indenter of known geometry, followed by a loading-unloading cycle while continuously re-cording the load, P, and indentation depth, h. The P-h curve obtained is a finger-print 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 ini-tial 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 proper-ties 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 impreg-nated 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 impregna-tion 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 statis-tically 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 dis-tribution curves (or Gaussian distribution) was produced using non-linear least

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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 impreg-nated 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 analy-sis [8]. The area under each curve also provides an estimate of the volume frac-tion for the hydrate/mineral phase it associated with. Table 1 presents a summary of results of the mechanical properties of individual phases and their volumefrac-tions 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 sand-stone 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 Resin impregnated 1-day hydrated cement paste

28-day hydrated cement without resin impregnation

Properties of phases E, GPa H, GPa V% E, GPa H, GPa V%

1 Loose-packed CSH 17.1 0.63 34.8 18.1 0.42 5.5

2 LD CSH 24.4 0.99 28.2 24.4 0.75 60.2

3 HD CSH 30.6 1.27 23.7 31.4 1.05 28.7

4 Ca(OH)2 plus 37.4 1.54 13.3 37.5 1.27 5.6

Sample Resin impregnated sandstone

Properties of phases E, GPa H, GPa V% - - -

1 Epoxy resin plus 6.7 0.40 21.5 - - -

2 Ankerite 98.9 5.57 17.0 - - -

3 Quartz A 108.9 14.2 28.3 - - -

4 Quartz B 119.5 15.7 33.2 - - -

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 ob-tained 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 val-ues 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 ce-ment 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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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 ce-ment sample suggested that the mechanical properties of the individual hydrate phases (e.g. LD-CSH, HD-CSH, etc) did not change with cement hydration.

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