NAT L INST. OF STAND & TECH R.I.C.

NBSIR 87-3504

Epoxy Impregnation of Hardened

Cement Pastes for

Characterization of Microstructure

L. Struble

E. Byrd

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

November 1986

Progress Report

Prepared for: -QC

100

. U56

87-3504

1986

tree Office of Scientific Research

tree Systems Command, USAF

g AFB, DC 20332

- C • 2
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20899
NBSIR 87-3504

EPOXY IMPREGNATION OF HARDENED

CEMENT PASTES FOR

CHARACTERIZATION OF MICROSTRUCTURE

L. Struble

E. Byrd

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

November 1986

Progress Report

Prepared for: Air Force Office of Scientific Research

Air Force Systems Command, USAF

Bolling AFB, DC 20332

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Methods were explored for drying of hardened cement paste

prior to impregnation with epoxy, in order to polish for microscopic

examination. All were shown to cause microcracking of the paste.

An alternative method has been developed for epoxy impregnation

of hydrated cement specimens without drying the specimens. The

method involves replacing pore water with ethanol, then replacing

ethanol with epoxy. The method appears to minimize the occurrence

of microcracks associated with drying.

TABLE OF CONTENTS

Page

ABSTRACT iv

I . INTRODUCTION 1

II. EXPERIMENTAL APPROACH 2

III. RESULTS 4

IV. CONCLUSIONS 5

V. ACKNOWLEDGEMENTS 6

VI . REFERENCES 7

LIST OF TABLES

Table 1. Methods used to dry specimens prior to epoxy

impregnation 8

Table 2. Procedure adopted for epoxy impregnation of

hardened cement paste by step-wise replacement

of pore water with epoxy 9

LIST OF FIGURES

Figure 1. BEI micrograph of vacuum-dried, epoxyimpregnated cement paste, large, widely-spaced

mirocracks 10

Figure 2. BEI micrograph of methanol-replaced, vacuumdried, epoxy- impregnated cement paste, showing

large, widely-spaced microcracks 11

Figure 3. BEI micrograph of freeze-dried, epoxyimpregnated cement paste, showing large,

widely-spaced microcracks 12

Figure 4. BEI micrograph of cement paste slice (1-mm),

whose pore water was sequentially replaced by

ethanol, then ethanol replaced by epoxy,

showing small microcracks 13

Figure 5. BEI micrograph of cement paste slice (1-mm),

whose pore water was replaced by ethanol in a single step, then ethanol replaced by epoxy,

with no microcracks visible 14

Figure 6. BEI micrograph at higher magnification of the

same region as Fig. 5, apparently free of

shrinkage cracks 15
.

‘
I . INTRODUCTION

Preparing a polished specimen of hydrated Portland cement,

or other porous material, requires that the pores first be filled

with a hard material such as epoxy. The hard material serves to

stabilize the delicate microstructure, providing physical support

to prevent damage to the microstructure during polishing. The

microstructure may then be examined directly, using such instruments

as an optical microscope, scanning electron microscope (SEM), or

transmission electron microscope.

Preparation of epoxy-impregnated specimens for polishing is

made difficult by the very small pores found in hydrated cement.

Much of the pore volume of hydrated cement paste occurs in

submicrometer-sized pores, and diffusion of liquid through these

pores tends to be slow. Therefore, filling of the pores by epoxy

or other liquid is generally achieved by vacuum impregnation,

i.e. by immersing the specimen in epoxy while under a vacuum,

then bringing the system to atmospheric pressure to force epoxy

into pores. However, the pores may contain a considerable amount

of liquid water or solution. The specimens must be dried prior

to impregnation, and removal of water from the small pores

produces shrinkage, which typically causes microcracking. For

this reason, specimens that have been dried, impregnated, and

polished tend to be highly microcracked, as noted by Chatter ji et

al (1).

The work described here is part of a wider study of microstructure and fracture. The mechanical properties of concrete,

particularly fracture and strength, are widely acknowledged to

depend on microstructure, and the principal microstructural

features affecting fracture appear to be microcracks-*- and pores

( 3 )

.

Due to the tendency of concrete to form microcracks when

exposed to the vacuum required for SEM, Ollivier (4) developed an

indirect method to study concrete cracks, examining by SEM an

acetylcellulose replica of the concrete surface.

The work described here was undertaken to develop an

impregnation method that does not induce microcracks, to allow

direct examination of the microstructure. Several methods were

explored: vacuum drying, vacuum drying after solvent replacement,

freeze drying, and gradual replacement of water by epoxy without

drying. Solvent replacement is a commonly used procedure in

which the water is replaced by another liquid with lower surface

tension. On drying, evaporation of the replacement liquid causes

significantly less stress due to its lower surface tension.

Parrott (5) discussed drying hydrated cement by a solvent

replacement process and concluded that it caused less alteration

-*-In this context, microcracks are considered to be cracks

0.1 mm or less in width (2).

1
to the microstructure than direct removal of the pore water.

Freeze drying is another technique used to reduce artifacts

caused by drying. At the transition from ice to vapor, there is

effectively no surface tension, and drying stresses are greatly

reduced. Scrivener and Pratt (6) used freeze drying, followed by

vacuum impregnation of epoxy, to prepare hydrated cement for SEM

analysis, but they did not assess drying cracks or other artifacts.

The final method, gradual replacement of water by ethanol

followed by replacement of ethanol by epoxy, is a procedure used

for preparing biologic samples.

Impregnated, polished specimens are required in order to

examine the microstructural features of hardened cement paste by

SEM using backscattered electron imaging ( BEX ) . The typical mode

of SEM operation is examination of fracture surfaces; SEM

provides a large depth of focus, and spatial resolution necessary

to resolve micrometer and submicrometer-sized features. Therefore,

our understanding of the microstructure of hardened cement paste,

as reviewed recently by Diamond (7), is derived principally from

examination of fracture surfaces using SEM. Diamond also

described an alternative method used recently to study the

microstructure of hardened cement, the examination of polished

surfaces using an SEM equipped with a backscattered electron

detector. SEM examination of fracture surfaces utilizes a detector that collects both secondary and backscattered electrons

and provides contrast principally by relief, or surface roughness.

Backscattered electron detection of a smooth sample, however,

provides contrast only by atomic number difference. Using BEI

allows one to differentiate between constituents on the basis of

chemical composition. The method has been shown to be effective

in studying the microstructure of clinker and of hydrated cement

(8,9,6). A particular advantage in the present study is that

epoxy-filled pores and cracks may be readily differentiated from

other constituents. An additional advantage is provided by the

planar specimen, since the image of a two-dimensional surface is

amenable to quantitative analysis, for example using a computer

image-analysis system.

II. EXPERIMENTAL APPROACH

Hardened cement paste samples were used to assess impregnation

methods for microstructure examination. The paste was prepared

using a Type I 2 portland cement and deionized water, at a waterto-cement ratio of 0.35 (by mass). These were mixed in a blender

as follows: mixed for 1 min, allowed to rest for 2 min, and

remixed for 1 min. The paste was cast into 10 mm cube molds,

allowed to cure in the molds for 24 h, then demolded and immersed

in saturated Ca(OH )2 solution for curing.

^According to ASTM Standard Specification for Portland Cement

( C150-81 )

2
After curing for several weeks, specimens were either dried

and vacuum impregnated with epoxy, or impregnated with epoxy

without drying. In some cases, the entire 10 mm cube was dried,

and in some cases a 1-mm slice of this cube specimen was used;

slices were cut using a low-speed, diamond saw^'^.

The dried specimens were prepared using the various drying

techniques listed in Table 1. After drying, specimens were

vacuum impregnated with a low-viscosity epoxy 5

, using the

following procedure; 1) the dried specimen was placed in a

reaction vessel, described elsewhere (10), and evacuated to a

pressure of ~3 kPa for ~30 min; 2) the specimen was immersed in

epoxy while still under vacuum, and the vacuum maintained for no

longer than 3 min; 3) the vacuum was released slowly and compressed

air added to provide a pressure of ~0.5 MPa for at least 10 min;

and 4) the epoxy was allowed to harden under ambient conditions.

The other process, in which pore water was replaced by epoxv

without drying, utilized a low-viscosity, ethanol-miscible epoxy°

that is thermally cured. The process involved two steps, first

replacing pore water with ethanol, then replacing ethanol with

epoxy. The process was applied both to the 1000-mm 5 cubes and to

1-mm slices. Two procedures were used to replace water with

ethanol. One was a sequential replacement procedure, in which the

specimen was progressively immersed in a series of ethanol and

water mixtures: first a 50:50 mixture, then a 75:25 mixture, and

finally 100% ethanol. The specimen was immersed for 24 hours in

each solution. The other was a single-step procedure, in which

the specimen was immersed in 100% anhydrous ethanol for 4 days.

After replacing water with ethanol, the ethanol was replaced with

epoxy by immersing the specimen in a 50:50 mixture of ethanol and

epoxy for approximately 24 hours, then in 100% epoxy for 24

hours. Finally, the epoxy was cured at 60 °C for 2 to 3 days

until hard. Once cured, it proved necessary to minimize contact

of the hardened epoxy with water vapor, since exposure for a few

days to ambient air caused the epoxy to soften.

^Certain trade names and company products are identified to

specify adequately the experimental procedure. In no case does

such identification imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the products

are necessarily the best available for the purpose.

^Isomet saw, Buehler, Ltd.

5 Epo-tek 301, Epoxy Technology, Inc.

^Spurrs ultra-low viscosity embedding medium. Polysciences,

Inc

.

3
For low-viscosity epoxies, such as the two used in these

experiments, it is important to note that there may be safety

hazards, and the manufacturer's instructions must be followed

precisely.

Impregnated specimens were ground and polished following

standard metallographic procedures, but using a nonaqueous

medium. Specimens were examined for microcracks using an SEM7

equipped with a Robinson backscattered electron detector.

III. RESULTS

SEM micrographs of all dried and vacuum- impregnated specimens

(Figs. 1 to 3 ) indicate considerable microcracking, regardless of

the drying method. The microcracks form a pattern comprised of

3-node cracks, ~120° apart; this pattern is believed to result

from drying shrinkage. The vacuum-dried specimen (Fig. 1)

contained a large number of large and widely-spaced microcracks,

typically 5 urn to 10 urn wide and a few hundred pm apart. The

specimen whose pore water was replaced with methanol prior to

vacuum-drying contained comparable microcracks (Fig. 2), and

freeze-drying likewise produced large and widely-spaced microcracks

(Fig. 3).

The specimen (10 mm cube ^ prepared by sequential replacement

of pore water by ethanol and replacement of ethanol by epoxy

contained microcracks, but they were considerably smaller than

the microcracks in the dried specimens described above. To

determine how deep the epoxy had penetrated, this specimen was cut

in half along a plane normal to the top surface, and one half was

polished to provide a side view. Examination by SEM indicated

that the epoxy had penetrated only to a depth of 0.2 mm to 0.3

mm. Therefore, a 1-mm slice cut from a cement paste specimen was

prepared using the sequential replacement process. (A 1-mm

thickness was considered sufficiently thin to be largely penetrated

by ethanol and epoxy, but sufficiently thick for cutting and

polishing.) SEM examination (Fig. 4) indicated that this 1-mm

slice, prepared by sequential replacement of pore water by

ethanol and replacement of ethanol by epoxy, contained only a few.

microcracks, no more than 2 urn wide.

To explore whether the sequential replacement of water by

ethanol is necessary for hardened cement paste, two specimens

were prepared using the single-step replacement. One specimen

(Fig. 5) was free of microcracks; the other specimen contained

only a few regions with small, widely spaced microcracks.

When examined at higher magnification, these specimens were

seen to contain very fine microcracks, ~0.5 urn wide and ~20 ym

7 ISI-40

,

International Scientific Instruments.

4
long. These finer microcracks, shown in Fig. 6, were much

shorter than the shrinkage cracks observed in the dried specimens,

and not branched. While these very fine cracks do not appear to

result from drying, it is possible that they are incipient drying

shrinkage cracks. Alternative explanations are that they are

caused by the cutting operation, or that they are a normal

feature of hydrated cement.

IV. CONCLUSIONS

A number of methods to dry cement paste prior to vacuum

impregnation of epoxy were explored, and all methods were found

to cause microcracking. When the pore water in a 1-mm slice of

cement paste was replaced by ethanol and the ethanol by epoxy

without drying the specimen, the occurrence of microcracks due to

drying was substantially reduced or eliminated. It appears that

pore water may be replaced by ethanol using either a sequential

replacement or a single-step replacement. Based on these

results, the impregnation procedure summarized in Table 2 has

been adopted in our laboratory to prepare epoxy-impregnated

specimens free of drying cracks for SEM examination.

5
V. ACKNOWLEDGEMENTS

The technical assistance provided by Mr. F. Davis and by

Mr. W. Roberts is gratefully acknowledged. The research described

was sponsored by the Air Force Office of Scientific Research, Air

Force Systems Command, USAF, under Grant Number AFOSR-155A86-

0057.

6
VI. REFERENCES

1. S. Chatterji, N. Thaulow, and P. Christensen. Formation of

shrinkage cracks in thin specimens of cement paste. Cement

and Concrete Research, 1^(1), 155-157 (1981).

2. F.O. Slate. Microscopic observation of cracks in concrete,

with emphasis on techniques developed and used at Cornell

University. In : Fracture Mechanics of Concrete, ed. by

F.H. Wittmann, pp. 75-83. New York: Elsevier (1983).

3. F.H. Wittmann. Structure of concrete with respect to crack

formation. In: Fracture Mechanics of Concrete, ed. by F.H.

Wittmann, pp. 43-74. New York: Elsevier (1983).

4. J.P. Ollivier. A non destructive procedure to observe the

microcracks of concrete by scanning electron microscopy.

Cement and Concrete Research, 1_5( 6), 1055-1060 (1985).

5. L.J. Parrott. Novel methods of processing cement gel to

examine and control microstructure and properties. Phil.

Trans. R. Soc. London, A, 310 , 155-166 (1983).

6. K.L. Scrivener and P.L. Pratt. Backscattered electron

images of polished cement sections in the scanning electron

microscope. In: Proceedings of the Sixth International

Conference on Cement Microscopy, pp. 145-155. Duncanville:

ICMA ( 1984) .

7. S. Diamond. The microstructures of cement paste in concrete.

In : Proceedings of the 8th International Congress on the

Chemistry of Cement, Volume I, 122-147 (1986).

8. A. P

.

Barker, P. Barnes, G.R. Fisher, and H. Jennings. Novel

microscopy techniques for studying the microstructure of

clinkers and hydrated cements - some preliminary results.

In : Proceedings of the Sixth International Conference on

Cement Microscopy, pp. 5-19. Duncanville: ICMA (1984).

9. A. P

.

Barker and P. Barnes. A review of techniques for phase

delineation within cement in the scanning electron microscope.

In : Proceedings of the Sixth International Conference on

Cement Microscopy, pp. 156-172. Duncanville: ICMA (1984).

10.

J.R. Clifton, J.E. Fearn, and E.D. Anderson. Polymer

impregnated cement pastes and mortars. National Bureau of

Standards (U.S.) Building Science Series 83, 16 pages.

Washington: U.S. Department of Commerce (1976).

7
Table 1. Methods used to dry specimens prior to epoxy impregnation.

Method No. Description of Method

1 Vacuum drying: cube, 10 mmx 10 mmx 10 mm,

exposed to modest vacuum ("30 kPa) for 1 day.

2 Solvent replacement: cube, 10 mm x 10 mm 10 mm,

immersed in methanol for 1 day, then vacuum dried

using Method No. 1.

3 Freeze drying: slice, 1 mm x 10 mm x 10 mm,

cooled to -75 °C, then exposed to a strong vacuum

(0.1 Pa) for "3 days.

8
Table 2. Procedure adopted for epoxy impregnation of hardened

cement paste by step-wise replacement of pore water

with epoxya .

Step No. Description of Step

1 Single-step replacement of water by ethanol, by

immersing specimen (1-mm thick slice) in 100%

anhydrous ethanol for 4 days.

2 Immerse specimen in a 50:50 mixture of anhydrous

ethanol and epoxy for 24 hours.

3 Immerse in epoxy for 24 hours

4 Cure epoxy at 60 °C for 2 days

a Spurrs ultra-low viscosity embedding medium. Polysciences,

Inc

.

9
200 /im

i-—— 1

Figure 1. BEI micrograph of vacuum-dried, epoxy- impregnated

cement paste, showing large, widely-spaced microcracks.

10
Figure 2. BEI micrograph of methanol-replaced, vacuum-dried,

epoxy- impregnated cement paste, showing large, widelyspaced microcracks.

11
Figure 3. BEI micrograph of freeze-dried, epoxy- impregnated

cement paste, showing large, widely-spaced microcracks.

12
Figure 4. BEI micrograph of cement paste slice ( 1 -mm )

,

pore water was sequentially replaced by ethanol, then

ethanol replaced by epoxy, showing small microcracks.

whose

13
100 uni

Figure 5. BEI micrograph of cement paste slice ( i-mm ) , whose

pore water was replaced by ethanol in a single step, then ethanol

replaced by epoxy, with no microcracks visible.

14
10 urn

Figure 6. BEI micrograph at higher magnification of the same

region as Fig. 5, apparently free of shrinkage cracks.

15
N B5-I1 4A [REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 87-3504 JANUARY 1987

4. TITLE AND SUBTITLE

Epoxy Impregnation of Hardened Cement Pastes for Characterization

of Microstructure

5. AUTHOR(S)

L. Struble and E. Bvrd

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards AFOSR-l 55A86-C057

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

Interim Proaress Report

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City , State, ZIP)

Air Force Office of Scientific Research

Air Force Systems Command, USAF

Bolling AFB DC 20332

10= SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-ward or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Methods were explored for dryina of hardened cement

paste prior to impregnation with epoxy, in order to polish

for microscopic examination. All were shown to cause microcracking of the paste. An alternative method has been

without drying the specimens. The method involves replacing

pore water with ethanol, then replacing ethanol with epoxy.

The method appears to minimize the occurrence of microcracks

associated with drying.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cement; concrete; epoxy; impregnation; microscopy; microstructure;

Scanning Electron Microscope

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