Epoxy Impregnation Procedure for

L. Struble and P. Stutzman

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

May 1988

Progress Report

Hardened Cement

Prepared for:

75 Years Stimulating America's Progress 1913-1988

100 srce Systems Command, USAF

. u^u

88-3702

>g AFB, DC 20332

1988

C.2

VI U f

NBSIR 88-3702

EPOXY IMPREGNATION PROCEDURE FOR

HARDENED CEMENT SAMPLES

L. Struble and P. Stutzman

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

May 1988

Progress Report

Prepared for: Air Force Office of Scientific Research

Air Force Systems Command, USAF

Bolling AFB, DC 20332

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

A method was developed previously for epoxy impregnation of hydrated

cementitious materials for microscopical examination without drying the

samples. The impregnation was achieved by sequentially replacing pore

solution with ethanol, then the ethanol with epoxy. During subsequent

application of the procedure, many specimens were found to be either

incompletely impregnated or inadequately cured. Studies were carried out

to identify the cause of these problems and to modify the procedure for more

reliable impregnation. Contamination with low levels (4 percent) of water

or ethanol was found to prevent proper curing. Modifications in the

procedure to prevent contamination, including monitoring the replacement of

pore solution by ethanol, were shown to provide consistent and reliable

impregnation.

iii
Table of Contents

Page

ABSTRACT ... iii

INTRODUCTION 1

EXPERIMENTAL APPROACH 1

RESULTS AND DISCUSSION ......... ............... 6

CONCLUSIONS 12

ACKNOWLEDGEMENTS 12

REFERENCES 13

List of Tables

Table 1. Chemical Composition of Cement 3

Table 2. Ultra-low Viscosity Epoxy Components 4

Table 3 . Modified Impregnation Procedure . 7

Table 4. Curing of Epoxy Contaminated by Ethanol or Water 8

List of Figures

Figure 1. Depth of penetration of dye in hardened cement paste

immersed in ethanol 6

Figure 2. SEM/BEI micrograph of hardened cement paste, 0.45 watercement ratio, cured for 6 weeks; images at low magnification (a) and at high magnification (b) showing unhydrated

clinker (A)

,

dense inner hydration product (B)

,

porous

outer hydration product (C)

,

and calcium hydroxide (D) . 10

Figure 3. SEM/BEI micrograph of hardened cement paste, 0.25 watercement ratio (containing ruperplasticizer ) cured for

4 weeks; images at low magnification (a) and at high

magnification (b) showing unhydrated clinker (A)

,

dense

inner hydration product (B)

,

porous outer hydration

product (C) , and calcium hydroxide (D) 11

iv
INTRODUCTION

Hydrated cementitious materials are highly porous, and some or all of

the pores are typically filled with an aqueous solution. When preparing a

polished section of such a porous material for examination by scanning

electron microscope (SEM) or optical microscope, it is often necessary to

impregnate first with epoxy, to provide physical support during sectioning

and polishing. Epoxy also facilitates the identification of voids and pores

using SEM with backscattered electron imaging (BEI) , since the low atomic

number of epoxy provides contrast with the other constituents of hydrated

cement.

In a previous report, Struble and Byrd [1] studied methods of drying

and impregnating hydrated cement. For epoxy impregnation, the aqueous

solution in the pores must be removed by drying, which causes considerable

shrinkage and cracking. Specimens dried and vacuum- impregnated with epoxy

were found to contain microcracks regardless of the drying method (vacuum

drying, critical point drying, or freeze drying). A method was described

in which pore solution was replaceed with alcohol and then the alcohol

replaced with epoxy without drying the specimen. This method produced

well- impregnated samples that were, in general, free of microcracks.

During subsequent application of the sequential replacement procedure

to large numbers of specimens, many were found to be inadequately cured,

especially around the hardened cement. It appeared from these observations

that a contaminant from the specimen was interfering with curing of the

epoxy. One impregnated specimen was sectioned perpendicular to the wafer

and examined using SEM. Pores and voids were filled with epoxy only in the

outer -100 ^m of paste, and the interior appeared to be free of epoxy,

suggesting that the specimen was only partially impregnated.

Studies were carried out to determine the causes for these problems,

and to modify the procedure so specimens could be reliably impregnated

without microcracks for microscopical examination. Incomplete replacement

of pore solution by ethanol or of ethanol by epoxy was studied as the most

likely cause both of inadequate impregnation and of inadequate curing if

epoxy were contaminated with water or ethanol. Therefore, methods were

explored to directly monitor each replacement step, and the diffusion rate

of ethanol was studied. Based on these studies, modifications in the

impregnation procedure were developed, and the performance and reliability

of the modified impregnation procedure were assessed.

EXPERIMENTAL APPROACH

The procedure described by Struble and Byrd [1] allowed impregnation

of 1-mm thick specimens of hydrated cement paste by ultra-low viscosity

(ULV) epoxy (described below). The procedure, similar to methods used in

biological studies [2], consists of the following sequence:

1
1) immerse the specimen in ethanol for 4 days;

2) immerse in a 50:50 mixture of ethanol and ULV epoxy for 24 hours;

3) immerse in the ULV epoxy for 24 hours; and

4) cure by heating at 60 °C for 2 days.

The sensitivity of epoxy hardening to contamination by water or

ethanol and to varying proportions of the individual components was

assessed. Because hardening was found to be sensitive to both water and

ethanol, a procedure was devised to monitor directly the replacement of

water by ethanol. Unfortunately, though no such procedure could be devised

for replacement of ethanol by epoxy. Based on these studies, a modified

impregnation procedure was developed to provide full impregnation of epoxy

and proper, reliable curing. The modified procedure includes a direct

measure of ethanol impregnation. The modified procedure was utilized for

samples of either normal or reduced permeability (water-cement ratio) to

assess the performance and reliability of the procedure.

Materials

Samples for impregnation were prepared using cement and water, with a few samples including a superplasticizer to provide reduced water-cement

ratio. The cement was a Type I portland cement, Cement No. 86 in the

Cement and Concrete Reference Laboratory program at National Bureau of

Standards. Its chemical composition is shown in Table 1. Sulfonated

melamine formaldehyde superplasticizer^ » ^ was used in preparing a few

samples

Reagent- grade , 200 proof, ethanol was used to replace pore solution in

hardened cement samples. Because water was expected to affect hardening of

the epoxy, the ethanol was dried using a molecular sieve^, a technique used

previously [3] to provide water- free methanol.

The ultra-low viscosity (ULV) epoxy, described by Mascorro et al. [2],

is available from various suppliers of electron microscopy accessories^.

It was selected for its very low viscosity and its miscibility with

1x This and other 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.

n^Melment, obtained from SKW, Atlanta, GA.

^Linde 3A, Union Carbide Corp., South Plainfield, NJ

^For example, ultra- low viscosity embedding medium obtained from

Polysciences Inc., Warrington, PA.

2
Table 1. Chemical composition of cement [4]

Constituent (%)

Si02 0.44

AI2O3 5.11

Fe 203 1.88

CaO 62.19

MgO 3.68

S03 3.31

Na20 0.25

K2 0 0.99

Loss on Ignition 1.8

alcohol. The dye is Oil Red EGN^ , a proprietary dye miscible in alcohol,

ULV epoxy, and other epoxies. Details on the components and mix ratios are

provided in Table 2

Potential hazards exist in handling the epoxy components, and safety

data sheets should be obtained from the manufacturer concerning the ultralow viscosity components. Materials should be stored and handled only in a well -ventilated fume hood, and gloves should be worn during mixing.

Materials for grinding and polishing impregnated specimens included

reagent-grade propylene glycol, SiC grinding papers, and synthetic diamond

paste

Methods

The normal cement paste was prepared using deionized water and

Portland cement at a water-cement ratio of 0.45. These were mixed as

follows in a blender^ operated at low speed and cooled using tap water:

mixed for 1 min, allowed to rest for 2 mins while scraping the sides of the

blender, and mixed for 1 min. Pastes of lower water-cement ratio containing

superplasticizer were mixed using a somewhat different method based on the

method used by M. Brown in our laboratory for preparing high-strength

concrete. Cement and water, at a water-cement ratio 0.22 to 0.35, were

first mixed using a mortar and pestle, scraping the sides of the mortar

often with a spatula; half the superplasticizer was added after 4 min and

the remainder after 6 min, continuing to mix using the pestle for a total

of 13 min; the paste was transferred to the blender and mixed for an

additional 10 min at low speed, stopping every 2 min to scrape the sides

and allow the mix to cool.

Paste samples were cast in 10-mm cube molds, which were vibrated for a few seconds to consolidate the paste; the molds were covered with a wet

^Obtained from Polysciences Inc., Warrington, PA.

^Commercial blender, model CB- 6 , Waring, New Hartford, CT.

3
Table 2. Ultra-low viscosity epoxy components.

Component Standard Mix [2]

(by weight)

Vinylcyclohexene dioxide

(VCD, Epoxy resin)

0 . 5 part

n-Hexenyl succinic anhydride

(HXSA, Hardener)

1.0 part

1 , 4-butanediol diglycidyl ether 0.075 part

(Araldite RD-2, Modifier)

Dimethylamioethanol

( DMAE , Catalys t

)

0.028 part

(1.0 percent by volume)

Dye -0.1 part (until color intensity is

satisfactory)

cloth and sealed in polyethylene, and stored at room temperature (-24° C)

for 24 hours . Samples were then demolded and immersed in a saturated

Ca(0H)2 solution for curing at room temperature. After curing in the

Ca(0H)2 solution, cubes were removed from solution and specimens prepared

for impregnation. A slice was cut for impregnation using a low speed,

diamond-blade wafering saw^, with the blade and specimen lubricated by

saturated Ca(0H)2 solution; approximately 1 mm of material was removed to

expose a fresh face, then a 1 -mm thick wafer was cut for epoxy impregnation.

The pore solution in the hydrated cement was replaced by epoxy in a

sequential two- stage replacement. Specimens were immersed in ethanol then

in epoxy until replacement was considered to be complete (discussed below)

The intermediate immersion in a 50:50 mixture of ethanol and epoxy used in

the previous procedure [1] was eliminated with no effect on the impregnation

The replacement of pore solution by ethanol was monitored using a dye

soluble in ethanol and insoluble in water (described below) . Penetration

of this dye was monitored in a companion specimen immersed in ethanol

containing the dye. Every 48 hours the monitor cube was cut to expose the

interior, which was examined visually to assess the extent to which the dye

and alcohol had penetrated. Because the dye is soluble in ethanol and

insoluble in water, the extent of penetration by the dye can easily be

visually assessed. Using this method, it was found that full replacement

of pore solution by ethanol in a 1-mm specimen required approximately 8

days, considerably more than the 4 days found in the previous study [1].

Elevating the temperature to 60°C during replacement of pore solution

by ethanol was explored as a method to reduce the time required for the

^Isomet saw, Buehler, Ltd.

Lake Bluff, IL.

process

4
diffusion process, whose rate is expected to depend on temperature. It is

recognized that heating the specimens may cause dehydration of hydrated

cement phases and thereby affect microstructure. The epoxy- impregnated

specimens must be heated to cure the epoxy, and in the previous study the

60 °C curing temperature was selected as a temperature low enough to

minimize any micros true tural effects while high enough to allow the epoxy

to cure within 48 hours. Therefore, it was not considered harmful to heat

the specimens at the same temperature while immersed in ethanol.

Epoxy components were proportioned by weight, dispensed using a

disposable plastic pipette in the proportions listed in Table 1. The epoxy

was stirred by hand until it appeared that the dye had dissolved and was

homogeneously blended.

No direct measure of epoxy diffusion could be devised for these

studies. Epoxy diffusion could not be monitored using the dye because the

dye is soluble in both the epoxy and ethanol. Changes in weight and

density as the epoxy replaces the alcohol were found to be inconclusive.

The epoxy replacement step consisted of immersing the ethanol -saturated

wafers in the ULV epoxy. The immersed specimen was stored at -2°C to

facilitate diffusion of the epoxy, because the epoxy exhibits a marked

increase in viscosity in 24 hours at room temperature [2] and storage at

2°C was considered to prevent this increase. In addition, the epoxy was

replaced every 48 hours with a fresh mixture to assure that the epoxy in

contact with the sample is low in viscosity.

Finally, specimens were immersed in fresh epoxy, prepared as described

above, and cured at 60°C for 24 to 48 hours. After curing the specimens

were stored under a vacuum or over a desiccant, because exposure to

moisture causes the epoxy to soften.

Impregnated specimens were ground and polished for SEM examination as

follows: ground using 240 grit and 400 grit SiC with no lubricant,

followed by 600 grit SiC using propylene glycol as a grinding lubricant,

then polished on a glass plate using a sequence of diamond pastes (6 ^m,

3 nm, 1 /xm and 0.25 /im) lubricated using propylene glycol. Microstruc tural

examination was caried out using SEM/BEI

Tests were conducted to examine the tolerance of the epoxy to added

ethanol or water. In addition, tests were performed to assess the curing

with variation in the amount of each of the four epoxy components; low

levels of ethanol were also added to these mixtures. To test the effect of

ethanol, an epoxy mixture (Table 2) was divided into four parts, to which

were added 0, 10, 20 or 50 percent (by weight) ethanol. To test the effect

of water, a mixture was divided into five parts, to which were added 0, 2,

4, 6 or 8 percent (by weight) water. To test the effect of slight variation

in the proportions of each component, mixtures were prepared in which each

of the four components was added at 90 percent or 110 percent of its

correct proportion. To each of these latter mixtures was added 0, 2, or 4

percent (by weight) ethanol. Each mixture was cured at 60 °C and examined

visually to assess its hardness.

5
A modified impregnation procedure, itself the result of studies

described here, was used to prepare specimens of normal and low permeability

to assess the performance and reliability of the procedure. The procedure,

summarized in Table 3, was followed to prepare specimens 1 mm in thickness

for examination using SEM/BEI. The specimens were immersed in ethanol at 60°C

until examination of the separate monitor cube indicated that ethanol

(containing dye) had penetrated >0.5 mm on each face, a period of

approximately 6 to 14 days. (When the monitor cube exhibits a 0.5 -mm band

of color, it is assumed that the wafers, 1 mm thick, are fully replaced

with ethanol.) Specimens were then removed from ethanol, immersed in

epoxy, and stored at ~2 a C for the same length of time required for the

ethanol replacement, replacing epoxy with a fresh mixture every 48 hours.

Finally, specimens were cured, ground, and polished as described above.

RESULTS AND DISCUSSION

Results concerning the epoxy sensitivity to contaminants and to mix

proportions are summarized in Table 4. Curing was suppressed by ethanol

addition in excess of 4 percent, and at 2 or 4 percent addition when the

proportions of some epoxy constituents were reduced slightly. Curing was

not suppressed by water at a 2 percent addition, but was suppressed at 4

percent or greater. After twelve days, the samples containing 4 percent or

more water had cured everywhere but the exposed surface, and when demolded

they shattered.

Epoxy prepared by varying each of the 4 components cured normally in

24 hours. The most pronounced change on varying proportions was in the

color of the cured epoxy, from a control color of slight brown tint to

altered blend colors of amber and dark brown.

Figure 1. Depth of penetration of dye in hardened cement paste immersed in

ethanol

0.6

O 0-5

O I 0.3

ccw 0.1

<

0

0 2 4 6 8 10 12

TIME (days)

o at 60°C

• at room temperature (~24°C)

6
Table 3. Modified impregnation procedure.

Step Description

1 Immerse specimen in water- free ethanol at 60 °C

2 Immerse monitor cube in water -free ethanol

containing dye at 60 °C

3 Examine monitor cube until dye indicates that

ethanol has penetrated at least 0.5 mm on each face

4 Immerse specimen in epoxy and stored at -2°C for

the same length of time required for the ethanol

replacement, replacing epoxy with a fresh mixture

every 48 hours

5 Immerse specimen in fresh epoxy and cure epoxy by

heating at least 24 hours at 60°C

6 Grind and polish for microscopical examination

7
Table 4. Curing of epoxy contaminated by ethanol or water.

Mixture Curing time

Control (Table 2) 3 days

Control + 10% ethanol 3 to 11 days

Control + 20% ethanol 3 to 11 days

Control + 50% ethanol 3 to 11 days

Control (Table 2) 1 day

Control + 2% water 1 day

Control + 4% water 1 day

12 days

Control + 6% water 1 day

12 days

Control + 8% water 1 day

12 days

Control + 2% ethanol 1 day

Control + 4% ethanol 1 day

90% VCD 1 day

110% VCD 1 day

90% HXSA 1 day

110% HXSA 1 day

90% ARD-2 1 day

110% ARD-2 1 day

90% DMAE 1 day

110% DMAE 1 day

90% VCD + 2% ethanol 1 day

90% VCD + 4% ethanol 1 day

110% VCD + 2% ethanol 1 day

110% VCD + 4% ethanol 1 day

90% HXSA + 2% ethanol 1 day

90% HXSA + 4% ethanol 1 day

110% HXSA + 2% ethanol 1 day

110% HXSA + 4% ethanol 1 day

90% ARD-2 + 2% ethanol 1 day

90% ARD-2 + 4% ethanol 1 day

110% ARD-2 + 2% ethanol 1 day

110% ARD-2 + 4% ethanol 1 day

90% DMAE + 2% ethanol 1 day

90% DMAE + 4% ethanol 1 day

110% DMAE + 2% ethanol 1 day

110% DMAE + 4% ethanol 1 day

Condition

hard (normal)

semi-solid, very viscous

liquid, viscous

liquid, very fluid

hard (normal)

hard (normal)

semi-solid, very viscous

hard, soft on exposed surface

semi-solid, very viscous

solid, soft on exposed surface

semi-solid, very viscous

hard, soft on exposed surface

hard (normal)

hard (normal)

hard (normal)

hard (normal)

hard (normal)

hard (normal)

hard (normal)

hard (normal)

hard (normal)

hard (normal)

solid, not hard

solid, not hard

hard (normal)

hard (normal)

hard (normal)

solid, not hard

hard (normal)

hard (normal)

hard (normal)

solid, not hard

hard (normal)

hard (normal)

hard (normal)

solid, not hard

hard (normal)

hard (normal)

8
The extent of ethanol diffusion into hardened cement paste was

measured both at room temperature and at 60°C (Fig. 1). After the first

day the diffusion rates at the two temperatures were similar, but the

extent of diffusion was greater at the higher temperature. Based on these

results, specimens were subsequently stored in a 60°C oven while immersed

in ethanol to facilitate the diffusion process.

Approximately 20 specimens, both normal and low water-cement ratio,

have been impregnated using the modified procedure summarized in Table 3,

and all specimens have cured normally. An example of normal water -cement

ratio paste is shown in Fig. 2, and and of low water-cement paste with

superplasticizer in Fig. 3. Both specimens appeared to be fully impregnated

and free of microcracks. Various features of the paste microstructure may

be identified by their contrast in the backscattered electron image. The

unhydrated clinker grains appear as the lightest constituent. Calcium

hydroxide is light gray and calcium silicate hydrate gel is slightly

darker. The darkest constituent is epoxy. Of particular interest in both

examples is the distinction between the relatively dense inner hydration product

and the more porous outer hydration product, in which the porous gel

appears to be impregnated with epoxy.

9
(b)

Figure 2. SEM/BEI micrograph of hardened cement paste, 0.45 water-cement ratio,

cured for 6 weeks; images at low magnification (a) and at high magnification

(b) showing unhydrated clinker (A)

dense inner hydration product (B)

porous outer hydration product (C)

and calcium hydroxide (D)

10
Figure 3. SEM/BEI micrograph of hardened cement paste, 0.25 water- cement

ratio (containing superplasticizer) cured for 4 weeks; images at low

magnification (a) and at high magnification (b) showing unhydrated clinker

(A)dense inner hydration product (B)

porous outer hydration product (C)

and calcium hydroxide (D)

11
CONCLUSIONS

The ultra- low viscosity epoxy was found to be tolerant of low levels

(2%) of ethanol and water, but did not cure properly when contaminated by

higher levels. Varying the proportion of each component by 10 percent did

not appear to affect curing.

The two step process of replacing pore solution with ethanol and

ethanol with ULV epoxy, assuring full replacement of pore solution by

ethanol and of ethanol by epoxy, was found to provide reliable impregnation

of hardened cement paste samples, both normal and low water-cement ratio.

Measuring the extent of penetration of ethanol using a monitor cube,

immersed in ethanol containing a dye provides assurance that ethanol has fully

replaced the pore solution, and provides an indication of the time required

for epoxy replacement of ethanol.

After curing, epoxy serves to fill existing cracks and pores and

provide support during subsequent grinding and polishing. As described

previously [1]the impregnation process allows preparation of polished

samples of hardened cement paste with little or no microcracking, and the

epoxy provides contrast when examining voids and pores using SEM/BEI

ACKNOWLEDGEMENTS

Technical assistance provided by B. Roe is gratefully acknowledged.

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

Air Force Systems Command, USAF, under Grant Number AFOSR-155A86-0057

12
REFERENCES

1. L. Struble and E. Byrd. Epoxy impregnation of hardened cement pastes

for characterization of microstructure. National Bureau of Standards

(U.S.), NBSIR 87-3504, 15 pp (1987).

2. J.A. Mascorro, M.W. Ladd, and R.D. Yates. Rapid infiltration of

biological tissues utilizing n-hexenyl succinic anhydride (HXSA) /vinyl

cyclohexene dioxide (VCD) , an ultra- low viscosity embedding medium.

Proceedings of the 34th Annual Meeting of the Electron Microscopy

Society of America, 34, 346-347 (1976).

3. L. Struble. The effect of water on maleic acid and salicylic acid

extractions. Cement and Concrete Research, 15,(4), 631-636 (1985).

4. R. Haupt. Personal communication from Robin Haupt, Cement and

Concrete Reference Laboratory (CCRL) , National Bureau of Standards

(U.S.), to all laboratories participating in the CCRL program,

September 25, 1987 (Concrete Sample No. 86).

13
NBS-114A (rev. 2 -ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 88-3702

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

MAY 1988

4

.

TITLE AND SUBTITLE

5. Epoxy Impregnation Procedure for Hardened Cement Samples

AUTHOR(S)

L. S truble and P. Stutzman

6.

PERFORMING ORGANIZATION (If joint or other than N8S, see instructions)

NATIONAL BUREAU OF STANDARDS

U.S, DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Con tract/ Gram t No.

S. Type of Report & Period Covered

9.

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

Air Force Office of Scientific Research

10.

SUPPLEMENTARY NOTES

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

11.

ABSTRACT (A 200-word or less factual summary of most significant in formation. If document includes a significant

bibliography or literature survey, mention it here)

A method was developed previously for epoxy impregnation of hydrated cementitious

materials for microscopical examination without drying the samples, by

sequentially replacing pore solution with ethanol, then the ethanol with epoxy.

During subsequent application of the precedure, many specimens were cured.

Studies were carried out to identify the cause of these problems and to modify

the procedure for more reliable impregnation. Contamination with low levels

(4 percent) of water or ethanol was found to prevent proper curing. Modifications

in the procedure to prevent contamination, including monitoring the replacement

of pore solution by ethanol, were shown to provide consistent and reliable

impregnation.

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

microscopy, ultra-low viscosity epoxy

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