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.*3S1R 82-2531

NATL INST. OF STAND & TECH

Aixiob 2s^MMs ivaluation of Hydrated

CaScium Aluminate Compounds

As Energy Storage Media

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

June 1 982

-QC100

.US6

32-2531

1932

Prepared for; U.S. Department of Energy

Office of Solar Heat Technologies

Conservation and Renewable Energy

Washington, DC 20585

NBSIR 82-2531

** AN EVALUATION OF HYDRATED

CALCIUM ALUMINATE COMPOUNDS

AS ENERGY STORAGE MEDIA

ATIOMAX. BUAAAtl o» BTAKDAKM

UBKART

'JUL 2 7 1^82 l

lJ Ooo

no. - /Ql ^

John B. Ings

Paul W. Brown

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

June 1 982

Prepared for; U.S. Department of Energy

Office of Solar Heat Technologies

Conservation and Renewable Energy

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Calcium aluminate hydrates and calcium aluminate hydrates containing other

ions were investigated to determine the feasibility of their utilization

as energy storage media. A series of these compounds were fabricated and

analyzed for purity. The energy liberated on hydration of each compound

was measured using conduction calorimetry and the dehydration temperature

was measured using differential scanning calorimetry. Of the compounds

investigated, 3CaO AI 2O3

3 CaS04 32H20 liberated the largest amount of

energy upon rehydration. Initially, this value was about 100 cal/g.

However, after 18 cycles of hydration and dehydration this value drops to

about 70 cal/g.

Key Words: Calcium-aluminum hydrates; calorimetry; dehydrating; energy

storage; rehydration; solar energy.

iii

TABLE OF CONTENTS

Page

Abstract ill

1 . Introduction 1

2 . Crystal Struc ture and Morphology 2

3 . Compound Preparation 2

3.1 4CaO«Al203* 13H20 2

3.2 CaO«Al203*10H20 3

3.3 2Ca0«Al203*8H20 3

3.4 3Ca0*Al203«3CaS04«32H20 3

3.5 3Ca0‘Al203«CaS04*12H20 3

3.6 3Ca0«Fe203*3CaS04*32H20 4

3.7 3CaO»Al203‘3CaC03*32H20 4

3.8 3Ca0«Al203*CaC03*llH20 4

3.9 3Ca0*Al203«3CaCl2*32H20 4

3.10 3Ca0«Al203*3CaCl2»llH20 4

4. Thermal Analysis 5

5 . Summary 8

6 . Acknowledgment 11

7. References 11
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1 . INTRODUCTION

Galcixjm aliominate hydrates and calcium aluminate hydrates modified by other

ions were investigated to assess the feasibility of their utilization as phase change storage materials (PCM's). This was based on literature

citations of low temperature reversible hydration-dehydration reactions

involving only small changes in crystal structure rather than melting and

freezing as is typical in salt hydrate PCM’s.

Compounds were selected for further study if the literature indicated a dehydration reaction in the temperature range of 30 to 80 “C and excluded if

the hydration reactions were known to be irreversible. However, the hydration

data in the literature were only valid for a preliminary screening of potential

compounds for phase change storage media. This is due to the fact that the

thermal decomposition behavior of these materials depends on the partial pressure of water vapor during dehydration. Almost all of the thermal dehydration

measurements cited in the literature are for ambient air of unspecified water

vapor pressure. Since compounds chosen for solar energy storage applications

may be used as slurries or under conditions of high relative humidity, the

literature values for the thermal dehydration could only be used as a guide

for selecting potential cement compounds for further investigation. In addition, little or no data on the kinetics of the hydration-dehydration reactions

are available from the literature. These data were ascertained experimentally.

Based on the data from the literature the following components were selected

for further study:

4Ca0*Al203«13H20

Ca0»Al203'10H20

2Ca0»Al203«8H20

3Ca0 •AI2O3 •3CaS04 O2H2O

3Ca0 •AI2O3 *CaS04 *12H20

3Ca0 •Fe203 •3CaS04 •32H2O

3Ca0 -A1203 *3CaC03 •32H20

3Ca0 •AI2O3 •CaC03 *1^20

3Ca0 ‘A1203 •3CaCl2 «32H20

3Ca0 *Al203 •CaCl2 *8H20

1
2 . CRYSTAL STRUCTURE AND MORPHOLOGY CHARACTERIZATION

The methods used to prepare the various compounds are described subsequently.

After preparation, the compounds were characterized for purity and completeness of reaction. Those not suitably pure were reformulated until a suitable

purity was obtained. X-ray diffraction (XRD) , and to a limited extent, scanning

electron microscopy (SEM) were used for the characterization.

Prior to x-ray diffraction analysis of the compounds, a procedure was developed

to prepare them for examination without changing their crystal structure as a result of drying, and for confirming that the XRD identification obtained for

a dry powder was valid for the damp paste.

It was found that hydrated pastes could be prepared for x-ray diffraction

analysis without altering their crystal structure by the following procedure:

The slurry was filter pressed to form a filter cake of low water content

(10 percent), followed by grinding the cake in absolute ethanol to form an alcohol-water-compound slurry. This was then vacuum dried at 30 ®C and a pressure of 91 kilopascals. The powder was then x-rayed and the patterns

compared to those obtained by the original original investigators.

To ensure that the vacuum drying procedure had not altered the crystal structure

of the compounds x-ray diffraction patterns were also obtained for undried

samples. This was done by intergrinding the filter cakes with a small amount

of glycerin and analyzing the damp paste. The glycerin acts as a viscous

thickener to prevent preferred orientation often found when performing x-ray

analyses on dry powders which have a platelet morphology. Although the peak

to background ratios of the damp patterns were smaller than those of the dry

patterns, it was confirmed that the drying procedure did not alter the crystal

structure or cause excessive preferred orientation.

3. COMPOUND PREPARATION

3.1 4Ca0*Al203« 13H20

This compound was reported to reversibly dehydrate to 4CaO* AI2O3 • IIH2O

at about 50 °C at a partial pressure of water vapor of 6 mm [1, 2]. On

dehydration a slight change in crystal structure occurs due to the simplification and deterioration of the order of stacking of the layers. This results

in a decrease in the basal spacings as the compound goes from the 13 hydrate

to the 11 hydrate and to the 7 hydrate at 120 “C [3]. 4CaO* AI2O3 • I3H2O

was prepared by the stochiometric addition of freshly ignited CaO to a metastable

monocalcium aluminate solution and shaking the mixture for several days at 25 °C.

Care was taken to exclude CO2 during preparation. The compound fabricated was a mixture of a and 0 pol3nnorphic forms of 4CaO» A1203* I3H2O and contained

a few percent of unreacted Ca(0H)2* The two polymorphic forms differ only by

a slight difference in the packing of the crystal lattice. There is little

difference in stability between the two forms. As Ca(0H)2 does not dehydrate

in the temperature range under investigation, the presence of a small amount

was not deemed detrimental.

2
3.2

CaO»Al2O3*10H2O

The compound CaO«Al203» IOH2O was reported to decompose to an amorphous calcium

aluminate if heated between 50 and 75 '’C in air [4], If heated to 110°C in the

presence of water vapor GaO»Al203* IOH2O irreversibly converts to a mixture of

3Ca«Al203»6H20 and Al203«3H20 [4]

.

The reversibility of the the reaction of

CaO»Al2O3*0H2O to amorphous reaction product was not cited. CaO«Al203 •IOH2O

is the main reaction product of hydrated calcium aluminate cement and was prepared by hydrating a commercially available calcium aluminate cement to

completion.

X-ray diffraction analysis of the compound CaO» Al203« IOH2O , revealed the presence

of minor amounts of 4Ca0»Al203« I3H2O due to the relatively impure commercial

calcium cement used as a reactant. It was, however, possible to account for the

4Ca0*Al203* I3H2O contribution to the total heats of transition measured [5].

3.3 2Ca0*Al203«8H20

The compound 2Ca0»Al203» 8H2O was reported to reversibly dehydrate to 2CaO«Al203 *7 .5H2O

at room temperature when dried in air having 34 percent RH and to further dehydrate

to 2Ca0«Al203 «5H20 when heated to 102‘*C [2]. Both the 7.5 and 5 hydrate compounds

will readily convert to the 8 hydrate when exposed to a damp atmosphere. A corresponding^ change in^the x-ray diffraction pattern for the longest basal spacing

of 10.7 A or 10.4 A depending on the polymorphic form of the 8 hydrate, to 10.6 A

for the 7.5 hydrate resulted, indicating some 'structural rearrangement had occurred.

The compound was prepared using a method similar to that used to prepare

4Ca0«Al203« I3H2O. However, 2Ca0«Al203«8H20 was found to be unstable with respect

to 4CaO»Al203* I3H2O and work on this compound was discontinued.

3.4 3Ca0*Al203«3CaS04«32H20

The compound 3Ca0«Al203»3CaS04«32H20 (ettringite) was reported to dehydrate to

3CaO« Al203«3CaS04«320H20 at 60 at an unspecified partial pressure of water

vapor [5]. X-ray diffraction has shown that very little change occurs in the

crystal structure during this dehydration. The dehydration reactions were

found to be reversible down to the 7 hydrate form [7]. Ettringite forms as the

result of slow reaction of 3Ca0»Al203 with GaS04»2H20 (gypsinn) in solution.

No secondary phases were detected in the 3Ga0»Al203«3GaS04»32H20 fabricated in

this manner. SEM analysis revealed well formed needles about 1 ym in length.

3.5 3Ga0*Al203«GaS04*12H20

3GaO* Al203»GaS04* I2H2O gradually loses water on heating transforming to

3Ga0«Al203*GaS04»7H20 by 110 ®G [7, 8]. No information was available on the

reversibility of the reactions. 3Ga0»Al203»GaS04»12H20 was prepared by adding

a saturated Ga(0H)2 solution containing sufficient GaS04«2H20 to obtain a GaS04/Al203 ratio of unity to a metastable calcium aluminate solution.

3
3Ca0*Al203*CaS04* 12H20 prepared in this manner was found to contain a small

amount of ettringite. However, since a pure ettringite sample was available,

the contribution of ettringite on the total heat of transition could be measured.

3.6 3 Ca0 *Fe203 * 3CaS04 * 32H20

The thermal dehydration behavior of 3Ca0»Fe203»3CaS04» 32H20 has been shown

to be similar to that of ettringite [ 7 ], as would be expected due to the

similiarity of their crystal structures. 3Ca0»Fe203«3CaS04*32H20 was prepared

by mixing a solution of iron aliam with Ca(0H)2 in suitable proportions [11].

SEM analysis revealed well formed crystals which exhibited a blocky habit.

3.7 3 Ca0 «Al 203 « 3 CaC03 * 32H20

The dehydration of this compound initiates at about 75 *0 and is essentially

complete at 135 ®C [ 9 ]. 3Ca0»Al203«3CaC03«32H20 was prepared by dissolving

CaO in an aqueous solution of sucrose. After the solution clarified by settling

it was decanted into a calcium aluminate solution followed by the addition of

an ammonium bicarbonate solution. 3Ca0«Al203*3CaC03»32H20 formed as a flocculant precipitate.

3.8 3 Ca0 *Al 203 *CaC03 «llH20

This compound was reported to dehydrate over a temperature range from 75 to

150 [ 10 ]. It was produced by the slow reaction of calcium aluminate cement

with Ca( 0H)2 and CaC03 in aqueous solution. SEM analysis revealed the

3 Ca0 » Al203*CaC03* IIH2O to form as thin hexagonal plates, approximately 2 ym

in width.

3.9 3Ca0 *Al203 » 3 CaCl2

* 32H20

This compound was reported to initiate dehydration at about 0 *0 and to decompose

above 20 ®C [ 13 ]. 3Ca0«Al203«3CaCl2*32H20 was formed by the reaction of

3CaO»Al203 with a CaCl2 solution at -10 "* 0 . On heating 3Ca0»Al203 •3CaC03 » 32H20

to room temperature, it was confirmed that this compound decomposed to

2Ca0«Al203«CaCl2*8H20. Further work on this compound was dropped due to its

instability.

3.10 3 Ca0 * AI2O3 • CaCl2* 8H2O

This compound was reported to initiate dehydration at about 120 ®C [ 13 ].

3Ca0»Al203»CaCl2*8H20 was produced by the slow reaction of a commercial calcium

aluminate cement with Ca( 0H)2 and CaCl2» SEM analysis revealed this compound

to form as 1 yia plates in width

4
4 THERMAL ANALYSIS

Two schemes were investigated for using hydrated calcium aluminate compounds

to store energy. The first involved storing the compounds as wet pastes in

sealed containers. As the containers are heated the compounds expel waters of

hydration from their crystal structures and absorb energy as latent heat.

Upon lowering the temperature of the containers, the compounds rehydrate and

release the latent heat.

The compounds were tested using a differential scanning calorimeter to determine

the temperature at which dehydration occurred and to determine the associated

latent heats. The key feature of the differential scanning calorimetric (DSC)

technique was the use of hermetically sealed sample pans which prevented the

samples from dessicating during heating. All the compounds formulated for this

study were tested using this method. Of these compounds only 3CaO •Fe203 •3CaS04

•

32H2O, CaO«Al203» IOH2O and 3Ca0»Al203»CaC03» IIH2O exhibited reversible

hydration reactions at temperatures below 140 "C. 3Ca0«Fe203*3GaS04 »32H20

exhibited a reversible hydration reaction at 64 ®C but the latent heat of

transition was only 5 cal/g. Similarly, CaO«Al203«10H20 and

3Ca0»Al203»CaC03* IIH2O had reversible reactions at 77 and 127 ®C, respectively,

with heats of transition of 6 and 24 cal/g. As a consequence of these results,

it was concluded that this method of energy storage using cement compounds was not feasible.

A second method for using hydrated cement compounds to store energy involved

removing the water of hydration by drying the compounds at a suitable temperature. Upon the subsequent addition of water, the compounds would rehydrate

and release their latent heats of hydration. This methodology involved the

determination of the temperature at which these compounds would dehydrate in

dry air. This was accomplished by testing the hydrated compounds in a DSC

utilizing a cell with a pinhole opening which allowed the escape of hydration

water. The thermogram for 3Ca0«Al203«3CaS04*32H20 is shown in figure 1 while that for 3Ca0*Fe203«3CaS04»32H20 is shown in figure 2. Three

endothermic dehydration peaks are observed for 3Ca0»Al203»3CaS04«32H20 . The

first peak shows an onset near 67 ®C and a small latent heat of dehydration.

The onset of the second peak occurs near 90 ®C and also shows as small heat of

dehydration. The onset temperature of the third peak is about 100 *C and a relatively large heat of dehydration is observed. The thermogram for

3Ca0»Fe203»3CaS04»32H20, figure 2, shows one endothermic dehydration peak

with an onset near 95 ®C.

Once the dehydration temperatures were determined using differential scanning

calorimetry the compounds were heated to temperatures above their dehydration

temperatures and their latent heats of hydration measured using a conduction

calorimeter. In this technique, the dried cement compound was placed in the

calorimeter, equilibrium established, and excess distilled water injected into

the calorimeter to hydrate the compound. The heat released as the compound

rehydrated was integrated to give the latent heat in cal/g. In addition,

information on the rate of heat release was also obtained.

5
ETTRINGITE 3§ 6

TEMPERATURE

(K)

Figure

1,

Thennograia

of

3CaO*3CaSO^*32H20

showing 3

endothermic

peaks

having

onset
3CA0.

FE203.

3CAS04.

32H20

a s §

ifi c\j ea <0GN3 G3S/1VGW

TEMPERATURE

CK)

Figure

2.

Thermogram

of3

CaO*Fe

203

-3CaS04

-32H20

showing

one

peak

with

an

onset

temperature

of

approximately

95“C.
Table 1 lists the onsets of the dehydration peaks as measured by differential

scanning calorimetry and the latent heats as measured by conduction calorimetry

for the compounds investigated. The temperatures at which the compounds were

dehydrated are also listed. The dehydration temperature is the temperature

at which the compounds were dried for 24 hours prior to rehydration. For

most of the compounds the latent heats were either too low or the temperature

required to dehydrate the compounds was too high for practical application.

However, compounds with significant latent heats were studied further. These

were 3Ca0»Al203*3CaS04»32H20 and 3Ca0*Fe203»3CaS04»32H20 with heats of

rehydration of 97 and 43 cal/g respectively after drying at 100 “G.

These compounds were alternately dehydrated at 100 “C for 24 hours and

rehydrated to measure the latent heat released on thermal cycling. Results are

shown in figure 3. Both compounds were observed to undergo a slow decrease in

total heat release with repeated cycling. An SEM examination of the morphology

of 3Ca0*Al203 •3CaS04 *32H20 after repeated rehydration did not suggest a reason

for the decrease in latent heat. Although the latent heats decreased with the

number of drying cycles, the time required for complete heat release remained

constant. 3Ca0»Al203»3CaS04*32H20 was readily wetted by water and its rate of

heat release was faster than the time constant of the calorimeter. This compound rapidly heated to a temperature of up to 60 *C giving off the majority of

its latent heat within 10 seconds. 3Ca0«Fe203«3CaS04*32H20 also readily wetted

by water but its rate of heat release was noticeably slower requiring at least

a minute to release the majority of its latent heat.

The time required to fully dehydrate 3Ca0*Al203«3CaS04«32H20 at 100 “C in order

to to achieve the maximum heat release on rehydration was 3 days. Drying 24

hr at this temperature yielded about 95 percent of the maximum drying while for

3 hr yielded about 75 percent.

5. SUMMARY

Of the compounds investigated in this study 3Ca0«Al203»3CaS04»32H20 , appears to

be the most promising phase change storage material.

The potential for the use of 3Ca0»Al203«3CaS04« 32H20 as an energy storage

medium results from several factors; 1) the ability to store energy at ambient

temperature; 2) low corrosivity and toxicity; and 3) the low cost of the compound.

Impure 3Ca0»Al203*3CaS04*32H20 may be made using waste products from coal

combustion including lignitic fly ash and scrubber sludge [12]. The disadvantages to the use of this compound are; 1) relatively high temperatures required

for dehydration; and 2) the observed decrease in latent heat on cyclic hydration

and dehydration.

8
Table 1. Latent Heats of Hydration for Selected Compounds

Compound Peak Onset

Temperature

Dehydration

Temperature

Latent heat of

Hydration cal/

g

3Ca0«Al203 •3CaS04«32H20 — 30 *C 2

67 *C 80 8

90 100 97

100 140 102

3CaO • Fe203 • 3CaS04 • 32H20 95 100 43

95 140 49

3CaO • AI2O3 •CaCl2* 8H2O 75 80 2

75 110 29

3Ca0«Al203*CaC03*llH20 118 60 12

• 118 110 12

118 160 26

3 CaO • A1 2O 3 • CaSO4 • 1 2H2O 57 100 14

Ca0*Al203«13H20 77 100 5

*CaO* AI2O3 • IOH2O 65 75 20

*Not reversible.

9
wva9/ivo a3ivn3an iv3h

10

Figure

3.

Plot

showing

the

losses

in

energy

storage

capacities

of

3CaO*Al203*

3CaS0^*

32H

2

O

and

3CaO*

Fe203^

3CaS0^'

32H

2

O

with

cyclic

hydration/dehydration.
6 ACKNOWLEDGMENT

The authors gratefxilly acknowledge the U.S. Department of Energy, Office of

Solar Heat Technologies, Conservation, and Renewable Energy for supporting

this investigation.

7

.

REFERENCES

1. Buttler, F. G., Dent Glasser, L. S«, and Taylor, H. R. W., J. Amer. Ger.

Soc., 121 (1959).

2. Roberts, M. H. , J. Appl. Chem. , 7^, 543 (1957).

3. Jones, F. E., Chemistry of Cement, Proceedings of the Fourth International

Symposium , Washington, 1960, p. 204. National Bureau of Standards Monograph

43, U.S. Department of Commerce.

4. Schneider, S., J. Amer. Cer. Soc., 42 , 184 (1959).

5. Berman, H. A. and Newman, E. S., Ibid, ref 3, p. 247.

6. Lerch, W., Ashton, F. W., and Bogue, R. H. , J. Res. Nat. Bur. Stand.

715, (1929).

7. Midgley, H. G. and Rosaman, D., Ibid. ref. 3, p. 259.

8. Turriziani, R. , in The Chemistry of Cements , Vol. 1, 262, Academic Press,

London (1964).

9. Carlson, E. T. and Berman, H. A., J. Res. Nat. Bur. Stand., 64A , 333 (1960).

10. Turriziani, R. and Schippa, G., Ric. Sci., 26 , 2792 (1956).

11. Jones, F. E., J. Phys. Chem., 344 (1945).

12. Ings, J. B. and Bauer, W. H. , Bull. Amer. Cer. Soc., 474 (1981).

13. Serb“Serbina, N. N., Savina, Y. A., and Zhurino, V. S., Dokl. Akad. Novk.

S.S.S.R., 111, 659 (1955).

11

NBS»n4A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See iastructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2531

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

July 1982

4. TITLE AND SUBTITLE

An Evaluation of Hydrated Calcium Aluminate Compounds as Energy

Storage Media

5. AUTHOR(S)

John B. Ings and Paul W. Brown

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions)

national bureau of STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report 3t Period Covered

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

U.S. Department of Energy

Office of Solar Heat Technologies

Conservation and Renewable Energy

Washington, DC 20885

10. SUPPLEMENTARY NOTES

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

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

bibliography or literature survey, mention it here)

Calcium aluminate hydrates and calcium aluminate hydrates containing other ions

were investigated to determine the feasibility of their utilization as energy

storage media. A series of these compounds were fabricated and analyzed for

purity. The energy liberated on hydration of each compound was measured using

conduction calorimetry .and the dehydration temperature was measured using

differential scanning calorimetry. Of the compounds investigated,

SCaO'AljO^ *2CaS04*32H20 liberated the largest amount of energy upon rehydration.

Initially; this value^was about 100 cal/gram. However, after 18 cycles of

hydration and dehydration this value drops to about 70 cal/gram.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Calciimi-aluminum hydrates; calorimetry; dehydration; energy storage; rehydration;

solar.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

rX] Unl imited

1

1 For Official Distribution. Do Not Release to NTIS 15

j ! Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

I ^1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161 56 .00

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