PUBLICATIONS

AlllDM imOSQ

NBS!R 84-2942

An Evaluation of Ettringite and

Related Compounds for Use in

Solar Energy Storage

L. J. Struble and ?. W. Brown

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

July 1984

Progress Report

issued October 1 9S4

Sponsored by:

U.S. Department of Energy

Office of Solar Heat Technologies

Passive and Hybrid Solar Energy Division

Washington, DC 20585

—QC

100

.U56

84-2942

1984

JfATIONAl BUHEAtf OF STANDABDS

LIBRARY

NBSIR 84-2942

AN EVALUATION OF ETTRINGITE AND

RELATED COMPOUNDS FOR USE IN

SOLAR ENERGY STORAGE

L J. Struble and P. W. Brown

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

July 1984

Progress Report

Issued October 1984

Sponsored by

U.S. Department of Energy

Office of Solar Heat Technologies

Passive and Hybrid Solar Energy Division

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

AN EVALUATION OF ETTR INCITE AND RELATED COMPOUNDS

FOR USE IN SOLAR ENERGY STORAGE

Leslie J. Struble and Paul W. Brown

Center for Building Technology

National Bureau of Standards

July 1, 1984

SUMMARY

This report describes an investigation of ettringite and related phases for

potential application in solar energy storage. The specific objective is to

evaluate the potential of ettringite dehydration and rehydration as a phase

change process for energy storage.

Synthesis procedures have been developed, and a number of ettringite-type

phases have been prepared. The heat capacity of each phase was approximately

0.3 calories per gram per degree Celsius. Studies of the dehydration of these

phases at atmospheric pressure indicate that the material has good potential

as a phase change material for solar energy storage. Dehydration occurred at

temperatures in the range between approximately 30°C and 55°C, with changes in

enthalpy ranging between 100 and 240 calories per gram of sample. In addition,

ettringite was found to have a reversible hydrothermal reaction at approximately

50®C, with an enthalpy change of approximately 4 calories per gram sample.

Future work during the remainder of this program will involve completing the

work described in the present progress report.

1
I. INTRODUCTION

Effective utilization of solar energy in the passive mode requires the

utilization of materials in which either sensible or latent heat may be stored.

One class of materials that holds promise for this application is hydrated

salts, an example of which is ettringite, also called Candlot's salt, a hydrated calcium alumi no-sulfate that forms during the hydration of Portland

cement. Therefore, a laboratory study is being carried out in order to evaluate ettringite and related phases for possible application in passive solar

energy systems. Preliminary studies indicated that ettringite dehydration has

potential for thermal energy storage (1). Further work, described in the present

report, has focussed on the synthesis of ettringite and several related phases,

and the determination of their thermal properties.

A. ENERGY STORAGE MATERIAL

Solar energy storage materials have two basic functions: to store solar energy,

and to release it at the appropriate time. To carry out these functions,

storage systems may rely on the specific heat of a material, and may also rely

on a phase change which occurs in the material. In general, the amount of

energy that a material is capable of storing and releasing is significantly

greater if a phase change is involved. On the other hand, the phase change

may bring about other changes, such as physical degradation, that limit the

usefulness of the material in this application.

Specific heat, or more precisely thermal capacity, is a fundamental and

measurable property (though it varies somewhat with temperature). Values of

specific heat, i.e. the thermal capacity of the material relative to that of

water, range considerably. The thermal capacity of water is 1 calorie per

gram per degree Celsius, while thermal capacities of solids are somewhat lower,

typically a few tenths calorie per gram per degree.

There is also a change in heat, or enthalpy, associated with any phase change —

including melting, changes in crystal structure, dehydration, and dissolution.

The enthalpy change may be positive, such that heat is consumed (endothermic)

or negative, such that heat is released (exothermic). If the phase change is

reversible, then the enthalpy change is also reversible. Therefore a reversible

phase change may be used for solar energy collection. The phase change utilized

most extensively is melting, and an example is the melting of Glauber's salt,

Na 2

S04 * 10H20 . The change in enthalpy due to a phase change is a measurable

property of the material (e.g., 80 calories per gram for the melting of ice,

and 60 calories per gram for the melting of Glauber's salt).

However, other parameters associated with phase changes are not fuiidamental

properties. The kinetics of the change, and therefore the precise temperature

at which the change occurs at a reasonable rate, may vary considerably with

the nature of the sample — the amount of material, its particle size, etc.

The temperature of a phase change is particularly important in passive solar

devices, where the operating temperature is limited. Therefore, the kinetics

2
and the temperature of a phase change, as well as the enthalpy, must be

considered in the evaluation of any material for use in solar energy collectors.

Finally, there are several other considerations when evaluating a material for

this application. For instance, the physical and chemical durabilities are critical. The material must be reasonably low in cost and readily available.

It must be easily and safely handled. While these factors were not the focus

of the present study, they were considered throughout the study.

B. ETTRINGITE

Ettringite is a hydrated calcium alumino-sulfate, [Ca5Al2(0H)i2](S04)3*26H20,

which typically forms from reaction of Ca3Al206 with gypsum (CaS04»2H20)

and water. The dehydration, or partial dehydration, of ettringite and related

phases is the particular focus of these studies. The material is known to

dehydrate partially at relatively low temperatures, and the dehydration is

expected to be reversible. Therefore, ettringite dehydration offers potential

as a phase change process for solar energy.

The crystal structure of ettringite was described by Moore and Taylor (2) in

1970, and the structures of ettringite and related phases were subsequently

reviewed by Taylor (3). The phase is hexagonal in structure, and typically

occurs as a prism or needle, elongated parallel to the c-axis. The basic

structural unit is a column of hydrated calcium aluminate hydroxide (Fig. la).

The columns, positively charged, are arranged parallel to the c-axis of the

crystal, with four such columns per unit cell (Fig. lb). The negative (SO4)

ions and the remaining water molecules are found within the channels formed

by these columns (Fig. Ic). The structural formula may thus be written

[Ca3Al (0H)6*12H20]4(S04)6*4H20.

The ettringite structure is very open, and considerable chemical substitution

may occur. Based on the crystal structure, however, certain restraints on the

substitution are expected. For instance, the substitution for the sulfate

ions would be limited by the need to balance the positive charge of the

columns, and by the available space within the channels. Substitution within

the columns, on the other hand, would be limited primarily by the 6-fold

coordination of the aluminum and calcium within the columns.

Several types of chemical substitution have been reported within the channel

sites. For instance, CO3 may replace SO4, either partially (3) or completely

(4). In an early reference, Flint and Wells (5) reported substitution of Si03

for SO4. Finally, substitution of Cl , a monovalent ion, for SO4 has been

reported (6). However, it is not clear from structural considerations whether

monovalent ions may substitute for SO4. The crystal structure analysis

suggests that there are not sufficient sites within the channel for the number

of monovalent ions that are required for charge balance.

Several substitutions have been described within the columns. Either Sr or Pb

may substitute for Ca (3). The following elements have been reported to substitute for Al: Ge (3); Fe, Mn (3,7); Ti (7); Cr (7,9,10); and Si (3,10).

The mineral thaumasite is a doubly substituted ettringite, in which CO3 is

3
partially substitutinq within the channel, and Si is substituting for A1

,

requirino 6-fold coordination of Si.

• The ettringite structure thus contains three distinct types of water molecules:

1) water within the channel, 4 molecules per structural unit, 2) water bound

to the columns, 48 molecules per structural unit, and 3) water as hydroxide

ions on the columns, 12 molecules per structural unit. It is reasonable to

expect the conditions for dehydration and the structural changes associated

with dehydration to be different for each of the three types of water.

Dehydration may be carried out at room temperature and atmospheric pressure by

reducing the partial pressure of water in the system. As reviewed by Lea

(11)

, this dehydration is reported to proceed with no change in X-ray diffraction pattern until there are 36 molecules of water per structural unit. This

loss presumably corresponds to the loss of the 4 molecules of water from within

the channel and the loss of 24 of the 48 molecules that are bound as water on the columns. Further dehydration causes a change in the X-ray diffraction

pattern, i.e. a shift in the basal peak from 0.97 nm to 0.73 nm, until 16

molecules of water remain, corresponding to loss of most of the remaining

water from the channels.

In some cases, dehydration at elevated temperature produces similar behavior.

Taylor (3) described changes on heating at 110®C, during which shrinkage was observed along the a-axis, until a = 0.84 nm. He suggested that dehydration

resulted in the individual columns coming closer together to produce the shorter

a-axis. However, a few studies have reported quite different results. Mehta

(12) found that heating in a dry environment to 94®C caused no changes in the

XRD pattern, including no shrinkage of the a-axis. Likewise, Sudoh et al . (13)

reported that heating at 100®C caused ettringite to become amorphous but produced no shrinkage of the structure. Thus there is some conflict in the

literature regarding the effects of thermal dehydration on the crystal structure.

The behavior is quite different when ettringite is heated under hydrothermal

conditions. In that case, the ettringite decomposes to hydrated calcium monosulfoaluminate, (12, 14). Hydrothermal dehydration was studied by Satava and

Veprek (14) using differential thermal analysis under hydrothermal conditions.

They found that in liquid water, ettringite converted to calcium monosul foal uminate and calcium sulfate hemi hydrate in an endothermic reaction at 150®C.

Far less information is available regarding thermal properties other than

temperature. Especially important in energy storage is whether the reaction

is reversible. Although no information was found, it is expected that if the

dehydration causes no change in crystal structure, it will be readily reversible.

No information was found regarding the enthalpy changes associated with any of

the dehydration reactions. One study of the heat capacity of ettringite reported

Cp values of 0.41 calories per gram (15)

The limited information supports the hypothesis that dehydration of ettringite

provides a potential phase change for solar energy storage. The dehydration

of ettringite occurs at a sufficiently low temperature, the reaction may be

reversible, and the large number of possible chemical substitutidns provides

4
opportunity for optimizing thermal properties. Therefore, ettringite and

related phases were synthesized and their thermal properties determined

experimentally.

II. EXPERIMENTAL PROCEDURE

During the preliminary studies (1), phases were synthesized by slow reaction

of Ca3Al206 or a similar material with water and some form of calcium

sulfate. For the more detailed studies, however, procedures were developed in

which the reactants were first dissolved, and then mixed. In this way, the

reactions proceed faster, typically complete within a day, rather than several

days or weeks. Furthermore, reactants in solution produce a more homogeneous

product and allow better control of the reacting constituents.

In general, the syntheses started with solutions of the desired reactants in

stoichiometric proportions, using as high a concentration as possible. One

solution contained one cation (aluminum, iron (III), etc.), and the desired

ion (sulfate, carbonate, chloride, etc.). The calcium solution was prepared

separately, by dissolving CaO (prepared by igniting reagent Ca(0H)2) in a 10% sugar solution. The sugar allowed significantly higher concentrations of

calcium (0.3 M with sugar, 0.02 M without sugar). When these two solutions

were mixed, precipitation usually began within an hour. Solutions were rotated

at room temperature for a few hours or a day, filtered, and the precipitates

washed with water and ethanol, then dried under reduced pressure.

Because CO3 is a possible substituting ion in ettringite, specific measures

were taken during synthesis to avoid unwanted carbonation. All solutions were

prepared in freshly boiled water. Reactions were carried out in closed containers, in which air was first displaced by N2 gas. The relatively short

reaction times made it easier to avoid carbonation.

Ettringite is known to be unstable — to undergo partial dehydration (or

rehydration) at room temperature, depending on the water vapor pressure.

Therefore, care was taken that all material be in equilibrium with some constant environment prior to the characterization and the thermal studies.

After synthesis and drying in a mild vacuum, each sample was ground to

-325 mesh. Specimens were then stored over a saturated ZnCl2 solution, at a nominal water vapor pressure of 28% RH.

Reaction products were characterized by X-ray diffraction. In a few cases,

qualitative chemical analysis was also necessary.

B. THERMAL STUDIES

Heat capacity (cp) was measured by Differential Scanning Calorimeter (DSC),

equipped with a microprocessor for control and computer software for data

collection and analysis. Scans for Cp were collected over a temperature

range below the dehydration reaction. The instrument was calibrated for Cp

using a standard sample of sapphire.

5
Thermal studies of the dehydration reaction at atmospheric pressure were carried

out in order to determine the temperature, the amount of water lost, and the

enthalpy change. These studies utilized both Thermal Gravimetric Analysis

(TGA), also with microprocessor and computer, and the DSC. Both these methods

are known to be highly sensitive to experimental conditions. In particular,

the transition temperature is highly sensitive to the rate of heating. In

order to determine the thermal properties precisely and accurately, the measurements were carried out at the very slow heating rate of 0.6° per minute. Both

TGA and DSC samples were run in open pans, in order to improve precision.

Finally, very small amounts of sample were used, typically 1 mg.

The DSC and TGA studies provided thermal data — dehydration temperature,

weight loss, and enthalpy changes — but no information about the kinetics of

the dehydration. Therefore one relatively large sample (several grams) was dehydrated at 60°C in order to determine its rate of dehydration. In addition,

partially dehydrated specimens from this experiment were analyzed by X-ray

diffraction, in order to determine whether partial thermal dehydration causes

any changes in the crystal structure.

Studies of the rehydration of partially dehydrated specimens were carried out

in a hydration calorimeter, which was designed and built at NBS (16). This

calorimeter allows much more sensitive and accurate measurement of enthalpy

changes than the calorimeter that was used for the preliminary studies (1).

Samples were first partially dehydrated in an oven, then mixed with water in

the calorimeter.

Some enthalpy changes were measured under hydrothermal conditions, using

high-pressure DSC cells, with the sample covered by water. This reaction

could be reversed while still in the DSC, so a single specimen could be

dehydrated and rehydrated repeatedly. This method avoids the experimental

problems described above, and provides a very precise estimation of the number

of dehydration/rehydration cycles before degradation for the hydrothermal

reaction.

III. RESULTS AND DISCUSSION

A. SYNTHESIS AND CHARACTERIZATION OF PHASES

Several procedures were explored for synthesis of ettringite and related phases,

most following the general method of mixing reactants in solution form. The

following substitutions were attempted: CO3 for SO4, Cl for SO4, NO3 for

SO4, Fe (III) for A1 , Cr (III) for A1 , and Si for Al. General aspects of

each procedure are provided in this section, and details are given in Appendix A.

The primary procedure for preparation of ettringite was to mix a solution of

Al2(S04)3 (1 M) with a solution of CaO (0.3 M). Precipitation began as soon

as the solutions were mixed, approximately 20% of the theoretical yield was obtained after reacting overnight, and approximately 30% of the theoretical

yield after reacting for several days. The only reaction product observed

by X-ray diffraction was ettringite.

6
A similar procedure was used to prepare several substituted phases. For

Fe-substituted ettringite, a slurry of Fe2(sp4)3 was used, since this phase

is much less soluble than the aluminum salt,* and this slurry mixed with a solution of CaO (0.3 M). Likewise, Cr-substituted ettringite was prepared

from a solution of CrK(S04)2 (4 M) and a solution of CaO (0.2 M). In each

case, the only reaction product observed by X-ray diffraction was an ettringitelike phase.

A similar procedure was tried for substituting monovalent ions for SO4. To

prepare the Cl-substituted phase, a solution of AICI3 (0.9 M) was mixed with a solution of CaO (0.3 M). In this case, the reaction product was not an ettringite

phase, but rather a Cl-substituted monosulfate, [Ca2Al (0H)5]2(S04) •6H2O.

Likewise, a solution of A1(N03)3 (0.8 M) mixed with a solution of CaO (0.3 M)

produced an amorphous reaction product. Partial substitution with each

monovalent ion was also unsuccessful. Ettringite-type phases were produced,

but qualitative chemical analysis of each phase showed it to be unsubstituted

ettringite.

Another successful procedure for preparation of ettringite was to mix a solution

of Na2Al204 (0.3 M) and Na2S04 (0.3 M), with a solution of CaO (0.3 M). This

provided a general procedure that was easily modified for the C03-substituted

phase, by replacing Na2S04 with NaHC03. Again, the only reaction product

observed by X-ray diffraction was an ettringite-1 ike -phase.

Preparation of thaumasite was attempted from a solution of Na2Si03 (0.5 M) with

Na2S04 (0.5 M) and NaHC03 (0.5 M), and a solution of CaO (0.3 M). The resulting

mixture was held at 5°C, as was suggested by van Aardt and Visser (17) in

order to achieve the required 6-fold coordination of Si. However, the precipitate

formed during this reaction was not thaumasite, but was amorphous to X-rays.

In summary, synthesis procedures were developed using solutions, which fulfilled

the initial objectives that the reaction be rapid, the product homogeneous,

and carbonation be prevented. These procedures allowed synthesis of ettringite

and ettringite-1 ike phases with various chemical substitutions, including CO3

for SO4, Fe for A1 , and Cr for Al. The procedures did not allow substitution

of Si for Al or Cl or NO3 for SO4.

B. HEAT CAPACITY

Measured heat capacities (Table I) were approximately 0.3 calories per gram

per degree Celcius. There was no significant difference in heat capacity for

any of the phases tested.

C. DEHYDRATION

TGA and DSC results are given in Table II. In general, the dehydration occurred

over a temperature range from approximately 30®C to approximately 55°C. This

upper temperature was lowest (49°C) for the C03-substituted ettringite phase

and highest (58°C) for the Fe-substituted phase.

7
Water loss averaged approximately 20 moles of water per mole of sample, or 40

moles relative to the structural formula. This corresponds to all the water

in the channels and approximately 75% of* the water bound to the columns.

The change in enthalpy for these dehydration reactions varied from 100 calories

per gram sample for the C03»substituted phase to 230 calories per gram sample

for the Fe-substituted phase. It should be noted that the changes in enthalpy

are extremely dependent on the experimental conditions — sample size, type of

specimen cell, etc. Enthalpy change of one material has been found to vary by

as much as 100% with different experimental conditions. Therefore, it is

important to assess the precision of the enthalpy change before concluding

that substituted phases have significantly different enthalpy changes. At any

rate, these enthalpy changes are significantly higher than the value (60 calories

per gram) observed for the melting of Na2S04, a phase change material used

in solar energy collection.

As expected, the dehydration at 56°C of a large sample occurred more slowly

than the dehydration of very small samples in the TGA and DSC experiments. The

TGA sample of 1 mg showed 27% loss by 51 °C (at 0.6°/min, this temperature was reached in 40 minutes). Several grams of the same material heated in an oven

at 56°C had only 14% loss in 72 hours (Fig. 2). However, heating at a higher

temperature increased the rate of dehydration dramatically. A large sample of

ettringite heated at 110°C showed a weight loss of 24% after 1 hour and 31%

after 2 hours.

Heated samples were analyzed by X-ray diffraction for any changes in the crystal

structure or unit cell parameters. Progressive dehydration caused the sample

to become increasingly amorphous to X-rays but with no observed changes in

unit cell parameters. The samples heated at 56°C were mixtures of ettringite

and amorphous material, and the samples heated at 110°C were entirely amorphous.

Ettringite and a sample of partially dehydrated ettringite were examined by

scanning electron microscopy (Fig. 3). The ettringite consisted of short,

stubby, prismatic crystals, approximately 1 urn in length. No difference was observed between the unheated sample and the partially dehydrated sample.

D. REHYDRATION

Partially dehydrated ettringite was rehydrated in a calorimeter, in order to

measure its heat of hydration. Enthalpy changes measured during rehydration.

Table III, ranged from 9 calories per gram sample for material dehydrated at

56°C (7% weight loss) to 106 calories per gram sample for material first

dehydrated at 110®C (31% weight loss).

In order to compare the change in enthalpy measured during rehydration with

the change in enthalpy measured during dehydration, the values must be expressed

relative to the amount of water gained or lost. Rehydration values are equivalent

to between 130 and 340 calories per gram of water. The changes in enthalpy

measured during dehydration by DSC, on the other hand, are equivalent to 460

calories per gram of water. Therefore, the heat of rehydration is somewhat less

than the heat of dehydration. This difference is attributed to the partial

8
prehydration of the sample during hydration calorimetry. More accurate

determination of the heat of rehydration will be an objective during the

remainder of the program.

E. HYDROTHERMAL DEHYDRATION/REHYDRATION

Hydrothermal reactions of the Fe-substituted ettringite were measured by DSC,

in which the sample was immersed in water in a high-pressure cell. These

results, summarized in Table IV, were quite different than the results in open

pans at atmospheric pressure. After the first hydrothermal heating, cycles

produced an exotherm on cooling and endotherm on heating, at a temperature of

approximately 50°C, and an enthalpy change of approximately 4 calories per

gram sample. The reaction was studied for 3 cycles, during which it was fully

reversible.

However, studies by Satava and Veprek (14) suggest that the hydrothermal

reaction is not dehydration, but rather conversion of ettringite to a mixture

of monosulfate and calcium sulfate hemi hydrate. If so, the reaction is not

congruent, in that solid ettringite is being converted to two other solids.

An incongruent reaction is less suitable for application in energy storage, in

that the reaction products may segregate, limiting the efficiency of the

material.

IV. CONCLUSIONS

Significant progress has been made on the study of ettringite as an energy

storage material. Although the program is still in progress, several important

conclusions have been reached:

(1) Procedures for rapid synthesis of ettringite have been developed, which

may be modified in order to synthesize ettringite-1 ike phases with various

chemical substitutions. Ettringite-1 ike phases were synthesized with the

following substitutions; CO3 for SO4, Fe for A1 , and Cr for Al. The following

substitutions were not successful: Si for Al , and Cl or NO3 for SO4.

(2) Values for heat capacity for ettringite and substituted ettringite-type

phases were approximately 0.3 calories per gram per degree Celcius. There was

no significant difference in values of heat capacity for the various phases.

(3) Dehydration of ettringite-type phases at atmospheric pressure occurred

over a temperature range from approximately 30°C to approximately 55®C. The

upper temperature was different for each substituted phase, lowest (49°C) for

the C03-substituted ettringite, and highest (58°C) for the Fe-substituted

ettringite. Measured enthalpy changes ranged between 100 and 240 calories per

gram of sample. Thus the material has good potential as a phase change material

for solar energy storage.

(4) Rehydration of dehydrated phases at room temperature produced an enthalpy

change of approximately 100 calories per gram sample, somewhat less than the

enthalpy change during dehydration of the same material. It is not clear

9
whether the discrepancy results from experimental difficulties or indicates

that (5) the rection is not fully reversible.

Hydrothermal reaction of ettringite occurred at approximately 50®C with an enthalpy change of approximately 4 calories per gram sample. The reaction

appears to be fully reversible in 3 cycles.

V. FUTURE WORK

During the remainder of the program, the following work will be completed.

Replicate measurements of the enthalpy change due to dehydration will be made,

in order to determine if the differences in enthalpy changes measured for

the different ettringite-like phases are significant. Rehydration experiments

will be continued, in order to determine how reversible is the dehydration at

atmospheric pressure. Finally, experiments on the hydrothermal reaction will be

continued.

VI. REFERENCES

(1) Ings, J.B. and Brown, P.W. 1982. An Evaluation of Hydrated Calcium

Aluminate Compounds as Energy Storage Media. NBSIR 82-2531. U.S. Department

of Commerce.

(2) Moore, A.E. and Taylor, H.F.W. 1970. Crystal structure of ettringite.

Acta Crystal lographica, B26 , 386-393.

(3) Taylor, H.F.W. 1973. Crystal structures of some double hydroxide minerals.

Mineralogical Magazine, 22. (304), 377-389.

(4) Bensted, J. 1983. Further hydration investigations involving portland

cement and the substitution of limestone for gypsum. World Cement Technology,

14(10), 383-386,389-390,392.

(5) Flint, E.P. and Wells, L.S. 1944. Analogy of hydrated calcium

si 1 i coal umi nates and hexacalcium aluminate to hydrated calcium sul foal umi nates.

Journal of Research of the National Bureau of Standards, 33, 471-478.

(6) Schwiete, H.E., Ludwig, U., and Albeck, J. 1968. Bindung von Calciumchlorid

bei Hydratation der aluminatisch-ferritischen Bestandteile des Portlandzementes.

Naturwissenschaften, 179.

(7) Bensted, J. and Prakash Varma, S. 1971. Studies of ettringite and its

derivatives. Cement Technology, _2(3), 73-76 and 100.

(8) Kuzel , H.J. 1971. Replacement of Al3+ by Cr3+ and Fe^'*' in ettringite.

Zement- Kal k-Gi ps , 24(2), 83-85.

(9) Bensted, J. and Prakash Varma, S.P. 1972. Studies of ettringite and its

derivatives; Part II, Chromate substitution. Silicates Industriels, 37, 315-318.

10
(10) Lukas, W. 1976. Substitution of Si in the lattice of ettringite. Cement

and Concrete Research, ^(2), 225-234.

(11) Lea, F.M. 1971. The Chemistry of Cement and Concrete. 3rd Edition.

New York: Chemical Publishing Company, Inc.

(12) Mehta, P.K. 1972. Stability of ettringite on heating. Journal of the

American Ceramic Society, ^(1), 55-56.

(13) Sudoh, G., Akiba, T., and Iwasaki , T. 1976. Some studies on calcium

sulfoaluminate hydrates. Review of the 30th General Meeting, Technical Session,

Cement Association of Japan, 42-44.

(14) Satava, V. and Veprek, 0. 1975. Thermal decomposition of ettringite

under hydrothermal conditions. Journal of the American Ceramic Society,

Discussions and Notes, _58, 357-359.

(15) Ederova, J. and Satava, V. 1979. Heat capacities of C3AH5, C4ASH12

and CgAS3H32. Thermochimica Acta, 126-128.

(16) Prosen, E.J., Brown, P.W., Frohnsdorff, G., and Davis, F. 1984. A

mul tichambered microcalorimeter for the investigation of cement hydration. To

be published.

(17) van Aardt, J.H.P. and Visser, S. 1975. Thaumasite formation: a cause

of deterioration of Portland cement and related substances in the presence of

sulfates. Cement and Concrete Research, _5(3), 225-232.

11
TABLE I

HEAT CAPACITY

Phase

CCa3Al(0H)6]2(S04)3.26H20

CCa3Fe(0H)6]2(S04)3.26H20

[Ca3Cr(0H)5]2(S04)3«26H20

CCa3Al(0H)6]2(C03)3.26H20

Cp

ca1ories/gram/°C

0.31

0.29

0.32

0.30

12
TABLE II

DEHYDRATION RESULTS

Phase Temperature

' O/

Loss aH

es H?0) (calories/gram)

[Ca3Al(0H)6]2(S04)3-26H20 40-51 20 120

[Ca3Fe(0H)6]2(S04)3-26H20 30-58 24 230

[Ca3Cr(0H)6]2(S04)3*26H20 30-55 13 120

[Ca3Al(0H)6]2(C03)3-26H20 32-49 23 100

* Lower temperature is the onset of the dehydration, determined by DSC, and

upper temperature is the end of dehydration, determined by TGA.

13
TABLE III

REHYDRATION RESULTS

Dehydration

Temperature Loss AH on Rehydration

(°C) (%) (calories/qram)

56 7 9

no 24 60

no 31 106

14
TABLE IV

HYDROTHERMAL REACTION OF Fe-•SUBSTITUTED ETTRINGITE

Peak Onset AH

Cycle (‘^O (calories/qram)

1 - heating 72 +0,5

103 -0.9

cool i ng 38 -4

2 - heating 61 +3

cool i ng 41 -4

3 - heating 60 +3

cool i ng 41 -5

15
Figure 1. Crystal structure of ettringite, from Taylor_(3); a) column of

hydrated calcium aluminate, projected on (1120), not showing the

water molecules that are bound to the column; and b) ettringite

channel viewed along the c-axis, with the triangles representing

the SO4 ions and water molecules within the channel.

16
Figure 2. Rate of dehydration of a large sample (several grams) of ettringite

at 56°C; dashed line prepresents the weight loss of the 1 mg TGA

sample.

17
Figure 3. Scanning electron micrographs of (a) ettringite, and (b) ettringite

heated 48 hours at 56®C (13% weight loss).

13
APPENDIX A

SYNTHESIS PROCEDURES

A-1. Ettringite.

Solution I: 13.63 grams Al 2

(S 04 ) 3 , dissolved in 40 ml H 2O.

Solution II: 13.40 grams CaO, dissolved in 890 ml aqueous 10% sugar solution.

Mix solutions I and II. Rotate for several hours or overnight. Filter.

A-2. Fe-substituted ettringite.

Slurry I: 15.92 grams Fe 2

(S 04) 3 , suspended in 1000 ml H 2O.

Solution II: 12.82 grams CaO, dissolved in 850 ml aqueous 10% sugar solution.

Mix slurry I and solution II. Rotate for one day or longer. Filter.

A-3. Cr-substituted ettringite.

Solution I: 38.26 grams CrK(S04 ) 2

* 12H 20 , dissolved in 20 ml H 2O.

Solution II: 12.89 grams CaO, dissolved in 1000 ml aqueous 10% sugar solution.

Mix solutions I and II. Rotate for several hours or overnight. Filter.

A-4. C 03-substituted ettringite.

Solution I: 9.66 grams Na 2Al 204 * 3H 20 , dissolved in 140 ml H 2O. Add and dissolve

11.15 grams NaHC03 .

Solution II: 14.91 grams CaO, dissolved in 990 ml aqueous 10% sugar solution.

Mix solutions I and II. Rotate a few hours or overnight. Filter.

A-5. Si- and C03 -substituted ettringite (thaumasite, unsuccessful).

Solution I: 9.80 grams Na2Si 03 , dissolved in 150 ml H2 O. Add and dissolve,

25.88 grams Na 2

S04 * 10H 20 and 8.51 grams Na 2C03 .

Solution II: 13.51 grams CaO, dissolved in 930 ml aqueous 10% sugar solution.

Cool solutions to 5°C. Mix solutions I and II. Store for a few hours at 5®C.

Filter. Product was amorphous.

19
A-6. Cl -substituted ettringite.

Solution I: 20.46 grams AlCl3«6H20, dissolved in 100 ml H2O.

Solution II: 14.26 grams CaO, dissolved in 950 ml aqueous 10% sugar solution.

Mix solutions I and II. Rotate for a few hours. Filter. Product was AFm

rather than ettringite.

A-7. N03"Substituted ettringite.

Solution I: 28.01 grams A1 (N03)3«9H20, dissolved in 100 ml H2O.

Solution II: 12.56 grams CaO, dissolved in 860 ml aqueous 10% sugar solution.

Mix solutions I and II. Rotate for a few hours. Filter. Product was amorphous.

20
APPENDIX B

X-RAY DIFFRACTION PATTERNS

This appendix contains X-ray diffraction patterns for ettringite and the various

substituted ettringite-type phases.

All XRD patterns were collected by powder diffractometer using CuK^ radiation.

21

IOlNT ; AA1L

r.niJMT rrME= 1.0 SEC.

RANGE= 6.00 -> 62,00 INCR= 0.02

ESTIMATED TIME OF COMPLETION IS 7:12

iniEnsity v5, two-theta FULL SCALE 3000. COUNTS/SEC.

0. 00

70.00

Figure B-1. Ettringite.

22
1

I DENT : BA3

COUNT TINE= K0 SEC.

INTENSITY VS. TWO-THETA

RANGE= 6.00 -> 62.00 INCR= 0.02

' ESTIMATED TIME OF COMPLETION IS 22 ; 6 ^ FULL SCALE = 3000. COUNTS/SEC, I

—3

3

i Figure B-2. Fe-substituted ettringite.

23
FILhMAME: LESCAl. SM SAMPLE: LESCAl 2/22/84

P]l TMAMES:

1000 -

40-

10 -

h f

• - ^ -t- •

+ 5.00 n.00 17.00 23.00 29.00

Figure B-3. Cr-substituted ettringite.
« * i\i r intensity

4E3L

iMe^ K0 sec.

vs. TWO-THETA

RANGE- B.00 -> 62.00 I NCR'-' Q.O

ESTIMATED TIME OF COMPLETION IS 2:1

FULL SCALE = 3000. COUNTS/SEC.

1

1

0.00

Figure B-4. C03 -substi tuted ettringite

25

i\)rj
APPENDIX C

TGA CURVES

This appendix contains TGA curves for dehydration in an open pan of ettringite

and substituted ettringite-type phases.

26
%

WEIGHT

LESLIE FILEtAAlLl. TC

DATEi 84/04/11 TIME* 13*41

TEMPERATURE CC5 TG

I PERKIN-EUMER [Thermal Analysis]

Figure C-1. Ettringite.

27
1

LJS FILEkBASL. TG

DATEi 84/05/17 TIMEi 02i 0S

TEMPERATURE CO TG

I PERKIN-gUS/IER [Thermal Analysi»|

Figure C-2 Fe- substituted ettringite
Figure C-3. Cr-substituted ettringite
WEIGHT

L

LJS FILEiAESL. TG TEMPERATURE <C) , TQ

PERKIN-EUVIEn Thermal Analysis DATEi 84/05/14 TIMEi 08i 41 ' ' '

Figure C-4. C03-substituted ettringite.

30
APPENDIX D

DSC CURVES

This appendix contains DSC curves for dehydration in an open pan of ettringite

and substituted ettringite-type phases.

31
MCAL/SEC

GALUK FILEtAAlNJ.QA

' TEMPERATURE <K)

OATEi 84/06/15 TIMEt 0St 54

SC

I PEWKIN-EL.MER [Thermal Anal yiis]

Figure D-1. Ettringite.

32
MCAL/SEC

DATEt 84/06/06 TIMEt 21i 49 PEnKIN-ELMER Thermal Analysis!

Figure D-2. Fe-substituted ettringite,

33
MCAL/SEC

(

OiLLlK FIL£iCA12.DA . TEMPERATURE DSC

DATE* 84/06/21 TIME* 11*23 | PEffKIN-gLMER [Thermal Analysis!

Figure D-3. Cr-substituted ettringite.

34
Figure D-4. COs-substi tuted ettringite.

35
APPENDIX E

HYDROTHERMAL DSC CURVES

This appendix contains the curves for hydrothermal reaction of Fe- substituted

ettringite.

36
E-1. First cycle, heating
MCAL/SEC

(

GALUK FILEi BAC2. DA

OATEi 84/06/aS TIHEs 13t 12

TEMPERATURE <K) SC

I PERKIN-g|_MER [Thermal Analysis

E-2 First cycle, cooling
MCAL/SEC

Aa

a

z

ut

GALUtC FILEt BAC3. DA

OATEi 84/08/ae TIMEi 14i 18

E-3. Second cycle, heating.

39
MCAL/SEC

(

GALUK FILEt BAC4. DA

ATEi 84/06/06 TIME* 14*54

TEMPERATURE <K) DSC

|PEnKltSt-gt-MER|Thefmal Analy*!*!

E-4. Second cycle, cooling.

^0
MCAL/SEC

E-5. Third cycle, heating.

41
MCAL/SEC

GALUK FILE»BACS.DA TEMPERATURE CK) DSC

DATEi 84/06/06 TIMEl 15l 42 | PERKIN-ELMER |Thermal Analysis]

E-6. Third cycle, cooling.

42

hBS'I I4A ncv. 2 >«c)

4. TITLE and subtitle

AN EVALUATION OF ETTRINGITE A^TD RELATED COMPOUNDS FOR USE IN SOLAR ENERGY STORAGE

5. AUTHOR(S)

Leslie J. Struble and Paul W. Brown

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

national bureau op STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

7. Con tract/ Grant No.

So Type of Report & Period Covered

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

Department of Energy

Office of Solar Heat Technologies

1000 Independence Avenue

Washington. D.C.

10. SUPPLEMENTARY NOTES

1 1 Document describes a computer program; SF-18S, FI PS Software Summary, is attached

U.*. 0*PT. OP COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2942

2. Performing Organ. Report No. 3. Publication Date October 1984

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) This report describes an investigation of ettringit

and related phases for potential application in solar energy storage. The specific

objective is to evaluate the potential of ettringite dehydration and rehydration as a phase change for energy storage.

Synthesis procedures have been developed, and a number of ettringite-type phases have

been prepared. The heat capacity of each phase was approximately 0.3 calories per gram

per degree Celcius. Studies of the dehydration of these phases at atmospheric pressure

indicate that the material has good potential as a phase change material for solar ener

storage. Dehydration occurred at temperatures in the range between approximately 30 “C

and 55“C, with changes in enthalpy ranging between 100 and 240 calories per gram sample

In addition, ettringite was found to have a reversible hydrothermal reaction at

approximately 50®C, with an enthalpy change of approximately 4 calories per gram sample

Future work during the remainder of this program will involve completing the work

described in the present progress report.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

AFt; enthalpy change; ettringite; phase change material; solar energy storage

13. AVAILABILITY

I yi Unlimited

I I For Official Distribution. Do Not Release to NTIS

i I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

i VI Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

44

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