5

Allioa 4T031b

All 10249031

6

Struble, L. J/lnorganlc compounds tor pa QC100 .U56 NO.86-3325 V1986 C.2 NBS-PUB- Inorganic Compounds for Passive

Solar Energy Storage -- Solid-State

Dehydration Materials and High

Specific Heat Materials

NBS

PUBLICATIONS

L.J. Struble and P.W. Brown

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

April 1986

Progress Report

Sponsored by:

U.S. Department of Energy

''k ": — of Solar Heat Technologies

and Hybrid Solar Energy Division

100 jton, DC 20585

• U56

86-3325

1986

C. 2

NBSIR 86-3325

> © C INORGANIC COMPOUNDS FOR PASSIVE

SOLAR ENERGY STORAGE - SOLID-STATE

DEHYDRATION MATERIALS AND HIGH

SPECIFIC HEAT MATERIALS

EESEflBCH l; C'~-:NPORMj

S2ES jON

u

S? Si C >

1 I. Struble and P.W. Brown

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

April 1986

Progress Report

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

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. MATERIALS STUDIED 3

3. EXPERIMENTAL PROCEDURES 6

3.1 Synthesis 6

3.2 Characterization 7

3.3 Thermal Studies 8

4. RESULTS AND DISCUSSION 11

4.1 Synthesis and Characterization 11

4.2 Thermal Properties 16

4.2.1 Dehydration 16

4.2.2 Rehydration 20

4.2.3 Heat Capacity 27

5. SOLAR ENERGY STORAGE 29

6. CONCLUSIONS 31

7. ACKNOWLEDGEMENTS 33

8. REFERENCES 34

iii
LIST OF TABLES

Page

Table 1. Chemical composition of four tr isubs ti tuted

phases 14

Table 2. Unit cell parameters of trisubsti tuted phases 16

Table 3. IR data for trisubsti tuted phases, wavenumbers

(cm

-1

) of bands assigned to species 18

Table 4. Results of replicaate DSC mesurements of some

trisubsti tuted phases 19

Table 5. Dehydration results for trisubstituted phases 19

Table 6. Enthalpy changes on dehydration of trisubstituted phases relative to the amount of

water lost 26

Table 7. Replicate heat capacity measurements for some

trisubstituted phases 28

Table 8. Heat capacity data for trisubstituted phases .. 28

Table 9. Heat capacity data for monosubsti tuted phases 28
List of Figures

Page

Figure 1. Crystal structure of ettringite, modified

from Moore and Taylor [13] 5

Figure 2. Specimen container for dehydration/

rehydration studies 9

Figure 3. Schematic diagram of dehydration/rehydration

test 10

Figure 4. SEM micrograph of ettringite 12

Figure 5. SEM micrograph of Fe-subs ti tuted ettringite 12

Figure 6. SEM micrograph of Cr-subs tituted ettringite 13

Figure 7. SEM micrograph of CO^-subs tituted ettringite 13

Figure 8. Dehydration data for tr isubstituted phase,

recalculation from TGA curves 17

Figure 9. Weight changes during cycles of drying and

wetting for ettringite at 44°C 21

Figure 10. Weight changes during cycles of drying and

wetting for ettringite at 25°C 22

Figure 11. Weight changes during cycles of drying and

wetting for ettringite at 40°C 23

Figure 12. Weight changes during cycles of drying and

wetting for Fe-substi tuted ettringite at

40 °C 24

Figure 13. Weight changes during cycles of drying and

wetting for CCK-substituted ettringite at

40 °C 7 25

Figure A-l. XRD pattern for thaumasite A-2

Figure A-2. XRD pattern for the Cl-monosubsti tuted

phase A-3

Figure A-3. XRD pattern for the S0

4

~monosubsti tuted

phase A-4

Figure A-4. XRD pattern for the NO ,-monosubs t i tuted

phase A5

Figure B-l. IR spectrum for ettringite B-2

v
List of Figures (Continued)

Page

Figure B-2. IR spectrum for Fe-subst i tuted ettringite .. B-3

Figure B-3. IR spectrum for Cr-substi tuted ettringite .. B-4

Figure B-4. IR spectrum for CO^-substi tuted ettringite B-5

Figure B-5. IR spectrum for thaumasite B-6

Figure C-l. TGA pattern (10° per min) for ettringite . C-2

Figure C-2. TGA pattern (10° per min) for Fe-subst i tuted

ettringite C-3

Figure C-3. TGA pattern (10° per min) for Cr-substi tuted

ettringite C-4

Figure C-4. TGA pattern (10° per min) for CCK-substi tuted

ettringite C-5

Figure C-5. TGA pattern (10° per min) for thaumasite .. C-6

Figure D-l. TGA pattern (0.6° per min) for thaumasite .. D-2

Figure D-2. DSC pattern (0.63° per min) for thaumasite D-3

Figure E-l. XRD pattern for ettringite after repeated

drying and wetting cycles at 25°, sampled

after the last drying cycle (see fig. 10) .. E-2

Figure E-2. IR spectrum for ettringite after repeated

drying and wetting cycles at 25°, sampled

after the last drying cycle (see fig. 10) .. E-3

Figure E-3. XRD pattern for ettringite after repeated

drying and wetting cycles at 25° , sampled

after the last wetting cycle (see fig. 10) . E-4

Figure E-4. IR spectrum for ettringite after repeated

drying and wetting cycles at 25° , sampled

after the last wetting cycle (see fig. 10) . E-5

vi
SUMMARY

Two classes of hydrated inorganic salts have been studied to

assess their potential as materials for passive solar energy

storage. The materials are part of the quaternary system

CaO-A^C^-SC^-H^O and related chemical systems, and the two

classes are typified by ettringite, a trisubstituted salt, and

Friedel's salt, a monosubstituted salt. The trisubstituted salts

were studied for their possible application in latent heat

storage, utilizing a low-temperature dehydration reaction, and

both classes were studied for their application in sensible heat

storage.

In order to assess their potential for energy storage, the salts

have been synthesized and characterized by several analytical

techniques, and their thermal properties measured. The dehydration

data of the trisubstituted salts vary somewhat with chemical

composition. The temperature at which dehydration begins range

from 6°C to 33°C, and enthalpy changes on dehydration range from

0.3 to 0.8 kJ/g. Heat capacity values for all trisubstituted

salts are approximately 1.3 J/g/K and for the monosubstituted

phases range from 1.0 to 1.2 J/g/K. Preliminary experiments indicate

that dehdyration of the trisubstituted phases is reversible,

though additional tests are required. These thermal data

demonstrate that the trisubstituted salts do have potential as

latent heat storage materials, and that both classes of salts

have potential as sensible heat storage materials. Furthermore,

it is noted that these materials may be contained in conventional

Portland cement concrete, making them particularly attractive for

thermal energy storage.

VI

1
PREFACE

In keeping with the national energy policy goal of fostering an adequate supply of energy at a reasonable cost, the United States

Department of Energy (DOE) supports a variety of programs to

promote a balanced and mixed energy resource system. The mission

of the DOE Solar Buildings Research and Development Program is to

support this goal, by providing for the development of solar

technology alternatives for the buildings sector. It is the goal

of the Program to establish a proven technology base to allow

industry to develop solar products and designs for buildings

which are economically competitive and can contribute significantly

to building energy supplies nationally. Toward this end, the

program sponsors research activities related to increasing the

efficiency, reducing the cost, and improving the long term

durability of passive and active solar systems for building water

and space heating, cooling, and daylighting applications. These

activities are conducted in four major areas: Advanced Passive

Solar Materials Research, Collector Technology Research, Cooling

Systems Research, and Systems Analysis and Applications Research.

The Advanced Passive Solar Materials Research includes work on new aperture materials for controlling solar heat gains, and for

enhancing the use of daylight for building interior lighting

purposes. It also encompasses work on low-cost thermal storage

materials that have high thermal storage capacity and can be

integrated with conventional building elements, and work on materials and methods to transport thermal energy efficiently

between any building exterior surface and the building interior

by non -me chan i c a 1 means.

The present report is an account of research conducted in the

area of Advanced Passive Solar Materials on low-cost thermal

storage materials. The report concerns a class of thermal

storage materials that are hydrated, inorganic salts, some of

which typically occur in hydrated portland cement and concrete.

The report describes research designed to assess the potential of

these salts as thermal storage materials.

v i i i
1 . Introduction

In general, the variety of materials proposed for storing

solar energy are based on one of two principal processes,

sensible heat storage or latent heat storage. Sensible heat

storage utilizes the specific heat capacity of a material, i.e.

the heat required to increase its temperature. Latent heat

storage utilizes the change in enthalpy, or heat content,

associated with a phase change of the material. Although each

method has advantages, latent heat storage generally offers the

important advantage of potentially storing a much greater amount

of energy per unit volume of material [1],

Latent heat storage is provided by a phase change, i.e. a

transition of the material from one physical or chemical state to

another. A phase change may involve transition between solid and

liquid, between liquid and gas, or from one chemical composition

to another. It is an inherent property of any phase change that

there be an associated change in enthalpy, or heat content of the

material.

Although melting appears to have been utilized more than other

phase changes for storing energy, others have been explored,

including changes in chemical composition and changes in crystal

structure. The phase change that is the subject of the present

report is dehydration, a chemical reaction of the type

A

^

B + H20 (1)

where A is a solid hydrated phase that dehydrates to form solid B

and water. Applications in passive solar energy storage using

dehydration reactions of this type appear to be uncommon. One

reference [2] was found describing an energy storage system based

on dehydration, using the reaction

Ca(OH) 2

^ CaO + H2 0 (2).

A similar reaction is the desorption of water by zeolites, whose

potential for energy storage has been demonstrated [3].

Materials must meet a variety of constraints in order to be

used in passive solar energy storage. In particular, the

following are considered to be important parameters for phase

change materials (PCM's):

1. The phase change should occur at a temperature

appropriate to passive applications, in general not

above 40 °C.

2. The latent heat associated with the phase change

should be large. *

1
3. The thermal performance of the PCM should not

degrade with thermal cycling.

4. The volume change associated with the phase change

should be minimized. This constraint usually

precludes a phase change involving a vapor.

5. The heat transfer characteristics of all phases

should allow adequate heat transfer through the

material. Therefore, the heat transfer

characteristics of the phases that exist above the

reaction temperature should not be significantly

different from the characteristics of the phases

that exist below the reaction temperature.

6. All phases in the system should be compatible with

their containment materials.

7. The PCM should be chemically inert, non-toxic and

non-flammable

.

8. The PCM should be inexpensive to produce.

While these constraints were not studied directly in the present

program, they influenced the experimental approach and were

considered throughout this study.

The present study evolved from an investigation of the system

CaO-A^C^-SC^-H^O, an important system in cement technology.

Some compounds in this system are known to dehydrate at temperatures as low as 110°C [4,5], and in some cases there are indications that the dehydration may be reversible [5-9]. Because a reversible reaction that occurs below approximately 40 °C might

have application in passive solar energy storage, a study was

initiated to explore whether hydrated inorganic salts in the

quaternary system CaO-A^C^-SC^-f^O and in related systems have

potential as materials for passive solar energy storage. The

experimental approach involved synthesis of compounds, evaluation

of their thermal characteristics, and exploration of their

response to thermal cycling.

There have been two previous reports on this project. The

first [10] described the preliminary studies, which indicated

that dehydration of ettringite might provide thermal energystorage . The second report [11] presented synthesis procedures

and some thermal properties of ettringite and related phases.

The present report covers synthesis procedures for additional

phases and studies of the ettringite phases during cycles of

dehydration and rehydration. Specific heat data, communicated

previously in a letter report [12], have been incorporated into

the present report.

2
2. Materials Studied

The two classes of materials included in this study are typified

by the two quaternary compounds in the system Ca0-Al 203 -S03 -H20 . The first compound is ettringite, [ Ca 3 Al ( OH) g

]

2

( SO4 ) 3

• 26^0 , a naturally occurring mineral also named Candlot ' s salt. Since it

contains three divalent anions for each two atoms of aluminum,

this class of compounds will be referred to as trisubstituted

salts

.

The trisubstituted salts contain a considerable amount of

water. As discussed previously, ettringite is known to lose some

of this water, in a reaction that which may be reversible, at

temperatures at least as low as 110 °C [4,5]. Therefore, the

thermal properties of the dehydration reaction and the specific

heat of the ettringite-type phases were studied.

The second compound, [Ca2Al(OH) ^

]

2

( SO4

)

6 H^O, is termed monosulfate

in cement literature; its chloride analog is Friedel's salt.

Since it contains one monovalent anion for each aluminum atom (or

one divalent anion for each two aluminum atoms), this class of

compounds will be referred to as monosubstituted salts. The

compounds of this type do not appear to dehydrate at temperatures

below 110 °C, or to dehydrate reversibly, so only their specific

heat values were determined.

The crystal structure of ettringite, as reported by Moore

and Taylor [13], consists of columns of empirical composition

[Ca 3 Al(OH)g« 12H203+

] , and channel sites (fig. 1). Three of the

channel sites are occupied by sulfate ions and the fourth by two

water molecules. This structure is amenable to various types of

substitutions [5]. Divalent anions, e.g. carbonate, may substitute for sulfate [14]. Substition of monovalent anions for

sulfate has also been reported, producing phases of the type

[Ca 3 Al (OH) g

]

2

( Cl ) g26H2 O [15,16]. However, Taylor [5] noted that

the existence of phases of this type appears to be in conflict

with structural requirements, since it would be necessary to

accomodate all six monovalent anions in only four available

sites. Attempts to prepare these phases in the present study

were unsuccessful [11]. Other trivalent anions can be substituted for aluminum, e.g. iron and chromium [17-19]. In some

instances tetravalent cations can be substituted for aluminum,

such as the carbonate- and silicon-substituted phase, thaumasite

[20-23]

.

As mentioned, the second class of compounds is typified by

monosulfate or Friedel's salt. The crystal structure of these

compounds is based on distorted, brucite-like layers of composition [Ca 2 Al(OH) g

]

+

, with interlayer sites and cavities in which

are found H 2 O and anions [24]. Some of the H 2 O molecules

occupywell-def ined , interlayer sites, while the remaining H 2 O

molecules and the anions occupy cavities and are only semi-ordered.

3
Divalent or monovalent anions readily substitute for the sulfate

.

.

4
Figure 1. Crystal structure of ettringite, modified from Moore

and Taylor [13].

5
3

.

Experimental Procedures

3 . 1 Synthesis

Synthesis procedures were developed to meet the overall objectives

of the project, and thus may not be suitable for preparation of

bulk quantities. The objectives of the synthesis procedures are

to allow reproducible preparation of pure phases that are

chemically homogeneous and uniform in particle size. These

objectives appeared best met by synthesis from solution.

Therefore, a general procedure was followed involving precipitation from a mixture of solutions, one solution containing calcium

and the other solution containing the remaining reactants in

their stoichiometric proportions, as detailed in the earlier

report [11]. When the solutions were mixed, a precipitate

typically formed immediately, and could be filtered after

approximately one hour to collect the product. Excess amounts of

Ca* 2 and SO4” 2 were not found to be necessary, although Jones

[26] and Lea [4] had reported that excesses of these ions were

necessary due to the incongruent solubility of ettringite.

Concentrations as high as possible were used, so as to maximize

yield. Concentration of the calcium solutions was increased by

dissolving by dissolving CaO in a 10 percent sugar solution, a procedure used previously by Carlson and Berman [14].

The following trisubstituted salts were synthesized by the

method described above:

1. ettringite, [Ca 3 Al(0H)g] 2 (S04

) 3 -26H 2 0,

2. iron-substituted ettringite, [Ca 3 Fe ( OH)

g

] 2

( SO4 ) 3

• 26H 2 0,

3. chromium-substituted ettringite, [ Ca 3 Cr (OH) g

]

2

( SO 4

) 3

• 26H 2 0,

4. carbonate-substituted ettringite, [Ca 3 Al ( OH ) g

]

2

( C0 3

) 3

• 26H 2 0

,

5. carbonate- and silicon-substituted ettringite, or

thaumasite , [ Ca3 Si ( OH ) g

]

2

( SO4 ) 2

( C03

) 2

•

2

4H2 0

.

Preparation of the first four phases was detailed previously

[11]. Thaumasite has only recently been synthesized successfully. The previous effort [11] produced a precipitate after a few hours at 5°C that was amorphous to X-rays. However, the

6
precipitate that formed after 5 months of mixing at approximately

5

0 C was identified as thaumasite. The synthesis presumably

requires extended reaction time at a low temperature to achieve

the 6-fold coordination of silicon [5].

The same general method allowed synthesis of monosubstituted

phases with a monovalent cation (Cl"), but not of monosubstituted

phases with a divalent cation (S0 4

" 2

). When solutions at or even

near the stoichiometric proportions for the sulfate monosubstituted salt were mixed, the precipitate that formed was the

trisubstituted, rather than the monosubstituted salt. Therefore,

an alternative procedure was used, analogous to the formation of

these compounds during hydration of Portland cement [27]. Solid

tricalcium aluminumate (Ca3Al20g) was mixed with a stoichiometric

proportion of the particular calcium salt and excess water. The

reaction appeared to be complete after several weeks or months of

mixing at room temperature. The following monosubstituted salts

were synthesized by this procedure:

1. chloride-substituted salt, [Ca2Al(0H) g

]

2 c l2 * 6H2O,

2. nitrate-substituted salt, [Ca2Al(0H) )2*6H20,

3. sulfate-substituted salt, [Ca2Al(0H)g] 2 S04

* 6 H20 .

3.2 Characterization

In order to determine what phases formed during each synthesis, samples were analyzed using one or more of the following

techniques: for qualitative phase composition, by X-ray diffraction (XRD) ; for bulk chemical composition, using inductively

coupled plasma-emission spectroscopy (ICP); for morphology, by

scanning electron microscopy (SEM); and by infrared spectroscopy

(IR). Two of the phases, the Cr-substituted ettringite and the

thaumasite, were not analyzed by every method. The qualitative

XRD analyses utilized normal techniques for specimen preparation

and data collection. The XRD analyses for determination of unit

cell parameters utilized much slower scan rates and, in some

cases, an internal standard. Chemical analyses followed normal

dissolution and analytical procedures required for ICP analysis.

The SEM examination of these materials employed standard procedures for specimen preparation and analysis, using an ultrasonic

bath to disperse each powder in alcohol, followed by evaporation

of the dispersion onto a carbon grid. The SEM was operated at a voltage of 10, 12, or 20 KeV as necessary to optimize the

micrograph. The IR analyses were made by Fourier transform

infrared ( FTIR) of specimens prepared as mulls of powder in

Nujol

.

7
3 . 3 Thermal Studies

As discussed previously [11], thermal studies were carried

out to determine the dehydration temperature, the amount of water

lost, and the enthalpy change associated with dehydration. These

studies utilized Thermogravimetric Analysis ( TGA) and Differential Scanning Calorimetry (DSC). Specific heat capacity (C

p

) was

also measured by DSC. Scans were collected over a temperature

range below any dehydration reaction. The instrument was

calibrated for C

p using a sapphire sample.

Experiments to determine reversibility of the dehydration

reaction were carried out on trisubstituted salts. These

experiments involved measuring changes in weight of samples

maintained at constant temperature while the relative humidity

was cycled. Sample chambers (fig. 2) allowed passing a stream of

gas through the specimen, typically 1 to 2 g of loose powder.

These sample chambers could be disconnected from the rest of the

apparatus (fig. 3) and weighed using an analytical balance. For

the dry cycle, the nitrogen gas was first dried by passing it

through a desiccant tube containing calcium sulfate. For the wet

cycle, the nitrogen gas was first bubbled through water. The

sample chambers and the water bubblers were placed in a water

bath for temperature control. In some experiments, they were at

the same temperature, so the humidity of the wet cycle was

assumed to be 100 percent relative humidity; in other experiments, the nitrogen gas was water-saturated at a slightly lower

temperature for a nominal 95 percent relative humidity.

8
GAS FLOW

IN

GAS FLOW

OUT

TEST SAMPLE GLASS WOOL

Figure 2. Specimen container for dehydration/rehydration

studies

9
a) Drying Cycle

Bubbler

b) Wetting Cycle

Figure 3. Schematic diagram of dehydration/rehydration test

10
4. Results and Discussion

4.1 Synthesis and Characterization

X-ray diffraction provided rapid analysis of the products to

assess each synthesis procedure. The XRD patterns obtained for

the four trisubstituted phases were presented previously [11],

and patterns for thaumasite and for the three monosubstituted

phases are included in appendix A of the present report. For

each phase reported, the XRD pattern indicated that little or no

contaminating phase was present, i.e., that in each case the

desired phase was the major crystalline constituent.

Bulk chemical compositions were determined (table 1) for comparison

to stoichiometric compositions. In all cases the water contents

were higher than expected. Otherwise, the compositions indicate

that ettringite and the Cr-substituted ettringite were synthesized with little or no impurity. The CO2 content of the

CC^-substituted phase is slightly higher than expected, and the

Fe content of the Fe-substituted phase is higher than expected,

suggesting possible impurities in these preparations.

The method used to synthesize materials for thermal studies,

precipitation from a mixture of two concentrated solutions,

allowed successful preparation of the trisubstituted phases with

divalent cations in the channel position, but not with a monovalent cation. However, this method could not be used to synthesize the sulfate monosubstituted phase. When solutions of

composition for the sulfate monosubstituted phase were mixed, the

only precipitate was the trisubstituted phase. There has been

discussion in the literature concerning the phase equilibria of

the sulfate trisubstituted phase, which is a stable phase, and

the sulfate monosubstituted phase, which may be only a metastable

phase [11]. Our results tend to support the conclusion that the

sulfate monosubstituted phase is metastable, at least at room

temperature. The reverse appears to be the case concerning

phases with monovalent cations in the channel position. It

appears that the trisubstituted phase is metastable and the

monosubstituted phase stable at room temperature.

The morphologies of the four trisubstituted phases, based on

the SEM micrographs (figs. 4-7), vary with chemical composition.

The ettringite (fig. 4) occurs as short, prismatic crystals,

variable in size from 0.2 to 3 urn in cross section, with an

aspect ratio of 2 or 3 . This morphology is different from the

characteristic morphology of ettringite as it occurs in hydrated

Portland cement, where it is present as prismatic, hexagonal

needles with aspect ratios between 4 and 10 depending on the

available space [27]. The Fe-substituted phase (fig. 5) is

uniform in size, approximately 3 urn in cross section, with an

aspect ratio of approximately 2. The Cr-substituted phase

11
Figure 4. SEM micrograph of ettringite

Figure 5. SEM micrograph of Fe-substituted ettringite

12
Figure 6. SEM micrograph of Cr-substituted ettringite

Figure 7. SEM micrograph of CC^-substituted ettringite

13
Table 1. Chemical compositions of four trisubstituted phases.

Fe- Cr- CO 3 - Ettringite substituted substituted substituted

ettringite ettringite ettringite

Weight percent01 Al2°3

S03

CaO

Cr 2°3

Fe2°3

C03

h2o

7.09

16.73

24.32

0

0.04

0

50.98

0

17.55

24.37

0

13.97

0

44.19

0

16.67

23.98

10.51

0.04

0

48.81

8.16

0.11

26.54

0

0.03

11.5

52.32

Calculated molar

Ca

ratios

6.0 6.0 6.0 6.0

Al 1.9 0.0 0.0 2.0

Fe 0.0 2.4 0.0 0.0

Cr 0.0 0.0 1.9 0.0

S03 2.9 3.0 2.9 0.0

co2 0.0 0.0 0.0 3.3

h2o 39.2 33.9 38.0 36.8

determined by ICP except CO3 and H2 O , which were determined from

TGA patterns ( appendix C )

.

(fig. 6 ) consists of plates with roughly hexagonal outline,

highly variable in size, ranging from 0.2 to 9 urn, lacking the

well-developed morphology of the other phases. Finally, the

C03 -substituted phase (fig. 7) is composed of clusters or

rosettes of prismatic crystals, 0.3 to 1 uni in cross section, and

has an aspect ratio of approximately 5

.

Though the XRD patterns vary slightly with chemical composition, each trisubstituted phase appears to have the same or

similar crystal structure. Unit cell parameters (table 2) were

determined by indexing the powder patterns using the trigonal

structure P31c reported for ettringite by Moore and Taylor [13],

or for thaumasite the hexagonal structure P 63 reported by Edge

and Taylor [28]. For ettringite and Cr-substituted ettringite,

the parameters in table 2 agree within 0.002 nm with values

reported by and by Buhlert and Kuzel [17]. Parameters for the

Fe-substituted phase, however, are larger than the reported

values [17], 0.002 nm larger in the a-direction and 0.005 nm

larger in the c-direction. Parameters for thaumasite in table 2 are slightly larger in both directions (0.003 nm in the a-direction

and 0.0024 nm in the c-direction) than the values reported by

Effenberger et al. [30].

14
As expected, the lattice parameters observed for the trisubs tituted

phases (table 2) vary with chemical composition. In the simplest

case, substitution of Cr or Fe for A1 is expected to expand the

lattice, since the ionic radii of Fe and Cr are larger than the

radius of Al. However, the lattice parameters with Cr or Fe

substitution are larger in the c-direction, but slightly smaller

in the a-direction. Thus the effect of the substitution appears

to be more complicated than may be explained simply by ionic

radii. Since the CO

3

anion is known to be smaller than than the

SO4 anion in thaumasite [29], the substitution of CO3 for S04 in

ettringite is expected to reduce the lattice in both directions.

This effect was observed (table 2).

The FTIR spectra (appendix B) were examined to determine the

effects of chemical composition on frequencies of the water

bands. It is generally accepted that a shift of IR band to

higher frequency indicates a higher strength bond. However, it

was argued by Ryskin [30] for the case of R3 SiOH...B complexes

(where R and B are organic species) that lowering the OH stretch

frequency is accompanied by an increase of the force constant of

the Si-0 bond. If the spectra for ettringite and substituted

phases are interpreted in the same way, a chemical substitution

that causes a shift of the OH stretch band to lower frequency is

expected to cause an increase in the force constant of the Ca-0

bond, and thus a greater enthalpy change with dehydration.

Based on [30], and with reference to the ettringite crystal

structure shown in fig. 1, OH bands were assigned to one of two

species: non-hydrogen-bonded hydroxyl species, in this case the

hydroxyl ions and presumably the water molecules on the column,

which are bonded to Ca atoms; or hydrogen-bonded hydroxyl,

i.e. water molecules within the channels, which are hydrogen-bonded

to the water molecules on the column. The frequencies of bands

assigned to hydrogen-bonded and non-hydrogen-bonded hydroxyl vary

with chemical composition of the phase (table 3). Compared to

the unsubstituted ettringite, both the non-hydrogen-bonded and

the hydrogen-bonded hydroxyl stretching bands are shifted to a

slightly lower frequency (20 cm

" 1 for each band) for the

Fe-substituted phase. The hydrogen-bonded hydroxyl stretch band

is shifted even further (170 cm" 1

) for the CC> 3 -substituted phase.

Thaumasite shows only one broad OH stretch band, assigned to

non-hydrogen-bonded hydroxyl stretch because the thaumasite

structure has no water in channel positions [ 5 ]

.

This hydroxyl

stretch band in thaumasite is shifted to a lower frequency

compared to the unsubstituted ettringite. Thus the FTIR data

suggest that the enthalpy change on dehydration will rank

according to the following: unsubstituted ettringite < Fe-substituted

phase < C03 -substituted phase < thaumasite.

15
Table 2. Unit cell parameters3 of trisubstituted phases.

Phase Structure a ( nm

)

c ( nm)

[ Ca3 Al ( OH

)

6 ] 2

( SO4

) 3

• 26H20 P31c 1.123 2.150

[Ca^Fe(0H) fi ] 9 (S04 ) v26H9 0 P31c 1.1182 2.2008

[ Ca3 Cr ( OH) g ] 2( S04 ) 3

• 26H20 P31c 1.119 2.177

[Ca 9 Al ( OH) ^

]

9 ( C0 9 ) 9 *26H9 0 P31c 1.0834 2.1250

[ Ca 3 Si ( OH

)

6 ] 2

( S04 ) 2 ( C03

T 2

* 2 4H2 0 P 63 1.106 1.042

^Determined from X-ray powder patterns. Parameters reported with

5 significant figures were determined using an internal

standard, and those with 4 significant figures were determined

using an external standard.

4 . 2 Thermal Properties

4.2.1 Dehydration

In addition to the low-temperature dehydration reaction, discussed

below, TGA data for each trisubstituted salt have been

collected over the full temperature range of dehydration and

decarbonation reactions (appendix C). From these patterns, the

mols of H2O per mol of each ignited phase have been calculated

and plotted (fig. 8 ). The major weight loss occurs below

approximately 200 °C under these conditions, and is probably due

to loss of crystalline water. As the temperature is increased,

further weight loss is probably due to loss of hydroxyl. These

TGA data agree to some extent with published data for ettringite,

the only phase with available data. The water contents at

intermediate temperatures (100-300°C) are higher in the present

study than reported by Lea [4] or Jones [26], but similar to

Taylor [31]. The water content of ettringite at room temperature

in the present study is approximately 40 mols per mol ignited

phase, while others report 32 mols [4,31]. It was anticipated in

the present study that the water contents of the trisubstituted

phases might not be stable, i.e., that the phases might dehydrate

in dry air and rehydrate in moist air. Therefore, as described

previously [ 11 ], samples for thermal studies were first equilibrated over a saturated ZnCl^ solution, at a nominal water vapor

pressure of 10 percent relative humidity. At this starting

condition, ettringite is expected to contain 32 mols of water

[4], although water contents as high as 36.5 mols have been

reported and attributed to adsorbed water [7]. The water content

of ettringite at room temperature in the present study is

approximately 40 mols.

There are clear differences in dehydration data for trisubstituted phases with different chemical composition (fig. 8 ).

The dehydration curves for the Fe-substituted phase and for the

C03 ~substituted phase appear to be very similar to the curve fcr

16
Figure 8. Dehydration data for trisubstituted phase,

recalculated from TGA curves

17
Table 3. IR data for trisubstituted phases, wavenumbers (cm-1

) of bands assigned to species3 . Q-H Stretch 0-H Bend

Phase Non-H- H- Hbonded bonded bonded

[ Ca 3 Al ( OH

)

6 ] 2

( S04 ) 3

• 26H20 3640 3420 1670

[ Ca^Fe ( OH )

]

9 ( SO* ) v 26H?0 3620 3400 1670

[Ca^Cr (OH) 6 ] 9 ( SO4 ) 3

• 26H9 O 3610 3430 1660

[ Ca3 Al ( OH

)

6 ] 2

( C03

) 3

• 26H20 3620 3250 1650

[ Ca 3 Si ( OH

)

6 ] 2

( S04

) 2

( C03

) 2

• 24H2 0 3410 1650

tentative peak assignments by inspection based on [30].

ettringite. The other two phases, the Cr-substituted phase

and thaumasite, retain more water at significantly higher temperatures than ettringite.

The DSC curves have been used to measure the onset temperature and change in enthalpy of the dehydration reaction [ 11 ].

Replicate DSC curves were run for ettringite, Fe-substituted

ettringite, and CC> 3 "Substituted ettringite to determine the

reproducibility of the low-temperature dehydration data. The

dehydration temperatures and enthalpy changes measured from these

curves (table 4) indicate the level of precision of these data.

The highest standard deviation of the dehydration temperature was

13 °C, and standard deviation of the enthalpy change ranged from

0.1 to 0.3 kJ/g.

Data regarding the low-temperature dehydration for all trisubstituted phases are presented in table 5 . The TGA and DSC

patterns for thaumasite, the only phase not reported previously,

are presented in appendix D. Some data in table 5 are different

from data reported previously [ 11 ], reflecting either the

replicate analyses (table 4 ) or a change in the procedure for

estimating dehydration temperatures. The dehydration temperatures reported previously [11] were estimated from both DSC and

TGA curves. However, the DSC curves are preferable, because the

DSC scans, contrary to the TGA scans, may be started below room

temperature. Therefore, results in table 5 were estimated from

only the DSC curves.

In order to determine whether the dehydration temperatures

and enthalpy changes vary significantly with chemical composition, the data in tables 4 and 5 were evaluated using a t-test

[32]. This statistical analysis indicated that some modifications in chemical composition produced significant differences in

temperature and enthalpy change. Both the CO 3 -substituted

ettringite and the Fe-substituted ettringite are significantly

lower in dehydration temperature than ettringite. The dehydration temperatures of thaumasite and ettringite are not

18
Table 4. Results of replicate DSC measurements of some trisubstituted phases.

Phase Onset Enthalpy

Temperature Change

(°C) (kJ/g phase)

[ Ca 3 Al ( OH

)

6 ] 2

( SO4 ) 3

• 26H2 0 36 0.62

36 0.59

35 0.42

17 0.73

27 0.53

Average 30 0.58

Standard Deviation 8 0.11

[ Ca 3 Fe ( OH

)

6 ] 2

( S04 ) 3

•

2

6H2 0 11 0.95

7 0.76

2 0.84

4 0.81

Average 6 0.84

Standard Deviation 4 0.08

[ Ca3 A1 ( OH ) g

]

2

( C03 ) 3

•

2

6H20 26 0.40

0 0.88

2 0.92

-2 0.96

Average 7 0.79

Standard Deviation 13 0.26

Table 5. Dehydration results for trisubstituted phases

Phase Lossa Onset

(mols Temperature^*

H 2 0 per mol phase) (°C)

1 •Enthalpy

Change^

(kJ/g)

[ Ca 3 Al ( OH ) £ ] ? ( SO4 ) 3

• 26H20 19 30±8 0 . 6 + 0.1

[ Ca 3 Fe ( OH ) ^ ] ? (

S

0 4 ) 3 *26H20 24 6 0.8

[ Ca 3 Cr ( OH) 5 ] 2

( S0 4 ) 3 *26H20 13 22 0.7

[ Ca 3 Al ( OH ) fi ] ^ ( CO^

)

3 -26H?0 23 7 0.8

[ Ca 3 Si ( OH ) g

]

2

( SO 4 ) 2

( CO 3

) 2

• 24H2 0 9 33 0.3

^Measured using TGA.

^Measured using DSC.

19
significantly different. When compared relative to the amount of

each phase, the enthalpy changes of the COg-substituted ettringite

and of ettringite are not significantly different. Both thaumasite

and the Fe-substituted ettringite are significantly lower than

ettringite in the enthalpy change associated with dehydration.

To compare the enthalpy changes of phases with various chemical

compositions independently of the dehydration loss , the change in

enthalpy of each phase was calculated relative to the amount of

water lost (table 6). On this basis, the enthalpy changes of

ettringite and thaumasite are similar, approximately 2.1 kJ/g

H2O. Values for both the Fe-substituted phase and the (^-substituted

phase are higher, approximately 2.6 kJ/g H^O, and the value for

the Cr-substituted phase is considerably higher, 3.7 kJ/g H 2 0.

These values bracket the heat of vaporization of water, which is

2.27 kJ/g.

4.2.2 Rehydration

As a preliminary method of assessing reversibility of the

dehydration reaction, weight changes of some trisubstituted

phases were measured during drying and wetting cycles. Data were

collected at 25, 40, and 44 °C. The initial measurements were of

ettringite at 44°C (fig. 9), which demonstrated that the system

could be used to study reversibility. On drying, the sample

dehydrated to approximately 70 percent of its starting weight,

similar to the dehydration levels observed in TGA studies ( table

5). The rehydration reactions appeared to be essentially

complete in approximately 1 to 2 days. Because the tests were

not allowed to go to completion, reversibility of the reaction

could not be demonstrated from this first measurement.

Weight changes of ettringite during a drying and wetting cycle at

25 °C (fig. 10) showed that dehydration and rehydration would

proceed at this low temperature, though at a much slower rate

than at 44 °C. On drying for approximately 20 days, the sample

dehydrated to approximately 70 percent of its starting weight.

On rewetting, the loss on dehydration appeared to be recovered in

approximately 8 days. Furthermore, on rewetting the sample

gained considerably more weight than was lost during drying, up

to 125 percent of its starting weight. This additional weight

has tentatively been attributed to adsorption of water by the

ettringite.

Weight changes of three trisubstituted phases, ettringite,

Fe-substituted ettringite, and C03 -substituted ettringite, were

measured during four cycles of drying and wetting at 40 °C

(figs. 11-13). The first two wet cycles utilized nitrogen gas

with a nominal water vapor pressure of 100 percent relative

humidity, as was used in the earlier experiments at 25 and 44 °C.

The remaining wet cycles utilized gas with a nominal water vapor

pressure of 95 percent relative humidity, to eliminate the

20
Figure 9. Weight changes during cycles of drying and wetting for

ettringite at 44°C

21
WEIGHT

(%)

Figure 10. Weight changes during cycles of drying and wet tin j for ettringite at 25 °C

22
WEIGHT

(%)

0 20 40 60 80

TIME (days)

Figure 11. Weight changes during cycles of drying and wetting

for ettringite at 40 °C

23
WEIGHT

(%)

Figure 12. Weight changes during cycles of drying and wetting

for Fe-substituted ettringtie at 40 °C

24
WEIGHT

(%)

Figure 13. Weight changes during cycles of drying and wetting

for (^-substituted ettringite at 40 °C

25
Table 6 . Enthalpy changes on dehydration of trisubstituted

phases relative to the amount of water lost.

Phase

[ Ca3 Al ( OH ) g

]

2

( S04

) 3

• 26H2 0

[ Ca 3 Fe ( OH

)

6 ] 2

( S04

) 3

• 26H2 0

[Ca 3 Cr (OH) 6 ] 2( S04

) 3

• 26H2 0

[ Ca3 Al ( OH

)

6 ] 2

( C03

) 3

• 26H2 0

[ Ca 3 Si ( OH

)

6 ] 2

( S04

) 2

( C03

) 2

• 24H2 0

Enthalpy Change

(kJ/g H20)

2.1

2.6

3.7

2.6

2.0

scatter in sample weights during the wet cycle, which wa

attributed to condensation in the sample tube. Due to this

reduction in humidity, the sample weights during the first two

wet cycles were somewhat higher than during the third and fourth

wet cycles.

The curves demonstrate reversibility of the dehydration reaction

for all three phases through the four cycles tested. During the

first drying cycle, the ettringite (fig. 11) decreased in weight

to approximately 70 percent of its starting weight in 2 to 3

days, and on rewetting required 2 to 3 days to rehydrate.

The Fe-substituted ettringite (fig. 12) dehydrated more slowly,

reaching approximately 70 percent of its starting weight in 4 to

5 days, and rehydrated in approximately 7 days on rewetting. The

C03 -substituted ettringite (fig. 13) dehydrated more rapidly,

reaching approximately 70 percent within 1 day, and rehydrated in

3 days.

The curves in figs. 11-13 suggest that both ettringite and

the CO^-substituted phase, but not the Fe-substituted phase, are

adsorbing water during rewetting cycles at 100 percent relative

humidity. Thus, on the first rewetting cycle, C0 3 -substituted

ettringite increased to 160 percent of its starting weight and

ettringite increased to 125 percent of its starting weight, while

Fe-substituted ettringite increased to 98 percent of its starting

weight.

The curves demonstrate reversibility of the dehydration reaction

for all three phases through the four cycles tested. Weights of

ettringite and the Fe-substituted phase in the dry cycles are

slightly higher for each subsequent cycle, an observation that

may be caused by a slight degradation during cycling. Finally,

the rates of dehydration and rehydration at 40 °C appear to be

most rapid for the C03 -substituted phase and least rapid for the

Fe-substituted phase. The order of ranking the samples by their

the dehydration and rehydration rates is the same as the order of

26
ranking the samples by their dehydration temperatures, reported

previously [ 11 ].

Some samples subjected to drying and wetting cycles have been

analyzed using XRD and FTIR (appendix E). Ettringite that

had been subjected to 2 cycles of drying and wetting at 44 °C

(fig. 9) was analyzed both after drying and after rewetting. The

dry ettringite was amorphous to X-rays, similar to the ettringite

dehydrated by heating at 110°C [11]. The dry ettringite was

slightly different than the original material in its IR pattern,

particularly in the broadening of one or more of the peaks in the

region of 3500 wavenumbers, tentatively assigned to OH stretching

of hydroxyl and water. When the dehydrated ettringite (fig. 9) was

rewetted and analyzed, both the XRD and the IR patterns were

similar to patterns of the original material.

4.2.3 Heat Capacity

Heat capacity values were measured for replicate samples of

two materials, in order to determine the precision of the

measurement (table 7). The coefficient of variation for measured

heat capacity was found to be approximately 10 percent for each

material. Analysis of data for all trisubstituted phases (table

8 ) using a t-test showed that heat capacity values for ettringite

and for Fe-, Cr-, and CO3 -substituted ettringite were not

significantly different. The average C

p

value for ettringite,

1.26 J/g/°K, is somewhat lower than the value of 1.55 J/g/°K

reported by Ederova and Satava [33].

The heat capacity values of the four trisubstituted phases,

reported previously [11], were approximately 1.26 J/g/°K. Heat

capacity values have also been measured for the three monosubstituted phases (table 9). These values ranged from 0.96 J/g/°K to

1.17 J/g/°K, generally lower than the values measured for the

trisubstituted phases. The C

p value for the sulfate phase was

1.17 J/g/°K, slightly higher than the value of 0.96 J/g/°K

reported by Ederova and Satava [33].

27
Table 7. Replicate heat capacity measurements for some trisubstituted phases.

Phase

[ Ca3 Al ( OH

)

6 ] 2

( S04

) 3

• 26H20 1.30

1.34

1.05

1.34

Average 1.26

Standard Deviation 0.14

[ Ca 3 Fe ( OH) g ] 2( S04

) 3

• 26H20 1.21

1.55

1.17

1.26

Average 1.30

Standard Deviation 0.17

Table 8 . Heat capacity data for trisubstituted phases.

Phase CD

(J/g7°K)

[ Ca3 A1 (OH) 6 ] 2

( SO4

) 3

• 2 6H20 1 . 3 ± 0 .

2

[ Ca 3 Fe ( OH) 6

]

2

( S04

) 3

• 26H20 1.3

[Ca 3 Cr(0H) 6 ] 2 (S0 4

) 3 -26H 20 1.3

[Ca 3 Al(0H) 6 ] 2 (C0 3

) 3 -26H 2 0 1.3

[ Ca 3 Si ( OH

)

6 ] 2

( S04 ) 2( C03

) 2

• 2 4H 2 0 1 .

3

Table 9. Heat capacity data for monosubstituted phases.

Phase CD

( J/g7°K)

[Ca2Al(0H) 6 ] 2 Cl 2 *6H2 0 0.96

[Ca 2 Al(0H) 6 ] 2 (N0 3

) 2 *6H2 0 1.00

[Ca 2Al(0H) 6 ] 2 S04 *6H2 0 1.17

28
5 . Solar Energy Storage

To evaluate the hydrated, inorganic salts for passive solar

energy storage, hydrated salts in the present study may be

compared to PCM's presently in use for passive solar energy

storage, e.g. Glauber's salt (Na 2 S04

• 10H2 O) and calcium chloride

hexahydrate ( CaCl 2

• 6H2 0) . Both materials utilize a melting

reaction for energy storage. Lane [1] reports that the melting

temperature of Glauber's salt is 32 °C, and its change in enthalpy

is 0.25 kJ/g; the melting temperature of calcium chloride

hexahydrate is 29 °C, and its enthalpy change is 0.19 kJ/g. The

enthalpy changes in the present investigation are between 0 . 4 and

0.8 kJ/g, twice the values of Glauber's salt and calcium chloride

hexahydrate. This comparison clearly indicates that the trisubstituted salts have potential as PCM's for passive energy

storage. Furthermore, within the general class of hydrated

inorganic salts included in the present study, the compositions

may be adjusted so as to vary the dehydration temperature or

enthalpy change as needed.

In the previous report [11], it was shown that the change in

enthalpy of ettringite on dehydration was considerably greater

than the change on rehydration as measured using a hydration

calorimeter. It now appears that this discrepancy was not due to

experimental difficulties, as was suggested in that report, but

rather to the major effect of the heat of vaporization of water.

The dehydration reaction in the DSC produces water in the vapor

phase. However, the rehydration reaction in a hydration calorimeter takes place between solid dehydrated ettringite and liquid

water. The enthalpy changes on dehydration in the DSC, when

expressed relative to the amount of water lost on dehydration,

are similar to the heat of vaporization of water. Therefore, the

enthalpy change with rehydration is expected to be considerably

less then the change on dehydration. Furthermore, it was shown

in the previous report that ettringite would dehydrate and

rehydrate while immersed in water with an enthalpy change of

approximately 0.02 kJ/g sample, which appears to be a reasonable

enthalpy change for this reaction under hydrothermal conditions.

A typical solid material for sensible heat storage is a rock

bed, often granite, with a C

p

of 0.80 J/g/°K [1]. Liquids have

appreciably higher specific neat values; for example, the C

p

of

water is 4.19 J/g/°K. The specific heat values determined for

the hydrated salts in the present study ranged from 1.0 to 1.3

J/g/°K. These values demonstrate that the hydrous inorganic

salts also have potential for sensible heat storage.

In order to assess fully the potential of the trisubstituted

salts as PCM's, it is necessary to demonstrate the reversibility

of the dehydration reaction. Experiments described in the

present report, though preliminary in nature, indicate that the

reaction is reversible, that ettringite and related phases

29
dehydrate and rehydrate during a number of successive cycles of

drying and wetting by controlling water vapor. The precise

kinetics of these reactions are important, and have been shown to

depend critically on the temperature of the experiment. Furthermore, the details of the dehydration process and its effects on

the crystal or molecular structure of the material will undoubtedly affect its reversibility. These issues will be explored

in much greater detail during the next phase of the project.

Other important criteria for PCM's were raised in the introduction. Although these criteria were not addressed directly in the

present study, they merit some discussion.

The first issue is that heat transfer be adequate and be similar

for phases above and below the reaction temperature. Thermal

conductivity of the hydrous inorganic salts in the present study

have yet to be explored. However, the chemical and structural

similarity of each phase whether fully hydrated or partially

dehydrated indicate that the thermal conductivity will be

similar.

A common problem with many PCM's is their containment in a material that is itself strong, durable, and compatible with the

PCM. Materials in this quaternary system are compatible with

cementitious systems and thus may be integrated into conventional

masonry or cast concrete units. This compatibility with concrete

makes these materials particularly attractive for thermal energy

storage.

Other criteria are that PCM's be inert, non-toxic, and non-flammable.

With the exception of the Cr-substituted phase, the materials in

the present study are expected to satisfy fully these criteria.

This expectation is based on widespread experience with concrete,

which contains ettringite and some of the related phases.

The changes in ettringite and ettringite-type phases during

cycles of wetting and drying have provided preliminary indication

that the materials may undergo desorption and adsorption, as well

as dehydration and rehydration. These sorption reactions raise

the possibility that the materials might have potential as

desiccants for use in solar-powered humidification systems. This

potential will also be explored during the next phase of the

project.

30
6 . Conclusions

Based on the experiments described in this report, the following

have been concluded:

1. A general synthesis method using a mixture of two

concentrated solutions allowed successful preparation of the trisubstituted phases with divalent

cations. The method did not allow preparation of a trisubstituted phase with a monovalent cation in

the channel position, or of a monosubstituted phase

with a divalent cation.

2. Morphologies of the synthesized phases vary with

chemical composition. Crystals are generally

hexagonal in outline, either short, prismatic

crystals or platey crystals.

3. The XRD patterns of four trisubstituted phases

could be indexed using a P31c structure, and of

thaumasite using a P 63 structure; the unit cell

parameters were shown to vary with chemical

composition.

4. The IR frequencies of bands assigned to hydroxyl

and water stretching for the trisubstituted phases

vary somewhat with composition. Compared to

ettringite, the bands are shifted to a slightly

lower frequency for Fe- and CO3 -substituted phases

and for thaumasite. These shifts are associated

with higher changes in enthalpy with dehydration

for Fe- and CO3 -substituted phases, but not for

thaumasite.

5. The amount of water lost, the onset temperature,

and the change in enthalpy associated with the

low-temperature dehydration of the trisubstituted

phases, vary with composition. Ettringite loses

approximately 19 mols of water, beginning at 30 °C,

with an enthalpy change of 0.6 kJ/g. The Fe- and

C03 -substituted phases lose more water, starting at

approximately 6 °C, with an enthalpy change of

approximately 0.8 kJ/g. The Cr-substituted phase

loses slightly less water, starting at a slightly lower

temperature, with a similar enthalpy change to the

ettringite. Thaumasite loses considerably less water,

beginning at a similar temperature, but with a lower

enthalpy change of 0.3 kJ/g.

6 . Considering the changes in enthalpy relative to the

amount of water lost, the values for ettringite and

thaumasite are slightly less than ‘the heat of

31
vaporization of water , 2.3 kJ/g, while values for

the other phases are greater.

7. Dehydration data indicate that the trisubstituted phases have potential as materials for latent

heat storage, though additional studies are

necessary to demonstrate this potential.

8. Heat capacity values for the trisubstituted phases

are approximately 1.3 J/g/°K, and for the monosubstituted phases range from 1.0 to 1.2 J/g/°K.

9. Weight changes measured during cycles of drying and

wetting give preliminary indication that the

dehydration is reversible, further demonstrating

potential of the materials for latent heat storage,

though additional studies are necessary to demonstrate reversibility over a large number of cycles.

« 32
7 . Acknowledgements

The project was funded by U.S. Department of Energy, Office

of Solar Heat Technologies, Passive and Hybrid Solar Energy

Division. We wish to acknowledge the contributions of K. Galuk,

E. Prosen and N. Waters, for their considerable technical

assistance, and of H.F.W. Taylor for many helpful discussions

concerning ettringite structure and dehydration.

33
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ettringite. Cement and Concrete Research, £(2)6., 225-234.

24. S.J. Ahmed, L.S. Dent Glasser, and H.F.W. Taylor. 1968.

Crystal structures and reactions of C 4 AH - 2 ana derived basic

salts. In : Proceedings of the Fifth International Symposium

35
on the Chemistry of Cement, Part II, pp. 118-127. Tokyo:

Cement Association of Japan.

25. H.E. Schwiete and U. Ludwig. 1968. Crystal structures and

properties of cement hydration products (hydrated calcium

aluminates and ferrites). Tn: Proceedings of the Fifth

International Symposium on the Chemistry of Cement, Part II,

pp. 37-67. Tokyo: Cement Association of Japan.

26. F.E. Jones. 1938. The calcium aluminate complex salts.

In : Proceedings of the Symposium on the Chemistry of

Cements, pp. 231-245. Stockholm: Ingeniorsvetenskapsakademien.

27. Brown et al. 1985. The hydration of tricalcium aluminate

and tetracalcium aluminoferrite in the presence of calcium

sulfate. Report of RILEM Committee 68MMH. Submitted for

publication in Materials and Structures.

28. R. A. Edge and H.F.W. Taylor. 1971. Crystal structure of

thaumasite, [ Ca 3 Si ( OH) g •

I

2 H2 O] ( SO4 ) ( CO 3

) . Acta Crystallographica, B27 , 594-601.

29. H. Effenberger, A. Kirfel, G. Will, and E. Zobetz. 1983. A

further refinement of the crystal structure of thaumasite,

Ca3 Si (OH) £C03 SC>4 • 12^0. Neues Jahrbuch fur

Mineralogie, Monatschafte, 2, 60-68.

30. Ya. I. Ryskin. 1974. The vibrations of protons in minerals:

hydroxyl, water and ammonium. In ; The Infrared Spectra of

Minerals, V.C. Farmer, ed. , pp. 137-181. London: Mineralogical

Society.

31. H.F.W. Taylor. 1984. Studies on the chemistry and microstructure of cement pastes. In : The Chemical and Chemically-Related Properties of Cement, F.P. Glasser, ed.

,

pp. 65-82. Shelton, Stoke-on-Trent: The British Ceramic

Society.

32. R.E. Walpole and R.H. Myers. 1972. Probability and

Statistics for Engineers and Scientists. 506 pages. New

York: Macmillan Company.

33. J. Ederova and V. Satava. 1979. Heat capacities of CiAH^,

C 4AS-H12 and CgAS- 3 H32 « Thermochimica Acta, 31 , 126-128.

36
Appendix A. XRD patterns for thaumasite and 3 monosubstituted

phases.
Figure A-l. XRD pattern for thaumasite.

AFigure A-2. XRD pattern for the Cl-monosubstituted phase.

A-3
Figure A-3. XRD pattern for the S04 -monosubsti tuted phase.

A-4
Figure A-4. XRD pattern for the NC^-monosubstituted phase.

Appendix B. IR spectra for trisubstituted phases.
TflA.NSM

I

TTRNCE

c

Figure B-l. IR spectrum for ettringite.

B-2
c

WAVENUMBERS

Figure B-2. IR spectrum for Fe-substituted ettringite.

B-3
RANSMI

TTANCF

o

IR spectrum for Cr-substituted ettringite. Figure B-3.

B-4
TFIANSM

T1

F

RNCE

c

L

_ 390 tiOO 2900 3^00 2900 ETgD 190C

WAVENUMBERS

TToC 900

Figure B-4. IR spectrum for CC> 3 -substituted ettringite.

B-5
TRANSMITTANCE

a

Figure B-5. IR spectrum for thaumasite.

B-6
Appendix C. TGA patterns (fast scans) for trisubstituted

phases.
%

WEIGHT

NAT FILE* AA2LJL TG TEMPERATURE CC) TG

Figure C-l. T*GA pattern (10° per min) for ettringite.

C-2
X

WEIGHT

Figure C-2. TGA pattern (10° per min) for Fe-substituted

ettringite.

C-3
mm -m +. —

4lm ulu zmm

GALUK FIL&CA22. TS

34. a 44. a 54.a 54. a 74. a

TEMPERATURE CO

iM.a

Figure C-3. TGA pattern (10° per min) for Cr-substituted

ettringite.

C-4
NAT FILE* AE3LR. TG TEMPERATURE CC) TG

Figure C-4. TGA pattern (10° per min) for CC^-substituted

ettringite.

05
%

WEIGHT

T6

11

WT. 1.4234 mg SCAN RATEi 10.00 d.g/mln

isa.ee

t

i

I

22. £2 -m 1 . —*-• 40.9 140.0 240.0 340.0 440.00 540.0 840.0 740.0

AWT FILEi 76. 7G TEMPERATURE CO

84A.S ft40.o TG

Figure C-5. TGA pattern (10° per min) for thaumasite.

C-6
Appendix D. TGA and DSC patterns for thaumasite.
WEIGHT

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00.ee -—* ,

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GALL* riL£* -52:. TG "EKPERAT’JRE CC)

Figure D-l. TGA pattern (0.6° per min) for thaumasite.

D-2
MCAL/SEC

VT« 0. 67 mq

SCAN RATE* I0. 63 d«g/nln

AO

q

£

1 PEAK FROM. 306. 01

TO. 352.65

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UK FIL £> 75?.?. TEMPERATURE CK)

Figure D-2 DSC pattern (0.63° per min) for thaumasite

Appendix E. XRD and FTIR patterns for ettringite that has been

subjected to drying and wetting cycles at 25 °C.
1000r

900

800

700

600 * 500

400

300

200

100 «*•

1000

900

800

700

600

500

400

300

200

180

40. 00

8.00 16.00 24.00 32.00 40.00

48.00

^^****«***V<«M 'M *i ><*>* T 1 56.00 64.00 72.00 80.00

Figure E-l. XRD pattern for ettringite after repeated drying and

wetting cycles at 25° , sampled after the last drying

cycle (see fig. 10).

E-2
nANSM

I

TTANCE

o

Figure E-2. IR spectrum for ettringite after repeated drying and

wetting cycles at 25° , sampled after the last drying

cycle (see fig. 10).

E-3
8.00 16.00 24.00 32.00 40.00

Figure E-3. XRD pattern for ettringite after repeated drying and

wetting cycles at 25°, sampled after the last

wetting cycle (see fig. 10).

E-4
TRANSMITTANCE

c

WPVENUMPFRS

Figure E-4. IR spectrum for ettringite after repeated drying and

wetting cycles at 25°

,

sampled after the last

wetting cycle (see fig. 10).

E-5
NBS-114A (rev. 2 «ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET ( See in struction s)

, PUBLICATION OR

REPORT NO.

TBSIR-86/ 3325

2. Performing Organ. Report NoJ 3. Publication Date

4. TITLE AND SUBTITLE

Inorganic Compounds for Passive Solar Energy Storage - Solid-State Dehydration

Materials and High Specific Heat Materials

5. AUTHOR(S)

L. Struble and P. Brown

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 December 1985

Progress Report

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

National Bureau of Standards and

U.S. Department of Energy, Office of Solar Heat Technologies

Passive and Hybrid Solar Energy Division

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

J 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. I f document includes a significant

bi bliography or literature survey, mention it here)

Two classes of hydrated inorganic salts have been studied to assess their potential

as materials for passive solar energy storage. The materials are part of the

quaternary system CaO-Al 202“SO_-H20 and related chemical systems, and the two classes

are typified by ettringite, a trisubstituted salt, and Friedel's salt, a monosubstituted salt. The trisubstituted salts were studied for their possible application

in latent heat storage, utilizing a low-temperature dehydration reaction, and both

classes were studies for their application in sensible heat storage. In order to assess their potential for energy storage, the salts have been synthesized, character:

by several analytical techniques, and thermal properties measured. The dehydration da

of the trisubstituted salts vary somewhat with chemical composition, with the temper it of the onset of dehydration ranging from 6°C to 33°C, and enthalpy changes on dehvdrat

ranging from 60 to 200 cal/g. Heat capacity is less variable with composition; values

for the trisubstituted phases are 30 cal/g/°C and for the monosubstituted phases betve

0.23 and 0.28 cal/g/°C. Preliminary experiments indicate that the dehydration is

reversible, and suggest that the materials might have additional potential as solar

desiccant materials. These thermal data demonstrate that the trisubstituted salts

ec

:areon

»n

have potential as latent heat storage materials, and that both classes of salts have

potential as sensible heat storage materials.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key Mords bv colons dehydration; enthalpy change; ettringite; Friedel's salt; latent heat storage;

passive solar energy storage; sensible heat storage; solar energy storage;

specific heat capacity

13. AVAILABILITY 14. NO. OF

PRINTED PACES

!

i Unlimited

I

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