BUILDING SCIENCE SERIES 6

Some Properties

of the Calcium

Aluminoferrite

Hydrates

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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, SecreUiry

NATIONAL BUREAU OF STANDARDS . A. V. Astin, Director

Some Properties of the Calcium

Aluminoferrite Hydrates

Elmer T. Carlson

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C.

Building Science Series 6

' • »

Issued June 1, 1966

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 20 cents
Contents

Page

1. Introduction 1

2. Materials and procedures 2

3. Experimental results and discussion 3

3.1. General observations 3

3.2. Effect of temperature 3

3.3. X-ray diffraction patterns 4

a. Hexagonal series 4

b. Isometric series (Hydrogarnets) 6

3.4. Thermal dissociation 7

3.5. Reactions of the hydration products with calcium sulfate solutions. _ 9

4. General discussion 10

5. Summary 11

6. References 11

II
Some Properties of the Calcium Aluminoferrite Hydrates

Elmer T. Carlson

Calcium aluminoferrite hydrates in two series, 4CaO • fAl203,Fe203). n H2O (hexagonal

plates) and 3CaO- (AloOj.Fe.Oa) -6 H2O (isometric), were prepared from the anhydrous aluminoferrites by hydration in the presence of Ca(0H)2. The hexagonal phase was stable below 15 "C, the isometric above 35 °C, in contact with solution. The end member, 2 CaO FciO^, did not produce an isometric hydrate, however. X-ray diffraction patterns of the hexagonal series in the

19 H2O stage of hydration were indistinguishable from patterns of 4 CaO • AI2O;, • 19 H2O.

After drying to the 13-hydrate stage, slight differences in the patterns were observed. The iso- metric series shows a definite shift in x-ray pattern with change in Fe20,i/Al203 ratio, the unitcell edge increasing from 12.573 for 3 CaO AhOs-G H2O to 12.716 for the member having

Fe203/Al203 close to 5. DTA traces show endotherms near 100 and 200 °C corresponding to dehydration stages of the hexagonal series and one near 300 °C representing the first dehydration stage of the isometric phase. The hexagonal hydrates are converted to the isometric during

the course of the DTA run. The peak attributed to the isometric phase shifts from 323 to 290 °C

going from the pure aluminate to the member richest in Fe203. All the aluminoferrite hydrates,

both hexagonal and isometric, reacted with CaS04 solution to give products of the monosulfate and trisulfate (ettringite) types, the latter predominating at later stages. The rate of reaction decreased with increasing Fe203 content. Key Words: Calcium aluminoferrite hydrates; calcium sulfate reaction with calcium

aluminoferrite hydrates; DTA of calcium aluminoferrite hydrates; x-ray patterns of calcium aluminoferrite hydrates.

1. Introduction

In the manufacture of portland cement clinker

of normal composition, the iron which is always

present in the raw materials enters into a ternary

composition with calcium oxide and alumina to form a reddish-brown crystalline calcium aluminoferrite. Although this phase is commonly re- ferred to simply as tetracalcium aluminoferrite,

4CaO • Al203 Fe203 (frequently abbreviated to C4AF1) the composition is known to be variable.

The existence of a solid-solution series between

the known C2F and a hypothetical C2A, extending

as far as C6A2F, has been shown by previous in- vestigators However, the range of composition to be expected in cement clinker is considerably narrower. A review of research in this field up to 1964 has been given by Welch [2]. In a more

recent paper, Majumdar [3] places the limit at 70 mole-percent C2A.

The calcium aluminoferrite phase resembles the

other major constituents of cement clinker in that

it reacts with water to form crystalline hydrated

compounds and thereby sets to a hard polycrystalline mass. In a previous paper [4] it was shown that

the rate of solution and hydration is a function of the composition (ratio of Fe203 to AI2O3, or F/A)

and that the composition of the hydrated phases

also varies with that of the anhydrous material.

' The abbreviated formulas frequently adopted by cement chemists will be used in this paper when there is no danger of ambiguity, C = CaO;

A. = AI2O3; F = FezOs; H = H2O.

Figures in brackets indicate the literature references at the end of this paper.

In the present study the composition and properties of the hydration products are explored in greater detail. Research on the preparation of the hydrated

calcium ferrites and aluminoferrites has, in general, followed the same lines that have been found

satisfactory for preparation of the hydrated cal- cium aluminates. The most important difference

is that the method based on precipitation from

clear supersaturated solutions, which is of great

utility in making the aluminates, is less satisfactory for the aluminoferrites and cannot be used

at all for the ferrites because of their low solubility.

It has been shown [1,5,6,7] that there is a hydrated

tetracalcium ferrite, crystallizing in hexagonal

plates, probably isostructural with tetracalcium

aluminate hydrate; also that there is a solid solu- tion series between the two. There is also a hydrogarnet solid solution series extending from C3AH6

almost to CsFHe, but in this case the existence of the ferrite end mem.ber is in doubt. It has not been

established that there are iron-bearing analogs of the known aluminates C2AH8 and CAHm. No

evidence of such compounds was observed in the

present investigation.

The purpose of this study is twofold: (1) to re- late certain identifying characteristics of the

hydrated compounds to their composition, and

(2) to ascertain the effect of composition on cer- tain properties of the materials which may be of significance because of their effect on the behavior

of cement.
2. Materials and Procedures

The calcium aluminoferrite hydrates were pre- pared by reaction of a series of powdered anhydrous calcium aluminoferrites with water and cal- cium hydroxide. Pertinent composition data for the anhydrous materials are given in table 1. Additional data and details of preparation have been

Table 1. Chemical composition of anhydrous aluminoferrite preparations

Designation CaO AI2O., FejOs

Molar ratio FezOs AhOa + FeaOa % % %

Fss-1 48.2 28.8 23.3 0.341

Fss-2 47.1 24.4 28.5 .427 Fss-3 46.3 21.0 32.8 .500 Fss-4 45.2 17.3 37.5 .580 Fss-5 44.3 13.5 42.1 .666 Fss-6 42.6 6.4 51.0 .837 C2F-I 41.1 0.0 59.0 1.000

reported previously [4]. In most cases, CaO or Ca(0H)2 was added either in excess or in the

amount required by the stoichiometry of the reaction. As reported previously [4], hydration in the

absence of added lime is accompanied by hydrolysis, and the hydrates are more basic than the an- hydrous materials, It was also observed that the

crystalline products generally contained less iron

than did the anhydrous materials and that the

rest of the iron separated as an apparently

amorphous form of Fe203. It was anticipated that

hydration in the presence of lime would result in crystalline products having the same ratio of iron

to aluminum as present in the anhydrous materials. Results compatible with this expectation

recently have been reported by Schwiete and

Iwai [7].

_ The hydration was accomplished in several

ways, none of which was entirely satisfactory in

all respects. The experiments were not conducted

in a glove box; however, contamination with

atmospheric carbon dioxide was minimized by

such precautions as sweeping out containers before use with air previously passed through a COoabsorbent, filtration in a closed system, and rapid

handling when containers were opened for any

reason. The x-ray patterns afforded a check on the

extent of exclusion of CO2, since the strong (001)

line of (S-tetracalcium aluminate hydrate could be

distinguished from that of the a-hydrate (currently believed to contain CO2) and from the cor- responding line of the monocarbonate. In many

cases there was no evidence of either of the latter compounds; nevertheless, it must be assumed that a slight amount of carbonation did occur in every

experiment.

The first method consisted of adding a weighed

amount of the anhydrous powder (about 2 g), plus

enough Ca(0H)2 to convert it all to tetracalcium

aluminoferrite hydrate, C4(A,F)H„, to 100 ml of saturated Ca(0H)2 solution in a polyethylene

bottle, and storing at 1 °C with occasional shaking. The low temperature was chosen to insure

formation of the hydrates of the hexagonal type.

The reaction rate varied with the composition,

becoming slower as the Fe203 content increased.

The sample highest in AI2O3 was completely con- verted to hydrate in a few months, except for a few consolidated lumps of anhydrous material.

Samples high in Fe203 were not completely hydrated after one year. The reaction products were

white and very finely divided. A sample of C3A

was put through the same procedure. This method

will be referred to in the following as method A. The next method used was similar except that

the reaction was accelerated by working at room

temperature, by frequent or continuous shaking,

and by adding a charge of plastic beads or small

steel balls. Storage at 45 °C was another variation

tried. These modifications, which will be referred

to collectively hereafter as method B, proved rela- tively unsuccessful, especially with the starting

materials high in Fe203. The steel balls were very

effective in speeding up the reaction, but the

products quickly became contaminated with ex- tremely minute particles of metallic iron resulting

from abrasion. The plastic beads were relatively

ineffective. Storage at 45 °C soon resulted in hydrolysis and liberation of Fe203, except for the

three members of the series lowest in this component. Despite these limitations, a few products

satisfactory for further study were obtained by

this general method.

Method C, which was in many respects the most

satisfactory of those used, consisted of grinding

the sample, mixed with .a slight excess of Ca(0H)2

and about 25 ml of water, with a motor-driven

mortar and pestle. To exclude CO2, a close fitting tent of sheet plastic was placed over the apparatus

and a stream of air previously bubbled through a

lime suspension was introduced directly above the

mortar. The operation was started at room temperature, but the heat developed by the motor

resulted in a rise to as high as 40 °C during the

grinding. Hydration of the materials highest in alumina was practically complete in 2 hours,

while the C2F was still incompletely hydrated after

7 hours. It must be assumed that some, solid material was abraded from the mortar and pestle.

However, only approximately 0.1 percent of silica was found gravimetrically in two of the hydrated

products selected for analysis.

The hydrated products were filtered through

fritted glass crucibles and dried over saturated

NH4CI solution (approximately 79% rh) in a

partially evacuated desiccator.

X-ray diffraction patterns of the powdered

preparations were made on a diffractometer using

filtered copper K„ radiation. Patterns of the platycrystalline materials show strong orientation

effects, since no special precautions were taken to avoid packing when mounting specimens. The

2
hydrogarnets, forming equidimensional grains,

present no problem in this respect.

The apparatus used for differential thermal

analysis (E)TA) consisted of a cylindrical block of aluminum about 4.5 cm in diameter and 5.5 cm in height, heated by a small cartridge heater located

axially. Two wells were provided for the sample

and the reference couple, respectively. The sample,

weighing about 50 mg, was placed in a glass tube

which was then placed in the block and the

thermocouple junction was set just below the sur- face of the sample. The upper operating limit was

about 400 °C. The heating rate varied somewhat,

starting at about 15 °C/min and decreasing to about 10 °C/min at the end of the run.

3. Experimental Results and Discussion

3.1. General Observations

A study of the relation between the composition

of the hydrates and their properties requires a knowledge of the composition. This cannot be

determined directly unless it is known that only a

single phase is present in the reaction product,

hence it is necessary to rely on indirect evidence.

We may consider a pair of idealized reactions,

selecting for example the anhydrous preparation

Fss-3 (table 1) having the composition C4AF:

4CaO-Al203 Fe203 + 4Ca(OH)2 + (2r?. - 4)H20 ^ 2(4CaO 0.5Al2O3 0.5Fe2O3 TCH,O) (1) 4CaO-Al203 Fe203 + 2Ca(OH)2 + lOHoO

->2(3CaO0.5Al2O30.5Fe2O3-6H2O). (2) Similar equations could be written for the other

preparations, differing only in the relative amounts

of AI2O3 and FciOs. The anhydrous compound is deep brown in color, varying in shade with the

proportion of Fe203 present. The reaction products

are white, or perhaps cream-colored at the highiron end of the series. Hence the disappearance of the brown color is a fair index of completeness of the reaction.

On the basis of data in the literature [1,2,5] and

the compatibility of results reported herein with

such data, the ratio of CaO to Al203+Fe203 in the product, namely, 4 to 1 in the first equation

and 3 to 1 in the second, may be accepted as correct. The number of molecules of water is, for present purposes, of little concern. The only uncertainty is in the ratio of AI2O3 to Fe203 in the

products. If the hydration reaction were to result

in a product richer in alumina than the parent

material, part of the Fe203 would have to appear

in some other form. Experience has shown [4] that

under similar conditions it readily separates as amorphous Fe203 (or possibly a hydrate) which is easily recognizable by its brick-red color. As noted

above, this type of coloration was observed in some

experiments, especially at 45 °C and with high

Fe203 content. It was not observed, however, in the hydrates prepared at 1 °C. The converse possibility, that of an increase in Fe203/Al203 ratio on

hydration with some liberation of AI2O3, would be

harder to detect, since either hydrated AI2O3 or any possible calcium aluminates formed would be

white. Inasmuch as the splitting off of part of the

Fe203 was observed to occur rather readily, it was

tentatively assumed that the liberation of AI2O3

would be unlikely to occur. As will be shown later,

this assumption is not necessarily valid under all circumstances.

3.2. Effect of Temperature

The hydrates produced at 1 °C were all of the

hexagonal -plate type. At room temperature the

products were mixtures of the plate crystals and

those of the hydrogarnet type, except in the case

of C>F which never produced any hydrogarnet

type. Hydration at 45 °C gave an increased pro- portion of hydrogarnet (again with the exception

of C2F). As previously noted, the preparations

higher in iron liberated Fe203 at this temperature.

Excess CaO was present as crystals of Ca(0H)2.

By changing the temperature of storage of the

aqueous suspensions, it was possible to convert

the hexagonal hydrates to the isometric, or vice

versa. A moderately narrow range of coexistence of the two phases was found for the calcium

aluminate hydrates and for the preparations low

in iron. The conversion became more sluggish and

less complete as the iron content increased. The

microscope proved very useful for these experiments. For example, in a suspension of calcium

aluminate hydrates containing both hexagonal

(C4AH19) and isometric (CsAHe) crystals, the

faces of the latter were observed to be clear and

sharp. After the suspension was kept 1 day at 10 °C the crystals were altered on the surface,

evidently through formation of plates of C4AH19.

After 2 additional days at 30 °C the C3AH6 crystals

again were clean. Three more days at 15 °C pro- duced the surface alteration again, 1 day at 30 °C

again removed it, after which 14 days at 20 °C

caused no change. On the basis of this evidence,

confirmed by x-ray diffraction analysis, the equilibrium temperature for the reaction C4A

hyd. ^=5 C3A hyd.+CH is placed between 15 and

30 °C. Seligmann and Greening [8], using an in- genious x-ray technique, have demonstrated the

reversibility of this reaction on varying the

temperature.

Conversion temperature ranges for the Fss

series, determined by the cycling procedure

described above, are given in table 2. The hydrates

used for these experiments were prepared by

method C, except for No. 1 which was prepared

by method A (see preceding section).

3
Table 2. Approximate temperatures of reversible reaction between hexagonal hydrates and hydrogarnets in

presence of saturated Ca(0H)2

F

No. Crystal form Reaction" Reaction'' A+F of orig. prepn. hg—>hex' hex—>hg'^

°C °C

^ Q 15 30

2 0.194 15 35

3 .341 hg 18 30

4 .427 hg+hex 18 25

5 .500 hg 20 30

6 .580 hg 18 35

7 .666 hg+hex 15 35

8 .837 hg -j-hex 15 35

9 1.000 hex

" Highest temperature at which this reaction was observed.

>> Lowest temperature at which this reaction was observed. hg = hydrogarnet. hex = hexagonal plate phase.

3.3. X-ray Diffraction Patterns

a. Hexagonal Series

Portions of the first series of hydration products

(prepared by prolonged storage at 1 °C) were fil- tered off and mounted in the x-ray diffractometer

while still moist. High humidity was maintained

in the chamber surrounding the mount. Under

these conditions, patterns similar to that of

the fully hydrated tetracalcium aluminate

(4CaO-Al203 l9H20 [9]) were obtained. However, the basal plane reflections were greatly magnified in relation to the intensities of the other re- flections because of preferred orientation of the

thin, platy crystals as mounted. As many as eight

orders of the strongest reflection were recorded,

while other expected lines were either weak or

entirely absent. Thus it was not possible to establish a relation between the Fe203/Al203 ratio and

the spacings or intensities of the weaker x-ray

lines. The d-spacing for the strong (001) line appeared to shift linearly from 10.72 to 10.59 A,

going from the C4A to the C4F hydrate. However,

this difference is so slight as to be within the error

of measurement at this low angle. As a check ex- periment to cancel out machine errors, a composite

of the C4A hydrate preparation with the hydrated

C4AF (preparation Fss-3) was prepared by mixing equal amounts of the suspensions and filtering.

The x-ray diffraction pattern of this mixture

showed a sharp (001) line corresponding to 10.66 A, with an equally sharp (002) line at 5.34 A. The absence of any hint of separation here indi- cates that the basal spacing probably is unchanged

as Fe203 replaces AI2O3.

All the preparations just described also showed

x-ray diffraction lines of a:-C4AHi3 which, according to recent research [8,10], may contain CO2.

The relative amount of this phase increased as

replicate x-ray runs were made on the same speci- men. A weak line corresponding to the basal spac- ing of the monocarbonate (3CaO-Al203-CaC03-

IIH2O) also was noted, in a few cases, after fur- ther exposure. These observations are compatible

with the explanation that the a-hydrate was

formed progressively by the action of atmospheric

CO2 on the moist C4(A,F)Hi9 phase. It is of in- terest to note that these carbonated phases (possibly also containing Fe203 in substitution for

AI2O3) form in the presence of liquid water,

whereas the 1S-C4AH13 type (see below) apparently

is produced only on drying.

For a study of the hydrates of the /3-C4AH13

type, samples equivalent to those described above

were filtered off and dried in a desiccator over

saturated ammonium chloride solution (79% rh,

approx.). X-ray diffraction patterns were made,

following the usual procedure. As in the case of the fully hydrated materials, the patterns were dominated by the (001) and (002) reflections. An

internal standard was introduced in order to make

a closer measurement of the position of these lines. Small portions of the dried hydrates were ground

with Ca(0H)2, which furnishes a strong diffrac- tion line between the (001) and (002) lines of the

hydrates. For comparative purposes, the (002)

line was chosen as affording better resolution than

the (001). The appearance of the peaks, corrected

as to position with reference to the standard, is shown in figure 1.

Fss-I -

Fs8-5 —

F8S-6 —

C2F-I -

22 23 24 25

ANGLE 2 6, DEGREES

Figure 1. Appearance of x-ray diffraction peaks corresponding to {002) reflection in series of C4(A,F,)Hi3 preparations.

4
The outstanding feature here is the separation

into a doublet in two of the intermediate members

of the series. Reading downward, the peak at first appears constant in position. With the third member a second peak is observed, which becomes

stronger in the fourth member. In the last three,

only the latter peak occurs and the position

appears to change slightly with composition. The

MOLAR RATIO ^—^ AljOj + Fez O3 Figure 2. Interplanar spacing, CI002, of C4(A,F)Hi3 preparations in relation to composition; calculated from peaks

shown in figure 1

.

relation between the position of the.se lines and

composition may be seen more clearly in figure 2. The significance of the doublet is not clear. There

may be two poly-types of the same composition,

or it may be that the 19-hydrate which is formed

initially breaks down on drying, forming two 13- hydrates differing in Fe20,-i/Al203 ratio. No ex- planation is offered for the small doublet in the

C2F peak.

The d-spacings corresponding to the x-ray dif- fraction lines for this series are given in table 3, last 7 columns. Column 3 gives corresponding data

for a preparation designed to contain half as much

Fe203 as Fss-1 and hydrated by wet grinding with a mortar and pestle as described previously.

Column 2 gives data for a sample of iS-dAHi.,

prepared in an earlier investigation, while the first column gives, for reference, the corresponding

spacings reported by Jones and Roberts [9]. Aside

from the basal spacings discussed above, only two

lines (not indexed) show a consistent trend, with

varying composition, of sufficient magnitude to appear significant. Figure 3 is a plot of the spacings for these lines as functions jof the composition.

They start at 2.87 and 2.37 A, respectively, for C4AH13, and increase with increasing Fe20i3 content. The second of these lines is of low intensity.

Table 3. Interplanar spacings for a series of calcium aluminoferrite hydrates of the |8-C4AHi3 type, computed from x-ray powder

diffraction patterns

^-C4AHi3» 18-C4AH13

Ci(A,F)Hi3 series; F A+F

CiFHi.

0 194 0 341 0 427 0 500 0 580 0 666 0 837

7.9 8 0 8 05 7 96 7 98 7 88 7 88 7 84 7 81 7.80 4.95 4 90 4 95 4 92 4 87 4.54 4 39 4 467 4 52 3.95 3 96 3 95 3 953 3 950 3 953 3 953

3 80 3 921 3 921 3 921 3 910 3.904

3.62 3 62 3 62 3 639 3 642 3 63 3 642 3 64 3.31 3 292 3 393 2.88 2 873 2 870 2 902 2 899 2 902 2 908 2 919 2 919 2.936

2.86 2 871 2 870 2 871 2 868 2.88 2 827 2 820 2 818 2 839 2 758 2.771 2.69 2 687 2 687 2 715 2 685 2 714 2 712 2 725

2.63 2 638 2 638 2 636 2 630 2 621 2 620 2 628 2.627

2.49 2 512 2 512 2 510 2 521 2 518 2 520 2 514 2.517 2.47 2 475 2 474 2 479 2 478 2 482 2 491 2 485 2 489 2.45 2 448 2 458 2 457 2 452 2 453 2 453 2 464 2.448

2.38 2 366 2 379 2 395 2 402 2 406 2 405 2 415

2.31 2 304 2 304 2 317 2 326 2 321 2 325 2 312 2.263 2.24 2 235 2 233 2 244 2 242 2 248 2 249 2 254 2 254 2.246

2.21 2 206 2 216 2 211 2 202 2 198 2 194 2.216 2.17 2 174 2 173 2 177 2 163 2 166 2 166 2.176 2.05 2 055 2 054 2 057 2 053 2 055 2 052 2 051 2.063

1.98 1 979 1 987 1 977 1 980 1 992 1 988 1 983 1 975 1.981 1.94 1 941 1 958 1 955 1 970 1 955 1.92 1 927 1 929 1 933 1 939 1 926 1.926 1.86 1 870 1 876 1 873 1 875 1.852

1.81 1 816 1 818 1 818 1 818 1 818

1.80 1 790 1 798 1 796

1.75 1 757 1 751

1 735

1 691 1 686 1 687 1 690 1 687

1 674 1 675 1 673 1 675

1.66 1 660 1 661 1 664 1 658 1 657

1.65

1.64

1.57 1 .584 1 582 1 589 1 579 1 587 1.55 1 563

" From Taylor [2], [d spacing in A]

5
Figure 3. Variation in d-spacings for two x-ray diffraction

lines with composition of C4(A,F)H,3 preparations.

b. Isometric Series (Hydrogarnets)

X-ray diffraction patterns of the isometric series were obtained on preparations hydrated at temperatures slightly above room temperature

(method C—see sec. 2). Samples were filtered off and dried over saturated NH4CI solution under

reduced pressure. The typical hydrogarnet pattern was observed in all cases except with the hydrated

C2F preparation, which exhibited only the hexagonal pattern. Lines due to Ca(0H)2 were also

present and served as a standard for correction

of the observed diffraction angles. The size of the

cubic unit cell was determined from the calculated

d-spacings.

Figure 4 shows the unit cell edge as a function

of composition. In this plot, the open circles repre- sent a series hydrated by wet grinding with mortar and pestle. The point at the F/A+F ratio near

0.2 represents a mixture of C3A and Fss-1 ground

and hydrated together, while the others are for the regular series (table 1). During each hydration

experiment the temperature rose from about 25 °C

to 35-40 °C because of the heat developed by the

motor and the insulating effect of the plastic tent. At this temperature the hydration product was a mixture of the isometric and hexagonal phases

(with Ca(0H)2), the former being strongly pre- dominant in the high-AloOs end of the series. The

three closed circles in figure 4 represent part of an

earlier series hydrated in the same way, but subsequently kept at 45 °C several days to insure complete conversion to the isometric hydrate. The rest of this earlier series was discarded because of evi- dence of separation of Fe203.

The two triangles were derived from another

series hydrated by shaking with plastic beads in plastic bottles followed by several days of storage

at 45 °C (method B). Here again, the rest of the

samples in the series turned red and were dis- carded. The plotted point for CsAHg, at 12.573 A, was taken from a previous report [4]. It is apparent

that there is an approximately linear relationship

between unit cell size and composition extending

from CsAHe at least to a point where five-sixths

of the AI2O3 has been replaced by Fe203.

X-ray patterns of several other preparations

gave values for ao that departed from this linear

relationship by amounts greater than could be

attributed to uncertainties of measurement. The

preparations yielding the anomalous results had

all gone through the transition, hexagonal ^

isometric, at least partially, one or more times.

The extent of the departures from the line may be

seen in figure 5, in which the straight line is copied

from figxire 4. The open circles and semi-circles in figure 5 represent a series prepared at 1 °C, con- verted to the isometric phase at 45°C, to the hexagonal again at 1 °C, and finally to the isometric at 35 °C. The filled circle and semicircles represent

two products, prepared at room temperature,

which contained both the hexagonal and the iso- metric phases. They were subsequently kept

several weeks at 1 °C, and finally converted to the

isometric form at 35 °C. The four points at the left (semicircles) represent two doublet x-ray pattern

peaks; a separate value of ao was calculated for each set. In order to verify the assumption that these departures from the norm were related in some way

to phase transitions, a sample of Fss-2 was hydrated by wet grinding with mortar and pestle, in the presence of a slight excess of lime, and then put

through a schedule of cooling and warming. Portions were filtered off for x-ray diffraction analysis at appropriate times. The schedule and results are given in table 4, and the calculated values of ao are shown in figure 5 (triangles).

It is evident that the unit cell of the hydrogarnet

phase increases slightly in size with changes in temperature across the transition zone between the

hydrogarnet and hexagonal phases. Also, although

in this case the evidence is scant, the change seems

to occur when the temperature is lowered, and

conversion to the hexagonal phase is favored.

Assuming that the change in dimensions results

solely from a change in composition, it is apparent

that the hydrogarnet phase becomes richer in Fe203 during the temperature cycling. As a consequence, it must be inferred that part of the AI2O3

is liberated. It is assumed to be present as amorphous hydrated alumina, since there is, in some cases, no other observable crystalline phase

to account for it.

6
Table 4. Effect of eyelet; of eooliny and warminy on the unit

cell edge length of a hydrogarnel preparation

.2 3 4 MOLAR RATIO

AI2O, + Fe, Oj

Figure 4. Variation of unit cell edge with composition of C3(A,F)H6 preparations.

Open circles: products prepared by method C (see sec. 2). Closed circles: same, with subsequent storage at 45 °C. Triangles: products prepared by method B. Square: from previous work [4].

Treatment and

temperature Phases present" Calculated . a, in hg phase A

Ground wet, 2 J/^ hr at about 30 °C

6 days at 15 °C

2 days at 36 "C

20 days at 1 °C

1 day at 38 °C

hg; Ca(0H)2; very little hex

hg; Ca(OH)2; increase in hex

hg; Ca(0H)2; very little hex mostly hex; hg; Ca(OH)2

hg; Ca(0H)2; very little hex

12.641

12.658

12.651

12.661 12.659

hg = hydrogarnet. hex = hexagonal plate phase.

3.4. Thermal Dissociation

The dissociation of the hydrates on heating was

studied by means of differential thermal analysis

with the apparatus described above and by determination of weight loss on heating at progressively higher temperatures in a covered crucible in an oven. Two series of samples were used for the

DTA study, the first having been made by method

C (see sec. 2) while the other consisted of the same

group after storage at 1 °C to convert the isometric

to the hexagonal hydrates. The samples had been

filtered off and dried over NH4CI under reduced

pressure. X-ray patterns showed the presence of minor amounts of the isometric phase in the

preparations predominantly hexagonal; also the

converse. These were, in most cases, detected by

the DTA, so that in a general way all the patterns were similar. There were, of course, differences in relative intensities of the heat effects, as well as

slight shifts in the temperatures at which these

effects were observed.

Typical traces for the predominantly hexagonal

series are shown in figure 6. The bottom trace in

.2 .3 4 MOLAR RATIO

AI2 Oj + Fe2 O3

Figure 5. Anomalous values of unit cell edge of C3(A,F)H6

preparations that had been converted to C4(A,F)H„

and back to C3(A,F)H6.

Open circles and semicircles: products prepared at 1 °C, subsequently held

at 45 °C, then at 1 °C, then at 35 °C. Closed circle and semicircles: prepared

at room temperature, subsequently held at 1 °C, then at 35 °C. Semicircles

indicate 2 distinct hydrogarnet phases in same preparation as shown by

doublet peaks. Triangles:^ single preparation put through successive cycles

of cooling and warming.

100 200 300

TEMPERATURE, DEGREES C

Figure 6. DTA traces of three calcium aluminoferrite hydrate

preparations.

Figures in parentheses relate to experiment numbers in table 5.
this group shows six separate endothermal effects (labeled A through F). The first five of these

appear to be associated with the hexagonal phase,

or phases, since they were either absent or greatly

reduced in magnitude in traces for the samples that were predominantly in the isometric form. The last endotherm, F, was strong in the traces of both the

isometric and the hexagonal hydrates, with the ex- ception of that of the hydrated C2F in which it was

entirely lacking.

Temperatures corresponding to these endotherms are given in table 5. In many cases the

Table 5. Temperatures of endotherms on DTA traces for hydrates of the calcium aluminoferrite series

No.

FezOs AhOs+FesOs A B c D E F

°C °c °c °C °C

1 0 127 194 323

2 0.194 C) (>>) 200 C) 310

3 .341 C) 105 191 208 265 311

4 .427 (") 97 186 203 264 304

5 .500 80 95 187 200 244 308

6 .580 (») 100 187 200 254 305

7 .666 C^) 93 183 C) 242 299

8 ,837 C)

C) C) 183 C) 240 290

9 1.000 110 180 C) 240 C)

» No endotherm observed.

•> Endotherm present but too slight for measurement.

figures given are averages of two or more readings,

since the data for the hexagonal and isometric

series were treated as a single group for this

purpose and a few duplicate runs were made.

Weight-loss determinations were made only on

the hexagonal hydrate series. The samples used

had been prepared by method A. The weight-loss

method was not sufficiently sensitive to permit

close correlation with the DTA results, but the

results were of some assistance in deducing the

significance of the various endotherms. Referring

again to figure 6, the numbers in parentheses

correspond to the same numbers in table 5. The

first endotherm, A, was observed only in traces 4

and 5, and was too small to permit even a guess as

to its origin. Endotherm B seems to be associated

with the loss of 2 or 3 mxolecules of water and was

detected in the DTA traces of all the hexagonalphase samples. There is a trend toward lower temperatures as the Fe203 content increases, but there

are also some irregularities (Nos. 6 and 9). Endotherms C and D appear to be closely related. The

former is absent from the trace of the hydrate containing no Fe203, while the latter is not exhibited

by the hydrates high in Fe203. Intermediate

samples show a progressive change in the relative

magnitudes, as may be seen in figure 6. It is possible that this double endotherm, as well as the

double x-ray diffraction peak discussed earlier, is an indication of the presence of two phases dif- fering slightly in composition or structure. The

temperature of C drops slightly but fairly consistently with increasing FeoOs content, while that

of D appears to go through a maximum.

Endotherm E was merely a shallow depression,

insufficient to warrant any conjecture as to its

significance. The weight-loss determinations failed to separate these heat effects. Endotherm F represents the first dehydration

step of the isometric phase, C3(A,F)H6. Preparation 1, table 5, was made for this experiment by

hydrating C3A by the same procedure used with

the aluminoferrite series. It was mainly CsAHg,

but contained a little of the hexagonal phase. For

this sample the temperature of endotherm F was

323 °C. A sample of CsAHg prepared hydrothermally showed a similar break at 300 °C. With

increasing FcoOs content, the temperature was

progressively lower, reaching 290 °C for preparation 8. No. 9, containing no AI2O3, gave no indication of an endotherm in this region. It is inferred

that no hydrogarnet phase was present in this

sample. Slight irregularities in the compositiontemperature relation probably result from slight

differences in rate of heating.

Endotherm F was the major heat effect in all samples (except No. 9), whether the material was

essentially in the isometric or the hexagonal form,

with no observable difference in temperature between the two types. This fact could be explained

either by a very close coincidence of dissociation

temperatures for the two phases, or by a trans- formation of the hexagonal phase to the isometric

during the course of the DTA. To check the latter

possibility, a sample of calcium aluminate hydrate,

initially about 90 percent C4AH13 and 10 percent

C3AH 6, was heated to 250 °C over a period of 15

min in a covered crucible, roughly simulating the

conditions present during the DTA run. An x-ray

diffraction pattern subsequent to this treatment

showed essentially complete conversion to CsAHe

and Ca(0H)2. Samples of several of the aluminoferrite hydrates were heated in the same manner,

with similar results. From the foregoing experiments it is evident

that in the DTA experiments with the hexagonal

hydrates, endotherm F is preceded by a trans- formation to the isometric phase. This conclusion

appears to be at variance with the results of other

investigators [10,11,12,13] who have studied the

thermal dissociation of C4AH13. However, it should be noted that the experimental conditions were quite different. In the present work the heating was of only a few minutes "duration, and the

water vapor pressure surrounding the crystals

must have been close to 1 atm. In the dehydration experiments of Buttler et al. [11], the pressure

of the water vapor was held at 6 mm.

Investigators who have studied the changes taking place in C4AH 13 on heating are not in complete

agreement as to the reactions that occur. However, the existence of two lower hydrates, C4AH11

and C4AH7, is widely accepted and amply supported by x-ray diffraction data. On this basis,

it appears reasonable to associate the endotherms

near 100 °C (A and B) with dehydration to the

11-hydrate and those near 200 °C (C and D) with

the loss of 4 additional molecules of H2O.

It may be noted that the loss should be the same

whether the product at this point is C4(A,F)H7

8
or C3(A,F)H6 + Ca(0H)2. However, the x-ray

patterns clearly indicate that the latter is the

actual condition.

The weight-loss data obtained were compatible

with the assumption of the loss of two molecules

of H2O at 100 °C and four additional molecules at 200 °C. The agreement was not quantitative,

probably because of overlapping of reactions,

among other factors. Approximately 2 molecules

of H2O remained after heating to 320 °C and most

of this was lost at 550 °C.

3.5. Reactions of the Hydration Products

With Calcium Sulfate Solutions

It is generally recognized that portland cements

of relatively high alumina content are more sus- ceptible to damage by sulfate waters than are cements low in alumina. It has been suggested

that this difference in behavior may be due at

least in part to differences in the nature or rate of reaction dependent upon the Fe903/Al203 ratio

in the aluminoferrite phase and in its hydration

products. It was therefore of interest to study the

reaction between calcium sulfate and some of the

hydration products obtained in the present work.

After some preliminary tests to ascertain the

approximate rate of reaction, an experiment was

set up to compare the reactions of the various

hydration products by measuring the reduction in concentration of a calcium sulfate solution after a

fixed period of reaction. The hydrated materials were the same as those used in the thermal dis- sociation study described above. They had been

dried over saturated NH4CI solution under reduced

pressure. There were two series, one in which the

hydrate was nearly all of the hexagonal type, the

other nearly all isometric. A weighed sample

(0.14 g for the isometric series, 0.20 for the hexagonal) was placed in a small plastic bottle with

100 ml of an aqueous solution saturated with

CaS04 and Ca(0H)2. The tightly closed bottles were stored at room temperature (25-28 °C) for 29 days, after which time the solutions were

analyzed for SO3. Analyses of companion samples

of the starting materials gave the quantities of AI2O3 and Fe203 present in the solids (the amounts

in solution being negligible) and the amount of SO3 taken up could therefore be computed in rela- tion to the sesquioxides. A molar ratio of 3SO3 to l(Al203, Fe203) would be required for complete

conversion to a product of the ettringite type,

while a 1 :1 ratio would suffice for the monosulfate

type, irrespective of the crystal species and

stoichiometry of the starting material. Reactions

leading to a product of the ettringite type may

be formulated as follows:

3CaO-(Al203, Fe203) -6H,0 + SCaSOj + 26H2O

-^3CaO (Al203, Fe203) -3CaS04-32HoO (3) 4CaO-(Al203, Fe203) 13HoO + 3CaS04 + 20H,O ^ 3CaO - (AI2O3, Fe203) -303804 •32H2O

+ Ca(0H)2. (4)

MOLAR RATIO

AljOs + FejOj

Figure 7. Removal of SO3 from solution by calcium aluminoferrite hydrate preparations during storage for 29 days.

Open circles: hexagonal hydrates. Closed circles: isometric hydrates.

The results are shown in figure 7. The decrease

in the amount of SO3 in solution, converted to moles and divided by the number of moles of AI2O3 plus Fe203 present, is plotted as the ordinate.

The abscissa expresses composition as in previous

graphs.

The results have no quantitative significance

aside from the particular samples used and the

conditions of the experiment. Nothing is known

as to surface area, an important factor in determining reaction rates. However, the hydrates were

prepared in the same manner, and particle size probably was fairly uniform within a given series. No difference in size was apparent under the

microscope. In comparing the hexagonal with the

isometric series, however, it is impossible to esti- mate the relative surface areas, since the isometric

crystals are nearly spherical, while the hexagonal

exist as very thin plates. Hence the fact that the

points for the two series fall close to a single curve

(fig. 7) may be fortuitous. It is clear, nevertheless,

that the relative reactivity toward CaS04 in solu- tion decreases with increasing Fe203 content in the hydrates.

Microscopic examination of the residues showed

almost complete conversion to needles, usually in spherulites, having the appearance of ettringite, for all samples high in AI2O3. Decreasing amounts

were found as the AI2O3 content decreased and

none at all in the last which contained no AI2O3.

These observations were confirmed by x-ray dif9
fraction. The lines due to a phase of the ettringite

type were predominant except near the high-FeoOs

end of the series. Other lines, corresponding to calcium aluminate monosulfate hydrate, were

found to increase in intensity toward the Fe^Os

end. Apparently the monosulfate phase formed

first, as no needles were observed within the first 5 days in the preliminary tests.

4. General Discussion

During the past few years there has been a great

deal of research on the tetracalcium aluminate

hydrates [8,9,10,11,12,13]. As a result of the

wealth of new data, the generally accepted ideas

of the relationships of the different hydrates have

been considerably altered. Although it would be

premature to say that all the previous uncertainties have been cleared up, certain conclusions

appear justified on the basis of the literature cited ; (a) that the compound heretofore called /3-C4AH 13, having a basal spacing about 7.9 A, is actually

C4AH12; (b) that there is a compound previously

designated a-C4AHi3 having a^ composition C4AHi:j

and basal spacing about 8.2 A; (c) that there is a

derivative, containing half a mole or less of CO2,

having the same basal spacing of 8.2 A. The present work has not included any experimental study designed to fix the precise water content of the various hydration stages of the calcium

aluminoferrites. It has been assumed, and confirmed at least approximately, that the water content is the same as that of the corresponding pure

aluminates after similar treatment. The formula

C4AH13 has been familiar in the literature for several decades; hence it has been adhered to throughout the present paper, even though the

works cited above make it appear highly probable

that the ;S-form should be called C4AH12.

One objective of this study was to determine the

relation between the Fe203/Al203 ratio of the

hydrates and certain measurable properties or parameters that might serve as identifying characteristics, affording a means of estimating the composition of the respective hydrated aluminoferrites in a complex material such as hydrated

Portland cement. Numerous investigators have re- ported the occurrence of a phase of the C4AH13

type in cement at certain stages of the hydration

process, but there has been no way of ascertaining

whether any Fe203 is present in solid solution. In the case of the isometric hydrate (CsAHe type)

there is still some doubt as to its presence in cement hydrated at ordinary temperatures, but it may exist in steam-cured cement.

From the data presented here, it is obvious that

the substitution of Fe203 for AI2O3 in the calcium

aluminate hydrates causes relatively slight changes

in their measurable characteristics, especially over

the range likely to be encountered in hydrated

cements. In the case of the hexagonal (C4AH13)

type of hydrate, the x-ray patterns show measurable shifts in certain lines, unfortunately not the

strongest. Whether the splitting of the (002) peak

near the middle of the range is a reliable and re- producible characteristic cannot be known without further study. The DTA traces also show slight

shifts of peaks with changing composition, but

these are even less useful than the x-ray patterns.

In the case of the isometric (CsAHe) series, the

x-ray pattern provides a better index of composition, since there is a distinct increase in computed

unit cell edge as the Fe203 content increases. The

DTA results are of less value in this case as well,

though the shift in dissociation temperature is somewhat greater than that for the hexagonal

series. It should be borne in mind that the

isometric hydrate, if it occurs in cement, is likely to contain silica in solid solution, resulting in fur- ther changes in unit cell size and probably in dissociation temperature.

Another useful index of composition in a solid solution series is the refractive index. During the

course of this study, numerous attempts were made to obtain accurate measurements of the in- dices of refraction of both the hexagonal and the

isometric series. Unfortunately the particles were

very small (mostly of the order of 2 microns) and

had a strong tendency to mat together on drying,

so that the optical properties could not be determined with the precision necessary to be of use in estimating composition. From a practical standpoint, it is likely that conditions in a complex

material like hydrated cement would be even less favorable.

Another objective was to ascertain the effect of substitution of Fe203 for AI2O3 on the reactions

of the hydrates with calcium sulfate in solution.

These reactions are of significance in connection

with the normal setting and hardening processes

of hydraulic cements and with the subsequent

behavior of cements in the presence of sulfates in solution.

From the results of the work described in section

3.5, it appears that all the aluminoferrite hydrates

reacted in the same manner, first forming the

monosulfate, which subsequently was changed to the trisulfate (ettringite type) with an increase in the amount of SO3 taken up. The rate of the reaction, however, decreased as the proportion of Fe203 increased. From these observations it is in- ferred that the superior resistance to sulfates ex- hibited by cements of high Fe203/Al203 ratio is not the result of a difference in the type of reaction

product, but may be related to the rate of reaction

with sulfate solutions.

No detailed study was made of the x-ray pat- terns of the sulfated products, so it cannot be

stated with certainty that the Fe203/Al203 ratio

remained the same as that in the unsulfated

hydrates. However, there was no change in color

of the solid residue; hence it may be inferred that no Fe203 was liberated during the reaction.

10
Certain results obtained in this study are of in- terest in relation to one of the more theoretical

aspects of the problem, namely, the question of the

composition of a-C4AHi3. The view advanced by

Seligmann and Greening [8] among others, that

this phase is really a "semicarbonate", has been

widely accepted. Alegre [12] and Lavanant [13], however, obtained a-C4AHi3 by careful dehydration of C4AH19 with rigid exclusion of CO2, and

consider it to be the only 13-hydrate. Still more

recently Dosch and zur Strassen [10] have con- cluded that there is an a-C^AUu and also a "quarter carbonate"^ the two having the same basal

spacing of 8.2 A. In the present work, the fact that the 8.2 A line appeared and grew in intensity during the course

of repeated x-ray diffraction runs on samples of moist C4(A,F)Hi9 is evidence that CO2 facilitates forniation of the compound represented by the

8.2 A spacing.

5. Summary

Calcium aluminoferrite hydrates in two series,

C4(A,F)H„ (hexagonal plates) and C3(A,F)H6

(isometric), were prepared by hydrating the corre- sponding anhydrous aluminoferrites in various

ways. Hydration of the end member, C2F, yielded

only the hexagonal type.

With all the aluminoferrite hydrates in contact

with Ca(0H)2 solution, a reversible reaction

occurred between 15 and 35 °C, the hexagonal

phase being stable below this temperature range

and the isometric (hydrogarnet) phase above.

X-ray diffraction analysis of specimens of the

hexagonal series, saturated with water, gave patterns like that of C4AH19. There was no detectable

shift in the basal spacing with change in Fe203/

AI2O3 ratio. Successive patterns obtained from the same mount showed gradual conversion to a-C4AH 13, attributed to reaction with atmospheric

CO2.

When dried to the 13-hydrate stage (/3-type)

the hexagonal series gave x-ray patterns varying

slightly with Fe.Oa/AloOs ratio. The (002) re- flection split into a doublet near the middle of the

series, while two of the less prominent lines showed

a consistent trend to larger d-spacings as the

Fe203 was increased.

The x-ray patterns of the isometric series showed

a linear relation between composition and unit cell

edge, going from 12.573 for CjAHe to 12.716 for the member having approximately % of the

AI2O3 replaced by Fe203.

Differential thermal analysis traces of the iso- metric series showed a large endothermic effect near 300 °C. The temperature ranged downward

from 323 °C for CAR, to 290 °C for the number

highest in Fe203. The hexagonal series gave more

complicated patterns, with peaks of moderate size near 100 and 200 °C and a larger one near 300 °C.

The first two varied only slightly in position with

change in Fe203/Al203 ratio. The third proved to be identical with the peak exhibited by the iso- metric series and it was found that the hexagonal

hydrate was converted to the isometric during the

course of the DTA run.

All the aluminoferrite hydrates reacted with

calcium sulfate solution forming mixtures of the

monosulfate and trisulfate (ettringite) types, the

latter predominating at later stages. The rate of reaction decreased as the Fe203/Al203 ratio of the

hydrate increased.

The author expresses his thanks to H. E. Swanson and J. H. deGroot for making the x-ray dif- fraction patterns, and to E. S. Newman for use

of the DTA apparatus which he had constructed.

6. References

[1] F. M. Lea and C. H. Desch, The Chemistry of Cement

and Concrete, 2d ed., Edward Arnold, Ltd., London

(1956). Ch. IV.

[2] The Chemistry of Cements, edited by H. F. W. Taylor,

Academic Press, New York (1964). Vol. 1, Chapter 2, by J. H. Welch.

[3] A. J. Majumdar, The ferrite phase in cements, Trans.

Brit. Ceram. Soc. 64, 105-119 (1965).

[4] E. T. Carlson, Action of water on calcium aluminoferrites, J. Res. NBS, 68A, 453-463 (1964).

[5] G. Malquori and V. CirilH, The hydrated calcium ferrites, and the complexes that originate from trical- cium ferrite by association with different salts of calcium (Note 4), (in Italian), Ric. Sci. 14, 78-84

(1943).

[6] S. Chatterji and J. W. Jeffrey, Studies of early stages of paste hydration of cement compounds, I, J. Am.

Ceram. Soc. 45, 536-543 (1962).

[7] H. E. Schwiete and T. Iwai, The behavior of the ferritic phase in cement during hydration, (in German)

Zement-Kalk-Gips 17, 379-386 (1964).

[8] P. Sehgmann and N. R. Greening, New techniques for

temperature and humidity control in x-ray diffrac- tometry, J. Res. and Devel. Labs., Portland Cement

Assn. 4, (2), 2-9 (1962).

[9] F. E. Jones and M. H. Roberts, The system CaO-AUO,-

H2O at 25 °C, Bldg. Research Current Papers, Research Series 1, Dept. of Sci. and Indus. Research,

London (1962).

[10] W. Dosch and H. zur Strassen, Investigation of tetra- calcium aluminate hydrate. I. The various hydrate

stages and the effect of carbonic acid, (in German)

Zement-Kalk-Gips 18, 233-237 (1965).

[11] F. G. Buttler, L. S. Dent Glasser, and H. F. W. Taylor,

Studies on 4CaO Al203 131120 and the related natural mineral hydrocalumite, J. Am. Ceram. Soc.

42, 121-126 (1959).

[12] R. Alfegre, Investigations on the hexagonal hydrated

tetracalcium aluminate, (in French) Rev. mater,

construe, trav. publ. No. 566, 301-314 (1965).

[13] F. Lavanant, Contribution to the study of some calcium aluminate hydrates, (in French) Rev. mater, construe, trav. publ. No. 592, 1-10, No. 593, 76-87, No.

595, 193-207, No. 596, 251-261, No. 597, 298-304

(1965).

11

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