The determination of crystallinity of tobermorite in autoclaved products

CEMENT and CONCRETERESEARCH. Vol. I0, pp. 213-221, 1980. Printed in the USA. 0008-8846/80/020213-09502.00/0 Copyright (c) 1980 Pergamon Press, Ltd.

THE DETERMINATION OF CRYSTALLINITY OF TOBERMORITE IN AUTOCLAVED PRODUCTS +

*

N. Hara and H.G. Midgley Building Research Station Garston, Watford, Herts., U.K.

(Refereed) (Received Feb. 12; in final form Dec. 7, 1979) ABSTRACT The "crystallinity" of the calcium silicate hydrate, tobermorite produced by autoclaving lime-silica mixtures has been defined as being equivalent to a mixture of amorphous and fully crystalline phases. The "crystallinity" can be measured from the peak height of either the 002 or 220 X-Ray diffraction reflections relative to an added internal standard or by the ratio of the peak heights of these reflections. The crystallite size measured from the line width of the 002 and 220 reflections sh~,s that the crystallite size increases with crys tallini ty. The surface area calculated from the c~dstallite volume shows a linear relation with the surface area measured by Nitrogen absorption.

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INTROD UCT I0N: There have been many reports which relate the physical properties of cement and concrete such as strength and shrinkage to the crystallinity of the calcium silicate hydrate formed by hydration (I ,2,3). The crystallinity +

Now at Government Industrial Research Institute, Kyushu, Japan

*

Now at Concrete Research Laboratory, Civil Engineering Division The ~atfield ~olytechnic, Hatfield, }{erts, U.K. 213

214

Vol. I0, No. 2 N. Hara,

H.G. Midgley

has been determined by X Ray diffraction (XRD), B.E.T. vapeur absorption, differential t2~ermal analysis (D.T.A.) and chemical analytical techniques. Lentz (4) determined the molecular weights of polysilicate derivatives obtained by esterification of the calcium silicate hydrates using hexamethyl/ disiloxane and found a relationship between the molecular weight and degree of crystallinity. The silicomolybdate methods of Funk (5) and Wieker and Stade (6) have also been used to determine "molecular weights", or the size of the individual particles. The calcium silicate hydrate group of minerals have a strong exotherm on a D.T.A. thermogram due to the heat of formation of wollastonite or larnite. However the simple relationship proposed by Kalousek (I) that the height of the peak is directly related to the degree of disorganisation has been questioned by Nurse and Taylor (7). In the present paper the authors have investigated the changes in the intensities and breadths of the characteristic reflections of the XRD patterns of the tobermorites produced by autoclaving lime-silica mixtures. They have also compared the surface areas calculated from the crystallite volume with the surface areas measured by BET nitrogen absorption. Crystallinity determination from X-Ray diffraction If a phase such as tobermotite shows some disorder due to misorientation of the layers then it may be described as lacking in crystallinity. The material with the most perfect crystal arrangement may be regarded as having 100% "crystallinity". If on the other hard a substance of the same or similar composition is amorphous then it can be said to have zero "crystallinity". Any stage between may be described as if it were composed of a mechanical mixture of the two extremes. It must be emphasized that this is an arbitary decision and will not distinguish between a degree of disorder and a mixture of two phases. The X-Ray diffraction patterns of well crystallised tobermorite and amorphous C-S-H are given in Fig. I. A and B. It can be seen that the 002 and 400 reflections at 11.3 and 2.82 ~ vary with "crystallinity"; they are strong for tobermorite and almost absent for C-S-H. It had been assumed originally that the 220 reflection would occur at the same place in the diffraction pattern (3.07 ~), but with high resoution diffractometry this reflection occurs at 3.08 ~ for tobermorite and 3.05 ~ for C-S-H. However at intermediate degrees of ordering there is only one distinct peak, Fig. I. D. Therefore the 002, 400 and 220 reflections may be used to describe the "crystallinity" of the calcium silicate hydrates present in autoclaved products. It may be assumed that there is a simple linear relationship between "crystallinity" and the peak height of the characteristic X Ray reflection relative to an internal standard, and so crystallinity may be defined in terms of relative peak heights. Quantitative X-Ray diffraction The samples were prepared for Q.X.R.D. by the internal standard method (9) by thoroughly m~Ting 10 parts by weight of ground fluorite with 90 parts of the various calcium silicate hydrates. Dyczek and Taylor (8) and Stokes (10) favoured faujasite and cordierite respectively as internal standards since these minerals have reflections near to 11 A. The present work has shown that fluorite is satisfactory. The X-Ray patterns were obtained by diffractometry using filtered Cu K~

Vol. I 0 , No. 2

215 TOBERMORITE, CRYSTALLINITY, XRD, CSH ",.OSA I],~A

I

"'lAIl,'"l'

2. ,,2A

12,79A I

~

C

2

'7

m

8

9

,

decree

28 2~'

22O

29

222

~0

40O

~1

'2

Cu ~

Fig.l- X.R.D. traces of mixtures of well crystallised tobermorite and C-S-H radiation at 40 kV 20 ma, scanning speed 0.25 counter with I° and 0.2mm slits.

2@ per min., proportional

The X-Ray diffraction intensities were measured as peak heights on the 002 (11~), 220 (3.O~) and 440 (1.53~)_reflections of tobermorite; the 220 (3.Q5~) reflection of C-S-H; the 101 (3.34~) reflection of quartz and the 111 (3.15~) reflection of fluorite. It would have been more correct to use the peak areas to measure the intensities of the reflections especially since the peak profiles were very ~ifferent; however the difficulties in defining what was peak area and the measurement led to such gross errors that peak height had to be used. Dyczek and Taylor (8) also found this difficulty and had to use peak height in their study of the quantities of calcium silicate hydrate produced in autoclaved products. If the amount of a calcium silicate hydrate present in a mixture is known by methods other than by the measurement of that phase by Q.X.R.D. then "crystallinity" (Yi) as defined above depends on the relative peak heights of the 002, 400 and 220 peaks of the X.R.D. pattern, and may be defined by the equation:-

Yi -- a

÷ b (hi/h s)

..............

(I)

where h i is the peak height for the characteristic reflection, h s for the

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H.G. Midgley

added standard, a and b are constants which depend on the geometry of the diffraction system and on the ratio of standard to -n~,own used. To test the validity of equation (I) two samples of well crystallised tobermorite were used, Q24 and T24 (see appendix I ). These were both considered to be of 100% crystallinity; however both contained some unreacted quartz. The quantity of quartz present was determined by Q.X.R.D. using calibration mixtures of pure quartz and alumina with calcium fluoride as internal standard. The weight percentage of the 100% crystallinity specimens were adjusted to allow for the quartz present. For material of zero crystallinity the C-S-H prepared by double decomposition was used. Mixtures containing various proportions of these two were prepared and examined by Q.X.R.D.; the results of the peak height of the 002, 400 and 220 reflections relative to the 111 peak. for fluorite against "crystallinity" are given in Fig. 2. 400 002 0()2 220

Proportion of well crys tallised Tobermorite to C-S-H (%)

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Peak height relative to 111 line of fluorite Fig.2.- Diffraction peak height of mixtures vs proportion of well crystallised tobermorite to C-S-H. There was a good correlation between "crystallinity" and relative peak height for the 002 And 400 reflections, the correlation coefficients being better than 0.99. There was not a simple relationship for "crystallinity" relative peak height for the 220 reflection, the reason is illustrated on Pig. I. The well crysta!lised tobermorite has its 220 reflection at 3.08 A, while C-S-H has it at 3.05~. In some of the mixtures the two peaks were resolved, Fig. ID. Dyczek and Taylor (8) state that the 400 reflection is overlapped by the 008 reflection at 2.82 A, but this is displaced to 2.85A in aluminium rich tobermorites, and so will appear as a shoulder on the main 400 reflection, Pig. ID. From the data for the 002 reflection of quartz:lime mixtures with 10% added fluorite given ia Fig. 2., the constants for equation (I) were a = 0.2 and b = 62.0. The data for samples of crystallinity greater than 20% gave constants of a = -47.1 and b = 79.6 for the 220 reflection although the results were of lower reliability. However it must be pointed out here that this method is only valid if the

Vol. I 0 , No. 2

217

TOBERMORITE, CRYSTALLINITY, XRD, CSH quantity of calcium silicate hydrate is known. Since the "crystallinity" may be measured by the two peaks the 002 and 220 deflections on the diffractogram, then by using a method equivalent to the "autoflushing" method of Chung (I I ) a simple relationship without an internal standard or without knowing the quantity of calcium silicate hydrate present will give the "crystallinity". From the constants a and b given above the "crystallinity" Yr may be determined. Yr = (0.25 + 4'7.1. hOO2/h220) I

(1.28 - hOO2/h220)

.........

(2)

To test this relationship specimens of unknown orystallinity (Q I-6, T I-6) were investigated. The amounts of the calcium silicate hydrates present were determined by measuring the unreacted quartz present and deducting this from the total weight. The crystallinity was measured both as YOO2 and Yr

(Table I )

TABLE I "Crystallinity" determined by peak height of the 002 reflection of calcium silicate hydrate relative to 111 reflection of fluorite internal and standard and the ratio of the peak heights of the 002 and 220 reflections Specimen TI T2 T3 T4 T5 T6 QI Q2 03

"crystallinity" YOO2 77 66 50 41 37 27 IO0 70 25

Yr 77 67 47 43 34 22 102 69 24

The correspondence between the two methods is good, a correlation coefficient of 0.996 was obtained for the linear relationship Yr = I .OS.Yo02 - 2.4. Since the samples were made with both pure quartz and tripoli the method will work for both pure and al,,m~nium substituted calcium silicate hydrates. Crystallite size by line broadening of X.R.D. reflections The crystallite size of a crystalline material may be determined by measuring the line breadth at half peak height of a diffraction ,~Yima of an X.R.D. reflection. There will be a correction necessary for the K doublet separation and for instrm,ental effects, Quartz 5 - 10mm diam. and faujasite 2 - 4mm diam. were used to establish the corrections necessary. The crystallite size, D, can be calculated from the Scherrer equation (9) as follows D ~ l --

K ~

............................. -

(3)

Where Dhk I is the mean crystallite size normal to the hkl reflecting plane, K a crystalline shape factor, the M-Ray wavelength used, @ the Bragg angle of the reflection, ~ the pure diffraction breadth due to the crystallite size. The diffraction breadth can be considered as being composed of two parts, that due to the instrument (b~ and that due to the crystalline material (~). The instrumental broadening (b) was determined for quartz and fauJasite and so

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H.G. Midgley

the measured width at half peak height (w) can be corrected to give # by the following relationship.

?

=

w - (0.314 - 0.118 l o g 2e) . . . . . . . . . . . . . . . . . .

(4)

Using the equation (3) with K = 0.9 and ~ = I .5405 the crystallite dis~meters (Dhkl) for the 220, 040, 002 and 400 reflections of the autoclaved lime-silica mixes (Q and T) were determined, Table 2. TABLE 2 Crystallite size from X.R.D. line broadening, crystal volume crystallinity Mix

Crystal size (~) from Dhk I

crystal vo l~me

crystallinity

220

040

002

400

X I O8~ 3

~

QI Q2 Q3 Q4 Q5 Q6

930 620 330 370 370 -

770 490 330 310 270 250

230 240 210 210 150 130

270 220 260 1 90 150 11 O

I. 56 0.73 O. 37 O.45 O. 14 0.06

IOO 66 38 36 25 12

TI T2 T3 T4 T5 T6

680 710 550 470 460 260

330 330 310 290 230

310 300 260 250 230 180

190 190 180 140 140 120

I .17 I .I 3 0.63 O.48 O.39 O. IO

77 66 50 41 37 22

The results show that for the tobermorites made from quartz (Q I-6) there is an increase of DO40 from 250 to 770 ~ with an increase in "crTstallinity" of 12 to I0~,%. For the tobermorites made from tripoli (T I - 6) the increase was from 230 to 3302 for a "crystallinity" increase of 22 to 77%. The results for D400 may be unreliable due to the superposition of the 008 diffraction reflection, while the DO40 may be affected by the superposition of the 112 reflection of the unreacted quartz 11~ tobermorite has a layer structure where the a - b planes are piled parallel to OO1. By electron microscopy (8) the crystals are ~een to be platy or lath like up to several micrometers across and about 200A thick. The DO20 for all the samples investigated have values from 180 to 310 ~, approximately the size observed by electron microscopy. The values for D4OO, D040 and D220 were very much less than those observed by electron microscopy, so it is considered that the calcium silicate hydrates have domain structures in which D4OO, DO40 and DOO2 correspond to the mean size of the single domain along the a, b and c axes. D220 was larger than the size expected from D4OO and DO40. The 220 line of the poorly crystalline tobermorite has a tendency to show an asymmetrical profile terminating abruptly on the low angle side but falling off gradually in intensity on the high angle side; this is considered to be characteristic of random layered structures. (9) From these results D220 may be considered to represent the size of the a - b plane consisting of a s ingle domain with a random layer surrounding the domain. Thus the volume (v) of the single crystallite may be calculated assuming the crystallite to be dics like with a diameter D220 and height DOO2.

v

=

~zt

(#220)2

])002

o o e o e o e o o o e e

(9)

Vol. I 0 , No. 2

219 TOBERMORITE, CRYSTALLINITY, XRD, CSH

The results are given in Table 2, and a simple linear relationship with a correlation coefficient of 0.972, was found between v and "crystallinity" YOO2 YOO2

=

5.31 x I~7v

+

15.9

.........

(5)

Although these t~o parameters of the calcium silicate hydrate are not of the same kind the relationship shows that as the order of regularity of the structure as represented by "crystallinity" increases so does the crystal si ze. Surface Area The surface area(s) of the crystallites may be calculated from the dimensions of the disc like plates as measured by >[.P..D. line broadening by the following equation S

-

~

(D220)2

+

~

D220

DO, 2

....

(6)

The number of crystallites per. gram (N) is given by ~+"

-

+ p.v

.

.

.

.

.

.

.

.

.

.

.

. . .

.

.

.

.

.

.

.

.

.

.

(7)

.

(p density, v volume) and so the specific surface area, s, (cm /g) may be calculated as S = 2 • 1 2 (8) ~" -..oeooo..o P DO02 D220 Using a density of 2.4 g/cm (9) for tobermorite the specific surface area has been calculated for the tobermorites QI - 6 and T I - 6 in Table 3. As a check on these results the specific surface area (SB) of the samples was determined using }[itrogen absorption and the B.E.T. isotherm method. (Table 3). There is a good relationship between the results using the two methods, for all the samples the correlation coefficient was 0.843, while for the samples T I-6 made with tripoli the correlation coefficient was 0.946. It is thought that contamination by xonotlite of the samples made with quartz may be the source of the errors. However the specific surface area as measured by the B.E.T. method is roughly half that calculated from the crystallite size by the X.R.D. line broadening method. TABLE Sample No.

3. B.~.T.

X.R.D.

QI Q2 0.3 Q4 Q5 Q6

76 51 65 37 41 45

135 105 76 72 41 34

TI T2 T3 T4 T5 T6

64 48 44 36 42 32

109 71 62 58 51 50

( Specific surface area ( calculated from (, crystallite volume and measured by nitrogen ( absorption. (m2/g)

220

Vol. I0, No. 2 N. Hara, H.G. Midgley

Conclusions :The "crystallinity" of the tobermorite phase produced by autoclaving lime-silica r&xtures may be determined from the relative intensities of the 002 and 220 reflections on the X.E.D. pattern provided that the quantity of tobermorite present is ]~own. The "crystallinity" may also be determined by the ratio of the 002 and 220 reflections using the auto flushing method of Chung (11). The crystallito size may be determined from the line broadening of the X.R.D. pattern using the Scherrer equation, and assumin~ that tobermorite has a disc like shape the cr~jstal volume may be calculated. It was found that as the "cr?~stallinity" increased so did the crystal volume. The specific surface area meast~ed from the crystal volume was compared with the specific surface area measured by nitrogen absorption, the correlation was good, but the specific surface area calculated from the c~jstal volume was greater than that measured by nitrogen absorption. Acknowl ed~en t_~s:We wish to thank the Science and Technology Agency of the Japanese Gove~sment for a fellowship given to N l~ra which allowed him to study at the Building R~search ~tablishment in 1972-73. The experimental work was carried out at the Buildin~ Research Establishment. References :I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

G.L. F~alousek, Prec. 5th, Int. Syrup. Chem. Cements, Tokyo 1968, 3,523(1969) J. Alexanderson, C.C.R. in press E.G. Midgley and S.K. Chopra, ~,~g. Concrete Research, 12, 19 (1960) C.I'J. Lentz, ~ g . Concrete Research, 18, 231, (1966) H. Funk, Prec. Syrup. Autoclaved Calcium Silicate Building Products, London, 1965, 122 (I 9 6 7 ) W. Wieker and H. Stade, ibid, 125, (1967) R.I~. Nurse and H.F.~:. Taylor, Prec. 3rd. Int. Symp. ChemistNy of Concrete, C. and C.A. London, 311, (1954) J.R.L. Dyczek and H.F.I~. Taylor, C.C.R. 1, 589 (1971) H.P. Klug and L.~L Alexander, X-Ray Diffraction Procedures for Polycrystalline & Amorphous ~iaterials,2nd Edit.p.618.John ITiley & Sons. N.York(1974) X.R. Stokes, Thesis, London University (1971) F.H. Chung, J. Appl. Crystall., 7 ,526, (1974)

A~pendix I. Specimen Preparation Two samples of well crystallised 11A tobermorite were prepared, Q24 b~ autoclaving at 17oOc for 24 hours a slurry/w/s (water to solids ratio) 3, of finely ground quartz and calcium hydroxide, with a Ca/Si molar ratio of 0.5; T.24 by autoclaving at 170°C for 24 hours a slurry, w/s 3, of tripoli and calcium hydroxide, Ca/Si molar ratio of 0.8. The quartz was of 99.O1% purity, specific surface (Blaine) 3000 sq. cm/g.; the tripoli contained 91.39~ silica s_nd about 8% alumina and a specific surface of 5200 sq.cm./g. The calcium hydroxide was prepared by calcining calci~ carbonate at IOOO 1 IOOC and then hydrating. Other samples Q I-6 and T I-6 were prepared by autoclaving similar quartz (Q) and tripoli (T) slurries for various periods of less than 24 hours. C,S-H, the ill crystalline material ~as prepared by double decomposition of

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TOBERMORITE, CRYSTALLINITY, XRD, CSH

221

calciu~u nitrate and sodium silicate (8). Fumed silica was dissolved in hot 0.33 M aqueous sodium hydroxide and 2 M aqueous calcium nitrate was then added. The molar ratios Na20: Ca0; Si02 were 1:4:1. The precipitate ~as washed with water followed by methylated spirit, which removed the sodium nitrate. All the hydrates synthesised were dried to a constant weight in a vacuum oven at 4000, then ground to less than I0-15~m. diameter.