Mat. Res . B u l l . , Vol. 22, p p . 1055-1062, 1987. P r i n t e d in t h e USA. 0025-5408/87 $3.00 + .00 C o p y r i g h t (c) 1987 Pe rgamon J o u r n a l s L td .

CHARACTERIZATION OF ALINITE CEMENTS THROUGH X-RAY DIFFRACTION AND

MAS 29Si NMR STUDIES

Pradip, E.C. Subbarao, P.C. Kapur

Tata Research Development & Design Centre I , Mangaldas Road, Pune 411 001

India

and

N.R. Jagannathan and C.N.R. Rao

Solid State and Structural Chemistry Unit Indian Ins t i tu te of Science, Bangalore 560 012

India

( R e c e i v e d March 17, 1987; R e f e r e e d )

ABSTRACT A l in i te cements have been synthesized using mining and steel plant wastes and pulverized fuel ash ( f l y ash) as raw materials and a cl inkering temperature of I150°C. The cements possess hydration character ist ics comparable to those of portland cements. X-ray d i f f rac t ion studies on these samples confirm the presence of a l i n i t e as the predominant phase. MAS :~Si NMR spectra have been used to dist inguish a l i n i t e and a l i t e cements. While both show resonances character ist ic of Q° type s i l i ca te species, the portland cements exhib i t three d is t inc t peaks corresponding to three inequivalent SiOa units present, while a l i n i t e shows a single sharp peak corresponding to the unique Si posit ion.

MATERIALS INDEX : a l i t e , a l i n i t e , cement (portland)

* Also at the Indian Ins t i tu te of Technology, Kanpur 208 016.

1055

1056 PRADIP, et al. Vol. 22, No. 8

Introduction

One of the essential requirements of conventional portland cement (OPC) manufacture is that the c l inker ing temperature should l i e in the region of 1450-1500°C in order to complete the reactions leading to formation of the well known a l i t e (3 CaO.Si02) phase. Recent work in the Soviet Union using pure oxides as raw materials has shown that in the presence of hal ide salts such as calcium ch lor ide, a phase s i m i l a r ~ a l i t e but incorporating chlor ide anions in i t s s t ructure, is formed at much lower temperatures, in the range of I000-1200°C ( I ) . This new a l i n i t e phase is a structural l e i tmo t i f of a l i t e and has been observed to consist of a considerable number of defects and vacancies ( I , 2). Cement c l inker conta in ing~in i tephase has been found to possess strong hydration character is t ics s imi lar to portland cement, even though i t s formation temperatures are much lower and the c l inker is much sof ter than OPC. Cements based on a l i n i t e phase, referred to as low temperature (LT) cements, thus o f fe r an in terest ing poss ib i l i t y of substantial energy savings in cement manufacture.

Extensive work has been carr ied out in our laborator ies to extend the above work to produce a l i n i t e phase in cement c l inkers manufactured from industr ia l and mining wastes as raw materials (as a source of Si02, AI203, Fe203, MgO and CaO), which are otherwise not amenable to u t i l i z a t i o n in OPC manufacture. This paper deals with the i den t i f i ca t i on and character izat ion of a l i n i t e in some of these cements.

Experimental

A l i n i t e cement samples were synthesized from f l y ash, limestone f ines , magnesite dust and mil l scale with calcium chlor ide serving as a s inter ing aid. A raw mix of these raw materials having a composition as shown in Table i was cl inkered in an e lec t r i c furnace. Cement samples coded C~ and C 2 were c l inkered at I150°C fo r 4 hours, and 2 hours respect ively. The a l i n i t e sample C 3 was prepared using waste ta i l i ngs sands of a gold cyanidation plant, limestone and calcium chlor ide, having s imi lar raw mix composition and c l inker ing done at I150°C fo r 5 hours. CaCI 2 content of these raw mixes was in the range of 5-8% by weight. Two d i f f e ren t samples of good qual i ty commercial portland cement coded as P~ and P2 were also invest igated fo r comparison. The sett ing and strength propert ies of a l i n i t e cements produced were found comparable to those of port land cements in a l l respects.

The X-ray d i f f r ac t i on patterns were obtained with CuK~ radiat ion on a Phi l ips X-ray d i f f ractometer .

29Si MAS NMR spectra were col lected at 59.62 MHz using a Bruker MSL 300 spectrometer with a 7.49 T wide-bore Oxford magnet. FT-NMR in conjunction with magic angle spining (and no decoupling) was used to obtain the spectra. Cyl indr ical rotors were used for co l lec t ion of the NMR spectra with typical spining speeds in the range of 3 to 3.5 kHz. A 45 ° pulse with 5 sec delay between the pulses were employed. Chemical sh i f t s are reported w . r . t , the

:~Si resonance of TMS. Gaussian resolut ion enhancement was employed to obtain bet ter resolut ion of the spectra.

Vol. 22, No. 8 ALINITE CEMENTS 1057

TABLE i

Comparison of Elemental Analysis of Synthesized Cement Samples and Reported A l in i te Type

Phases

Element ALINITE BELINITE 2C2S. This Work C *** Ref 3* Ref 4** Ref 2 Ref 5 CaCI 2 Calcu- Exptl.

Caa Mg Ref 6 lated determined (SiO~) 4 C12

Ca

Si

A1

Fe

Mg

0

C1

LOI

50.3 46.7 46-47 40.9 42.4 40.5 38.4

9.6 11.2 10-11 14.3 9.9 8.3 8.6

3.1 2.0 1.5-3 - 4 4.4

. . . . 3.5 4.5

- 2.3 1-1.5 3.1 5.1 4.9

32.9 33.8 35-36 32.7 22.6 32.4 34.1

4.1 4.18 2.5-3 9.1 25.1 5.9 2.3

. . . . 2.5

@c Call (Si0.75 AI0,25) 4 018CL

Ca9. 9 Mgo. 8 r~O.3[Si3. 4 AIo. 6 016101. 9 C1

Chloride assay in C 2 and C 3 samples observed to be 3.69% and and 3.4% respectively.

Results and Discussion

X-Ray Dif f ract ion Studies

The X-ray d i f f rac t ion patterns obtained w~th CuKm radiation are shown in Fig. I . I t can be seen that a l l the character ist ic peaks of ~ -a l i n i te as reported by Nudelman e t .a l . ( I ) are also present in the patterns of the present a l i n i t e cements CI , C2 and C3. MgO, C3A and C3S (a l i te ) are also found in these cements as minor phases. This may be at t r ibuted to the presence of s ign i f i cant ly higher amounts of Mg and somewhat larger amount of A1 in the compositions studied as compared to the single crystals of a l i n i t e (2, 3 and 4) and be l in i te (5) reported in the l i te ra tu re (see Table 1). As expected, the peaks corresponding to a l i t e and bel i te phases of portland cement are not prominent in a l i n i t e cements. As i l l us t ra ted in Fig. I , the X-ray patterns of portland cements are d i s t i nc t l y d i f fe rent , consisting of a l i t e (C3S) as the predominant phase with C3A (3 CaO. AI203) as a minor phase. A l in i te cements thus can readily be distinguished from portland cements by X-ray d i f f ract ion studies.

1058 PRADIP, et El. Vol. 22, No. 8

CEMENT C-I, C-2 ,C-] --.ALINIT E CEMENT

P-I, P-2 --*PORT LA NO CEMENT

1 -- I -AL IN ITE

2 - A L i T E ( 3CmO,Si~2 )

] -I,C&O# AI20 ] ,F$203 { C / A I r )

4 - 3CaO,AL203 {C3A)

S - W90

I I I j,

c-~

c-.z2

z w

- = ~ = , , c.j

I

z 2 ~

2 1

2 2 l

l 0 S I 31 $/ . 33' SO ~.l 41 /,4 &2 &0 ] l N ) & 31 3 1 I I M i & n M i I zj

F I G . 1

X-Ray Diffraction Patterns of Alinite Cements (Ci, C 2, C 3) as compared to Portland Cement (PI'~P2 ~

Elemental Analysis of Cements

The elemental analysis as per the structures predicted in literature has been compared with the chemical analysis of the alinite cement Ci in Table 1. Lampe et.al, (4, 5) have recently indicated that alinite is stable only in the presence of MgO and hence the structure of alinite should be Cag, 9 Mgo.• ~I~3[Si~. ~ AI~,~ 0.~ I O~.q Cl instead of Ca~ (Sieq~ AIo.~)~ O~a Cl as reported by Ilyukhln et.al 3 . I t is interesting to note that in either case the maximum amount of chloride substitution in the alinite lattice structure is of the order of 4.1% by weight. In fact, the chemical analysis of

Vol. 22, No. 8 ALINITE CEMENTS 1059

pure a l i n i t e crystals (2) compares quite well with the results reported by Lampe e t .a l . (4). Just as a l i n i t e is a le i tmo t i f of a l i t e , the corresponding phase be l in i te , which is a le i tmot i f of bel i te phase in portland cement, has also been synthesized recently (5). This phase can accommodate higher quanti t ies of chloride, that is , 9.1% by weight in i t s l a t t i ce (Table I ) . I t has, however, been observed that unlike a l i n i t e , the hydration character ist ics of be l in i te , are not better than the corresponding portland cement phase be l i te (5). Another phase in th is series, which has been synthesized and characterized is C2S. CaCI 2 (2CaO. SiO 2. CaCI2) which contains 25% by weight of chloride (6). This phase is not as stable as a l i n i t e . In fact , i t has been reported as an intermediate phase in the formation of a l i n i t e (2). The last two columns of Table I present the results of chemical analysis of sample C£ as well as the theoret ica l ly computed composition of f inal cement, based on the analysis of raw mix af ter taking into account the loss on igni t ion during c l inker ing. I t appears from the results that a portion of CaCl 2 added to the raw mix evaporates during cl inkering and therefore both Ca and chloride assays in the f inal cement are lower. In terest ing ly , in a l l the three samples, Ci , C 2 and C 3, the f inal chloride content is of the order of 3% as compared to 4.1% chloride content of pure a l i n i t e crystals (3, 4).

I~S - 29Si NMR Studies

In order to characterize and dist inguish the a l i n i t e phase from that of a l i t e phase in portland cement, a high resolution :gSi magic angle spinning NMR study (8) was carried out on these samples. 2~Si MAS NMR spectra of three a l i n i t e samples Ci , C 2 and C 3 are shown in Fig. 2. A single sharp resonance in the -71.0 to -72.0 ppm range is observed for a l l the samples. This suggests that s i l icon has a unique position in the structure of a l i n i t e phase and the local environment of s i l icon is same in a l l these samples. A comparison of the observed chemical sh i f t values (around -71 ppm) with those reported in the l i te ra tu re for similar s i l i ca te compounds (9) suggests that s i l icon in a l i n i t e phase is in a Q° d is t r ibu t ion , that is , Si is in a purely tetrahedral arrangement as in SiO~ . The crystal structure of pure a l i n i t e as reported by l lyukhin e t .a l . (3), also indicates one unique position of Si in isolated s i l i ca te tetrahedra. A single sharp si l icon resonance peak around -70 ppm, is therefore expected, as indeed observed in our investigations (Fig. 2).

29Si NMR chemical sh i f ts are very sensit ive to changes in the geometry and the local environment of the s i l icon atom (9). Distort ion from a pure tetrahedral arrangement of Si04 also affects :eSi chemical sh i f ts . I f one ignores the effects of the SiOSi bond angle and the cation, the influence of structural factors on the s i l i ca te ion can be readily seen in the NMR signals. :SSi MAS NMR spectra of the two portland cements PI and P2 (Fig. 3), consisting of predominantly a l i t e phase c lear ly show differences from the spectra of the a l i n i t e samples. Both these portland cement samples show three s i l icon resonances in the Q° region at -70.1, -72.3 and -74.5 ppm correspon- ding to the three nonequivalent SiO~ tetrahedra noticed by Jeffrey (I0) in his X-ray structural work. The spectrum of P2 shows better resolution possibly because i t is of a superior qual i ty . The observation of 3 sharp resonances in our samples of portland cement (PI and P2) is to be compared with the 9 resonances reported by Magi e t . a l . ( 9 ) in the spectrum of pure synthetic tr icalcium s i l i ca te . These workers have suggested 9 non-equivalent posit ions for Si in support of the structure proposed by Golovastikov e t .a l . ( i i ) . The f i r s t resonance in the nine l ine spectrum appears around -69 and

1060 PRADIP, et al. Vol. 22, No. 8

I J I • 50 0 -SO

I I I -IO0 -ISO -2OO

Chemical shift |ppm)

FIG. 2

29Si MAS NMR Spectra of A l in i te Cements a) C 1 b) C 2 and c) C 3

f i i i . , J -60 -65 -70 -75 -110-85

Chemical shift (ppml

- 2 ; ' " 4 ~ ' ' '-~'o' ' '-~o' ' "~;o'-~o Chemical shif t [ppm]

FIG. 3

29Si MAS NMR Spectra of Portland Cements a) PI and b) Pz, recorded with Gaussian resolution enhancement

the last at -75 ppm, the most intense resonance appearing at -74 ppm. In our spectrum, the most intense signal is around -72 ppm. I t therefore seems P1 and P2 real ly possess much less than 9 non-equivalent sites, probably three.

I t is to be stressed that a comparison of the 29Si NMR spectra obtained for the a l in i te and a l i te (portland cement) samples (Fig. 2 and 3) clearly shows the remarkable difference between the two phases. MAS-NMR can therefore be effect ively used as a powerful technique for characterizing the s i l icate structure in cements.

Vol. 22, No. 8 ALINITE CEMENTS 1061

Conclusions

The following conclusions are evident from the data presented in the preceeding sections on a l i n i t e cements:

1. Low temperature cements produced from industr ial and mining wastes such as f l y ash and ta i l ings sands essent ial ly consist of a l i n i t e phase, a phase qua l i ta t ive ly d i f ferent from phases in portland cement and d i s t i nc t l y ident i f iab le by X-ray studies.

2. X-ray investigations unambiguously prove the presence of a l i n i t e as a predominant phase in these samples. These X-ray results on a l i n i t e cements are identical to the data ear l ie r reported by Nudelman e t .a l . ( I ) for a l i n i t e crystals synthesized from pure oxide compounds.

3. MAS29Si NMR is most useful for dist inguishing the a l i n i t e structure from that of a l i t e . The NMR results confirm the presence of Q° d is t r ibut ion of s i l i ca te ions in a l i n i t e samples. The spectra of portland cements show greater structure due to at least three inequivalent s i l icon positions in the a l i t e structure.

Acknowledgements

The authors thank Mr. D. Vaidyanathan, Dr. B.N. Singh, Mr. H.N. Prasad and Mr. B.G.K. Baliga for the valuable help during the course of this work.

i .

2.

3.

4.

5.

6.

7.

References

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M. Bikabou, ib id. IV-371.

V.V. l lyukhin, N.N. Nevsky, M.J. Bickbou and R.A. Howie, Nature, 269, 397 (1977).

F.V. Lampe, W. Hilmer, K.H. Jost, G. Reck and A. I . Boikova, Cement and Concrete Research, 1__6, 505 (1986).

F.V. Lampe, K.H. Jost, B. Wallis and P. Leibnitz, Cement and Concrete Research, 16, 624 (1986).

W. Kurdowski and K. Miskiewicz, Cement and Concrete Research, 15, 785 (1985).

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1062 PRADIP, et al. Vol. 22, No. 8

8. E. Oldf ield and R.J. Kirkpatr ick, Science, 227, 1537 (1985).

9. M. Magi, E. Lippmaa, A. Samson, G. Engelhardt and A.R. Grimmer, J. Phy. Chem. 88, 1518 (1984).

i0. J.W. Jeffrey, Acta Cryst. _5, 26 (1952).

11. N.I. Golovastikov, R.G. Mateeva and N.V. Belov, Soviet Physics - Crystallography 20, 441 (1975).