Evidence of calcium silicoaluminates in hydrated mixtures of tricalcium silicate and tricalcium aluminate

CEMENT and CONCRETERESEARCH. Vol. 6, pp. 733-740, 1976. Pergamon Press, Inc Printed in the United States.

EVIDENCE OF CALCIUM SILICOALUMINATES IN HYDRATED MIXTURES OF TRICALCIUM SILICATE AND TRICALCIUM ALUMINATE

Mo Regourd, H. Hornain and Bo Mortureux D4partement Microstructures, C.E.R.I.L.H. 23 rue de Cronstadt

75015 Paris - France

(Communicated by H. F. W. Taylor) (Received July 16, 1976)

ABSTRACT Calcium monosilicoaluminate C3A.CaSi03o12 H20 and calcium trisilicoaluminate C3A.3 CaSi03o3] H20 have been identzfied in pastes of C3S + C3A. Electron probe microanalysis and scanning electron microscopy with energy X-ray dispersive analysis show that silieoaluminates are both formed around C3A grains by diffusion of silicate ions from C3S grains through solution. In samples containing gypsum, these silicate ions diffuse through the sulfoaluminate zone.

Le monosilicoaluminate C3A.CaSi03o12 H20 et le trisilicoaluminate C3A.3 CaSi03o31 H20 ont 4t@ identifi@s dans des p~tes de C3S + C3A. L'analyse ~ la microsonde 41ectronique et au microscope @lectronique i balayage 4quip@ d'un spectrom~tre X dispersif en 4nergie montre que ces silicoaluminates se formeni autour des grains de C3A par diffusion des ions silicate depuis les grains de C3S, i travers la solution. Dans les @chantillons contenan% du gypse, les ions silicate diffusen% ~ travers la zone de sulfoalumina%eo

733

734

Vol. 6, No. 6 M. Regourd, H. Hornain, B. Mortureux

Introduction The existence of silicoaluminates in hydrated mixtures of calcium silicates and aluminates and in hydrated Portland cement is generally considered as doubtful° However, Flint and Wells (I) synthesized such phases by precipitation from solutions of calcium silicate and aluminate with various amounts of solid calcium oxide to satura~ the solutions. In some cases, a finely divided precipitate appeared that was entirely composed of very small hexagonal plates occurring in ~ph~ulitic groups; on standing for 3 years in contact with lime solution, these slowly converted into needle-shaped prisms. In other cases, hexagonal plates coexisted with amorphous material° Chemical analysis of the plates after 27 days gave molar ratios corresponding to %he formula C3A.CS.H12. The needles gave lhe formula C3A.3CS.H31 ; this %risi~coaluminate was not obtained pure. I% was also detected im mixtures made with ~-C2S + C4AF , ~-C2S + C3A , or C3S + C4AF but not with C3S + C3A. Silicoaluminates appeared to be th! silica analogs of the two hydrated calcium sulfoaluminates C3A.xCSoyH found in hydrated Portland cements° In certain mixtures made with CaO, Si02 and A1203, Mohri (2) identified a compound which may have been_C3A.3CS.H31. Its X-ray diffraction pattern resembled that of C3A.3CS.H31 but electron microscopy showed only thin hexagonal plates, and not the needle morphology typical of ettringite. Mituzas, Kaminskas and Mituzas (3) deduced from chemical analysis that a phase of approximate composition C3A.3C~.a q. existed in hydrated Portland cements. It was not possible to extract it with hot saturated H3BO 3 solution. Tricalcium silicate and %ricalcium aluminate are the first minerals to hydrate when Portland cement is mixed with water° We have investigated the mutual interactionofC3S,C3A and water, and detected calcium silicoaluminates by electron probe mieroanalysis (EP}~), scanning electron microscopy (SEM) and X-ray diffraction (4).

Experimental Minicylinders (volume I cm 3, section I em 2) of mixtures of C3S + C3A with marying granulometry (25-40 um or 40-63 bm) were prepared with water/solid ratios of 0.35 or 0.50~ kept in a moist atmosphere for one day, and then immersed in fresh water at room temperature. Nitrogen circulation was used to prevent carbonation of %he samples° Except where otherwise stated~ the mineralogical composition of the mixtures was 76 % C3S + 19 % C3A + 5 % gypsum (triclinie C3S and cubic C3A ). The composition represented the mean amounts of silicates (C3S + C2S)~ aluminates (C3A + C4AF ) and gypsum in Portland cements. At predetermined times (2, 7, 28 days; 3, 6, 10 months), 3 minicylinders were fraLtured in tension-compression (Brazilian Test), freeze-dried, and examined by EPMA ( C ~ C A MS 46), SEM (JEOL 50 A) with X-ray energy dispersive analysis (TRACOR XS 880) and X-ray dif-

Vol. 6, No. 6

735 CALCIUM SILICOALUMINATES, PASTE HYDRATION

fraction (Guinier camera and Siemens monochroma±or)o Exoerimental

diffractometer

equipped with a

Resu!±s

Examination of polished sections, after different
76 C3S+19C3A + 5G 7days

hydration C3 C 6 AS3 C-S C,

10 Hm

absorbed

Ca

electrons

Si

S

Ai

FIG.

I

EP>k& elemental distribution for a C3A grain hydrated for 7 days at w/s = 0°35

736

Vol. 6, No. 6 M. Regourd, H. Hornain, B. Mortureux

C6AS3H31 always forms the i n t e r n a l zone° The d e p t h of the h y d r a t e d layer v a r i e s from one g r a i n of C3A to another, The smallesi crystals in the sample (25-40 ~m) are a l m o s t wholly i~ydrated, C h e m i c a l reactions are faster for w a t e r / s o l i d : 0,5 than for ~/s : 0°35° %q~en there are no more free S042- ions, ~olo~%es of i:~ono=u~oal ~ m• l~ ~,I ~o ~ e.. -. ~ are formed° The C3S grains are s u r r o u n d e d by a layer of C-S-H (C/S : I ,5) w h i c h contains 2 ~ AI205 (by weight) coming from the C3A,

7GC3S + 1 9 C 3 A + 5 28 days hydration C3A. C6AS3H C4A~H1 lOi,t absorbed

Ca

S

FIGo EPMA

electrons

Si

AI

2

: e l e m e n t a l d i s t r i b u t i o n for a C3A g r a i n h y d r a t e d for 28 days at w/s = 0.35

A% 3 months, both s i l i c o a l u m i n a t e s are c l e a r l y o b s e r v e d around large grains of C3A (Fig. 3). On p a s s i n g away from the C3A grain there can be seen s u c c e s s i v e l y ~ t r i s i l i c o a l u m i n a t e (zone I), monos i l i c o a l u m i n a % e (zone 2)~ C4AH13 (zone 3) and m o n o s u l f o a l u m i n a t e (zone 4)° The m o n o s i l i c o a l u m i n a t e contains about 2°5 ~ $O 3 (by weight) while only I ~ SO 3 is p r e s e n t in the t r i s i l i c o a l u m i n a t e . At 6 months, all C3A grains are s u r r o u n d e d by both s i ! i c o a l u minates and m o n o s u l f o a l ~ i n a t e coexists with C4_Aj{13 o The values of %he m o l a r ratios for h e x a g o n a l p l a t e s in c o n t a c t w i t h m o n o s i ! i c o al~ina%e (CaO/AI203 = ~o4~ C a O / S O 3 = 5.7, S 0 3 / A 1 2 0 3 = 0.6 a~ree w i t h a m i x t u r e of C 3 A , C S , H 1 2 and C4AH13 , and this was c o n f i r m e d by X-ray diffraction,

Vol. 6, No. 6

737

CALCIUM SIL[COALUMINATES, PASTE HYDRATION I

;l'

I i t

-.Y t 1,,

I

t f

q. • ,, ~&:!

'{

] '-------~ t . it I

.

.

.

.

t: ;i :. ~

, I

1

.I L I '<----~............. :41i:,m

w~' I I

/f~,

I l

' t t

j~,_

AI

Si

$

FIGo

:i,!<"

i

I: ! ~.

[

,-i": .....

/

.

i S

/

f

i

.....2

I ,q".

i Ca K=

KI]

3

(a) SEM of a polished section of a paste (76 % C3S , 19 % C3A , 5 % gypsum) hydrated for 3 months at w/s = 0.35, showing (I) trisilicoaluminate, (2) monosilicoaluminate, (3) C4AH15 and (4) monosulfoaluminateo (b) Energy-dispersive analytical results for regions (1)-(4)

At 10 months, almost all of the C3A grains are hydrated in the samples with particle size beiween 40 and 65 bm. The concentraiion of silicon in the m o n o s i l i c o a l ~ i n a t e decreases from the contact with C4AH15 and monosulfoaluminate towards the center of the C3A grain (Table I)o Minicylinders of the same mixture immersed in sea-water gave much ettringite and no silicoalmminate. No silicoaluminates were detected when SO42- ions were present and we therefore investigated two new compositions : one with more gypsum than C3A and the other with no gypsum. With pastes made with 64 ~ C3S , 16 % C3A and 20 gypsum, no silicoaluminate was observed at any time. Ettringite and C4AH]3 were identified at 34 days together with C-S-H which contained 1.5 ~ S03 (by weight) but no A1203o With pastes containing no gypsmm (80 % C3S and 20 % C3A), %risilicoaluminate formed at 7 days around some grains of C3A and a broad zone was observed at 34 days (Fig° 4). At this latter age, X-ray diffraction showed %he presence of C4AH13 , Ca(0H)2, C-S-H and monosilicoal~minate. This last phase~ the presence of which is of particular interest gave a pattern similar %0 tha} of monosulfoaluminate (characteristic reflexions at 8°92, 4°46, 2.88 i) (Fig° 5)° No X-ray lines attributable to the Si analog of the %risulfoaluminate were detected° The amounts of monosilicoaluminate CdASH12 , found by X-ray diffraction and by SEM were low compared with that of C4AHI3. Using X-ray energy dispersive

738

Vol. M. Regourd,

H. H o r n a i n ,

TABLE

I

, , o - a r ..... i o s f o r S i l l : o - ~ , L , < : t : a t e s P r e s e n t i n ..... <} i . , b - : ! . L. 3A t l c~ , :~.:-d O.)-:fsu::~ (5 -~) a t ., ., .a.~. r Sta~.:_~rd Ez'rors ( . A b s , ~ . " ~ i e l A , . e r : ~ g e 0 , 2 <, ,

5, No. 6

B. Mortureux

~

P : t s t e s :'Jade " ¢ i t l - U3S : Solids Ratio = '~ 35

Pito.se

7ixe

C/'S

C,A

S/A

Y r i s i l i c o a i u m [r~ate

9:< [ , , < <

9 1

5 &

9,7

!

3 m';~ihs

1~7

4.4

2.5

1,2

3 ~::o..ut'>-:

_°9 4

6 o~

~°,6

i ,7

26 d u v s

4,0

3.6

0.9

1.9

3 r-:~nt h e

3o7

3~3

0.9

i.7

6 months

5 ~

3 8

0.7

P P

10 monihs

6°3

3.~

0~.~

1oi

10

4.4

3.3

0.$

i .9

(in!ernai

,:one)

Hono s i i i coal urn±nat e ('~

-4 ~ x,_ernal zone)

H o n o s i I.icoal urn±mate (center) (edge)

:::o~lths

anal'sis, ihe m o n o s i l i c o a l u m i n s t e the ~ r z s l l i c o a l u m i n a t e a~q <4AH13

a p p e a r e d .~s a ihin layer which ~sn co '~ "'-'

C ( S + A)

5

%=tween

Discussion E P H A and SEM have p e r m i t t e d ihe identifir, a t i o a in ~-a~T=s made z. . . . from C3S , C 3 A and gvpsur.,., of two silicoal,a~.~,.~.-~ ..... .~s made by.. Flint and Wells (I) from s o l u t i o n s of c a l c i u m s i l i c a t e s and a l u m i n a < e s . H o w e ver, even using both these ± e e h n i q u e ~ we could no±. confirm
.

.

.

Vol. 6, No. 6

739 CALCIUM SILICOALUMINATES, PASTE HYDRATION

C3S

80% C3A

20%

34 days hydration C4AH13

S Z0,~'

..f-.

C4ASH12 IL92~ ,

C4ASH12

~

"•

. . . ". , . "'"

,,", .

C3A

..'.:

FIGo 4

S~f of polished section of a paste (80 ~ C3S , 20 }~ C3A) hydrated for 34 days at w/s : 0°35. x : trisilicoaluminate~ A = monosilicoaluminate~ o : C4AH13

9.5o

l0 a

10.$ ~

FIGo

11 °

20 °

29C~JK~1

11 5 ~

2~) C ~ K a |

5

X-ray step-by-step diffrac
licoaluminate, i.e. the max±myra silica concentra±ion is furthesx from the C3S grain° This external zone of monosilicoaluminate
740

Vol. 6, No. 6

M. Regourd, H. Hornain, B. Mortureux zes and shapes° In these heterogeneous products, the diffusion vectors are more random° Both the formation and the observation of sili co~Im::inates &re likely to be more difficult but the deductions of >ii~uzas, Kamiskas ~nd }[ituz~s (3) suggest that they must occur. Our experiments have shown on the one hand, the great affinity of silicate ions for C3A and on the other, the intense and rapid diffusion of silicate through the solution° These results can bring so=e ne~" conceptions into cement hydration. They are a point in favor of xhe dissolution of C38 according to Le Chateiier's theory. The present results concern pastes made with cubic C]A. We h~ve made simil~r studies on the formation of silicoaluminates in samples using solid solulions of C3A containing Na, the results of which will be published in a second paper° Ackno~ led~ement The authors gratefully cri±ical comments°

ackno~'ledge

Professor

H.F~W.

Taylor

for his

References Io EoP.

Flint

2. Jo Mohri,

and L.So Wells, CAJ Revo

Jo Res.

Nat.

12 th Gem. Meeting,

Bur.

Stand,

9, Tokyo

(1958)~

3° Yu. I. Mituzas, Ao Yuo Kaminskas and ~.Yu.>Iituzas, Cl~emistry of Cement, Moscow (1974)o 4. M. Regourd~ Ho Hornain Chaux, 687, 69 (1974),

et B. Mortureux~ 693, 87 (1975)o

__

__

6 %h Into Congo

Cimen~s-B@tons-Pl&~res-

5o WoAo Corstanje, HoNo Stein and JoMo Stevels, 3 791 (1973); 4, 193 (1974); 4, 417 (1974) __7

33, 471 (1944)

Cement °

Concre±e

Res.