Internal morphology of a fiber-reinforced cement

CEMENT and CONCRETERESEARCH. Vol. 4, pp. 327-333, 1974. PergamonPress, Inc Printed in the United States.

INTERNAL MORPHOLOGYOF A FIBER-REINFORCED CEMENT Michel Murat Laboratoire de Chimie Appliqu~e et de G~nie Chimique Universit~ Claude-Bernard Lyon I, France

(Communicated by R. G. L'Hermite) (Received Nov. 23, 1973; in final form Jan. lO, 1974) ABSTRACT The fracture of a fiber-reinforced cement, investigated by scanning electron microscopy, shows different types of areas with emergence and imbrication of fibers. Particular wellmonocrystallized ettringite ("flower-like" crystallization), generally not encountered in hydrated pure portland cement paste, can be observed in some voids or cavities. This phenomenon is due to the macroporous texture of the material.

La fracture d'un ciment arm~ de fibre (fibrociment), observ~e par microscopie ~lectronique ~ balayage, pr~sente diff~rents types de surfaces avec emergence et imbrication des fibres. De l ' e t t r i n g i t e particuli~rement bien cristallis~e (cristallisation en "fleur"), g~n~ralement non observ~e dans les ciments portlands hydrates ~ l ' ~ t a t de pate pure, peut 8tre observ~e dans certains vides ou cavit~s. Ce ph~nom~ne est du ~ la texture macroporeuse du mat~riau.

327

328

Vol, 4, No. 2 SEM, MORPHOLOGY, FIBER REINFORCEMENT, CEMENT

For the l a s t five years, the scanning electron microscope has been largely used for studying the morphology of materials.

Numerous papers and p a r t i c u l a r l y

those published in this Review, deal with the setting of pure phases encountered in the Chemistry of Cements, tl-4)'' the study of hydrated portland cements

(5-8)

hot-pressed cements, (9) cements with additives, (10) jet-cements, (11) hydration products of blast-furnace slags,(12-13) adhesion between cement and surrounded iron, t14)"" hydrated plasters,(15-19) and concrete carbonation. (20) We want to describe some observations made with scanning electron microscopy on the fracture of a f i b e r - r e i n f o r c e d cement (french "fibrociment"). This material is generally obtained by a process employed in the manufacture of paste. board.

The raw materials are portland cement without additives (and with low

gypsum content) and fibers (usually asbestos).

Some treatments are necessary

for obtaining a good forming of the plates, such as hot pressing which accelerates the setting. The sample studied in this work has been broken, and a thin layer of gold has been deposited on the fracture by a vacuum process, before observations. The aspects of this fracture, observed with small or middle magnification, are of two types: some large areas are free of fibers (Fig. I) and formed of hydrated cement plates with an apparently bad c r y s t a l l i z a t i o n .

The encapsulat-

ion of CSH by the portlandite crystals is not evident and a great number of large-size holes or voids can be seen between the blocks of hydration products. On other areas, the imbrication and emergence of fibers are easily observable (Fig. 2) and with a larger magnification, theadhesion between hydrated products

FIG. 1 SEM observation of an area of the fracture. Photo b is obtained by magnification of the central part of photo a.

Vol. 4, No. 2

"

~....

I

lOOM"

SEM, MORPHOLOGY,FIBER REINFORCEMENT,CEMENT

a

"

329

"

FIG. 2 SEM observation of another area of the fracture and fibers, and the encapsulation of fibers are clearly seen in Fig. 3.

Some

large-size fibers have been broken during the sample preparation for SEM observations (Fig. 4) and some hydration products are particularly well-crystallized in the cavities.

That is the case for the central part of photo (a)-Fig. 4:

when the magnification of this part is larger, "flower-like" crystallization can be observed (photo a Fig. 5) among large spherulitic grains, the surface of which is covered with very small crystallites (photo b Fig. 5). This "flower-like" crystallization is certainly due to the formation of ettringite: crystals are acidular and have grown around nuclei.

"20 ~

a

,

Similar crystal-

• lO/~,

FIG. 3 Magnification of the upper right part of photo b of Fig. 2.

330

Vol. 4, No. 2 SEM, MORPHOLOGY, FIBER REINFORCEMENT, CEMENT

.

.

.

.

50 ~

.

.

.

.

.

.

.

.

a

. . . . .

FIG. 4 SEM observation of the emergence of a large f i b e r . l i z a t i o n and growth process occur for instance in the case of the hydration of plasters t18)"" or during the formation of some synthetic calcium sulfate dihydrates. (21)

I t appears generally when the material contains many holes or voids

in i t s texture. Another characteristic example is given in Fig. 6.

The central Dart of

this area has been largely magnified in Fig. 7: some details of the supposed e t t r i n g i t e crystals become apparent.

i

Every crystal c o n s t i t u t i n g the "flower-

,it

Photo a : Photo b:

FIG. 5 Magnification of the c r y s t a l l i z a t i o n seen on the central part of photo a of Fig. 4. Magnification of the s p h e r u l i t i c grain seen on the upper r i g h t part of photo a.

Vol. 4, No, 2

331

SEM, MORPHOLOGY,FIBER REINFORCEMENT,CEMENT

•

, so

a

. . . .

2o1

FIG. 6 SEM observation of another area of the fracture. Photo b is obtained by magnification of the central part of photo ~. m

l i k e " product seems to be fomed of long monocrystals which have the same orien. tation,

The outside surface of such crystals is not perfectly smooth but ap-

pears as stepped in the d i r e c t i o n perpendicular to the elongation axis.

Such a

morphology of e t t r i n g i t e ("long l a t h - l i k e " crystals) has been observed by electron microscopy t22)'" in the case of hydrated super-sulphated cements and is chara c t e r i s t i c of a growth from solution. The c r y s t a l l i z a t i o n

is very d i f f e r e n t around the emergence of f i b e r s , as

i t can be seen in Fig. 7 (magnification of the r i g h t upper part of photo a of

FIG. 7 Magnification of the " f l ower-I i ke" crystal I i zation seen on the central part of photo b, Fig. 6.

332

Vol. 4, No. 2 SEM, MORPHOLOGY, FIBER REINFORCEMENT, CEMENT

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

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

"

S

FIG. 8 Magnification of the emergence of the fibers seen on the r i g h t part of photo a, Fig. 6. Fig. 6).

The hydrated cement grains are covered with very small crystals (less

than 0,I ~m length) with a great d i s o r i e n t a t i o n .

Their c r y s t a l l i z a t i o n state

does not seem to be very good. (Fig. 8) In conclusion, we can make some general remarks: during the hydration of the f i b e r - r e i n f o r c e d cement, the calcium sulphare i n i t i a l l y

present in the port-

land cement, becomes concentrated probably in the aqueous i n t e r s t i t i a l and especially in voids or cavities.

phase

Such a phenomenon can explain the particu-

l a r " f l o w e r - l i k e " c r y s t a l l i z a t i o n of e t t r i n g i t e from seeds in the l i q u i d phase. The same c r y s t a l l i z a t i o n phenomenon has been observed with other types of hydrated phases in the Chemistry of Cements. That is the case for portlandite which appears sometimes as long c r y s t a l s , with hexagonal section, at the i n t e r face between cement and surrounded iron, ~14)'' that is to say in a place where voids or cavities are present.

Another example is that of aluminous cements.

Beautiful crystals of hydrated aluminate appear during hydration.

CAHIo develops

from seeds and c r y s t a l l i z e s with the time in the voids as " p e n c i l - l i k e " crystals. (23)

These phenomena are d i f f e r e n t of those observed in the case of the hydrat-

ion of portland cement paste, that is to say: bulky formation and CSH on the surface of C3S grains, and c r y s t a l l i z a t i o n of portlandite as thin plates which cover slowly the CSH. The presence of w e l l - c r y s t a l l i z e d e t t r i n g i t e in voids is not systematic, in the case of the f i b e r - r e i n f o r c e d cement studied here.

This observation is in

agreement with the low gypsum content of the portland cement generally used in the manufacture of these reinforced materials.

Vol. 4, No. 2

333 SEM, MORPHOLOGY,FIBER REINFORCEMENT,CEMENT

Acknowledgement We thank Miss C. Bardot, Ing~nieur C.N.R.S., who has realized the photographs of scanning electron microscopy. References I. M. Daimon, S. Ueda and R. Kondo, Cement and Concrete Res. ! , 391 (1971). 2. F. V. Lawrence, Jr., Cement and Concrete Res. 3, 149 (1973). 3. B. Cottin, Cement and Concrete Res. ~, 177 (1971). 4. A. Negro, Rev. Mat. Constr., Fr., (1973 nov.) (in press). 5. S. Chatterji and J. W. Jeffery, Nature, G.B., 209, 1233 (1966). 6. S. Chatterji and J. W. Jeffery, Nature, G.B., 214, 552 (1967). 7. S. Diamond, Proc. of the Third Annual Scanning Electron Microscope Sjnnpos. ium, IIT Research Institute, Chicago, I l l i n o i s , p. 385 (1970). 8. S. Diamond, Cement and Concrete Res. 2, 617 (1972). 9. D. M. Roy, G. R. Gouda and A. Bobrowsky, Cement and Concrete Res. 2, 349 (1972). lO. V. S. Ramachandran, Cement and Concrete Res. 3, 41 (1973). I I . H. Uchikawa and K. Tsukiyama, Cement and Concrete Res. 3, 263 (1973). 12. R. Dron and P. Petit, C. R. Acad, Sci., Fr., 274, 1275 (1972). 13. M. Murat, M. Charbonnier and A. Negro (in press). 14. M. Moreau, Rev. Mat. Constr., Fr., 678, 4 (1973). 15. N. H. Harbord, D. K. Manikon and R. Smeeton, J. Roy. Microsc. Soc., 87, I05 (1967). 16. A. J. Majumdar, J. F. Ryder and D. L. Ra~nnent, J. Mat. Sci. 3, 561 (1968) 17 G. Jeandot and M. Murat, Rev. Mat. Constr., Fr., 674/475, 313 (1971). 18 M. Murat and G. Jeandot, Mat. and Struct. 32, 129 (1973). 19 20 21 22 23

A. Z. M. H. B.

N. Knauf, Tonind. Ztg. 97, 3 (1973). ~auman, Cement and Concrete Res. ~, 645 (1971). Murat, Tonind. Ztg., (in press). G. Midgley and K. Pettifer, Cement and Concrete Res. ~, lOl (1971). Cottin, Cement and Concrete Res. ~, 273 (1971).