Mechanical properties in relation to the microstructure of fibre reinforced portland clinker pastes

CEMENT and CONCRETE RESEARCH. Vol. 8, pp. 765-774, 1978. Printed in the United States.

Pergamon Press, Inc

MECHANICAL PROPERTIES IN RELATION TO THE MiCROSTRUCTUREOF FIBRE REINFORCED PORTLAND CLINKER PASTES R. Sh. Mikhail, M. Abd-EI-Khalik and A. Hassanein Department of Chemistry, Faculty of Science Ain Shams University, Cairo, Egypt D. Dollimore and R. Stino Department of Chemistry and Applied Chemistry University of Salford, Salford M5 4WT, U.K. (Communicated by J. Skalny) (Received July 27; in final form Sept. 15, 1978) ABSTRACT Portland clinker was mixed with various water/clinker ratios ranging between 0.7 to 0.2 covering the range for both "normal" and "low" porosity pastes. These pastes were reinforced with five kinds of fibres, all with weight percentages of 0.5 and 3.0. All the samples were tested for compressive and tensile strengths, total porosity, microstructure and degree of hydration. Fibre reinforcement led to an increase in the total porosity and a decrease in the degree of hydration as compared with the neat pastes. Also, compressive strength has to be sacrificed to a certain extent in order to obtain better flexural strength. SEM furnished direct evidence that fibre reinforcement could affect the pore structure, the habit and shape of the hydration products, as well as their spatial distribution. The indication gained is that the mechanical behavior of the mix is due mainly to changes in the physicochemical properties induced by the presence of fibres.

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Vol. 8, No. 6 R.Sh. Mikhail, M. Abd-EI-Khalik, A. Hassanein, D. Oollimore, R. Stino

Introduction Strength is the property most sought-after during fabrication of hydrated portland cement products, and much work has been done in an effort to understand the various parameters governing i t (1,2). I t is generally known that hardened cement pastes and concrete are weak in tension in comparison with their strength in compression (3). The weakness was conventionally overcome by strengthening their matrices with steel and, more recently, by reinforcement with fibrous materials. Over the last decade, extensive research on the properties of fibre reinforced cements has been conducted in several laboratories (4), with particular interest focused on the type and chemistry of the fibre used, type of material, age, fibre content and their effect on the mechanical properties of the fabricated composites. The relationships and dependence of the mechanical properties of cement pastes and concrete on porosity was summarized earlier by Powers (5), and on microstructure was pointed out by Feldman and Beaudoin (6). Fibre reinforcement is expected to affect both porosity and microstructure, but unfortunately no detailed systematic studies were conducted to investigate this effect. The present study is an attempt to c l a r i f y the effect of fibre reinforcement on the microstructure of both normal and low-porosity portland clinker pastes. Clinker and not cement is used in this investigation, as i t represents a less complex system upon which academic studies could be done and preceding that on cement, which undoubtedly is of more practical interest. Experimental Procedures Clinker (NB) was supplied by the Blue Circle Company (Nottingham, U.K.) and is of "Type I" according to ASTM designation. The Blaine surface areas, after laboratory grinding in porcelain j a r mills, was 5300/5600 cm2/g. Five types of fibres were used for reinforcement, namely, crocidolite (blue) asbestos, chrysotile (white) asbestos, E-glass, "cem-FIL" glass and polymer coated glass fibres. In all types, the length of fibres was limited to about 0.6 inch. E-glass will be denoted in this work as type I glass and glass "cem-FIL" type I/ glass. Table I summarizes the main characteristics of the various fibres used. Table l The main characteristics for the various fibres used Type of fibre

Diameter (pm)

Specific gravity

Tensile strength (Ib/in2xlO 3)

Elongation at b r e a k ~

Availability of the fibres

Suppliedof/ manufactured by

brittle/ plastic

limited mainly in South Africa

RefractorsCo, Helwan.Koegas mines.

brittle

unlimited

Z I - N a s Glass r factory (fibre branch,Dokki)

Crocidolite (blue)

01-I

3.37

500

Glass (E)

10-15

2.56

300-500

Chrysotile (white)

0.02-I

2.25

450

3

brittle/ plastic

unlimited(mainly Canada& USSR)

CapeAsbestos Fibres Ltd. Uxbridge

Glass 'cem-fil'

30-60

2.71

300-400

2-3

brittle

Pilot s t a g e

Pikington Brothers Ltd.

.

5-10

Polymer coated filaments glass fibre of 12 rect. x-section

.

.

.

2-3

Stress/strain character, at room temp.

2-3.5

brittle & fairly plastic

readily available

Vol. 8, No. 6 767 CLINKER PASTES, FIBRE REINFORCEMENT, POROSITY, MECHANICAL PROPERTIES

Table 2 Sample Nomenclature NB WL W2

Neat clinker, from Blue Circle Company, U.K. Neat clinker reinforced with 0.5% White asbestos (Chrysotile). Neat clinker reinforced with 3.0% White asbestos (Chrysotile).

GII 1

Neat clinker reinforced with 0.5% Glass fibres type ( I I ) .

GII 2 Gp 1

Neat clinker reinforced with 3.0% Glass Fibres type ( I I ) . Neat clinker (NB) reinforced with 0.5% of Polymer coated glass fibres. Neat clinker (NB) reinforced with 3.0% of Polymer coated glass fibres.

Gp 2

Mixes were made with different water/cement ratios ranging between 0.2 and 0.7, for both the neat and the fibre reinforced samples. Reinforcement was carried out using 0.5% and 3% by weight of each fibre. Details of mixing are given elsewhere (4). Sample nomenclature is given in Table 2, and a number in parenthesis, ranging between 2 and 7, would indicate a water/clinker (W/C) ratio ranging between 0.2 and 0.7, respectively. Compressive strength measurements were made on 2" cubes using an "Amsler" lO tons testing machine with a rate of loading 5-6 mm/min (Z779/5). Flexural strength measurements were made on I . I / 2 . 6 inch prisms, using a "WPM" testing machine, with a rate of loading equals 20.8 mm/min. Scanning electron microscopy was carried out on silver coated samples, with coating approximately 200 A thick, using a "Cambridge Type 96113 Mark 2 A" instrument. The degrees of hydration and the total porosities of the various hardened pastes were determined according to methods described earlier (7). Results A.

Degree of Hydration and total Porosity:

The degree of hydration of the various pastes cured f o r 180 days was found to increase with the increase in the W/C r a t i o . The i n t e r e s t i n g r e s u l t obtained is that, at a fixed W/C r a t i o , addition of fibres s i g n i f i c a n t l y reduces the degree of hydration and, the higher the percentage of f i b r e , the lower is the degree of hydration. Total porosity, ~, was calculated from the equation

E = We/Vp

where We is the weight or volume of evaporable water with density one, and Vp is the -volume of the paste. Fig. 1 shows the r e l a t i o n between the degree of hydration and total porosity for neat and f i b r e reinforced (NB) c l i n k e r pastes. As expected, higher W/C r a t i o produces pastes of higher total porosity, which results in the accomodation of a larger quantity of hydration products and, accordingly, would lead to higher degree of hydration. Keeping the W/C r a t i o constant, f i b r e addition leads to a decrease in the degree of hydration accompanied by an increase in the total porosity. The solid lines in Fig. 1 indicate t h i s r e l a t i o n s h i p . These lines were obtained by least square analysis of the data (Regression method), and they show negative slopes. As a f i r s t assumption, one can assume that f i b r e addition affects mainly the total porosity, e.g. by affecting a l l types of pores, namely, micro, meso and macro-pores, as well as t h e i r d i s t r i c u t i o n , consequently affecting i n d i r e c t l y the degree of hydration. The major d i f f i c u l t y with this assumption is that a mere increase in porosity, considering the fibres to be chemically i n e r t materials,

768 Vol. 8, No. 6 R.Sh. Mikhai], M. Abd-El-Khalik, A. Hassanein, D. Oollimore, R. Stino 'X% I

I

I

W/C=O.70

~00 -

FIG. l Total porosity versus degree of hydration for various clinker (NB) pastes of d i f f e r e n t fibre reinforcement at constant W/C ratio.

95

90

W/C: 85

should also bring an increase in the degree of hydration, B0 which is obviously not the case. Another alternative is to assume that the fibre addition affects 75 independently both the pore structure (8,9) as well as the degree of hydration. The l a t t e r 70 effect is most l i k e l y to be a specific e f f e c t . In a previous publication by the authors ( I 0 ) , i t was shown that fibre addition leads to considerable reduction in the specific surface area as measured by water, which was inI I I I , 1 I terpreted to be due to formation 1o 15 20 25 30 35 Z,0 ~ % of thicker tobermorite sheets, and the higher total porosity r, C h r y s . f i b e r s 0.5"/, E~ GI. f i b e r S ( t y p e II)0.5"/, due to fibre reinforcement proO Pl. cog, ted glass fibersO.5'/, • C h r y s . f ; b e r s 3.0 '/, vides the room needed for sheet • G[. f i b e r s ( t y p e I I ) 3 . 0 " / , • P I . c o a t e d glass f i b e r s 3 . 0 % thickening to take place. SEM ® Neat clinker. pictures presented in this paper w i l l give some indication of t h e i r effect on both the wide pores present, and also on the hydration products affecting t h e i r habits and shapes, and t h e i r spatial d i s t r i b u t i o n .

B.

Scanning Electron Microscop~:

SEM pictures were taken in order to throw some l i g h t on the microstructure of both neat and f i b r e reinforced c l i n k e r pastes cured for 180 days. The term microstructure, however, can have d i f f e r e n t meanings, depending on the level of sophistication, from the crystal structure to the three-dimensional arrangement of the hydration products in the set paste. Obviously, the l a t t e r d e f i n i t i o n is mainly concerned with the spatial d i s t r i b u t i o n of the solid matter, thus creating a definite pore structure, which affects many of the physicochemical as well as the engineering properties of the paste. A great deal of the spatial arrangement depends on the shape and habit of the crystals, and how these are packed together in the limited volume of the paste, as pre-determined by the water/clinker ratio. SEM of both neat and fibre reinforced pastes of W/C ratios of 0.2 and 0.7, cured for 180 days were studied, and those of W/C ratio of 0.2 are presented in Plates I-IV.

B.I.

Neat (NB) paste:

Plate [a and b show that the structure is mainly composed of densely populated hydration products, which are formed of i l l - c r y s t a l l i z e d material, which

Vol. 8, No. 6 769 CLINKER PASTES, FIBRE REINFORCEMENT, POROSITY, MECHANICAL PROPERTIES

Plate ( I ) :

Neat c l i n k e r (NB); W/C = 0.20; curea for 6 months.

l~sM

/O

(a)

Plate ( I I ) :

(a)

Plate ( I I I ) :

Clinker (NB) reinforced witn 3.0% c h r y s o t i l e , W/C = 0.20; cured for 6 months.

~b)

Clinker (NB) reinforced with 3.0~ glass f i b r e s W.C 0.20; cured f o r 6 months.

(II);

770 Vol. 8, No. 6 R. Sh. Mikhail, M. Abd-EI-Khalik, A. Hassanein, D. Dollimore, R. Stino

[o) Plate (IV):

Clinker (NB) reinforced with 3.0% polymer coated glass; W/C = 0.20; cured for 6 months.

is mainly CSH gel with random o r i e n t a t i o n throughout the volume of the paste. Within the dense matrix, r e l a t i v e l y wide "holes" are scattered throughout the paste; these holes are f i l l e d with larger, well c r y s t a l l i z e d hexagonal c r y s t a l s , assumed to be calcium hydroxide c r y s t a l s , also of random o r i e n t a t i o n . In this regard i t should be mentioned that the whole structure reproduces in an outstand. ing manner the picture predicted by Powers (5). B.2.

Fibre reinforced pastes:

Plate IIa and b represents the electron micrographs for the NB paste reinforced with 3.0% Chrysotile fibres. Evidently the main features of the microstructure of the neat paste (Plate I ) , are s t i l l preserved, with the additional existence of chrysotile f i b r i l s , making the crimping and meandering of the f i b r i l s a c h a r a c t e r i s t i c feature of the sample. Beside t h e i r predictable effect on the pore structure (mainly the macro-pore f r a c t i o n ) , the habit of the i l l c r y s t a l l i z e d hydration products suffer from some changes as well. Plate I l l shows the microstructure of the same NB paste reinforced with 3.0~ glass fibre (Type I I ) . Plate l l l a shows a bundle of glass f i b r e s , oriented almost parallel to each other, with no crimping or meandering, reinforcing the mesh of the hydration products with t h e i r elements going in criss-cross orientation. Plate I I I b shows under high magnification the surface of an exposed glass fibre in a hydrated specimen, and the interesting feature is the presence of a thin layer at the f i b r e - m a t r i x interface which looks d i f f e r e n t from the rest of the hydration products. This layer had a fine porous texture of amorphous material, which adheres quite strongly to the f i b r e . For the NB paste reinforced with polymer coated glass f i b r e s , two main features are observed. The f i r s t is reflected in Plate IVa, which shows that coated glass fibres did not affect the habits or shapes of the c r y s t a l l i t e s of the hydration products as did the other two kinds, of fibres mentioned e a r l i e r . The second main feature is shown in Plate IVb, namely the apparently weaker bond between the f i b r e and the paste matrix in which the f i b r i l could be pulled out leaving almost a "clean" smooth interface behind. Along the length of the fibre almost no hydration products are deposited, but at the broken ends, more adhering material and mo~e hydration products are always noticed.

Vol.

C)

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6 771 CLINKER PASTES, FIBRE REINFORCEMENT, POROSITY, MECHANICAL PROPERTIES

No.

Mechanical Properties:

For a l l the neat and f i b r e reinforced pastes, both the compressive and f l e x ural strengths increase with the increase in time of hydration, and with decrease in the w a t e r / c l i n k e r r a t i o . This is the expected behavior and has been explained repeatedly in the l i t e r a t u r e . The increase of strength with age can be related to an increase in the degree of hydration, whereas the decrease of strength with W/C r a t i o is related to an increase in porosity (8). In order to compare the strength values of the fibre reinforced samples, with the neat samples, the strength values were recalculated as r e l a t i v e values with respect to the neat pastes mixed under identical conditions. This could simply be done by dividing the strength value of the f i b r e reinforced paste by that of an identical paste containing no reinforcement. I I I I I 1 1 I I 1 l I I I~ Typical set of relative compressive strength values t2 (R.C.S.) is shown in Fig. 2 for W/C ratios of 0.70. In I.o general, and with the exception of few cases, fibre add i t i o n leads to a reduction o8 in compressive strength as compared with the neat c l i n o.6 ker. In high porosity pastes (W/C = 0.7), a s l i g h t gain o.~ Z~ C h r y s o t i l e f i b e r s O.S'l, • Glass f i b e r s (typell)3.O'l, in strength is noticed (Fig. O Pl.coated glass fibersO.S'Id 2), for samples cured for 28 o.2 -.A C h r y s o t i l e f ; b e r s 3.0"I, Glass f;bers(type|l)O.5"l, • P l . c o a t e d glass f;bers3.0"lo days. This is noticed for I 1 ! 1 1 I ~ l I I I 1 1 L 0.5% white asbestos and 0.5% o 2 4 6 8 10 12 14 16 1~ 20 22 24 26 Z~ polymer coated glass f i b r e AGE. D a y s samples, and this indicates that the strength depends FIG. 2 somewhat on the specific type Relative compressive strength for various of the f i b r e used; t h e i r chemc l i n k e r pastes of W/C = 0.70. i s t r y and t h e i r mechanical behavior. Asbestos and polymer coated glass fibres are f a i r l y p l a s t i c , and t h e i r effect in i n h i b i t i n g crystal growth of the hydration products, would be much less than f o r other f i b r e s . However at a higher f i b r e concentration, or in denser pastes, a l l fibres lead to reduction of strength. The differences between the various reinforced pastes are larger in the high-porosity than in the low-porosity pastes, and these differences diminish with age. For the dense paste (W/C = 0.30), reduction in strength f o r the 28 days cured samples is limited within 20%, whereas for a leaner mix (W/C = 0.70), i t can be as large as 30%. Relative flexural strength values are shown in Fig. 3 f o r W/C r a t i o of 0.70 and these values show that f i b r e addition leads in every case investigated to an increase in the flexural strength. The increase in flexural strength for the lean paste of W/C : 0.7 can be as high as 45% a f t e r 28 days, and is d e f i n i t e l y greater than for the dense paste of W/C r a t i o : 0.30. In the l'atter case, an increase as high as 38% can be reached in some cases. The above results show that some compressive strength has to be sacrificed in order to obtain certain required improvements in flexural strength. The microstructure of c l i n k e r pastes could be related with i t s physical properties by taking into consideration certain hypotheses made by Chatterji and Jefferey ( I I ) . The paste can be considered as a random packing of nearly ideal CSH needles in which larger p a r t i c l e s are embedded. These needles are bonded together

772

Vol. 8, No. 6 R.Sh. Mikhail, M. Abd-EI-Khalik, A. Hassanein, D. Dollimore, R. Stino

-~'~

I

I

I

2.5 --,5. C h r y s o t 4 e 2,z,

A

I

l

]

I

I

]

i

I

]

t

I~

f i b e r s 0.5°/o

• Glass f b e r ' s ( t y p e I [ ) 3 . 0 " / ~

C h r y s o t i l e f i b e r s 3.0~I,

0 Pl coated glass fibers0.5°/,

,~. Glass f,berS(type[I}0.5°/o

• Pl.coated glass flbers3.0°/,

22 Z0

O

2

z.

6

8

~0

12

1~

16

l~J

20

22

2z.

26

28

AGE.Days

FIG. 3 Relative flexural strength for various clinker pastes of W/C = 0.70.

by a t t r a c t i v e forces arising from the free surface energy. In compressive f a i l u r e , CSH needles have to be sheared, whereas in tensile f a i l u r e the needles are pulled apart (12). This allows to calculate the relationship between the volume density of CSH needles and the compressive strength of the paste (13), which was found to be identical to Feret's law (14), and very similar to Power's empirical relation (15). A similar relationship was also derived for the tensile strength of the paste (16), taking into consideration the i n t e r p a r t icle separation.

The equations of Chatterji et al (13,16), derived o r i g i n a l l y for work based on set plaster of Paris, were t r i e d in t h i s investigation on both the neat and f i b r e reinforced NB pastes cured for 6 months. Their application on the various pastes was found to be completely s a t i s f a c t o r y as discussed elsewhere by the authors ( I 0 ) . The s a t i s f a c t ory application of both equations indicate that for the incorporation of discontinuous f i b r e s , and at least w i t h i n the concentration l i m i t s used, the mechanical behavior of the mix is s t i l l governed mainly by the mechanical properties of the cement matrix. In other words, the mechanical properties of the fibres themselves are not d i r e c t l y involved, and the bahavior of the mix is due mainly to change in the physicochemical properties induced by the f i b r e s . Specific action of the type of the fibre used, as well as the nature of the i n t e r f a c i a l bond between the f i b r e and the cement matrix is now under study. References I.

H.F.W. Taylor, Ed., The Chemistry of Cements." Academic Press, New York,1964.

2.

G. Verbeck and R.A. Helmuth. Structures and Physical Properties of Cement Pastes. Vol. I I I , p. 8, Proc. F i f t h International Symposium on the Chemistry of Cement, Tokyo, 1968.

3.

F.M. Lea and C.H. Desch, The Cemenistry of Cement and Concrete, second e d i t ion (revised by F.M. Lea), Edward Arnold, London, 1965.

4.

Review given in Ph.D. thesis in R. Stino, University of Salford, U.K., 1976.

5.

T.C. Powers, Proc. Fourth I n t l . Symp. Chem. of Cement, Wasnington, D.C.,1960, N.B.S. Monograph 43, Vol. I I , p. 601, and reference cited there.

6.

R.F. Feldman and J.J. Beaudoin, Cem. Concr. Res. 6, 387 (1976).

7.

M. Yudenfreund, I. Odler and S. Brunauer, Cem. Concr. Res. 2, 313 (1972); and 2, 463 (1972).

8.

R.Sh. Mikhail and G. Oweimreen, Cem. Concr. Res. 3, 561 (1973).

9.

R.Sh. Mikhail, D. Dollimore, R. Stino and A.M. Youssef, IL CEMENTO4, 177 (1976). R,Sh. Mikhail, D. Dollimore and R. Stino, IL CEMENTO, accepted for publication.

I0.

Vol. 8, No. 6 773 CLINKER PASTES, FIBRE REINFORCEMENT, POROSITY, MECHANICAL PROPERTIES

II.

S. C h a t t e r j i , J.W. Jefferey, Nature 214, 559 (1967).

12.

S. C h a t t e r j i , J.W. Jefferey and N. Whitney, Cement Tech. ~, 171 (1970).

13.

H.S. Mamedov, S. Chatterji and J.W. Jefferey, Indian Concrete J., 42, 66 (1968).

14.

F.M. Lea, The Chemistry of Cement and Concrete, E. Arnold Ltd., London~ 1956, p. 339.

15.

T.C. Powers, The Chemistry of Cements, ed. by H.F.W. Taylor, Vol. I , (Academic Press: London 1964), p. 414.

16.

H.S. Mamedov, S. Chatterji and J.M. Jefferey, Indian Concrete J., 42, 281 (1968).