Structure of compacted cement pastes

CEMENT and CONCRETE RESEARCH. Vol. 13, pp. 239-245, 1983. Printed in the USA. 0008-8846/83/020239-07503.00/0 Copyright (c) 1983 Pergamon Press, Ltd.

STRUCTURE OF COMPACTEDCEMENT PASTES Adolf Bajza Slovak Technical U n i v e r s i t y , B r a t i s l a v a , Czechoslovakia

(Communicated by D.M. Roy) (Received Aug. 20, 1982)

ABSTRACT The e f f e c t of w/c, compaction pressure and curing time on the i n i t i a l and t o t a l p o r o s i t y and pore size d i s t r i b u t i o n , form of hydration products, and morphology of the high-pressure compacted cement paste is discussed. The engineering properties of concrete - - such as s t r e n g t h , modulus of elast i c i t y , creep, and d u r a b i l i t y - - depend on the pore s t r u c t u r e and morphology of the hardened cement paste. The main condition of producing a dense product is a dense i n i t i a l s t r u c t u r e . This can be done by physico-chemical methods ( e . g . , by decreasing the w/c using high-range water reducers) or mechanical methods ( v i b r a t i o n , compaction, e t c . ) . The presented data were obtained on pastes made by high-pressure compaction. Experimental All experiments were performed using ordinary portland cement, at w/c = 0.08*, 0 . I 0 " , 0.15" and 0.30. Compacted samples were made also in absence of water. The samples were compacted at O, 45, I01 and 207 MPa for 3 minutes; the pressed samples were then vacuum treated for 30 minutes, and f i n a l l y cured at room temperature for I , 7, 28, 60 and 400 days. The paste s t r u c t u r e was evaluated by mercury porosimetry and scanning electron microscopy (5). Discussion of Results As expected, the i n i t i a l density of the cement paste increased with compaction pressure used (Table I , Figures 1 and 2a-d). This is shown by calculated values of the t o t a l i n i t i a l p o r o s i t y , t o t a l volume of pores having a radius of 3.75 to 7500 nm, and average pore radius.

* optimum water-to-cement r a t i o s (2,5) 239

15.2 16.3 15.4 -J-k-L-

16.3 15.6 15.6 15.2

* Average of six separately made compacts.

--w ;5:0 25.1 25.0 25.2

28.3 28.3 28.2 28.4 . 24.3 24.2 24.3 24.5

19.7 18.4 18.2 17.6

C

F+73:8 69,3

712

~ 617

0,02161

66,O

E+8014

11,3 9,x

--E1113

14,5

11,3 5,7 53,3 2,8

68,8

0,02680 0.02328 0,13656 0,04167

d-g

2; 130 10,l

0,02913 0.01556 0,09902 0,03232

_

11,3

44,l 8-6

18,7 72,8

5-3 4917

0,02992 0,01538

290

10,o 690

76,7 38,3

ll,o

0,11278 0,05274

.__M__ 41,6 12,0

102- lo3 nm

(%I 18,7 69,9

10-10 nm

istributi

(o/o) 190

3.7510 nm

-PO re Size

0,16454 0,06967

__

21.8 20.6 20.1 19.7

___

( cm3.g-')

(%)

(9)

31.7 31.7 31-7 31.7 31.7

nitial

Total Pore Volume 3.75-7500 nm

*

I

of Pressed Cement Pastes

After Curing

-PO *

Propeldies

TABLE I

llJ__

1277

15,3 __lLL 12,0 737

15,3 6;rl 25,0 637

35,2 7-3

890 630

(%I 38,7 7,l

103-7.5 c 103nm

1

27,5

470,o 28,5

26,5 10,o

(nm) 530,o 32,0

r'

--TX-

733___

22,0

3310

28,5

---Z-k 2710

__-~

T

Vol. 13, No. 2

241 COMPACTED PASTES, STRUCTURE, POROSITY

"'~'.,.. "'-..~ .

.....

Initial porosity

~

Porosity after curing

FIG. 1 Influence of compaction pressure on the porosity of cement paste pressed at the optimum w/c.

/~]

45

day 7days 28days

10l

20/

COMPACTING PRESSURE (MPa~

An increase of compaction pressure from 45 to lOt MPa resulted in a decrease of average i n i t i a l porosity of by 3.42%, that from lOt to 207 MPa by 3.94%. In other words, to decrease the porosity by I%, pressures of 16.4 and 27.9 MPa were needed, respectively. Previously reported data on different cements show that the increase of compaction pressure from 207 to 423 MPa resulted in a need for 42 to 58 MPa for I% of decreased porosity (2). The lubricating effect of water (at the optimum w/c) is evidenced by the fact that, at a compaction pressure of 207 MPa, far lower i n i t i a l porosity was obtained at w/c = 0.08 than in the absence of water (Table I ) . During curing, the greatest densification of paste was achieved at early stages of hydration. After l and 60 days of curing, the porosity decreased to 6769% and 60-62% of the i n i t i a l porosity, respectively. Consequently, the volume of pores having radius of 3.75 to 7500 nm is 30-47% and 9-23% of i n i t i a l value after l and 60 days of curing, respectively. At the same times, the average pore radius decreased to 6.0-8.0% and 1.9-5.7 of the original values. The difference in porosity of the compacted paste can be evaluated also from LOI between 105° - tO00° C (5). I f we consider, for s i m p l i c i t y , the LOI as a measure of the degree of hydration then, at a constant LOI, the total pore volume decreases with the compaction pressure (Figure 3). For example, at a LOt value of 8%, the pore volumes (3.75-7500 nm) at 45, lOt and 207 MPa compaction pressures are 0.063, 0.035 and 0.015 cm3/g, respectively. Additional information about the character of the paste structure can be obtained from microscopic observations (Figure 4). After l day of curing, both the compressed and control samples showed presence of Type I and Type I l l C-S-H (9); in pores, calcium hydroxide and e t t r i n g i t e crystals are v i s i b l e . Compacted pastes with largest amount of pores (lowest compaction pressure) show well developed calcium hydroxide crystals (Figure 4a); those compacted at high pressures contained pores with few c r y s t a l l i n e deposits (Figure 4c). As the degree of hydration increases, the c a p i l l a r y pores are f i l l e d by reaction products. After 28 days (Figure 4d), the predominant phase v i s i b l e is Type I l l C-S-H, with imbedded clinker particles and calcium hydroxide and Arm crystals. The presence of these hydrates was confirmed by X-ray analysis and

242

Vol. 13, No. 2 A. Baiza iO0

100

80

Z

60 40 20

I 'R.

J

">,"

__~.f S'/ 0

i0

8O

///

=~

6o

~

4o

.o

s~sj

20

!

103

I0

104

lO0

100

80

8O

~-ld

I

/

ii ,~Ds

O I0

sj

~o

~-

4O

2

103

104

,LD

TM

~" 6Od

/

20

// !

~s-- "-"'T~'s

I

102 PORE RADIUS(nmp

c Fi gure 2.

J

g

~'- 28d /#~'~'- D ~ " 60d / /

2C

102 PORE RADIUS (nml

PORERADIUS InmP

40

~" D

,

102

60

sf j

103

104

10

102 PORERADIUS

I 103

104

mm~

d

Pore size d i s t r i b u t i o n curves of compacted cement pastes: (a) w/c = 0.12 (45 Mpa), (b) w/c = 0 . I 0 (I01 MPa); (c) w/c = 0.08 (207 MPa); (d) dry cement (207 MPa). D-immediately a f t e r demolding.

DTA. There is some X-ray evidence that in high-pressure compacted samples hydrated for longer periods, a hydrogarnet, probably a s i l i c a - c o n t a i n i n g member of the hydrogarnet series, was formed. I f this interpretation is c o r r e c t , •Formation of phase similar to C6AFS2H8, reported by Copeland et a l . ( I 0 ) , is a probability, especially in th~ view of very low w/c. No new forms of hydration products were detected ( 5 ) , t h i s being•in line with the findings of other authors (4, 8). Changes in the paste structure are closely connected to the changes in the amount of chemically bound water, wn. For different compaction pressures and at all curing times, Wn is smaller for pastes having lower i n i t i a l porosity. For pastes compacted at 207 MPa (w/c = 0 and 0.08) this relationship is valid only in samples cured at l day. This is caused by the high density and low permeability to water of the samples compacted from dry cement. For samples compacted at 45 MPa (w/c = 0.12), the decrease in porosity at 60 days by II.99% is equivalent to wn = 5.44%. For pastes compacted at lOl MPa (w/c = 0 . I 0 ) , the 60-day porosity decreased by I0.63% and Wn= 4.62%. The same values for

Vol. 13, No. 2

243 COMPACTED PASTES, STRUCTURE, POROSITY

O.Ol

0.06

"-'-

FIG. 3 Relationship of total pore volume to the degree of hydration (LOI).

pi!Po o

0.05

E

~ 0.04 >o j

o_

0.03

0.0~

O0.1p~ !

I

I

I

I

I

6

7

8

9

10

ll

LOSS ON IGNITION (%)

dry cement, compacted at 207 MPa, are I0.09% and wn = 4.00%. Thus, decrease of porosity during curing by I% corresponds to an average increase of bound water by 0.454% (for samples compacted at 45.MPa), 0.435% (lO] MPa), D.406% (207 MPa at w/c = 0.08), and 0.396% (207 MPa at w/c = 0). This is an indirect evidence showing that lower i n i t i a l porosity leads to formation of hydration products having lower water content, and is in agreement with results of Jambor (8). The effect of porosity on the strength of compacted and cured paste depends on the compaction pressure ( i n i t i a l porosity). Until 28 days, the relative increase in strength per I% of decreased porosity increases with decreasing i n i t i a l porosity. After this time, probably due to larger permeability of the samples compacted at lower pressure, the reverse is true (Table I I ) . At lower porosity, a unit volume of newly formed hydration products has a greater effect on mechanical properties than at higher porosity. In other words, i t s "specific binding capacity" increases.

Summar~ Production of very dense hardened cement pastes by high-pressure compaction is possible. The product density increases with increasing compaction pressure and, at a constant pressure, with increasing w/c from 0.00 to an optimum. This is due to the corresponding increase of solids (cement) per unit of compressed paste (2, 5). With curing time, the total porosity, the pore volume detectable by mercury porosimetry, and the average pore radius decrease. The larger the i n i t i a l porosity, the larger the structural changes during curing. Pastes with low i n i t i a l porosity, whose structure was adequately densified already during the compaction and the early stages of curing, hydrate at later stages at a

244

Vol. 13, No. 2 A. Bajza

a

b

~i~¸

c

Figure 4.

d

Scanning electron micrograph of cement pastes: (a) w/c = 0.3 I day (b) w/c = 0.12 (45 MPa) l day; (c) w/c = 0.08 (207 MPa) l day; (d) w/c = 0.2 (45 MPa) 28 days.

Vol. 13, No. 2

245 COMPACTED PASTES, STRUCTURE, POROSITY TAuLE II Some Properties of Compacted Cement Pastes

Compaction Pressure

w

c

(MPa)

Initial Porosity

ct 2 _°t I

(%)

(MPa x % Hydr. Prod.

Pt I

Pt 2 i

0-I Day

1-28 Days

28-60 Days

0-60 Days 1.9,42 23,84 29,03 27,43

45

0,12

31.70

II,42

61,66

37,07

I01

0,I0 0,08

28.28 24.34 25.06....

15,65 21,22 19,20

66,0] 138,86 68,44

32,79 12,75 14,38

207

-bT

L.

Otl

-

strength at time t 1

ot2

-

strength a f t e r curing at time t 2

pt I

-

porosity at t 1

pt 2 -

porosity a f t e r curing at t 2

r e l a t i v e l y low rate. Understandably, densification of cement pastes during hydration is due to formation of increased volumes of hydration products (decrease in porosity of paste). Average water content needed to increase the volume of hydration products by I% decreases with decreasing i n i t i a l poros i t y . However, the average "specific binding capacity" of the hydration products increases. References I. 2. 3. 4. 5. 6. 7. 8. 9. I0.

C.D. Lawrence. The Properties of Cement Paste Compacting under High Pressure. Res. Rep., 19, London, Cem. and Concr. Assoc. 1969. A. Bajza. The Physical Properties of Cement, Cement Paste and Mortar Compacted by High Pressure: Influence of Compacting and Curing Procedure. Tech. Rep. London, Cem. and Concr. Assoc. 1970. J. Skalny and Z. Bajza. ACl J . , 67, 3, 221-227 (1970). D.M. Roy, G,R. Gouda and A.R. Bob~wsky. Cem. Concr. Res. 2, 3 (1972). A. Bajza. Staveb. Cas. 30, 4, 319 (1982). I. Soroka and P.J. Sereda. The Structure of Cement Stone and the Use of Compacts as Structural Model. Proc. Fifth I n t l . Symp. Chem. Cement. The Cement Assoc. of Japan, Vol. 3, pp. 67-73, Tokyo, 1969. R.Sh. Mikhail and G.A. Oweimreen. Cem. Concr. Res. 3, 561-573 (1973). J. Jambor. Staveb. Cas. 24, 3, 227-247 (1976). S. Diamond, in Hydraulic Cement Pastes: Their Structure and Properties. Cem. and Concr. Assoc., London, 1976. [oE. Copeland, et al. Chemistry of Cement. Proc. Fourth I n t l . Symp., NBS Monograph 43, V . I . , p. 429, Washington, 1960.