1-s2.0-s0950061897000615-main

Constntction and Building Materials, Vol. 11, Nos 7-8. pp. 383-393, 1997 0 1997 Published by Elsevier Science Ltd

Printed in Great Britain. All rights reserved

09so-U618/97 $17.00 + 0.w

PII:SO950-0618(97)00061-5

The influence of different curing conditions on the pore structure and related properties of fly-ash cement pastes and mortars

C. S. Peon*, Y. L. Wong and L. Lam

Concrete Technology Group, Department of Civil and Structural Engineering, The Hong Kong PolytechnicUniversity, Hung Horn, Hong Kong

Received 3 February 1997; revised 19 August 1997; accepted 4 September 1997

The influence of two different curing conditions (in water at 27”C, and in air at 15°C and 60% relative humidity) on the mechanical and durability properties of fly-ash cement pastes and mortars are studied. Cement pastes and mortars at two water/cement or binder ratios were prepared in the laboratory and tested for compressive strength, chloride and water penetration. The mercury intrusion porosity of the samples is monitored to provide mechanistic explanations for the measured results. The results show that fly ash has significantly different influence on the strength, porosity and durability parameters of cement pastes and mortars when the cementitious materials are subjected to different curing conditions. 0 1997 Published by Elsevier Science Ltd.

Keywords: cement pastes; fly ash; curing

Introduction

Fly ash is being increasingly used as a common mineral admixture in concrete to obtain improved properties of the fresh and hardened products’,*. The use of fly ash modifies the composition of the cement pastes, influ- ences the hydration and microstructure of the pastes and mortars, and the strength and permeability of concretes”-“‘.

Montgomery et al.4 found that the ash particles act as nucleation sites for the hydration of cement. Wai et aL5 stated that fly ash has retarding effects on the hydration of cement. Xu et al.83Y indicated that fly ash has both enhancement and retardation effects on cement hydration, and may also provide nucleation sites for the growth of hydration products. A research study carried out at Intron”’ indicated that fly ash improves the particle packing of the cementitious matrix and contributes to the strength development even when it is not active as a pozzolana but acting as a so-called ‘filler’. The research also investigated the effects of the addition of fly ash on the interfacial zone between the aggregate and the paste, and found that the rate of the pozzolanic reaction is faster in the interfacial zone than in the bulk of the paste. Gopalan” found that the

*Correspondence to Dr C. S. Peon

gel/space ratios of fly-ash mixes estimated by the strength data are higher than those calculated from the water/cement (w/c> ratios of the mixes, and he at- tributed it to the equivalent cementing effect of fly ash. He concluded that the strength contribution of fly ash is the sum of nucleation and pozzolanic factors. Marsh et al.‘* found that the pozzolanic reaction of fly ash in blended cement-pastes can cause substantial reduction in permeability. This reduction may be as large as three orders of magnitude when compared to an identically cured Portland cement paste of the same water/solid ratio.

Pore structure is considered to be one of the major factors controlling the durability and strength of hydrated cement products. A measure of the porosity and pore-size distribution can lead to more basic under- standing of many physical and durability parameters of the material’“. Feldman’” found that at early ages, the porosity of fly-ash cement pastes determined by mercury intrusion porosimetry (MIP) is greater than that of the comparable Portland cement pastes although the pore-size distributions of the two types of pastes are similar. Bijen’j further indicated that the pore size distribution, measured with MIP of a cement prepared with ASTM class F (ASTM C618) fly more coarse than the plain cement paste at the curing period, but later the opposite is the case.

paste ash is initial

383

384 Porestructure and related properties of fly-ash cement pastes and mortars: C. S. Poon et al.

Malami et al3 demonstrated that at a w/cc +f) ratio of 0.5, the additions of 4% and 15% fly ash (ASTM class C) did not significantly influence the porosity of the mortars, but higher percentage additions resulted in an increase of the porosity. Xu et ~1.~ indicated that equal weight replacement of cement by fly ash lowered the total initial volumetric porosity of the mortar. Al-Amoudi et al.” presented the results of permeability and helium porosimetry tests for concrete mixtures at w/c from 0.35 to 0.55, and with the replacement of cement by fly ash (ASTM class F) from 0 to 40% by mass. They found that at the initial curing period of 50-75 days, the plain cement concretes were the least permeable and the least porous. But after 1 year of curing, the best performance was shown by 20% fly-ash cement concrete followed by 30, 10 and 40% fly-ash concretes.

The above studies showed beneficial effects of fly ash on the long-term durability properties and microstructure of concrete. However, these results were usually obtained from specimens cured in water or moisten environments. It should be noted that the fly-ash concrete is more sensitive to the curing condition and requires a longer curing time because its hydration rate is slower than that of the plain concrete”. The benefits of using fly ash in concretes may be diminished due to inadequate curing.

The authors previously studied the influence of different curing conditions on the strength of normal- strength (w/c = 0.49) and high-strength (w/c = 0.3) fly-ash concretes . ” The results showed that in water curing, direct replacements of cement by fly ash in normal-strength concrete reduced the 28-day strength although the long-term strength was comparable to the plain concrete. Extra (10%) addition of fly ash in concretes decreased the early strength but enhanced the latter strength. The strength of the high-strength mixes was enhanced by the extra fly ash addition irre- spective to the age of curing. When cured in 15°C and 60% Relative Humidity (RH), fly ash addition to concrete was shown to be detrimental to concrete strength. It has also been shown that the main contribution of fly ash to concrete strength at the age of 28 days was likely due to the improvement of the bonding between the cement paste and the aggregate, rather than the enhancement of the strength of the cementitious matrix.

Objectives

The present paper tries to determine the influence of curing conditions on the mechanical and durability properties of fly-ash cement pastes and mortars by a systematic study of the pore structure and porosity of the cementitious materials. At the same time, the rela- tionships between the compressive strength, porosity and two durability parameters (water permeability and chloride penetration) are also quantified.

Experimental

Materials

The materials used were locally available ordinary Portland cement meeting the requirements of British Standard 12 (Green Island Cement) and fly ash meeting the requirements of British Standard 3892 (China Light and Power), crushed granite and river sand. Table I gives the chemical analysis of cement and fly ash whereas Table 2 shows the properties of aggregates. A commercially available naphthalene based super-plasticizer (RBllOO) was used for the mixes with low water/binder ratios to give a consistent workability.

Midure design

Two series of plain cement pastes with water/cement ratios of 0.49 (Series 1) and 0.3 (Series 2) were prepared (Table 3). Some mixtures were prepared with the cement partially replaced by 15 or 25% of equal (direct replacement) or extra 10% (extra addition) mass of fly ash. The normal-strength mixtures (Series 1) are denoted by ‘N’ and the high-strength mixtures (Series 2) are denoted by ‘H’. The codes of the mixtures have three components. For example, N-15-10 represents a normal-strength mixture prepared with a water/binder ratio of 0.5, with 15% of cement replaced by fly ash and an extra 10% by weight of fly ash added. In parallel, concrete mixtures with fly-ash/cementitious materials and water/cementitious materials ratios identical to those of the corresponding pastes were prepared. Addi-

Table I Properties of cement and fly ash

Cement (%)

SiO, 20.77

CaO 65.07

MgO 0.89

Fe& 3.23

AU’, 6.20

SOS 2.47

NazO 0.17 K,O 0.43 LO1 0.97 Insoluble residue 0.21 Chloride < 0.05 Specific surface area (g/cm’) 3519 Density (g/cm’) 3.15

Fly ash (%)

44.92 5.69 1.24 4.89

35.39 0.71 0.58 0.64 5.61

63.32 < 0.05

3860 2.3 1

Table 2 Properties of aggregates

Coarse aggregate

IOmm 20 mm

Relative density (S.S.D.) 2.58 ‘2.60 Water absorption (o/o) 0.78 0.69

Sand

2.64 0.87

Porestructure and related properties of fly-ash cement pastes and mortars: C. S. Poon et al. 385

tionally, mortar samples were prepared by removing pastes and the mortars from the compression tests the coarse aggregate particles using a 5-mm sieve dur- were collected and immersed in acetone to stop the ing concrete mixing. The mixture design is given in cement hydration and the samples were then used for Table 4. the measurement of pore structure.

The samples were prepared, cast with appropriate compaction, demoulded (after 24 h of initial curing) and then cured for the specified periods (27 and 90 days). The curing conditions were either in water at 27°C or in an environmental chamber at 15°C and 60% relative humidity (RH). In the latter environment, the specimens were first placed in the environmental chamber in sealed plastic bags for 3 days and then exposed to the chamber environment.

Water permeability and chloride penetmtion. The water permeability test was performed on concrete cylinders, 200-mm diameter x 220-mm long, using an automatic permeameter at a constant water pressure of 2.0 &- 0.05 MPa. The rate of chloride ion penetration into concrete was tested on 150-mm cubes. Separately prepared 150-mm cubes after the specified curing were immersed either in (i> a 5 M sodium chloride solution which had been saturated with calcium hydroxide, or (ii) a control solution containing saturated calcium hydroxide only, for 28 days. Powdered samples were obtained by drilling to different concrete depths (25 mm, 50 mm and 75 mm). A mixture of 5 g of the powdered sample and 100 ml of diluted nitric acid (10 ml of nitric acid + 90 ml of distilled water) was digested in a 300-ml beaker on a hot plate for 15 min. The mixture was filtered through a fast ashless filter paper. The chloride ion concentra-

Test procedures

Compressiue strength. Compressive strength tests of the paste and the mortar samples were performed on 70.7- mm cubes and that of the concrete was performed on lOO-mm cubes using a Denison 7231 compression test- ing machine. The results of the concrete strength have been reported previously , I7 The fracture pieces of

Table 3 Mixture proportion of pastes

Paste Notation Mixture proportion (g) Ratios Super-plasticizer series Water(w) Cement cc> Fly ash (f) w/cc +f) f/cc +f) [ml per 1000

g (c +fI

1 (normal a N-O-O 49 100 0 0.49 0 0 strength) N-15-0 49 85 15 0.49 0.15 0

N-25-O 49 75 25 0.49 0.25 0

b N-0-10 49 100 10 0.44 0.091 0 N-15-10 49 85 25 0.44 0.227 0 N-25-10 49 75 35 0.44 0.318 0

2 (high a H-O-O 30 100 0 0.30 0 5 strength) H-15-0 30 85 15 0.30 0.15 5

H-25-O 30 75 25 0.30 0.25 6

b H-O-10 30 100 10 0.27 0.091 8 H-15-10 30 85 25 0.27 0.227 9 H-25-10 30 75 35 0.27 0.318 10

Table 4 Mixture proportion of concretes

Concrete series

Notation Mix proportion (kg/m31 w/cc +f) Super- Water Cement Fly ash Coarse aggregate Sand plasticizer

(w) (c) (f) 10 mm 20 mm (I/m’)

1 (normal a N-O-O strength) N-0-15

N-25-O

b N-0-10 N-15-10 N-25-10

2 (high a H-O-O strength) H-15-0

H-25-O

b H-0-10 H-15-10 H-25-10

220 450 0 357 713 398 0.49 220 383 67 357 713 575 0.49 220 338 112 357 713 561 0.49

220 450 45 357 713 546 0.44 220 383 112 357 713 523 0.44 220 338 157 357 713 509 0.44

168 560 0 357 713 643 0.3 168 476 84 357 713 615 0.3 168 420 140 357 713 597 0.3

168 560 56 357 713 578 0.27 168 476 140 357 713 551 0.27 168 420 196 357 713 532 0.27

2.73 3.64 3.27

5.18 5.82 6.36

Porestructure and related properties of fly-ash cement pastes and mortars: C. S. Poon et al.

tion of the sample solution was determined using an ion selective electrode (Orion). The chloride content of the powdered sample was calculated and expressed in mg of chloride per g of sample.

Mercury porosimetry. The cement paste samples after the specified curing ages were first broken into smaller pieces and immersed in acetone for not less than 7 days. The samples were then dried at 105°C for 24 h. A micromeritic mercury intrusion porosimeter (MIP) with a maximum mercury intrusion pressure of 210 MPa was

Table 5 Compressive strength of pastes and mortars

used. A cylindrical pore geometry and a contact angle 8 of 140” were assumed’s*19. The mercury intruded pore diameter d at a pressure of P was calculated by d = - 4y cos 8/P, where y = 0.483 N/m, the surface tension of mercury 19.

Results and discussions

Compressive strength of cement pastes and mortars

Table 5 gives the compressive strength values of (i) the

Compressive strength of pastes

(N/mm*)

Cured in water Cured in air at 15°C at 27°C and 60% RH

28 days 90 days 28 days 90 days

Compressive strength of mortars

(N/mm*) Cured in water Cured in air at

at 27°C 15°C and 60% (28 days) RH (28 days)

N-O-O N-15-0 N-25-O N-0-10 N-15-10 N-25-10

H-O-O H-15-0 H-25-O H-0-10 H-15-10 H-25-10

67.8 72.4 59.4 63.6 55.2 74.8 71.0 88.6 58.2 84.8 49.8 74.8

115.7 115.0 115.5 115.2 106.0 119.5 122.2 135.1 117.6 134.5

98.5 108.9

49.4 47.5 54.2 46.3 39.2 34 52.5 44.6 33.2 31.2 48.4 38.0 52.1 46.9 60.6 51.5 39.7 37.6 56.3 40.8 30.9 34.4 47.9 29.5

86.9 89.3 93.8 79.1 77.2 80.4 94.5 73.5 66.6 71.8 83.0 61.9 92.4 88.5 98.3 83.8 81.5 79.8 105.3 72.3 68.6 66.7 100.5 61.3

Table 6 Results of water permeability test of concrete samples

Water permeability (m/s)

Cured in water at 2PC Cured in air at 15°C and 60% RH


N-O-O 1.02 E-l 1 5.00 E-12 5.08 E-10 2.73 E-10 N-l 5-O 9.89 E-l 2 3.91 E-12 5.11 E-10 3.12 E-10 N-25-O 1.08 E-l 1 3.92 E-12 5.51 E-10 3.43 E-10 N-0-10 8.62 E-12 1.63 E-12 4.59 E-10 2.27 E-10 N-15-10 8.50 E-l 2 1.03 E-12 5.07 E-10 2.78 E-10 N-25-10 9.00 E-12 1.43 E-12 5.48 E-10 2.89 E-10

Table 7 Results of chloride ion penetration of concrete samples

Concentration of chloride penetrated at different depths (mg/g)

Cured in water at 27°C Cured in air at 15°C and 60% RH


25 mm 50 mm 75 mm 25 mm 50 mm 75 mm 25 mm 50 mm 75 mm 25 mm 50 mm 75 mm

N-O-O 3.23 0.55 0.46 2.03 0.27 0.22 11.78 1.85 1.03 8.07 1.44 0.74 N-15-0 3.26 0.69 0.52 2.01 0.33 0.25 11.57 2.48 1.03 9.02 1.69 0.73 N-25-O 3.43 0.78 0.53 2.02 0.36 0.26 12.30 3.8 1.42 8.33 2.55 0.97 N-0-10 2.62 0.39 0.30 1.01 0.19 0.15 9.88 1.85 0.68 5.88 1.23 0.45 N-15-10 2.67 0.40 0.38 0.87 0.19 0.18 9.84 1.84 0.59 5.93 1.20 0.35 N-25-10 2.72 0.54 0.40 0.91 0.21 0.19 11.11 2.12 0.82 7.13 1.43 0.55

H-O-O 1.61 0.39 0.29 I .09 0.18 0.10 4.76 0.68 0.31 2.53 0.37 0.25 H-15-O 1.16 0.13 0.08 0.52 0.05 0.03 3.98 0.19 0.13 4.07 0.62 0.44 H-25-0 1.43 0.21 0.14 1.02 0.11 0.08 4.54 0.34 0.14 4.64 0.76 0.56 H-0-10 0.75 0.1 0.02 0.24 0.03 0.01 1.47 0.10 0.05 1.58 0.19 0.12 H-15-10 0.88 0.1 0.07 0.23 0.02 0.01 1.52 0.11 0.11 1.00 0.21 0.16 H-25-10 1.14 0.12 0.07 0.27 0.03 0.01 2.38 0.20 0.12 3.09 0.53 0.38

Tab

le

8 R

esu

lts

of m

ercu

ry i

ntr

usi

on p

oros

imet

ry

test

for

pas

tes

at 2

8 an

d 90

day

s

28 d

ays

90 d

ays

Por

osit

y (p

) A

vera

ge p

ore

% P

ore

frac

tion

s in

tot

al p

ore

volu

me

Por

osit

y(p)

A

vera

ge p

ore

% P

ore

frac

tion

s in

tot

al p

ore

volu

me

(%

v/v)

di

amet

er (

nm

) <

10

nm

lo

-100

n

m

>lO

On

m

(% v

/v)

diam

eter

(n

m)

< 1

On

m

lo-1

OO

nm

>

1O

On

m

Cu

red

in w

ater

at

27°C

N

-O-O

24

.68

N-l

5-O

25

.45

N-2

5-O

26

.61

N-0

-10

22.2

5 N

-15-

10

26.3

N

-25-

10

27.4

30.8

26

.9

26.6

27

.9

26.3

23

.2

6.4

79.2

14

.4

22.2

9 28

.0

8.5

75.8

15

.7

11.2

76

.0

12.8

21

.47

23.6

13

.9

74.7

11

.4

10.3

74

.7

15.0

22

.45

19.6

18

.7

68.7

12

.6

16.3

72

.4

11.3

18

.62

22.7

14

.7

74

11.3

12

.3

75.4

12

.3

23.0

7 24

.8

11.8

71

.5

16.7

14

.1

77

8.9

23.1

0 24

.8

13.5

70

.5

16.0

H-O

-O

16.5

8 38

.8

4.5

85.3

10

.2

13.0

5 35

.0

4.5

79.8

15

.7

H-1

5-0

16.6

5 29

.1

9.7

75.6

14

.7

13.6

27

.1

13.5

69

.9

16.6

H

-25-

O

17.2

7 26

.2

11.4

72

.2

16.4

15

.6

27.0

10

.8

69.8

19

.4

H-0

-10

15.7

4 33

.9

6.1

73.9

20

.0

13.3

30

.9

6.5

78.1

15

.4

H-1

5-10

15

.3

26.1

12

.7

71.3

16

.0

14.4

5 26

.4

11.6

67

.0

21.4

H

-25-

10

16.7

5 23

.4

12.5

72

.3

15.2

16

.66

23.1

17

.1

67.8

20

.1

Cu

red

in a

ir a

t 15

°C a

nd

60%

R

H

N-O

-O

28.8

4 N

-15-

0 29

.81

N-2

5-O

31

.4

N-0

-10

27.4

N

-15-

10

30.2

6 N

-25-

10

31.5

3

45.2

6.

1 45

.9

48

29.5

3 58

.1

46.7

7

38.2

54

.8

30.5

6 75

.2

57.9

5.

1 29

.5

65.4

33

.75

78.1

44

.3

5.9

47.3

46

.8

27.7

1 62

.0

40.0

7.

9 44

.5

47.6

31

.43

56.5

51

.0

6.1

33.6

60

.3

32.5

1 65

.6

3.5

2 37

.3

59.2

32

.9

65.1

26

.8

70.0

37

.5

59.5

40

.3

56.2

32

.3

64.8

3.2

3 3.5

2.9

H-O

-O

20.7

4 46

.9

3.1

72.5

24

.4

20.1

4 54

.7

2.3

67.2

30

.5

H-1

5-0

22.9

1 48

.3

4.3

57.2

38

.5

23.1

1 55

.6

3.3

54.0

42

.7

H-2

5-O

22

.66

44.8

4.

6 57

.2

38.2

22

.13

47.0

3.

5 64

.1

32.4

H

-0-1

0 22

.57

45.9

3.

6 65

.7

30.7

21

.15

54.1

2

66.5

31

.5

H-1

5-10

22

.87

45.8

3.

5 63

.8

32.7

23

.52

50.9

2.

8 61

.3

35.9

H

-25-

10

23.9

7 43

.1

3.6

65.4

31

.0

24.2

9 44

.6

3.1

64.1

32

.8


pastes at 28 and 90 days and (ii) the mortars at 28 days. The results show that with 27°C water curing, replacement of cement by equal weight of fly ash reduced the 28-day strength for both the pastes and the mortars. At 28 days, extra addition of fly ash increased the strength of the mortars but decreased the strength of the pastes. At 90 days, all the fly-ash mixtures exhibited strength values comparable to or higher than the control plain cement mixes.

Air curing at 15°C and 60% RH had the negative

effect of fly ash on early strength more significant. Contrary to the case of water curing, the extra addition of fly ash did not improve the strength of both the mortars and the pastes.

Water permeability and rate of chloride penetration

The results of the water permeability test on concrete cylinders are given in Table 6 and the rates of chloride penetration expressed as the chloride concentrations at

Table 9 Results of mercury intrusion porosimetry test for mortars at 28 days

Cured in water at 37°C

Porosity Average pore diameter (% v/v) (nm)

Cured in air at 27°C and 60% RH

Porosity Average pore diameter (% v/v) hm)

N-O-O 21.63 37.6 N-15-0 17.2’1 30.6 N-2.5-O 21.62 28.7 N-O-IO 17.81 32.8 N-15-10 17.75 28.4 N-25-10 20.45 26.0

H-O-O 14.03 38.6 14.25 41.8

H-15-0 11.01 28.6 14.86 48.2

H-25-O 13.88 25.6 17.56 43.1

H-0-10 10.97 33.5 14.68 44.7

H-15-10 10.70 29.3 15.83 49.7

H-25-10 10.51 21.6 17.21 65.9

23.04 19.52 23.57 19.22 22.27 22.19

48.7 51.4 61.4 44.2 69.1 78.8

-Serkla

20 / 0 10 20 30 40

Fly ssh to totnl ccmentitiotrr mstcrial ratio

W)

a. Pastes without extra fly ash b. Pastes with extra fly ash

“. -Serbs la

_.-._Series2e

\

\ ‘\

‘*t \ .

-_._

0 10 20 30 40

FIy a_& to’total ccmentitiolrs material ratio

W

20, / 0 10 20 30 40

Fly uh to total cnnentitiola material ratio

W

T -Serieslb

_.-._Sdss2b -\

10 20 30 40

Fly ash to total ccmentitiour mtierial ratio

W)

c. Mortars without extra fly ash

Figure 1 Average pore diameters of water-cured samples at 28 days

d. Mortars with extra fly ash


different concrete depths are given in Table 7. For the penetration. It can also be noted that the water perme- normal-strength water-cured specimens, when com- ability of the concrete samples with inadequate curing pared with the control mix (N-O-O), mixes with direct is two orders of magnitude higher than that with water

replacement of cement by fly ash (N-0-10, N-15-10 and curing.

N-25-0) showed equivalent and lower water permeability coefficients at the ages of 28 days and 90 days, Pore structure respectively, but slightly higher rates of chloride penetration at both ages. However, mixes with extra fly ash The results of the MIP test for the cement pastes are (N-15-10 and N-25-10) showed consistently lower water summarized in Table 8 and those for the mortars are in permeability coefficients and rates of chloride penetra- Table 9. Both tables give the total mercury porosity and tion at both ages. the average pore diameters. In Table 9, the pore frac-

For the water-cured high-strength concretes, only tions with diameters < 10 nm, 10-100 nm and > 100 the results of the chloride penetration test are avail- nm are also given. able, because the water permeability was too low to be measured by the available equipment (the test never- theless showed that the air-cured samples had consis-

Porosity and average pore diameter. For a specific w/cc

tently higher water permeability than the water-cured +f) ratio, the porosity and the average pore diameter

samples). varied with the fly-ash contents and curing conditions.

With water curing, most normal- and high-strength In 27°C water curing, the addition of fly ash reduced

fly-ash concretes had lower rates of chloride penetra- the average pore diameter of the pastes and the mor-

tion than the control mixes (N-O-O and H-O-O) at both tars at 28 days (Figure I). However, it can also be observed in Figure 2 that the addition of fly ash up to a

curing ages, especially for those with extra fly ash level of 25% of the cementitious materials increased addition. For both the normal- and high-strength mixes, the porosity of the pastes but reduced the porosity of when the fly-ash concrete samples were cured in air at the mortars. When cured in air at 15°C and 60% RH, 15°C and 60% RH, addition of fly ash to concrete did the addition of fly ash increased the porosity and the not show significant improvement of concrete perfor- average pore diameter of both the pastes and the mance in terms of water permeability and chloride mortars (Tables 8 and 9).

30 7 30

25

2 _Series la

.o 20

-T

T 25 --

s e / _Serieslb

_._._Sar@s2a

.I 2o

-._._saries2b

E _-._._.-_----- 15 15 --

10; lOI 0 10 20 30 40 0 10 20 30 40

Fly ash to total cemclltitious mated ratio Fly ash to total cemcntitious material ratio w9 C4

a. Pastes without extra fly ash b. Pastes with extra fly ash

-Serissla

_._._Sarii2a -Series lb

_.-._Serk2b

51 51 0 10 20 30 40 0 10 20 30 40

Fly ash to’total cemcntitious material ratio Fly ash to total cemcatitiou, material ratio

W) cr.)

c. Mortars without extra fly ash d. Mortars with extra fly ash Figure 2 Porosity of water-cured samples at 28 days


120

t

loo -.

I_ 80..

.ia w

f” 3 40 --

20

t

Lbear regression:

y=303.34(1-X)"W

R=O.9741

OJ t----t--i

0 0.05 0.1 0.15 02 0.25 0.3 0.35

POrOSity

Figure 3 Compressive strength of pastes vs. porosity

mwerregrersinn:

y =214.96(1-x)".m

R-O.9419 .

20 0 L : ---I

0 0.05 0.1 0.15 0.2 0.25

Porosity

Figure 4 Compressive strength of mortars vs. porosity

The increase in porosity of the fly-ash pastes might be due to the higher water/cement ratio and lower gel/space ratio when cement was partially replaced. This is in agreement with Feldmani and Malami et aL3 The decrease in porosity of the water-cured fly-ash mortars might be due to the better filling effect of the fly ash in the mortars. This observation supported the theory that fly ash improves the interfacial bonding between the paste and the aggregate in concreteio9”.

It can be observed from the regression analysis (Fig- ures 3-6) that the MIP porosity correlated well with the strength, permeability and rate of chloride penetration values. With respect to strength, there are a num- ber of expression used to relate the porosity and the strength of porous solids, such as”):

CT= a,(1 -p>”

or

u= a, - kp

where p is the porosity, a0 is the strength at zero porosity and n or k are empirical constants.

For the range of porosities covered in this paper, a linear expression is thought to be more appropriate (see Figures 3 and 4). Also, the power regression analysis of the porosity against the permeability and the rate of chloride penetration values shown in Figures 5 and 6 indicate good correlation.

y = I)._‘“”

R=0.8218

1-I: I

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

PWCdy

Figure 5 Water permeability of concretes vs. porosity of pastes

I4 T

12 1

10

8

6 ._

4

2

t 0 c__

0

25m y =429,32x""g

R=O.&315 .25m 0 rnsm 00

5omlK y=185.~."' l

R=0.8574 A75mn . .

75mny=7&939x'-

R=0.8334

,

0.05 0.1 0.15 0.2 0.25 0.3 0.35 m=lty

Figure 6 Chloride penetration concentrations vs. porosity

On the contrary, the average pore diameter appears not to be correlated with the strength and permeability values.

Pore-size distribution. The results in Table 8 indicate that the water-cured fly-ash pastes have larger pore fractions with diameters < 10 nm (gel pores) when compared with the water-cured plain cement pastes, indicating the fly-ash pastes had smaller average pore sizes. Also, the pore sizes were influenced significantly by the curing conditions. When exposed to air at 15°C and 60% RH, the pore fractions with diameters > 100 nm were increased by several times.

The pore-size distribution of the pastes and the mortars are further characterized by expressing the relative volumes as dV/d logD, the differential incre- ment in pore volume with respect to the pore size. The results are plotted against the pore diameter in Figures 7-10. The pore-size distribution curves show that the water-cured plain pastes and mortars at the age of 28 days had peaks between 10 nm and 100 nm (Figure 7). The addition of fly ash shifted the peaks to the left along the x-axis indicating a reduction in pore size. For the high-strength mixes, this effect was more significant (Figure 8).

The pore-size distribution curves of the air-cured pastes and mortars samples were very different from

Pore structure and related properties of fly-ash cement pastes and mortars: C. S. Poon et al. 391

+ N-25-O

100 IOW IOWLl

Porn diamtcr (am)

-H-o-o 3 0.2

+J_ H-250

9 +l+O-10 6 0.15 s _&_ H-25-10

J 0.1

i -: 0.05

t

0

Pore damter (am)

a. pastes

a Pastes

0.12

=r 1 -N-o-a

3 3 _o- N-25-O 6 0.09

h

J _&_ N-25-10

0.06

i 8. -: P 0.03

0

lb 100 -1WWJ

Pore diamtcr (nm)

b. Mortars

Figure 7 Pore-size distribution of normal-strength samples cured in water at 27°C

those of the water-cured specimens as shown in Figures 9 and 10. Curing in air shifted the peak to the right of the lOO-nm mark. The addition of fly ash, whether as a direct replacement or as an extra addition, further increased the pore size and the peaks at approx. 1000 nm became significant for both the high- and normal- strength mixes. It should also be noted that the lOOO-nm peaks of the mortars are much larger than those of the pastes. This might be due to the presence of larger pores between the paste and aggregate interface when the mortars were subjected to drying shrinkage.

Conclusions

The above study indicates that the fly ash being studied has significantly different infh_rences on the strength, porosity and durability parameters of cement pastes and concretes when the cementitious materials are subjected to different curing conditions. Other fly ashes from different sources might show similar or different results and warrant additional study.

Cured in water at 27°C

In water curing, the pastes and mortars had the most probable pore sizes between 10 and 100 nm, and more

100 1000

Pore dhmter (nm)

b. Mortars

Figure 8 Pore-size distribution of high-strength samples cured in water at 27°C

than 80% of the pore volume comprised of pores with diameters smaller the 100 nm. The addition of fly ash increased the porosity of the pastes but reduced the porosity of the mortars at the age of 28 days. But adding fly ash rendered the pore size of both the pastes and the mortars smaller. This observation explains the different effects of fly ash on the strength perfor- mances of cement pastes and concretes and supports the theory that fly ash improves the interfacial zone between the pastes and the aggregates.

The addition of fly ash substantially reduced the water permeability and the rate of chloride penetration of the water-cured concrete specimens. The effect was not significant for normal-strength concretes [w/cc + f) = 0.491 with only direct fly-ash replacement at 28 days. In contrast, the effect of extra addition of fly ash was significant for normal- and high-strength concretes [w/cc +f> = 0.44 and 0.271 at the ages of 28 and 90 days.

Air-cured at 15°C and 60% RH

When compared with the results of the samples after water curing, the exposure in air considerably increased the pore size and pore volume of the pastes and the


~H-o-0

n -c,- H-25-O

+HOlO

Pore diameter (nm) Pore diamter (nm)

a. Pastes a. pastes

-N-O-O

--Q-K250

+N-O-10

_.+-K2510

0.12 i

9’ i? 0.09

-5’

E z d On9 : k -: 0.03

2

0 -_t_------- 100 1Mx)

Pore diamter (nm)

b. Mortars

Figure 9 Pen-size distribution of normal-strength samples cured in air at 15°C and 60% RH

mortars, particularly for those samples containing fly ash. The pore size distribution results of the mortars indicated that the pore fraction with diameters approx. 1000 nm became significant, which is considered to be

resulted from the drying shrinkage at the interfacial zone.

Subject to inadequate curing, the water permeability of concrete specimens were two orders of magnitude higher than that of concrete with water curing; the rate of chloride penetration was also increased twofold.

Addition of fly ash to the concrete specimens did not show any improved resistance against water permeation and chloride penetration.

The porosity of pastes and mortars correlated well with the compressive strength, water permeability and rate of chloride penetration of the samples. On the

contrary the average pore diameter was less well correlated with strength and water permeability.

References

Dunstan, M. R. H., Fly ash as the ‘fourth ingredient’ in concrete mixtures. Fly Ash, Silica Fume, Slag, atrd Natural Pozzolanics in Comrete. AC1 SP-91, Detroit, 1986, p, 171 Ho, D. W. S. and Lews, R. K., Effectiveness of Ily ash fol strength and durability of concrete. Cement and Concrete Re- search, 1985, 15. 793

0.12

I -H-O-O

+ H-25-0

-.-H-O-10

+ H-25-1 0

100 1000

Pore diamter(nm)

b. Mortars

Figure 10 Port-size distribution of high-strength air at 15°C and 60% RH

3

4

5

h

7

8

9

IO

I1

12

13

samples cured in

Malami, C., Kaloidas, V., Batis, G. and Kouloumbi, N., Carbona- tion and porosity of mortar specimens with pozzolanic and hydraulic cement admixtures. Cement and Concrete Research. 1994, 24, 1444 Montgomery, D. G., Hughes, D. C. and Williams, R. I. T., Fly ash in concrete - a microstructure study. Cement and Concrete Research. 1981. 11, 591 Wai, F. J., Grutzeck, M. W. and Roy, D. M., The retarding effects of fly ash upon the hydration of cement pastes: the first 24 hours. Cement and Concrete Research, 1985, 15, 174

Plowman, C. and Cabrera, J. G., Mechanism and kinetics of hydration of C,A and C,AF, extracted from cement. Cement and Concrete Research, 1984, 14, 238 Ogawa, K., Uchikawa, H. and Takemoto, K.. The mechanism of the hydration in the system C3S-pozzolania. Cement and Cow crete Research, 1980, 10, 683

Xu, A., Sarkar, S. L. and Nilsson, L. 0.. Effect of fly ash on the microstructure of cement paste. Materials and Structures, 1993, 26, 414 Xu, A. and Sarkar. S. L., Microstructural development in high- volume fly-ash cement system. Journal of Materials in Civil Engi- neering, ASCE, 1994, 6, 117

Bijen, J. and Van Selst, R. F/y Ash as Addition to Concrete, Research by Intron, Institute for Material and Environmental Research B.V.A.A. Balema. Rotterdam, 1992

Gopalan, M. K., Nucleation and pozzolanic factors in strength

development of Class F fly ash concrete. AC1 Materials Journal. 1993.90, 117 Marsh. B. K.. Day, R. L. and Bonner, D. J.. Pore structure characteristics affecting the permeability of cement paste containing fly ash. Cement and Concrete Research, 1985, 15, 1027 Feldman, R. F. and Beandoin, J. J., Pretreatment of hardened

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hydrated cement pastes for mercury intrusion measurements. Cement and Concrete Research, 1991,21, 297 Feldman, R. F., Effects of fly ash incorporation. In Cement and Concrete, ed. S. Diamond. Mater. Res. Sot, University Park, PA, USA, 1981, p. 124 Bijen, J., Benefits of slag and fly ash. Construction and Building Materials. 1996, 10, 309 Al-Amoudi, 0. S. B., Maslehuddim, M. and Asi, I. M. Cement, Concrete and Aggregates, 1996, 18, 71 Wong, Y. L., Poon, C. S. and Lam, L., Strength development of normal and high strength PFA concrete under different curing

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environments. In Concrete ‘97, Proceedings of 18th Biennial Con- ference. Concrete Institute of Australia, 1997, p. 145 Day, R. L. and Marsh, B. K., Measurement of porosity in blended cement pastes. Cement and Concrete Research, 1988, 18, 63 Taylor, H. F. W., Cement Chemistry. Academic Press, London, 1990 Robler, M. and Odler, I., Investigations on the relationship between porosity, structure and strength of hydrated Portland cement pastes, 1. Effect of porosity. Cement and Concrete Re- search, 1985, 1.5, 320

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