Early longitudinal dimensional changes of fresh fly ash mortar exposed to drying conditions

CLMENT and CONCRETE RESEARCH. Vol. 16, pp. 902-910, 1986. Printed in the USA 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Journals, Ltd.

EARLY LONGITUDINAL DIMENSIONAL CHANGES OF FRESH FLY ASH MORTAR EXPOSED TO DRYING CONDITIONS

DAN RAV!NA Department of Civil Engineering Building Research Station Technion - Israel Institute of Technology Technion City, Haifa 32000, Israel

(Communicated by R.E. Philleo) (Received July 31, 1986)

ABSTRACT The present work examines the effect of fly ash (ASTM Class F) on early longitudinal dimensional changes in fresh cement-fly ash (i0, 25 and 40 percent cement replacement) mortar mixes exposed to drying conditions immediately after casting at elevated temperature (30 C,86 F). It was found that the fly ash affects expansion in the fresh mortar mix: the larger the cement replacement percentage (i.e. increased amount of fly ash in the mix), the higher the expansion values and the longer its duration. However, the effect of the fly ash on plastic shrinkage during drying has no clear trend, as it is governed both by the preceding expansion and by the duration of the shrinkage phase (both influenced in turn by the specific chemical composition and physical properties of the fly ash and the cement).

Introduction Fresh Cement mixes undergo volume changes (1,2) which may be either contraction due to external or internal drying, or expansion due to chemical reactions. Fly ash, as a cementitious mineral admixture, can affect these volume changes either physically, by its particle fineness, or chemically, according to its specific composition.

Scope The present work examines the effect of fly ash on the early longitudinal dimensional changes of fresh cement-fly ash mortar mixes exposed, to drying conditions immediately after casting at elevated temperature (30C, 86 F).

902

Vol.

16, No. 6

903 DIMENSIONAL

CHANGE,

EARLY STAGE, DRYING,

FIY ASH

Materials Cement

- Two ordinary Portland cements were used. Their chemical and (calculated) mineralogical composition, and the setting time are given in Table 1.

Table

1 - Chemical composition

Cement

A

Oxide

B

A

Percent

CaO SiO 2 AI203 Fe203 MgO SO 3 Free lime Loss on ignition Insoluble residue

Fly Ash - Three used.

62.0 19.9 6.4 3.0 1.9

60.1 20.7 6.0 2.8 2.8

2.3 1.0 2.2 1.0

2.3 0.6 3.3 1.4

Mineralogical Composition

C3S C2S C3A C4AF

Setting Initial Final

B Percent

47.21 21.44 i1.88 9.13

36.48 31.83 ii.16 8.52

108 157

128 180

time, Min.

fly ashes, ASTM Class F, from different sources, Their chemical analyses are given in Table 2.

Table 2 - Chemical Oxide, Percent

SiO 2 A1203 Fe203 CaO MgO Na20 K20 SO 3 Ignition loss

Aggregates

and setting time of the cements

were

analyses of fly ashes

Fly ash designation 1

2

3

44.9 29.2 ll.1 4.2 1.6 i.i 3.4 i.i

43.8 29.1 5.3 8.0 1.8 0.3 0.8 0.9

37.0 32.2 4.2 9 .i 2.2 0.2 0.5 0.7

2.5

7.4

14.8

- Limestone quarry sand and natural siliceous fine sand were used. The Fineness Modulus of the quarry sand was 4.6 and that of the natural fine sand was 1.4.

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D. Ravina

Mortar Mixtures - The mix proportions of the mortars, made with fly ash 2, are given in Table 3. The cement r e p l a c e m e n t p e r c e n t a g e was i0, 25 and 40 by weight. To m a i n t a i n the same yield, the d i f f e r e n c e in absolute volume b e t w e e n the cement and the fly ash (due to the difference in their specific gravity) was adjusted by a=~ropriate m o d i f i c a t i o n of the ~mount of fine sand. The c o n s i s t e n c y of all mixes was plastic, with a slump of ii + icm (4 + 1/2 in.). -- The initial temperature of the mixes was 30 + 1 C (86 + 2 F), same as that of the air.

Table

3 - Mix p r o p o r t i o n s

of m o r t a r s

Cement

made w i t h

replacement,

fly ash 2

percent

Materials

Quarry Sand Natural Sand Cement Fly Ash Water

Test

0

i0

25

40

880 590 550 250

880 580 495 55 250

880 540 410 140 255

880 490 330 220 260

Specimens

and Procedure

All tests were c o n d u c t e d under c o n t r o l l e d conditions in a climatic laboratory. The w e i g h t - b a t c h e d m a t e r i a l s were mixed for 5 min, the m o r t a r was placed in the molds, t h o r o u g h l y c o m p a c t e d by hand and steel %roweled. The specimens were then placed in a w i n d tunnel (inside the climatic laboratory). The early l o n g i t u d i n a l d i m e n s i o n a l changes were m e a s u r e d on mortar prisms 7x7x28cm (2-3/4x2-3/4xllin.) at a gauge length of 23cm ¢9 in.) and at a depth of 0.7cm (0.27 in.) below the s p e c i m e n u p p e r surface, exposed to the drying atmosphere. Readings w e r e taken by a strain-gauce based transducer f o l l o w i n g the m o v e m e n t of studs inserted into the fresh mortar prism at both ends (for a more d e t a i l e d d e s c r i p t i o n , see (3), Figs. 2-3, except for the type of the dial gauge), and r e c o r d e d by a Vishay 220 data logging system. Zero time of the r e a d i n g s was set on a p p l i c a t i o n of the wind, was one hour after the b e g i n n i n g of mixing. Readings were taken min. intervals up to 4 hours.

which at 15

The air t e m p e r a t u r e was 30+1 C (86+2 F), the relative humidity 50 percent and the evaporation rate--in the F i n d tunnel about 1 kq/sq.m/hr (0.2 lb. per sq. foot/hr.).

6

Vol. 16, No. 6

905 DIMENSIONAL CHANGE, EARLY STAGE, DRYING, FLY ASH

Test Results and Discussion The early longitudinal dimensional change of the fresh mortar mixes exposed tc drying conditions immediately after casting, is plotted in Figs. 1 and 2; Figs. IA, IB, IC, represent mortars made with cement A and fly ashes 1,2 and 3, respectively, and Fig. 2 represents mortars made with cement B and fly ash 2. It can be seen that all curves have a common characteristic pattern, except that the values and duration of expansion or shrinkage differ at the various levels of cement replacement and according to the fly ash and cement used. The pattern'comprises three distinct stages. In the first stage bleeding outweighs evaporation and the surface of the specimen remains saturated; during this stage the mortar undergoes e:rpansion. The second stage sets in - after a short transition - when evaporation is from within the still plastic mix; during this stage shrinkage increases linearly with time, with the slope of the straight line mainly dependent on the evaporation rate and mix properties. The third stage sets in as the mix gains rigidity due to the hydration processes and loss of water; the increased resistance to volume changes brings about a pronounced reduction in shrinkage as reflected by the asymptotic flattening of the upper (third) stretch of the shrinkage curve. For better clarity of the analysis, the early longitudinal dimensional changes were separated for expansion and shrinkage.

f

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O%

o.6~-

~

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/

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0.2

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OX.

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1.L

1 11

(IAI

0.61

0

0%

S

0.2)

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OZt i L 96 021

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FLYASH

1 2 3 EXPOSURE TIME,HOURS C1B)

0.6t 0.81 1.01 1.21

FLYASH3

1.Lf 1.6

1.B

1

I

(.IC)

Fig. 1 - Early Longitudinal Dimensional Curves for Mortars made with Cement A and Fly Ash 1 - fig. IA, and Fly Ash 2 Fig. IB, and Fly Ash 3 - Fig. IC, at replacement percentage of 0, 10, 25 and 40.

906

Vol.

16, ~o. 6

D. Ravina

The expansion of the fresh mortar is clearly influenced (see fig. 3) by the amount of cement replacement and the specific fly ash used. As the cement replacement percentage increases (i.e. the amount of the fly ash in the mix increases) so do the the expansion values: at !0 percent replacement ~he increase is very small, but at 25 percent the average expansion is doubled, and at 40 percent nearly tripled compared to ~he Reference mix. It should be mentioned that at 40 percent replacement, expansion sets in 15 minutes later than for the other variants.

CEMENT A

0.2

0.z. 0.6 0%

.~E 0.8 £ £ 1.0 o --3 O' Z 13_

x

u.J

~'10% FLY ASH 1,2,3

~_

25% FLY ASH 1

2 5 % FLY ASH 2

1.2 25a/o FLY aSH 3 1.l 1.6

,1.8

40% FLY ASH 1 L0°2o FLY ASH 2

2.0

1 2 EXPOSURE T I M E , H O U R S

1 2 3 EXPOSURE TIME,HOURS Fig.

2 - Early Longitudinal Dimensional Curves for Mortars made with Cement B and Fly Ash 2 at replacement percentage of 0, 25 and 40.

Fig.

3 - Expansion Curves for Mortars made with Cement A and Fly Ashes i, 2 and 3, at replacement percentages of 0, i0, 25 and 40.

The chemically-induced swelling mechanism is still imperfectly understood at present (4). It is generally recognized that formation of ettringite is the cause of the expansive force, through either dissolution or topochemical reaction. Four components, CaO, A1203, SO 3 and H20 , are involved; the first three components originate normally from the cement. The aqueous phase (the solution formed when the mixing water reacts initially with the materials) must, however, contain sufficient concentrations of them for the ettringite formed to be stable. This

Vol. 16, No. 6

907 DIMENSIONAL CHANGE, EARLY STAGE, DRYING, FLY ASH

requirement is satisfied so long as SO 3 is available to the solution in amounts equal to, or exceeding the solubility of the product. Ettringite may begin to form actually soon after water has been added to the cement; addition of fly ash was found (5) to enhance the process. Two main factors influence the reaction during the early hydration of cement (6): the rate of dissolution of calcium sulfate and the hydraulic reactivity of the clinker minerals, especially alite, C ~ and C 4 AF. However, apparently, the saturation ratio of the Ca(OH) 2 a n d C a S O 4 in the liquid phase is a much mo~e important factor (7) in the early hydration process than reactivity of the clinker minerals. In addition to those from th~ cement, sulfates may also originate from the fly ash; in fact, sulfates (alkali, and perhaps also calcium) have been found to be deposited on the surface of the fly ash particles (6,8) and it may be suggested that perhaps also on those of unburned carbon (which have a high internal surface), from the vapor phase in the boiler, as the particles cool. The sulfates are fully soluble and, being freely available to the mix water in their deposited state, augment both the alkalies and sulfate brought into solution from the cement. The above can explain the higher expansion values measured with fly ash mixes as more ettringite was formed, and at a faster rate.

the

The specific chemical composition and physical properties of the cement used should, obviously, affect the early longitudinal changes as well. Indeed fig. 4 shows that both the expansion and shrinkage of cement B were higher than that of cement A (both of them ASTM Type I). Accordingly the expansion of the mortars made with the same fly ash, at different cement replacement percentages, was higher, (fig. 5) in the mortars made with cement B than with cement A. In addition to the higher values of expansion, the duration of expansion phase is also longer, (figs. 1-3) as the amount of fly ash increases, i.e. the shrinkage phase sets in later. The shrinkage pattern is, as explained before, similar for the various mixes. Moreover, it can be seen (fig. 6) that under the constant climatic conditions, the shrinkage rate (the slope of the straight stretch of the curve) of the various mixes (same cement and consistency) is about the same. Accordingly, the shrinkage value is a function of the duration of the second stage of the longitudinal dimensional change pattern, i.e. the time elapsed from the beginning of the shrinkage phase until attainment of rigidity of the mix. The point at which a mortar mix starts to develop rigidity (namely the transition point from stage II to stage III) is chartacterized by asymptotic flattening of the shrinkage curve and is marked with a small circle, fig. 6. It can be seen that, generally, the higher the fly ash percentage, the later the onset of rigidity. At the same time the corresponding shrinkage values of the mixes are not necessarily higher, as they depend also on the moment of onset of the shrinkage phase. It can be seen, fig. 6, that the shrinkage phase in all fly ash mixes sets in later than in the reference mix, due to the expansion phase, which continues for 15 minutes longer at i0 and 25 percent replacement and for 30 minutes longer at 40 percent replacement.

908

Vol. 16, No. 6 D.

Ravina

ASH 2 'lENT A 4EN'T B

0.2 0/-. ,,, 0.6 06

1_9

£

ok

~ 08

mz 0.2

Z

o

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1.0

Z

s. 1.2 x i,,IJ

%

1.L 1.6

~-0.5

1.8

0.8 I

l

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t

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1 EXPOSURE Fig.

l

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20~ , ,

l

~%

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2 EXPOSURE T IHE,HOURS

2 TIME,HOURS

4 - Early Longitudinal Dimensional Curves for Mortars made with Cement A and C e m e n t B.

Fig.

5 - Expansion C u r v e s for Mortars made with Cement A and Cement B a n d w i t h F l y Ash 2 at replacement percenta g e s of 0, 25 and 40.

l/-

0%

0%

1.2

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0.8 05 0L 02

1

2

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Fig.

6 - S h r i n k a g e C u r v e s f o r M o r t a r s m a d e w i t h C e m e n t A a n d F l y Ash 1 Fig. 6A, F l y A s h 2 - Fig. 6B, a n d F l y A s h 3 - Fig. 6C, at replacement percentages of 0, I0, 25 a n d 40.

Vol.

16, No. 6

909 DIMENSIONAL CHANGE, EARLY STAGE, DRYING, FLY ASH

The fact that the various factors affecting shrinkage play different and/or even opposite roles, precludes a definite trend in the shrinkage curve for the mixes examined. However, the shrinkage values of the mortars made with the same fly ash are higher, fig. 7, in the mixes made with cement B than those made with cement A (shorter initial setting time). 22 LO%

2.0 I.E /

1.6

/

,.f I { I

I

LO%

E E

Fig. 7 - Shrinkage Curves for Mortars made with Cement A and Cement B and Fly Ash 2 at replacement percentages of 0, 25 and 40.

L:3 <~

~ 1.0 r¢ ,1-

u~0.~

?

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FLY ASH 2 ~CEMENT A

0~

02

/// ,l YlI Y .

--CEMENTB .

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2

.

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.

.

.

3

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L

EXPOSURE TIME,HOURS

The above findings bring up an additional aspect regarding the effect of fly ash on the behavior and properties of a fresh cement paste or mortar mix. As was mentioned, an important role is apparently played in this context by the minor constituents, which are even not always tested or reported. Moreover, the fact that the same fly ash, but with different cements - may have different effects, particularly in magnitude (due to the interaction between the cement and the fly ash), all means that the determination of the effect of fly ash on fresh mix is neither simple nor straightforward. Conclusions i.

Fly ash increases expansion of fresh mortar mixes.

2.

Increased cement replacement percentage increases both the expansion values and duration.

3.

The shrinkage of the mortar mixes under drying conditions is also influenced by the fly ash, the effect being influenced by the preceding expansion and by the duration of the shrinkage phase. This precludes a definite trend in the values of the shrinkage phase.

4.

Both expansion and shrinkage (under constant environmental conditions) are affected by the specific chemical composition and physical properties of the fly ash and cement used.

910

Vo!. 16, No. 6 D. Ravina

References 1)

International R.G. L'Hermite, "Volume Changes of Concrete", Fourth Symposium on the Chemistry of Cement, II, 659-661 (1960).

2)

"Properties of set concrete at early ages" - State of the .Art Report, RIL~M 42 - CEA, Materials and Structures 14, 399-450 (1981).

3)

D. Ravina and R. Shalon, "Plastic Shrinkage Cracking", Concrete Institute, V.65, No.4, 282-292 (1968).

4)

"Expansive cement concretes - present state Committee 223, J. A m e r i c ~ Conzrete Institute, (1970).

5)

H. Jun-yuan, B.E. Scheetz and D.M. Roy, "Hydration of Fly Ash-Portland Cements", J. Cement and Concrete Research, 14, 505-512 (1984).

6)

W. Richartz, "Zusammensetzung und Eigenschaften yon Flugaschen" (composition and properties of fly ashes), Zement-Kalk-Gips, 3 7 (2), 62-71 (1984).

7)

H. Uchikawa, S. Uchida, K. Ogawa and S. Hanehara, "Influence of CaSO 2H 0, CaSO I/2H 0 and CaSO on the initial hydration of clinker having different burning degrees"j J. Cement and Concrete Research, 14, No. 5, 645-656 (1984).

8)

S. Diamond, "The characterization of fly ashes", Symposium N on: Effects of fly ash incorporation concrete, 12-23 (1981).

J. American

of knowledge" - ACI V. 67, No. 8, 583-610

Proceeding in cement

MRS and