Acceleration of strength gain of lime-pozzolan cements by thermal activation

CEMENT and-CONCRETERESEARCH, Vol. 23, pp. 824-832, 1993. Printed in the USA. 0008-8846/93. $6.00+00. Copyright © 1993 Pergamon Press Ltd.

ACCELERATION OF STRENGTH GAIN OF LIME-POZZOLAN CEMENTS BY THERMAL ACTIVATION Caijun Shi and Robert L. Day Department of Civil Engineering, The University of Calgary Calgary, Alberta, Canada T2N 1N4 (Refereed) (Received June 1, 1992; in f'malform March 8, 1993)

ABSTRACT This paper deals with the effect of curing temperature on: (a) the time for development of appreciable strength, (b) ultimate strength and (c) strength development rate of lime-natural pozzolan cement pastes with and without chemical activator (4% Na2SO4 and 4% CaClv2H20). The results indicate that the initial hardening time (time for pastes to achieve significant strength) decreases noticeably as the curing temperature rises. The ultimate strength of these cement pastes indirectly shows good linear correlation with the curing temperature; the rate of strength development fits the Arrhenius equation well. The addition of 4 % Na2SO4 does not change the initial hardening time, but can increase both the strength gain rate and ultimate strength by 50 to 90%. The presence of 4% CaC12.2H20 greatly improves the ultimate strength - by 2 or more times compared to the control pastes; initial hardening time, however, increases. Based on the strength development rate constants and the Arrhenius equation, the apparent activation energies of the control pastes, pastes with 4% Na2SO4 and the pastes with 4% CaC12.2H20 are 66, 75 and 99 KJ/mol respectively. INTRODUCTION One obvious disadvantage of lime-pozzolan cement is its slow strength development during room temperature curing [1]. Experimental results indicated that an increase in apparent activation energy could result in a slower rate of reaction and strength development at room temperature [2-4]. Accordingly, it can be inferred that lime-pozzolan cement has a much higher hydration activation energy than Portland cement and the hydration of lime-pozzolan cement will be more sensitive to curing temperature. Many researchers have confirmed this. Lea found that the strength differences of Portland pozzolan cements under two different curing temperatures increases as the pozzolan content goes up [5]. Also, the strength of 1:1:6 (lime:pozzolan:sand) 824

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825

mortars increased as the curing temperature rose. Where mortars with Santorin Earth did not show any strength at 7 and 28 days at 0°C, they did show high strength when cured at 35°C [5]. Day [6] obtained similar results. The lime-pozzolan pastes cured at ambient temperature and tested at 7 days gave strengths of about 0.8 MPa, but gave 6 MPa when the pastes were cured at 52°C. For practical use of lime-pozzolan cements it is essential that they obtain satisfactory strength in an acceptable time period. Heating curing is usually necessary. However, different pozzolans show different response to temperature rise. Alexander & Wardlaw [7], for example, observed that the mixtures of opal, opaline shale and diatomite with lime have the lowest modulus of rupture/compressive strength ratio at 43°C. Thus, in the evaluation of proposed pozzolan sources, it is important to estimate the curing temperature-strength function. This paper comprises an examination of how curing temperature affects the initial hardening time, ultimate strength and the rate of strength development of lime-pozzolan cement pastes with and without chemical activator (4% Na2SO4 or 4% CaC12.2H20).

EXPERIMENTATION Natural pozzolan (NPB) from the vicinity within a 30 Km radius of LzPaz, Bolivia was used. Its chemical composition is shown in Table I. The commercial hydrated lime was provided by Summit Lime Works Ltd, Coleman, Alberta. The lime-pozzolan cement consisted of 20% hydrated lime and 80% natural pozzolan by mass. The dosages of Na2SO4 and CaCI2.2H20 were 4% based on the mass of lime-pozzolan cement. The water to cement ratio was 0.5. These parameters were determined from an extensive series of optimization tests.

Table I Chemical Composition of Natural Pozzolan NPB SiO2

A1203

FeaO3

CaO

MgO

66.48

15.41

0.66

10.29

0.31

SO3 0.23

Na20 1.39

K20

I.L

5.64

5.10

Mixing and specimen preparation were performed at room temperature. Prior to mixing, however, the temperature of the water, activator (if any), lime-pozzolan cement and apparatus were elevated to ensure that the initial temperature of the pastes were near the expected curing temperature. The pastes were cast into small glass vials (~b25x50 mm), capped and then placed into different water baths having temperatures of 23, 35, 50 and 65__+2°C. The glass vials were stripped when specimens had hardened significantly; this process was very rapid and did not substantially affect the curing process. At every testing age, three specimens were taken out one hour before the testing time to cool. Ends were polished to make the two bearing surface flat and parallel. The specimens were tested in compression and the strength results given are an average of the three specimens. The average coefficient of variation of these results is less than 10%.

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EXPERIMENTAL RESULTS AND DISCUSSION Effect of Curing Temperature on Strength Development of Lime-pozzolan Cement Pastes Fig. 1 shows the effect of curing temperature on strength development of the control cement pastes without any activator. A rise in curing temperature accelerates early age strength development greatly. The pastes do not show measurable strength until about 4 days at 23°C, but have a strength of about 2.5 MPa after 1 day at 65°C. Strength levels-off to a shallow slope (plateau) at about 75 days at 23°C, about 20 days at 35°C, 15 days at 50°C and 10 days at 65°C. The rate of initial strength development increases as curing temperature increases. The addition of Na2SO4 does not change the trends just noted (as shown in Fig.2), but the pastes with 4% Na2SO4 show higher strength than the control pastes at a given curing temperature and age. Characteristics of the strength plateau are also similar to that of control pastes at the same curing temperature. A rise in curing temperature has a more pronounced effect on the strength development of the hardened cement pastes containing 4% CaC12.2H20 (see Fig.3). For 23°C cure, the strength gain is shallow over all ages; there is no plateau. At elevated temperature, the plateau develops at about 75 days at 35°C, about 20 days at 50°C and about 10 days at 65°C. The changes of the initial strength development rate are also much more prominent than the control pastes or the pastes with 4% Na2SO4. At the same time, both of the chemically activated cement pastes exhibit much higher strength than the control pastes. For curing at elevated temperature, CaCI2 gives higher strength than Na2SO4 during the first 50 days of curing.

Strength-Age Relationship Several authors have confirmed that the strength-age relationship of cement pastes, mortars and concrete can be described by the equation [8-10]:

Kr(t-to) S:S

o.

~

1 +Kr(t-to)

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

(1)

where: S - compressive strength (MPa); So - ultimate compressive strength at infinite age (MPa); KT - strength development constant at curing temperature T (dayl); t - actual curing age at temperature T (days); to - theoretical initial hardening time (days).

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25

Based on the data in Fig 1, 2 and 3, the theoretical best-fit values of So, KT, and to are outlined in Table II. The curves in Fig. 1, 2 and 3 are derived from equation (1) and the regression results in Table II.

=

15

-~ 10

! d

O:

0

Initial Hardening TimeTemperature Relationship Fig.4 illustrates the relationship between the initial hardening time to and curing temperature. The relationships for the control pastes and the pastes with 4 % Na2SO4 are essentially the same. The different line for the pastes containing CaC12 reflects the a p p a r e n t retardation of very early strength gain that a CaC12 activator has.

40

80 120 Age (days)

160

200

Fig. 1 Effect of Curing Temperature on Strength Development of Control Pastes 2520C I) 0

t

S

J

1510- ~

~"!

E 0 o 0

Strength Development Rate Constants The Effect of curing temperature on Kr is shown in Fig.5. From the present data Na2SO4 appears to be the more efficient activator at 50°C. This may be caused by different pozzolanic reaction and different reaction products occurring above 50°(2. CaC12 is more efficient at temperature above 50°C. Research to determine a detailed explanation is in progress.

o

I

,

40

80 120 Age (days)

160

200

Fig.2 Effect of Curing Temperature on Strength Development of Pastes With 4% Na2SO4 25

~20

j

.C

o o

~ ' 7 ~

5

} 0

/

~"

j r

g

J

40

80 120 Age (days)

160

200

Fig.3 Effect of Curing Temperature on Strength Development of Pastes With 4 % CaC12.2H20

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Table II Strength Development Characteristics of Lime-pozzolan Cement Pastes sample

curing temperature

(MPa)

al*

(day1~

a2

(days)

a3

23

14.3

0.87

0.019

0.003

4.65

1.01

35

11.0

0.36

0.060

0.007

1.12

0.35

50

8.5

0.31

0.197

0.032

0.50

0.35

65

5.6

0.27

0.520

0.226

nil

nil

23

24.8

0.99

0.029

0.003

4.71

0.63

35

18.4

0.67

0.096

0.014

1.34

0.30

50

14.0

0.51

0.377

0.094

0.50

0.31

65

9.8

0.52

0.456

0.205

nil

nil

23

39.1

4.14

0.005

0.000

6.24

1.25

35

25.0

1.12

0.029

0.003

2.11

0.51

50

18.9

0.68

0.185

0.032

1.22

0.23

65

15.7

0.32

0.817

0.145

0.66

0.12

(oc)

control

control -t4 %Na2S04

control + 4 %CaC12.2H20

So

KT

to

* - estimated standard error of the regression parameters; 1 =So, 2=Kr, 3 =to.

7

\

6 "0 v o

5

I

I

t control -

I

+4% Na2S04

E

1.7. 4 .¢::

+4% CaCI2.2H20

3 "I-

2

°~

_=

1 0

10

20

30 40 50 Curing Temperature (C)

60

70

Fig.4 Relationship Between the Initial Hardening Time and Curing Temperature

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STRFNGTH,THERMALACTNATION,CHEMICALACXVATION

Curing Temperature

829

(C)

Fig.5 Effect of Curing Temperature on Strength Development Rate Constants

Curing Temperature (C)

Fig.6 Relationship Between the Ultimate Strength and Curing Temperature Ultimate Strength-Temperature Relationship The values of the theoretical ultimate strength (Sd) of the pastes in Table II are @lottedas a function of curing temperature in Fig.6. The figure indicates that S, of control pastes decreases linearly with the curing temperature over temperature between 23 and 65°C; this is valid for Na$O, and CaCl, pastes between 35 and 65°C. The regression results are summarized in Table II. All these pastes, in fact, between the temperature 35 and 65 “C, behave similarly, except for the approximate 5 Mpa offset between the control and the pastes with 4% Na$O,, and between the pastes with 4% Na.$04 and 4% CaC1,.2H,O. CaCl, activator has the maximum

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effect on the theoretical ultimate strength. Note, however, that the beneficial effect of CaC12 is most pronounced at 23°C curing. Table II Regression Results for Ultimate Strength-Temperature Relationship pastes

equations

correlation coefficient

control *

So = 18.6 - 0.20T

0.996

control + 4% Na2SO4 **

So = 28.4 - 0.29T

1.000

control + 4% CaC12.2H20 ** So = 35.3 - 0.31T • between 23 and 65°C, ** between 35 and 65°C.

0.985

Apparent Activation Energy The Arrhenius equation can be employed to depict the effect of curing temperature on cement hydration or the strength development of cement pastes [2, 3, 8, 11]: Kr=A.exp(~-~) ..................... (2) where: A E, R T

- frequency constant (day1); - apparent activation energy (J/mol); - gas constant (8.314 J/K.mol); - Kelvin temperature (K).

Fig.7 is an Arrhenius plot of KT VS 1/T (K-l). Clearly, the relationship demonstrates excellent linearity within the studied temperature range except for the paste with 4% Na2SO4 at 65°C. Table III outlines the regression results of these straight lines within the studied temperature range. Compared with the hydration activation energy of portland cement (about 45 KJ/mol) [2], the hydration activation energies of lime-pozzolan cement pastes are high (66 KJ/mol), particularly the pastes with 4 % CaClz.2I-I20 (100 KJ/mol). The addition of 4 % Na2SO4 influences hydration by increasing the activation energy by about 15 %. The curing temperature has more effect on the strength development of the pastes with 4% Na2SO4 than on the control pastes. Compared to the control pastes, the pastes with 4% NaESO4 achieve a larger % of ultimate strength at early age and then approach the ultimate strength more gradually. The addition of CaC12 increases the hydration activation energy from 66 to 99 KJ/mol (more than 50%). These pastes are much more susceptible to curing temperature than the other pastes tested. The strength of these pastes with 4% CaC12.2H20 develops slowly at 23°C, but very quickly at 65°C. The strength development rate at 65°C is much higher than the control pastes or the pastes with 4% Na2SO4. Meanwhile, the "ultimate strength" of the pastes with 4% CaC12.2H20 is 2.5 times or more, that of control pastes. CONCLUSIONS This paper has examined the effect of curing temperature on the strength development

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1-

0-

,

I I

-1

--

-----------~ - -

I--- -2 --

-,3-

-6 2.8

2.9

3

i

i

'

-5-

3.1

i

3.2 3.3 1/'T (x 1000)

I ~ 3.4

3.5

3.6

Fig.7 Correlation Between Strength Development Rate Constant and Curing Temperature Table III Regression Results for Strength Rate Constant-Age Relationship pastes

A (day"1)

correlation coefficients

activation energy Ea (KJ/mol)

control

8.71 x 109

0.999

66

control + 4% Na2SO4*

5.74 x 1011

1.000

75

control + 4% CaC12 * between 23 and 50°C.

1.62 x 101~

0.999

99

characteristics of lime-pozzolan cement pastes with and without a chemical activator. The following conclusions can be made based upon the above analysis: 1. A rise in curing temperature can shorten the initial hardening of lime-pozzolan cement pastes. The addition of Na2SO4 does not influence the initial hardening time, but CaC12 prolongs it. 2. The strength development rate constants (KT) increase as the curing temperature increases. These constants fit the Arrhenius equation well within the studied curing temperature range except that of the pastes with 4% Na2SO4 at 65°C. The activation energies of limepozzolan cement pastes are higher than that of Portland cement pastes. The introduction of Na2SO4 or CaC12 increases the activation energy of lime-pozzolan cement pastes even more. The results confirm the greater sensitivity of lime-pozzolan cement pastes to elevated curing temperature when compared to plain Portland cement pastes. 4. The addition of Na2SO4 increases the ultimate strength by 50 to 90%, and the pastes with 4% CaCI2.2H20 boost the ultimate strength 2.5 or more times, when compared with the control pastes. The ultimate strength of these cement pastes decreases linearly with the curing temperature in the range 35-65°C.

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ACKNOWLEDGEMENT The financial support from the International Development Research Centre of Canada and the Natural Science and Engineering Research Council of Canada are gratefully acknowledged. REFERENCES .

.

3.

.

.

6. .

.

9. 10. 11.

Day, R. L., Natural Pozzolans for Building in Latin America, Proceedings of Project Identification Meeting on Local Building Materials, Nairobi, Kenya, British Science Council, 1988. Wu, X, Langton, C. A. and Roy, D. M., "Hydration of Slag Cement at Early Stage", Cement and Concrete Research, Vol. 13, No.2, pp277-286, 1983. Roy, D. M. and Idom G. M., "Hydration, Structure, and Properties of Blast Furnace Slag Cements, Mortars and Concrete", Journal of American Concrete Institute, Proceedings, Vol.79, No.6, pp444-457, 1982. Regourd, M., "Structure and Behaviour of Slag Portland Cement Hydrates", Proceedings of 7th International Congress on the Chemistry of Cements, Vol.I, ppIII-2/11 - 2/26, Pads, 1980. Lea, F. M., The Chemistry of Cement and Concrete, 3rd Edition, Edward Arnold, 1974. Day, R. L., P0zzolans for Use in Low-Cost Housing: A State of the Art Report, Department of Civil Engineering, The University of Calgary, Research Report No.ce921, January, 1992. Alexander, K. M. and Wardlaw, J., "Limitations of the Pozzolan-lime Mortars Strength Test as Method of Comparing Pozzolanic Reactivity", Australian Journal of Applied Science, Vol.6, pp334-342, 1955. Knudsen, T., "On Particle Size Distribution in Cement Hydration", Proceedings of 7th International Congress on the Chemistry of Cements, Vol.II, ppi-170-175, Pads, 1980. Roy, D. M. and Idorn, G. M., "Relationships Between Strength, Pore Structure and Associated Properties of Slag-Containing Cementitious Materials", Proceedings of Materials Research Society, Vol.42, pp133-142, 1985. Cadno, N. J., "Maturity Method: Theory and Application", Cement, Concrete and Aggregate, Vol.6, No.2, pp61-73, Winter, 1984. Shi, C., Tang, X. and Li, Y., "Thermal Activation of phosphorus Slag Cement", IL Cemento, Vol.88, No.4, pp219-225, 1991.