Potential expansion of cement mortars in the presence of K2SO4 and pozzolan

Pergamon

Cement and Concrete Research, Vol. 24, No. 4, pp. 728-734, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0008-8846/94 $6.00+.00

0008-8846(94)E0016-Q POTENTIAL EXPANSION OF CEMENT MORTARS IN THE PRESENCE OF K2SO 4 AND POZZOLAN M. P. Luxfin*, M. Frias* and F. Dorrego (*) Instituto C. C. E. Torroja. Apartado 19002.28080-Madrid (Spain) (Refereed) (Received July 15, 1993; in final form March 5, 1994)

ABSTRACT The hydration of cements is a very complex process and the alkalis have important influence on its properties. A study is presented on the influence which the incorporation of Na2CO3 and K2SO4 has on different types of portland cement, with high and low expansion effect being observed due to the presence of the alkalis. The addition of natural pozzolan in proportion of 5 and 15 % modifies the results obtained, due to its reactivity.

Introduction The presence of alkalis in cement influences the hydration reactions, so its properties are modified. Many excellent papers have been published on the phases composition, morphology, hydration rate and technical properties such as setting time, shrinkage, strength development, alkali-aggregate reaction,...(1) and on different aspects of durability (2). In general, the incorporation of natural pozzolans and fly ashes to the high-C3A portland cements improves the performance of blended cements to sulfate attack. The mechanisms by which this improvement occurs, are not fully understood (3). Researches on the effect of fly ashes composition, low and high content, on sulfate resistance of mixed cements have been realized (4). In this paper aspects on potential expansion of portland cement mortars, by using ASTM C 452 standard, and the presence of alkaline salts and natural pozzolan. The first part deals with the incorporation of Na2CO3. To avoid problems with setting, this salt was substituted by K2SO4 in the second part.

Experimental Four portland cement clinkers: two normal portland ones and two portland with low C3A content (< 5 %) were chosen, to prepare two type I cements and two type V cements respectively, in accordance with ASTM, The gypsum for preparing the cements is a 728

Vol. 24, No. 4

K2SO4, Na2CO3, EXPANSION, POZZOLANS, MORTARS

729

natural material containing 98 wt.% of Ca SO4. 2H20, using 5 % by weight for the mixtures. Alkali salts (Na2CO 3 and K2SO4) of analytical grade were added to each of the cement types (I and V) in the following proportions by weight of cement/alkali salt: Cement: I-1 and V-1 / Na2CO3 = 100/0; 99/1 and 98/2. Cement: I-2 and V-2 / K2SO4 = 100/0; 99/1; 98/2 and 97/3. In order to discover the effect of the presence of active additions to the cement containing K2SO4, a natural Spanish pozzolan was chosen and 5 and 15 % was added to cement mixtures containing 2 % K2SO4. The materials were ground separately, a process controlled by Blaine final specific surface value around 3500 cm2/g for each clinker and gypsum and 6000 cm2/g for natural pozzolan. Test methods

The chemical composition of the portland cements and natural pozzolan are listed in Table I. Setting time was determined according to ASTM C 451 and Vicat's method. TABLE I Chemical Composition of the Materials ,SAMPLE

S

A

F

5,9 2,6 5,4 4,1 15,3

4,2 5,7 4,1 6,2 12,2

C

M i

I-1 V-1

18,0 20,5 I-2 19,7 V-2 19,3 N. POZZ. 37,5

62,7 62,6 64,4 62,2 15,2 i

1,6 0,7 1,1 2,8 11,5

s

N

3,4

0,3

ii K

0,8

LOI 2,5

2,6

0,2

Ins.R. l'Free C 0,3

0,3

0,6

3,2

0,4

0,3

2,9 0,4 0,7 3,4 ] 0,1 ] 0,5

1,3 2,9

0,1 0,2

0,5 0,2

o,6 I 1,31o,5

5,1

-

-

Potential expansion test of portland cement mortars exposed to sulphate: The experimental method was realized conforming to ASTM C 452 specifications. For the I-1 and V-1 cements and their mixes with sodium carbonate the results are exposed in the Fig. 1 (a and b). A group of these specimens was cured at 40°C instead of 23°C after 28 days up to 360 days (Fig. 2: a and b). For the V-2 cement and its mixes with potassium sulphate and natural pozzolan the results are shown in Fig. 3. In every diagram the experimental curve for a portland cement with C3A content = 5 wt.% is indicated. The values were obtained from P. Garcia de Paredes's research, (5). Strength characteristics: Compressive strength results for portland cement and its mixes with potassium sulphate and natural pozzolan are in Fig. 4 (a and b), (ISORILEM-CEMBUREAU method). Test results and Discussion

Effect of the addition of Na2CO3 on the properties of cement Potential expansion test (Fig. 1): The incorporation of Na2CO3 ( 1 % and 2 %) always reduces the expansion of the mixed cement in relation to the cement without alkali salt. This effect is specially noticeable after 90 days for portland cements and after 180 days for lower C3A portland cements. For both cements, the addition of 2 % Na2CO3 leads to shrinkage after 360 days. The increase in the proportion of added Na2CO3, from 1 to 2 %, produces a decrease of the expansion at high ages for the type I cement (Fig. la).

730

M.P.Lu,xanetal.

Vol.24,No.4

0,30

o~TYP____EE I_ CEMENT(I- 1)

/

A~,

0.26t

0'201 : o:eI

°"'I

-~ °"'t

i'1

I // ...:," //t"/"

CEMENT CEMENT*,% No, CO, . . . . CEMENT+2%N02CO, -

-

-

/

-

.... - , EXPERIMENTALCURVEFORP.C.(C,A C . {=5C%) , ~ . . . i. .

o.oaI

....-,~.,~

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i

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~6

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,~0 . . . .

.,o

rio lo'&

AGE(doys)

/

b. TYPEV CEMENT(V-I)

0,30' O,Ze 0.26 0,24

O,tZ 0,20 o~s ~ 0,14. <

w

..-- CEMENT ° •-- CEMENT+, Y.No,CO, . . . . CEMENT'+2%No2CO~ EXPERIMENTALC U :

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s~,o

",~o 1o6o

Vol. 24, No. 4

K2SO4, Na2CO 3, EXPANSION, POZZOLANS, M O R T A R S

0,30

731

o TYPE I C E M E N T ( I - I )

0.28 0.2~ 0,24

O,2Z 0,20

o~ o.le z

CEMENT

o,~ 0.14

w

•~ ....

00.12 ,10

o

/

CEMENT ~ I '/° No, C03 CEMENT + 2 % No2 CO~l

/ " /

EXPERIMENTAL CURVE FOR P C ( C 3 A = 5 % )

J

,

0.08,

0.04, 0,02. 0,00.

.

.

3

.

~-~40

7

C

28

.

.

56

. 90

. 180

~0

AGE (ClOyS)

0,30

b. TYPE V CEMENT(V-t )

02e 0.26. 0,24"

0.22. 0,20 o~ o.la CEMENT ~.w z

uJ

0.14.

....

CEMENT+ 1% No2 C03 CEMENT + 2%NO2 C03

/ " /

0.12 0.10 0.08 (~06 0,04 0.02 0.00

3

7

28

56

90

180

AGE (doys)

FIG. 2 (a and b) Potential Expansion (ASTM C 452) of Portland Cements (from 28 days at 40 °C)

360

loToo

732

M.P. Luxan et al.

Vol. 24, No. 4

Curing at 40 °C (Fig. 2): The main effect of thermal treatment is the stabilization of expansion values, independent of the type of cement; this influence is small at early ages (28-56 days). The expansion decreases by increasing the percentage of added alkalis. The observed dispersion of the experimental values is due to small number of samples and lower frequency of the measures. Most spectacular results for curing at 40°C are by normal portland cements, and their values are always below the experimental curve for portland cement with C3A content = 5 %. The portland cement with lower C3A content shows a similar development with or without thermal treatment. Setting time: The effect of Na2CO3 on the setting of cements and the formed compounds was already described (6). The hydration reactions are altered by the addition of Na2CO3 and abnormal setting (false or quick set) takes place. By incorporating a natural pozzolan, the alkalis are fixed and the setting is normal again. To avoid problems with this salt, it was substitued by K2SO4 since it has a different effect on hydration reactions. TYPE V CEMENT( V - 2 )

}2o t o~ o,m1

--.-....

°~"1

..... ~-----

CEMENT CEMENT+ 1% K2SO4 CEMENT*-2%K2 S04 CEMENT+3%K2 S04 CEMENT* 2%Kz S04 + 5% NAT.POZZOLAN CEMENT~2°/oK2 SO, * 15% NAT POZZOLAN

/ / ~" j

0.02, oooI ,

•

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- -i. ~-"~--.~-----------" ".'ZC..'" .~-~--~-------:T. - - - -

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(d0ys)

FIG. 3 Potential Expansion (ASTM C 452) of Portland Cement (C3A < 5%) Effect of the incorporation of K2SO4 on the properties of cement Potential expansion test: The addition of K2SO4 reduces the expansion of the type V cement. The most effective proportion of the added K2SO4 is 2 % (Fig. 3). The cement mixes with addition of K2SO4 and containing natural pozzolan have a higher expansion than without addition, although below the experimental curve for portland cement with C3A = 5 %. Prior studies of the pozzolanic reaction show that the addition of natural pozzolans to cement causes a decrease in the concentration of alkalis in the medium, particularly ROtassium (7). By increasing the percentage of added natural

Vol.

24, No. 4

80-

70o

~

K2SO4, NaCO3, EXPANSION, POT'IOLANS, MORTARS

CEMENT ~ ' - " - ' CEMENT'i- 1% .... CEMENT÷2% ..... CEMENT+ 3 % " " - ' " - ' CEMENT÷ 2 % ~ CEMENT ÷ 2 %

733

o. TYPE I CEMENT ( 1 - 2 ) Kz K2 Kz Kz

S04 SO4 SO,l S04 * 5 % NAT. POZZOLAN

K2 SO4 + 15% NAT. POZZOLAN

60,

z I-ID z t~J nl-~n

50

~

30

u0 20-

10-

O,

3f,o AGE (days)

b TYPE V CEMENT (V- 2 ) 80'

70'

~.m

CEMENT CEMENT ÷ I %

....

CEMENT t 2 % K2 SO4

. . . .

CEMENT

.!- 5 %

K2 SO4 ~

CEMENT t 2 % Kz S04 * 5 % NAT POZZOLAN CEMENT * 2 % Kz SO4 ~,I5%NAT. POZZOLAN

o

"

K 2 SO4

.... / " ~

J

..... _: .~.~~"~

60 ¸

•

-r" t,,.D Z 50" tAJ rr l-t/)

,---

2'----

LU 4 0 ' > (n u) ¢ n

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u0 20. f J }0'

3~,o AGE ( cloys ) FIG. 4 (a a n d b) Oompressive Strength of Cements

734

M.P. Luxan et al.

Voh 24, No. 4

pozzolan, the expansion increases. It can be explained: since the C/S molar ratio of the hydrated cement is lower due to the presence of the pozzolan and more alkalis are retained by the hydrates, therefore leading to a reduction in the amount of soluble alkali

(8). Setting time: is normal for the cements containing 1, 2 and 3 % in K2SO4. The abnormal setting of the cements does not happen by the incorporation of K2SO4 (1, 2 and 3 %), however it accelerates the setting of the cements. In contrast, incorporation of K2SO 4 (<1%) delays both the initial and the final sets, (9). Strength development: In general, it was noticed that the incorporation of alkali salts (K2SO4: 1, 2 and 3 %) in the portland cements decreases compressive strength development, both at early and late ages (Fig. 4: a and b). This reduction is corroborated by I.A. Kryzhanovskaya (10) and is due to alteration of the hydration of the clinker minerals by the alkalis in the liquid phase. Conclusions The principal findings of the study on the potential expansion test of portland cement mortars with added alkali salts are the following: * The incorporation of alkali salts (Na2CO3:1 and 2 wt.%; K2SO4:1,2 and 3 wt.%) reduces the expansion of portland cement mortars, mainly when C3A content is low. * The amount of added alkali salts has an optimum value for each portland cement. * The expansion remains stable for samples cured at 40°C instead of 23°C alter 28 days. * The presence of a natural poz.zolan causes a reduction of the effect of the alkali.~ in the potential expansion test, due to their retention by the solid phase. References

1. I. Jawed, J. Skalny, Cement and Concrete Research, 8, n.1,37, (1978). 2. R.S. Gollop, H.F.W. Taylor, Cement and Concrete Research, 22, n.6, 1027, (1992). 3. P.K. Mehta, P, Schiessl and R. Raupach, 9th Int.Cong.Chem.Cem. (New Delhi), 1, 571, (1992) 4. P.K. Mehta, ACI Journal, 83, n.6, 994, (1986). 5. P. Garc|a de Paredes, Working Paper (IccET), n.12, (1967). 6. M.P. Lux~n, T. V~zquez, Mater. Constr., 23, n.149, 54, (1973). 7. Lux~n, M.P., Soria, F., Cement and Concrete Research,5, n.5, 461, (1975) 8. V. Sabatelli and G.L. Valenti, Proc. 7th Int. Symp. Chem. Cem. (Paris), IV, 256, (1980) 9. P.L. Rayment, Cement and Concrete Research, 12, n.2,133, (1982). 10. I.A. Kryzhanovskaya, Cem. Lime Manufacture, 5, 97, (1969).