Curing temperature and humidity effects on the strength of an aluminous cement

CEMENT and CONCRETERESEARCH. Vol. I0, pp. 491-497, 1980. Printed in the USA. 0008-8846/80/040491-07502.00/0 Copyright (c) 1980 Pergamon Press, Ltd.

CURING TEMPERATURE AND HUMIDITY EFFECTS ON THE STRENGTH OF AN ALUMINOUS CEMENT

T. M. Kula, M. D. Meiser and R. E. Tressler Ceramic Science and Engineering Section Department of Materials Science and Engineering The Pennsylvania State University University Park, PA 16802

(Refereed) (Received Oct. 8, 1979; in final form March 20, 1980) ABSTRACT The effects of curing temperature (21 and 60°C), time (1,7,28 days) and relative humidity (25 to 87%) on the strength and phase compositions of specimens prepared from neat Fondu paste with water/aluminous cement ratio of 0.30 were investigated. The room temperature strengths of the materials aged at 60°C, containing C3AH 6 and AH3, were higher than pastes cured at 21°C, containing CAHI0. Conversion at 60°C of the specimens aged at room temperature resulted in strengths comparable to materials isothermally cured at 60°C. The effects of these curing conditions on the high temperature strength were also investigated.

L'action de la temperature de durcissement (21 ° et 60°C), la dur@e (i, 7, 28 jours) et l'humidit~ relative sur la r~sistance et la composition de phase des eprouvettes " preparees " d ' une p~te de rapport eau/ciment alumineux de 0.30 a @t@ ~tudi~e. La r~sistance, calcul~e ~ la temperature normale d'int~rieur, des mat~riaux ~g~s ~ 60°C et contenant C3AH 6 et AH3, ~tait plus fortes que la resistance des p~tes ~g~s ~ 21°C, contenant CAHIo. La conversion ~ 60°C des ~prouvettes ~g~es a la temperature normale d'int~rieur a obtenu une r@sistance similaire aux mat~riaux ~g~s isothermiquement ~ 60°C. Les rapports entre les conditions de durcissement et la r~sistance ~ haute temperature ont aussi ~t~ etudi~s. Introduction Reports in the literature concerning the effects of curing temperature and humidity on the strength of high alumina cement (HAC) and HAC concretes have appeared to be conflicting in the past (1,2). The CAHIo phase which forms first under normal ambient conditions is metastable and converts to C2AH 8 and AH 3 or at higher temperatures to C3AH 6 and All3 (3). This conversion reaction often results in drastic strength reductions of the cement or concrete (1,3,4,5). By reducing the water to cement ratio (6), and carrying the reaction out at high temperature or by doing the conversion slowly (3) the drastic strength loss can be minimized.

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In fact Ramachandran and Feldman (6) have suggested that the CA may be hydrated directly to C3AH 6 resulting in a dense, fine grained material with higher hardness than the materials that convert from CAHI0 to C3AH 6. It has also been proposed that alumina gel is the important bonding phase in HAC, and that the strength loss results from dehydration and crystallization of the gel to AH 3 (7). The effect of curing conditions, and thus the hydrated phase composition on elevated temperature strength has not been systematically addressed in published experimental studies. This investigation was undertaken to examine the effects of humidity at room temperature and higher temperatures on the strengths and phase compositions of samples prepared from neat Fondu* paste with an intermediate water to cement ratio. The effect of subsequent conversion on the strength of the room temperature aged specimens was of interest. In addition, the effects of these curing variables on the high temperature strength was of interest for the intermediate to high temperature regime. Experimental Procedure The cement chosen was a HAC Fondu with the chemical composition shown in Table i. The primary phase in the as-received powder is CA with minor phases of C12A7, C3A , C4AF , and Fe203. The water to cement ratio chosen was 0.30, twice that used by Ramachandran and Feldman (6) to show the lack of a serious conversion effect on hardness and just into the range of the ratios shown by Midgley (2) to cause drastic strength reductions upon conversion. The paste was cast into bars 3" x 1/2" x 1/2", covered with a damp cloth and cured in a sealed (plastic) bag at 21°C for 24 hrs or at 60°C for 30 min. Those samples which were given the 21 °C initial cure were aged (after removal from the mold) at 21°C either at 30% or 87% relative humidity for i, 7 or 28 days. Those initially cured at 60°C were aged for i, 7 or 28 days at 60°C either at 25% or 77% relative humidity. In addition, a group of samples was initially cured at 21°C, 87% relative humidity, for 1 week, then transferred to 60°C, 77% relative humidity for 3 weeks to compare the strengths of these materials which had undergone the conversion reaction to the strengths of those which were predominantly composed of the CAHI0 or C3AH 6. The strengths of the bars were determined with a three point bend test using a span of 1.28" and a strain rate of 0.009 min -I in an Instron mechanical testing machine. Five specimens were tested for each condition. The elevated temperature testing was performed in the same machine with an electrical resistance furnace attached. The samples were inserted directly into the furnace at temperature and held 5 min at the test temperature before beginning the test, giving the sample a total residence time at the test temperature of i0 to 15 minutes. Powder X-ray diffractometry was used to analyze for the crystalline phases. A semi-quantitative analysis for the amounts of the various phases was done by analyzing a fixed volume of material each time and comparing the integrated peak areas of the major diffraction peaks which were scanned at 1/4 ° 20/min. Results and Discussion Effects of Curing Conditions on Room Temperature Strength In Figure 1 the bend strengths as a function of curing time under the four different conditions are presented as well as the data for the group which was * Lone Star Lafarge Inc., Norfolk, VA

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493 TEMPERATURE, HUMIDITY, ALUMINOUS CEMENT, STRENGTH

TABLE 1 Chemical Analysis of Fondu Calcium Aluminate Cement in Weight Percent (8)

A1203 Ca0

39.0% 38.5

Fe203 Fe0

12.0 4.0

Si02

4.5

Ti02

2.5

first aged at 21°C and then at 60°C. The room temperature aged materials appear to follow the idealized strength development curve described by Midgley and Midgley (3) in which the initial increase is attributed to the formation of increasing amounts of CAHI0 and the fall to the conversion reaction with the final rise, which is not apparent in the curves from the 21°C cured samples, due to formation of C2ASH 8 or perhaps other bonding phases. 2400 • o • D A

2000

60°C, 25°/° RH 60oc, 77%RH 2 f °C,50%RH 2 I°C,87%RH 21oc,s7%RH/60oc,

7 7 % RH

1600 I I(.9 Z1200

4

800

f400

I

I 28

I 7 DAYS OF S T O R A G E

FIG. 1 Room temperature bend strengths as a function of curing time. Error bars represent 95% confidence intervals of the means.

The data from the samples cured at higher temperatures shDw the highest strengths after 1 day of curing, an apparent minimum around 7 days, and a rise between 7 days and 28 days. From Midgley's model one may suspect that the ideal ized curve is shifted to much shorter times at the higher temperature with only the fall and slow rise portions documented here. The surprising aspect of these data is that the higher temperature cured cement showed significantly higher strengths for all ageing times which is not consistent with Midgley's relationship for strength as a function of the rate of conversion. Even more surprising is the result for the samples cured under mixed conditions first at lower temperature then at the higher temperature which produced the highest average strength of all groups tested. The phase analysis presented below points out other discrepancies between the present results and those previously published.

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Crystalline Phase Composition Results The semi-quantitative analyses for the phases present in the specimens cured under the four different conditions are presented in Figure 2. The 60°C curing conditions resulted in much more rapid hydration of the CA than the 21°C cure, but no CAHIo was detected in the former specimens even at 1 day. This result suggests that the large strength drop between 1 day and 7 days is not due to the conversion of CAHI0 to C3AH 6. In fact the observed strength decrease for the 60°C cured materials appear to correlate with increasing AH3, which tends to support the hypothesis that amorphous alumina gel is an effective binder and that the decrease in strength is the result of dehydration and crystallization of the alumina gel. The other phase which tends to increase in amount with ageing at 60°C is the C3A.CaC03-11H20 (4). There was no evidence of the C2AH 8 phase in any of the specimens. CA

AH 5

C3A-t

6

CAHto

CsA

LLI ill

6 O O C , 2 5 % RH

CoCO 5 1 1 H 2 0

J,,_

6 0 ° C , 7 7 % RH

2 I °C , 3 0 % RH

21oc ,87°/oRH

lii' r

7

28

'" III :"

_.,!

i

7

28

r

7

28

i

7

28

i

7

28

DAYS OF STORAGE

FIG. 2 Semi-quantitative x-ray diffraction analyses for the phases present in the specimens cured under the four different conditions.

There was no C3AH 6 detected in the 21°C cured materials after one month. Therefore, one may well expect the strength to decrease further as these materials are cured for longer times, and as the conversion occurs. The experiment with specimens cured first at 21°C for one week and then cured at 60°C for 3 weeks was done to accelerate the conversion and measure the effect on strength. In Figure 3, the phase compositions for these specimens are compared to those cured under isothermal conditions. As expected the CAHI0 is nearly completely converted to C3AH 6 and AH 3. In fact, the major difference in phase composition between the isothermal 60°C, 28 day specimen and the mixed condition specimen is a significantly smaller amount of AH 3 in the latter. This result again leads one to suggest that the amorphous alumina gel is an important binding phase. Clearly from the strength results and phase composition results conversion of CAHIo even when done fairly rapidly can result in significant strengthening rather than the widely held attitude that weakening is inevitable. Presumably,the intermediate water to cement ratio used in this study yields fairly fine grain C3AH 6 and fine scale porosity.

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495 TEMPERATURE, HUMIDITY, ALUMINOUS CEMENT, STRENGTH

CA

AH3

FIG. 3 Phase composition for specimens aged at 21°C and then at 600C compared to those cured under isothermal conditions.

:i

C3AH 6

CAH10

I

C3H - CaCO 3- 11H20

II

I 13" TIT

Elevated Temperature

•

21°C , 87 °/~ RH 7doys 60°C,77% RH 2 8 d o y s 21°C,87% RH 7days / 6 0 ° C , 7 7 % RH 21clays

Strength

Presented in Figures 4 and 5 are the strength versus test temperature data for the samples cured at 21°C and 60°C, respectively, with the 95% confidence intervals of the means. The confidence intervals of the data for the various groups all overlapped. Samples aged at 21°C did creasing temperature that the cured specimens contained the less disturbed when tested at

not show the large decrease in strength with insamples aged at 60°C exhibited. Room temperature most unreacted CA. Thus, the microstructure was elevated temperatures.

The samples cured at 60°C showed very minor changes in the amounts of phases present after testing at 100°C. The 3000C exposure resulted in reduced amounts of C3AH 6 and AH 3. After 500°C exposure, only trace amounts of the hydrated phases were present; the major phase was C12A7 . The samples cured at 21°C contained small amounts of C3AH 6 and AH 3 after testing at IO0°C due to the conversion of CAHI0. The 300°C exposure resulted in still larger amounts of C3AH 6 and AH 3 and lesser amounts of CAHIo. These samples also contained very little hydrated phase after 500°C exposure, but a significant amount of CI2A 7 • Regardless of the curing conditions 300 ° and 500°C.

the strengths were all similar at

Conclusions This study has shown that the widely held belief that cement pastes cured to give CAHIo yield stronger materials than ones containing C3AH 6 and AH 3 and that the conversion is detrimental to the strength of aluminous cement is not always the case. For the conditions used in this study, the curing conditions affected the room temperature bend strengths of the cement pastes. The pastes aged at 60°C, containing C3AH 6 and AH3, were as strong as or stronger than pastes cured at 21°C which contained CAHI0. The conversion of CAHI0 did not cause a reduction in the strength of the cement. Presumabl~, the 0.30 water to cement ratio used in this study yielded fairly fine grain C3AH 6 and fine scale porosity. The results of this study lend support to the hypothesis that amorphous alumina gel is an effective binder and loss of strength in the

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Vol. lO, No. 4 T,M. Kula, et a l .

1 6 0 0 [-

21°C,30% • I day • 7 days • 28 days

21°C, • I m 7 • 28

1600

RH

87%RH day days days

1200 O.

{3.

T 0 7 800 hi

T F

'

I

800 rr b-

0") 4 0 0 1

400

0

I 5O0

J

I00 500 TEMPERATURE

0

I

(°C)

FIG. 4 Bend strength as a function of temperature the specimens aged at 21°C. 2 2 0 0 -q

J

I

I00 3OO TEMPERATURE

5OO (°C)

for

~-20C

60°C ,25% • • •

1800

6 0 ° C , 7 7 % RH

RH

I dQy 7 days 2 8 days

• • •

1800F

1400

40C

'i

I day 7 days 28 dGys

| /

[3_

I I- I 0 0 0 c9 z ~J t600

200

IO00F

I I I00

I

300 T E M P E R A T U R E (o C )

600H I

500

I

I00

i

500 T E M P E R A T U R E (°C)

i

d

500

FIG. 5 Bend strength as a function of temperature for the specimens aged at 60°C. aluminous cements could be correlated with the development of crystalline AH 3. At elevated temperatures (300 to 500°C) the strengths were similar, regardless of the curing conditions. References i. 2.

D. D. Double and A. Hellawell, Sci. Am. 237 82 (1977). H. G. Midgley, Trans. Brit. Ceram. Soc. 6 6 161 (1967).

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497 TEMPERATURE, HUMIDITY, ALUMINOUS CEMENT, STRENGTH

3. 4. 5. 6. 7. 8.

H. G. Midgley and A. Midgley, Mag. Concr. Res. 27, 59 (1975). T. D. Robson, The Chemistry of Cements, p. 3 (ed. H. F. W. Taylor) Academic Press, New York (1969). A. M. Neville, Proc. Inst. Civil. Eng. i0 185 (1958). V. S. Ramachandran and R. F. Feldman, Cem. Concr. Res. 3 729 (1973). L. S. Wells and E. T. Carlson, J. Res. Nat. Bur. Std. 57 335 (1956). Technical Bulletin, Lone Star Lafarge Company, Norfolk, Virginia.