c ratio and curing temperature on the permeability of hardened cement paste

CEMENT and CONCRETERESEARCH. Vol. I I , pp. 575-579, 1981. Printed in the USA. 0008-8846/81/040575-05502.00/0 Copyright (c) 1981 Pergan~n Press, Ltd.

THE EFFECTOF W/C RATIO AND CURINGTEMPERATUREON THE PERMEABILITY OF HARDENED CEMENTPASTE Seishi Goto* and Della M. Roy Materials Research Laboratory The Pennsylvania State University University Park, PennsylvJnia 16802

(Refereed) (Received April 22, 1981) ABSTRACT The importance of concrete permeability is discussed relative to its effect on durability. Experimental studies were made of the effect of water/cement ratio and curing temperature on the porosity, pore size distribution and permeability of cement pastes. Although total porosities of samples cured at 60°C are smaller than in those cured at 27°C, the pore volume larger than 750A radius is greater in the 60°C samples and is related to higher permeabilities also in the latter.

Introduction Permeability is one of the very important properties of concrete relating to its durability, and especially when concerned with its use in special applications such as in the isolation of waste materials. Establishing the pathways and mechanisms for fluid flow through cementitious materials is an important part of development of the understanding of mechanisms of ionic trans. port. Although the two mechanisms are not identical, they have certain common features (1). Powers (2) earlier discussed the importance of the pore volume,

*Current address: Department of Inorganic Materials, Tokyo Institute of Technology, Tokyo, Japan. 575

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Vol. I I , No. 4 S. Goto, D.M. Roy

and its influence on the permeability of hardened cement paste. However, the permeability is influenced not only by the total pore volume, but also by the pore size distribution. Very recently, Nyame et al. (3) and Mehta et al. (4) presented some experimental results showing the existence of a relationship between permeability and pore size distribution. The experimental parameters, water/cement ratio (w/c) and hydration time were investigated. According to Nyame the "maximum continuous pore radius" is representative of the size of the pores through which the (permeability) water flows, while Mehta indicated that a "threshold diameter" is very important in their relationship. Both papers indicated that the pore size distribution was changed with w/c ratio and curing time. In the present paper, results of experimental studies are presented which relate the water/cement ratio and curing temperature to the porosity, po~e size distribution and permeability of cement pastes. Experimental Sample Preparation: Pastes of type I cement (I-5) with w/c ratios of 0.35, 0.4 and 0.45 were mixed using high-speed API mixing procedures and were degassed using an aspirator for 15 minutes. These pastes were cast in l inch diameter cylindrical molds. They were demolded after l day curing at room temperature under ca. I00% RH conditions. They were cured for 4 weeks at either 27°C or at 60°C in a Ca(OH)2 saturated solution. Porosity Measurement: After curing, the cylindrical samples were sliced to make sections about l cm height. The weights of water saturated samples were measured in water (Wl) and in air (W2). The weights of samples dried in a vacuum oven for lO hours were also measured (W3). The porosity (T.P.I) was calculated by the following equation: porosity (T.P.I) = (W2 - W3)/(W2 - Wl).

(1)

Permeability Measurement: The sliced samples for permeability measurement were stored in deionized water at room temperature for ca. 50 days. Samples were set up in the apparatus which has been described previously (5), as shown in Fig. I. The water pressure used in the permeability measurements was 500 psi. Pore Siz e Distribution Measurement: Pore size distribution was measured with mercury intrusion porosimetry, carried out in the Quantachrome Co. Ltd. Results Porosity: The porosities of hardened cement pastes (T.P.I) are plotted in Fig. 2, as a function of w/c ratio. Numerous previous studies, including (6-8) have shown the phenomenon of decrease in porosity with decreasing w/c ratio. In the present study the samples which were cured at the higher temperature had lower porosities. This is reasonable because the percentage hydration for samples cured at the higher temperature is larger than for those cured at a lower temperature, for early age samples and the porosity decreases with increased percentage hydration. However, there is evidence that the degree of hydration for samples cured at higher temperatures will eventually be smaller thaninones cured at the lower temperature after longer times (9,10). Permeabilitx: The permeability values for the same sets of samples are shown in Fig. 3. The permeability values decreased exponentially with decreasing w/c ratio, at a constant curing temperature. Sampleswhich were cured at the higher

Vol. l l, No. 4

577 PERMEABILITY, W/C RATIO, CURINGTEMPERATURE, PORESTRUCTURE

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Apparatus for permeability measurement!l l ) temperature, however, had higher permeabilities. The permeability for the sample with the w/c ratio of 0.35 cured at 27°C could not be calculated exactly, because the minimal amount of water coming through the specimen was the same as, or less than, that which was evaporated in the given experimental setup. Pore Size Distribution: The pore size distributions of sampl.es with w/c ratio of 0.40 cured at 27°C and at 60°C are shown in Fig. 4. The mercury intrusion porosimetry technique did not measure pores larger than 7.5 um in radius. The sample cured at 27°C had a peak in the pore size distribution at 75 ~ 140X. and revealed no pores larger than 4300X in radius, while the sample cured at 60°C had two peaks, at 75 140A and at 1400 ~ 2300A.

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I t should be noted that the values of total pore volume (T.P.2, cm3/gdried sample) measured by mercury intrusion porosimetry were 0.190 and 0.135, instead of 0.339 and 0.379 for samples cured at 60°C and at 27°C, respectively. The differences existing between the values of T.P.I and T~P.2 reflect the volumes of larger pores, greater than 7.5 ~m in radius.

578

Vol. I I , No. 4 S. Goto, D.I4. Roy Discussion

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T.R2 Iml 60°C 0.190

I t would be expected, as a f i r s t approximation, that a samp]e with a higher porosity also would have a larger permeab i l i t y . However, this generality does not 2.0 !~ 27°C 0.135 take into consideration the possibility of and potential effects of differences in 1.5 pore size distribution. In comparing the results of samples with a w/c ratio of 0.40 cured at 60°C with samples having the same w/c ratio but cured at 27°C, the samples cured at the higher temperature 0.5 have a smaller T.P.I but a larger T.P.2 and also a larger permeability. The main difference therefore is that brought about o 0 mm~o0ooo00000 by the existence of pores in the range N ~ " f"- ~ " ~ ~ ~ 0 0 0 0 O0 from 750~ to 2300~ in radius, in the sample cured at 60°C. This result is in agreeradius (~,) ment with the effects of heat curing on cement hydration observed by Kondo et al. for C3S hydration. They stated that the FIG. 4 degre~ of hydration of C3S cured at a higher temperature was larger than in Pore size distribution of samples cured at a lower temperature at an hardened cement paste. early age and that i t becomes proportionately smaller at later ages. They also indicated that the peaks in pore size distribution of samples cured at 20°C changed with age to smaller values of radius while in samples cured at 80°C the peak remained at a large size.

Very recently Nyame et al. (3) and Mehta et al. (4) also studied the relationships between permeability and pore size distribution for samples with different w/c ratios and at different curing times. The concept of "maximum continuous pore radius" (3) is very important to permeability. The concept of "threshold diameter" (4) measured by mercury intrusion porosimetry is essentially similar to that of the maximum continuous pore radius. Mehta showed the existence of two components of pores. One is the same in the samples with different w/c ratio, whil~ the other changes with w/c ratio. The two pore ranges were divided at 1320A in diameter. He concluded that pores greater than 1320A in diameter had a close relationship with permeability. From our current results, i t was shown that samples with substantial volumes of pores greater than 75oA in radius had larger permeabilities, in general agreement with Mehta's observations. The effect of curing temperature on the pore size distribution is similar to the effect of curing time, but is not identical. The mathematical expressions proposed by Nyame or Mehta do not f i t well in the case with temperature effects. Conclusions The conclusions of the present study may be summarized as follows: l.

When porosity was calculated from measurements of evaporable water, i . e . using weights of water saturated sample and dried sample, the porosities of samples cured at 60°C were lower than in those cured at 27°C.

Vol. I I , No. 4 579 PERMEABILITY, W/C RATIO, CURINGTEMPERATURE, PORESTRUCTURE .

3. .

Porosities calculated from the results of mercury intrusion porosimetry, however, gave values in those samples cured at 60°C, which were larger than in those cured at 27°C. Permeabilities of samples cured at 60% vmre larger than in those cured at 27°C. Pores in which the radius values range from 750 to 7500X have a very important influence upon the permeabilities of hardened cementitious materials. Acknowledgements

This study was carried out at the Materials Research Laboratory of The Pennsylvania State University, under support from the U.S. Department of Energy. Assistance of personnel in the Materials Research Laboratory is gratefully acknowledssd, especially Drs. E.L. White and M.W. Grutzeck. References I.

S. Goto and D.M. Roy, in "Borehole Plugging and Shaft Sealing Systems," Ed. D.M. Roy, ONWI/SUB/78/ESI2-O4200-3 (Jan. 15, 1980).

2.

T.C. Powers, J. Amer. Ceram. Soc. 4_Zl, I-6 (1958).

3.

B.K. Nyame and J.M. l l l s t o n , Proc. 7th I n t l . Cong. Chem. Cement, Paris, 1980, VI-181 - 185.

4.

P.K. Mehta and D. Manmohan, Proc. 7th I n t l . Cong. Chem. Cement, Paris, 1980, VII-l - 5.

5.

Della M. Roy et a l . , "Annual Progress Report, Oct. l , 1977 - Sept. 30, 1978," BATTELLE/ONWI Report, A5.1 - A5.9 (]978).

6.

D.M. Roy, G.R. Gouda, Proc. 6th I n t l . Cong. Chem. Cement, Moscow, 1974, Vol. I I - l , 310 - 315.

7.

R.F. Feldman, J.J. Beaudoin, Proc. 6th I n t l . Cong. Chem. Cement, Moscow, 1974, Vol. I f - l , 288 - 294.

8.

A.A. Staroselsky, A.G. Olginsky, Yu. A. Spirin, Proc. 6th I n t l . Cong. Chem. Cement, Moscow, 1974, Vol. I I - 2 , 192- 195.

9.

S. Brunauer, D.E. Kantro, The Chemistry of Cement, Ed. H.F W. Taylor, 305. Academic Press, London (1964).

lO.

R. Kondo, S. Goto, M. Daimon, G. Hosaka, C.A.J. Review of the 27th General Meeting, 1974, 41 - 43.

II.

E.L. White, B.E. Scheetz, D.M. Roy, K.G. Zimmerman, M.W. Grutzeck, pp 471478, in Scientific Basis for Nuclear Waste Management, Vol. l , Ed. G.J. McCarthy, Plenum Press, NY (1979).