Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 349-351
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A Thermal Comparator Method for Measuring Thermal Conductivity of Cementitious Materials Akram A. Rousan+ and Della M. Roy' Materials Research Laboratow, The Pennsylvania State Unlversliy, Unlverslty Park, Pennsylvania 16802
The thermal comparator method is introduced as a suitable method to measure the thermal conductivity of cementitious materials, as a rapid and nondestructive method which supplies no Significant heat to the sample. Thermal conductivity of neat cement paste and cement plus additives is reported and discussed. It tends to reach constant values (1-1.8 Wm-' K-' for pastes and 1.2-3.8 for composites) as the material approaches maturity, depending on the initial w/c (water/cement) moisture contents and the constituents of the sample and their
proportions.
Introduction The thermal conductivity of cement pastes, mortars and concrete, like all composites, is a function of the conductivities of the constituents. The major factors influencing the conductivity of such materials can be listed as: (1)void space and relative moisture content, (2) type of aggregate, admixtures, and additives, (3) mixture proportions, (4) type of cement, and ( 5 ) temperature of the sample. Characteristics of pores in cement pastes vary according to moisture content from empty pores of approximately zero conductivity to saturation where the conductivity of pores approaches that of water. Therefore, the conductivity of pores ranges from zero to about 0.52 W/mK. That makes the choice of the thermal conductivity measurement method a crucial one because any heat supplied to the samples changes the moisture content and hence lowers its thermal conductivity. Thermal Comparator Method The idea of the thermal comparator was introduced by Powell (1957). It is a relative method in which different standards are used to establish a calibration curve from which the values of the unknowns can be obtained. An improved comparator designed and constructed in this laboratory, Figure 1,based upon two thermocouples connected differentially (Rousan, 1982). The two junctions are mounted in the base of a cylindrical container through which a fluid (water) at fixed temperature is being circulated by a circulating controller which regulates the fluid temperature with an accuracy of *0.01 "C. One of the junctions, the contact junction, protrudes slightly to touch the material under investigation, while the other remains in contact with air. In operation the circulating fluid is maintained at a constant temperature in excess of the sample's temperatures (30 "C in this investigation). When contact with the sample is made, the two junctions will be cooled at different rates because one will be cooled by the sample and air and the other only by air. The differential emf is recorded by a chart recorder. This emf is a function of the thermal conductivity of the material. Typical output is shown in Figure 2, where the reading taken into consideration is that when the curve reaches a constant value. When there is no contact the output is constant at zero as shown in Figure 3. This method does not supply significant heat to the sample because the area of contact is 'Physics Department, Yarmouk University, Irbid-Jordan. 0198-432118311222-0349$QI.5OlQ
very small (less than 0.2 mm2) and the time of measurement takes less than 2 min. Sample Preparation Sample size is important in this method, and for every range of thermal conductivity there is a minimum sample size (Powell, 1969). In case of cementitious materials a sample 1.3 cm (0.5 in.) thick and 5 cm (2 in.) in diameter is larger than the minimum size required, and this size was used in this investigation. The reading of the thermal comparator is also a function of surface finish; therefore all samples should be ground to the same surface finish which is similar to that of the standards, to ensure accuracy. The grinding in case of cementitious materials was performed under running water to prevent drying. Results and Discussion (1) Neat Cement Paste. Samples of API Class C cement were prepared with different w/c ratios and tested both in as-cured condition and after being oven-dried for 90 min in a 100-110 "C oven. The curing conditions for all samples reported in this paper were at 27 "C in a saturated Ca(OH), solution for the predetermined curing time. Thermal conductivity values of neat cement were found to have the range of 1-1.8 W/mK for different w/c ratios and curing times, while Missenard (1965), Lentz and Monfore (1966),and Harada et al. (1972) reported values in the range 0.84-1.21 W/mK. The major reason for the higher values found in this study may be explained by the state of moisture content during the measurements, which was high in this study because no significant heating was involved, while in the other methods heat was introduced in carrying out the experiments. To show the importance of moisture content, in Figure 4 thermal conductivity of the as-prepared samples is compared to that of the same samples oven-dried in a 100-110 "C oven for 90 min. Oven drying reduced the thermal conductivity by 28-40%. This decrease is probably not entirely due to loss of moisture, but also it is possibly due to cracking created by drying. The effect of w/c ratio on thermal conductivity is more pronounced in the results of the oven-dried samples; Figure 5 shows that thermal conductivity is lower the higher the w/c ratio. This was also found to be the case by Zoldners (1971) and Campbell-Allen and Thorne (1963). This observation goes along with the fact that pores are usually the result of water in excess to that required for hydration. Therefore, it is expected that as porosity is higher, the thermal conductivity is lower, the higher the w/c ratio. It 0 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
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Figure 4. The effect of moisture content on the thermal conductivity of neat cement paste.
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Figure 1. The thermal comparator: (a) side view; (b) base view.
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Figure 2. Typical output of the thermal comparator for different materials of different thermal conductivities.
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Figure 6 represents the relation between thermal conductivity and weight percentage of quartz for different curing times. It is clear that thermal conductivity increases with increasing weight percentage of quartz. Since quartz does not react appreciably with cement at ordinary temperature, the sample can be considered a two-phase composite, and consequently its thermal conductivity is determined by the conductivities of the constituents. If the sample consisted entirely of quartz, the value of thermal conductivity would approach that of quartz irrespective of curing time; Le., all curves in Figure 6 should meet at the value of quartz at 100% quartz. All curves in the figure fit very well to second-degree polynomials as follows one week samples: X = 1.203
+ 5.696 X 10-3w + 4.530 X 10-4w2
Ind. Eng. Chem. Prod. Res. Dev., Vol.
22, No. 2, 1983 351
four week samples:
+
X = 1.343 2.459 X 10-3w + 4.686 X 10-4w2 eight week samples: X = 1.564 + 1.376 X 10-3w 4.610 X 10-4w2 where w is the weight percent of quartz. If w = 100, then for the one-week samples, X = 6.303, for the four-week samples X = 6.275, and for the eight-week samples X = 6.312 W/mK. The three values are very close with an average of 6.297 K/mK, which is in the range of thermal conductivity of quartz mentioned earlier. Another example of having inert additives to cement is the case of a mortar, Figure 7; and sand/cement ratio is 2.75. The dynamic phase, being cement paste, is of small proportion; therefore, the increase in thermal conductivity with increasing curing time should be small, which is the case as is evident in the f i e . Also, it is clear that thermal conductivity tends to reach a constant value as the material approaches maturity. (3) Cement Plus Fly Ash Pastes. Samples of cement plus different weight percentages of low lime fly ash were also prepared and tested. Fly ash is reactive to some extent and the conductivity of the composite is not determined by the conductivity of the dry fly ash and cement paste but instead by the products of the reaction. Fly ash has lower density than cement, consisting mostly of glassy spheres, some of which may be hollow. Addition of fly ash, therefore, causes the thermal conductivity to decrease. Figure 8 represents the relation between thermal conductivity and fly ash weight percentage at different curing times. Because of the reactivity of fly ash, the lines in this figure should not converge to the same value when extrapolated to 100% fly ash; instead, the value which each line reaches represents the thermal conductivity of the hydrated fly ash at the respective curing time. This gives a method to determine the thermal conductivity of hydrated fly ash in the presence of cement which cannot be determined otherwise. Samples cured four weeks have lower thermal conductivity than at the other curing times including one week. This implies some type of anomalous hydration behavior in this time period. Grutzeck et al. (1981) described the hydration of fly ash mixed with cement and came the conclusion that radiating bundles of fibrous C-S-H are produced, which are relatively few and possess an open structure about this curing time. It is possible that this structural change affects the conductivity and reduces it. Also, as expected, thermal conductivity tends to approach a constant value with increasing curing time. Conclusions The thermal comparator method described here is a rapid and nondestructive method which supplies no significant heat to the sample, making it ideal to measure thermal conductivity of hydrous cementitious materials. Several conclusions concerning the thermal conductivity of cementitious materials are summarized below. 1. Similar to all properties of cementitious materials, thermal conductivity depends on curing time and approaches steady values as the material approaches maturity. 2. Thermal conductivity of cementitious materials is sensitive to moisture contents, and it is lower in the case of oven-dried samples by up to 40% than that of saturated samples; the thermal comparator method is capable of performing the measurement without causing significant drying in the process. 3. Thermal conductivity of hardened cement pastes depends on water to cement ratio, and is lower the higher
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the w/c ratio (and hence porosity). 4. In case of composites, thermal conductivity is a function of the constituents and their proportions. In case of inert additives, cement acts as a binding material whose properties change with time. In case of reactive additives, both phases change with time, and the resultant thermal J conductivity is a function of the hydration products. The thermal conductivity of the hydrated reactive additive can be estimated from the experimental results. Acknowledgment The authors express their appreciation for helpful discussions with Drs. P. H.Licastro and H.A. McKinstry. Literature Cited Campbell-Allen, D.; Thorne, C. P. Mag. Concr. Res. 1983, 15 [43], 39. Clark, S. P., Jr. I n "Handbook of Physical Constants"; Clark, S. P., Jr., Ed.; The Geological Society of America, Inc.: New York, 1966; p 462. Grutzeck, M. W.; Roy, D. M.; Scheetz, 8.E. I n "Effects of Fly Ash Incorporatlon in Cement and Concrete"; Dlamond, S., Ed.; Proceedings, Symposium N, Materials Research Society Annual Meeting: 1981, p 24. "Handbook of Chemistry and Physics", 53rd ed.; Weast, R. C., Ed.; The Chemlcal Rubber Co.: Cleveland, 1972-1973. Harada, T.; Takeda, J.; Yamane, S.; Furumura, F. I n "Concrete for Nuclear Reactors"; Kesier, C. E., Ed.; ACI Special Publication 34: 1972; Vol. 1, p 377. Lentz, C. P.; Monfore, 0. E. J . PCA Res. Dev. Leb. 1986, 8, 27. Missenard, A. Ann. i'lnst. Techn. BaNment Trav. Pub. (Paris) 1965, 18, 950. (Cited in ZoMners, 1971). Morgan, M. T.; West, 0 . A. "Thermal Conductlvity of Rocks in the Bureau of Mlnes Standard Rock Suite", Rept. No. ORNLITM-7052; Oak Ridge National Laboratory: Oak Ridge, TN, 1980. Powell, R. W. J. Sci. Instrum. 1957, 3 4 , 485. Powell, R. W. I n "Thermal Conductivlty"; Tye, R. P., Ed.; Academic Press: London and New York, 1969; Vol. 2, Chapter 6. Rousan, A. Ph.D. Thesis, The Pennsylvania State University, May 1982. Zoldners, N. G. I n "Temperature and Concrete"; ACI Special Publication 25: 1971; p 1. R e c e i v e d f o r r e v i e w S e p t e m b e r 27, 1982 A c c e p t e d N o v e m b e r 15, 1982
