Thermal properties of selected sandstones

DANA KOŇÁKOVÁ, EVA VEJMELKOVÁ, ROBERT ČERNÝ Department of materials engineering and chemistry

Czech technical university in Prague - Faculty of civil engineering Thákurova 7, 166 29 Praha 6

CZECH REPUBLIC dana.konakova@fsv.cvut.cz, eva.vejmelkova@fsv.cvut.cz, cernyr@fsv.cvut.cz

Abstract: - Rocks are, generally, one of the oldest materials used in the human history. The origin of rocks application dates back to the prehistoric times. One of the best known and most used stones is sandstone. From the geological viewpoint, it is a clastic sedimentary rock. Nevertheless, this definition includes many very different kinds of stones. This study is focused on different kinds of sandstones which are quarried in the Czech Republic. Basic physical characteristics were measured and evaluated. The bulk density of selected sandstones ranges between 1868 kg m-3 and 2651 kg m-3. The matrix density shows almost equal values in all cases, and it achieves approximately 2630 kg m-3, and the open porosity ranges from 1% to 29%. However, the main part deals with thermal properties of sandstones. Nowadays, since heat transfer is one of the most discussed topics, it is necessary to determine thermal properties not only of insulating materials, but of all materials in constructions. Rocks are still one of the basic building materials. Thermal properties were measured in order to get input data for a numerical analysis of thermal transport. The thermal conductivity in the dried state attains values between 1.230 W m-1 K-1 and 3.665 W m-1 K-1. The specific heat capacity in the dried state ranges from 646 J kg-1 K-1 to 801 J kg-1 K-1. Key-Words: Sandstone, basic physical properties, thermal conductivity, specific heat capacity 1 Introduction

Rocks have been used as building materials since the prehistoric age. Mass application of sandstones began in the 14th century. That was caused by the necessity of more complicated and modern methods for technological processing of sandstones. Nowadays, stones are used mainly for reconstruction processes of historical buildings and monuments. When constructing new civil engineering projects, stones are being used just as a decorative material. At present, the utilization of rocks as a loadbearing material is not economical. The most common application of stones is in flagstone pavements, supporting walls, stone facing, and others.

From the geological point of view, sandstone is a clastic sedimentary rock. However, this definition of sandstone is too general. In fact, various sandstones differ in color or their properties (physical, mechanical, thermal, hygric, etc.). All of them depend mainly on the mineral composition of the specific sandstone type and also on the granularity.

There are several functioning quarries in the Czech Republic that produce sandstones. As it was mentioned above, they differ in their

properties. Eight functioning quarries were selected for this study. Basic physical properties of the selected sandstones were determined.

When choosing building materials, one prefers those that provide energy economy, which is one of the most important criteria. To be able to make correct analysis of construction energy demand, thermal properties of all buildings material have to be known. Due to this reason, the aim of this study was the determination of thermal characteristics, namely, the thermal conductivity and the specific heat capacity of the selected sandstones. 2 Studied materials

Sandstones are the materials studied in this article. Geologically speaking, sandstones are clastic sedimentary rocks, but they can have different chemical compositions. The structure of sandstone is formed by grains (with different dimensions and shapes) and a binder. Siliceous, argillaceous, calcareous, and marlaceous binders are the most common in sandstones. Chemical and structural diversity is the reason why sandstones have so different properties.

Advances in Modern Mechanical Engineering

ISBN: 978-960-474-307-0 100

The studied materials were sandstones quarried in the Czech Republic. The number of quarries is changing every year, depending on the economic situation and possibilities of quarrying. For this study eight functioning quarries were chosen. In table 1 we give basic information on the selected sandstones.

Table 1: Studied sandstones [1]

attribute colour quarry

grain size

S1 quartzose / feldspathic hoar Božanov coarse

S2 quartzose / feldspathic

greyish blue ochre edges Bzová fine

S3 quartzose grey white up to yellow

Podhorní Újezd fine

S4 quartzose grey white up to yellow Kocbeř fine

S5 quartzose kelly green, yellow-green Záměl medium

S6 micaceous - green

greenish grey up to teal Řeka fine

S7 feldspathic hazel Úpice (Lány) medium

S8 quartzose Grey white Mšené-lázně fine

3 Experimental methods

As was mentioned in the introduction, the measured properties belong to two categories. Measurements of basic physical and thermal properties were performed by the methods that we shall now describe. 3.1 Basic physical properties

First, the basic physical characteristics were measured. For the determination of the bulk density (ρ), the matrix density (ρmat), and the open porosity (ψ0) the vacuum water saturation method [2] was used. This method is based on weighing samples in different moisture states. Five samples of each type of sandstone with dimensions of 50 × 50 × 50 mm were used for this measurement. The first measured moisture state was the dried one (md). To remove physically bounded water, samples were put in a drying box kept at a temperature of 105˚C. The second state was

the saturated one. Samples were put into a desiccator with boiled distilled water and air was evacuated with vacuum pump (Fig.1). After removing samples from desiccator, their mass in saturated state was measured (mw). Finally, the value necessary for calculating the basic physical properties is the mass of saturated samples under the water, so called the Archimedes mass (ma). The basic physical characteristics were determined from the equations

, (1)

, (2)

, (3)

. (4)

Fig. 1: Vacuum pump with desiccator 3.2 Thermal properties

The second group of characteristics is the thermal properties. Using the apparatus ISOMET 2104 [3], the thermal conductivity (λ) and the volumetric heat capacity (cρ) were measured. This commercial device applies a dynamic measurement method so that the time of measurement reduces to dozens of minutes. The measurement process is based on the analysis of a temperature response of the analyzed material to heat flow impulses. The heat flow is induced in a resistor of probe by a distributed electric power. The temperature is recorded and evaluated from the polynomial regression.

ISOMET 2104 is verified by special etalons with known values of thermal conductivity. This verification of measuring ability had been performed before the actual measurement started.

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In the case of the thermal conductivity the error of measurement was, in all cases, less than 3 %. However in the user manual for this device [3] the accuracy of the thermal conductivity is 5% of reading + 0.001 W m-1 K-1. In the case of the volumetric heat capacity the accuracy is 15% of reading 1 J m-3 K-1.

Measurements by ISOMET 2104 are performed using special probes. These probes are of two kinds – needle probes or surface probes. In this study surface probes were used. According to the chosen probes, the height of samples had to be more than 15mm and their surface had to be circular with a minimal diameter less than 60 mm. Both thermal properties were determined in dependence on the moisture content. Three samples with dimensions of 70 × 70 × 70 mm were prepared. The first measurement of the thermal conductivity and volumetric heat capacity was performed under laboratory conditions. Then the samples were put into a drying box with a temperature of 105˚C to remove physically bounded water. Thermal properties were measured in the dried state. For another few weeks sandstones were kept in water to achieve the fully saturated state (without using an external pressure). The measurements using ISOMET 2104 were performed again. Then the samples where left in the open air so they could dry out naturally. During the drying process thermal properties of sandstones were determined in a specific moisture state. Before measuring by ISOMET 2104, the samples were, for a certain time, kept in a plastic bag due to stabilization of the moisture field.

Fig. 2: ISOMET 2104 ISOMET 2104 measures the volumetric heat

capacity (cv), but in the civil engineering practice the most common and most used is the specific heat capacity (Cu). To evaluate the latter, we applied the equation

. (5)

The moisture content needed to be calculated as well. To this end, we used the well-known equations

, (6)

. (7)

4 Experimental results and discussion

In this section the measured data on the basic physical properties (the bulk density, the matrix density and the open porosity) and thermal properties (the thermal conductivity and the specific heat capacity) are presented and analyzed.

4.1 Basic physical properties In table 2 we present the results

from the vacuum water saturation measurement, namely, the bulk density ρ, the matrix density ρmat and the open porosity ψ0 of the selected sandstones.

Table 2: Basic physical properties

ρ

[kg m-3] ρmat

[kg m-3] ψ0

[%]

S1 2182.99 2612.55 16.44

S2 2650.87 2685.82 1.30

S3 1994.31 2609.91 23.59

S4 2223.94 2614.02 14.92

S5 2047.29 2634.30 22.28

S6 2498.88 2622.43 4.66

S7 2433.64 2620.60 7.14

S8 1867.95 2617.83 28.64

It is evident that the matrix density attained almost the same values in all cases. On the other hand, the open porosity exhibits a rather large difference of values. Sandstone S2 has a smaller open porosity than sandstone S8 by about 27%. Since the open porosity is so diverse, the bulk density has also very different values. Sandstone S8 has the bulk density smaller than sandstone S2 by about 29%.

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4.2 Thermal properties The results from the measurement of thermal

properties, namely the volumetric moisture content w, the thermal conductivity λ, and the specific heat capacity Cu, are shown in Tables 3 – 10. As it was mentioned this measurement was performed using the apparatus ISOMET 2104.

Table 3: S1 – Thermal properties w

[%m3m-3] λ

[Wm-1K-1] Cu

[Jkg-1K-1]

0.00 2.348 672.62

1.76 3.233 706.06

4.51 4.303 757.19

7.81 4.493 817.07

11.09 4.540 874.73

Table 4: S2 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 3.292 649.79

0.74 3.312 661.48

1.01 3.603 665.60

Table 5: S3 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 2.575 712.03

0.63 2.658 725.20

4.59 3.018 806.03

9.14 3.197 895.17

12.89 3.666 965.82

16.89 3.763 1038.48

Table 6: S4 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 3.227 759.91

0.21 3.688 763.93

2.61 4.067 808.38

5.48 4.612 860.39

7.81 5.335 901.67

10.64 5.680 950.69

Table 7: S5 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 2.255 674.06 6.03 2.925 793.74

12.23 3.354 909.61 14.54 3.995 951.28 17.33 4.167 1000.25

Table 8: S6 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 2.535 645.62 1.64 2.968 672.82 4.98 3.705 727.28 6.20 4.043 746.90

Table 9: S7 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 3.665 646.49 0.30 4.183 651.64 3.55 5.752 706.59 6.21 6.388 750.49

Table 10: S8 – Thermal properties

w [%m3m-3]

λ [Wm-1K-1]

Cu

[Jkg-1K-1]

0.00 1.230 801.23 0.32 1.650 808.28 5.33 1.730 917.18

10.37 1.958 1021.14 15.65 2.108 1124.36 19.96 2.444 1204.82 25.21 2.648 1298.28

Sandstone S8 has the smallest value of the thermal conductivity in the dried state, while sandstone S2 has one of the largest values. The largest value was attained by sandstone S7. Its thermal conductivity is almost three times higher than in the case of S8. As far as the specific heat capacity is concerned, in the dried state sandstone S8 has the largest value which is by 20% more than the smallest value in the case of S6.

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Both studied characteristics were measured in dependence on the moisture content. It is obvious that as the moisture content increases, the thermal conductivity and the specific heat capacity also increased. This tendency is a common phenomenon caused by different values of the thermal conductivity and the specific heat capacity of water and air. 5 Conclusion

In this study the basic physical and thermal properties of several sandstones were determined. The studied materials were selected from functioning quarries. Eight types of sandstones were studied.

Basic physical properties were measured by the water vacuum saturation method. The matrix density was almost the same for all sandstones – 2630 kg m-3. On the other hand, in the case of the open porosity the results were rather different. The obtained values were in the range between 1% and 29%. Therefore, the bulk density also varied much, from 1868 kg m-3 to 2651 kg m-3.

Thermal properties were determined by a special commercial device ISOMET2104 which applies a dynamic measurement method. Thermal characteristics were determined in dependence on the moisture content. In the dried state the thermal conductivity attained values from 1.230 W m-1K-1 to 3.665 W m-1K-1. The specific heat capacity ranged in dried state from 646 J kg-1K-1 to 801 J kg-1K-1.

As it is obvious from the obtained results, the measured properties of sandstones are very different. This is caused mainly by their mineral composition and granularity. Both of these properties depend on the location of a quarry. Hence, when applying sandstone, it is really important to know where the specific sandstone comes from and to measure its specific properties.

Acknowledgements:

This research has been supported in the Czech Republic under project SGS12/105/OHK1/2T/11 and SGS13/165/OHK1/3T/11. References: [1] Česká geologická služba: Dekorační kameny

[online], april 2013 <http://dekoracni-ameny.geology.cz/index_cz.pl>

[2] Kumaran, M. K.: Moisture Diffusivity of Building Materials from Water Absorption

Measurements. IEA Annex 24 Report T3-CA-94/01, Ottawa 1994.

[3] Applied Precision - ISOMET. [User manual], Bratislava, 1999

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