The ASTM C97-02 "Standard Test Methods for Absorption and Bulk Specific Gravity of Dimensional Stone" is commonly used by engineers, architects and designers when selecting natural stone for various projects. This simple test is performed by weighing dried samples before and after 48 hours of water immersion at constant temperature. Besides flexural and compressive strength data, it is one of the most important and widely used criteria the professional has available to access the durability before the stone is placed into service. For example, a type of stone with a high water absorption value may not be suitable for outdoor projects even with application of a coating or penetrating sealer.

In this paper, we examine several granites and limestone/marbles (herein "limestone") by BET surface area analysis, mercury porosimetry, and profilometry. In selecting the granite and limestone samples, ASTM C97-02 ratings were reviewed and each type of stone was arbitrarily divided into low, medium and high water absorption. A representative sample of granite and limestone was then selected from each absorption level. The mercury porosimetry provides pore volume and size, while the profilometry gives a cross section of the surface profile and calculates average surface roughness values. These data are compared to ASTM C97-02 water absorption values to better understand how this characteristic describes natural stone.

Experimental

The surface area is deduced by adsorbing a monolayer of an inert gas on the material of interest. This quantity is measured, compared to the molecular size of the gas, and converted to a surface area. Porosity is determined by first placing a sample of the material in a container, evacuating all air, and then forcing mercury into the voids and pores under pressure. The volume of mercury absorbed yields the porosity which is then mathematically converted to pore-size distribution. In contrast, profilometry is performed by either by a stylus-type or optical instrument. Here, we use an optical method where the intensity and pattern of the reflected light provide dimensional surface features.

All samples of the granite and limestone were polished were obtained from local dealers in Northern California (Table 1). Mercury porosimetry was conducted using a Micromeritics Autopore IV 9500 with 199A 5-bulb penetrometer. Samples were evacuated to 50 mmHg and mercury filling pressure was increased from 1 to 60,000 psi. A Micrometrics ASAP 2405 physical adsorption analyzer with krypton gas was used to measure surface area, and a Stil MicroMeasure Profilometer equipped with a CHR 150N/0 to 300 mm range probe were used for surface roughness analysis. Analysis was performed by the equipment manufacturers.

Results

Low, medium, and high water absorbing granite and limestone were selected as shown in Table 1. Measured surface area, porosity, and surface roughness are shown in Table 2. Our surface area values are similar to values for Candoglia marble (0.01-0.03 meters squared per gram [m2/g]), but lower than those for a "porous" Calcarenite limestone of 1.6 - 2 m2/g.8 Our porosity values were in reasonable agreement for a fine grained granite (0.2%) and diorite (0.4%)9, and an extremely dense granite (0.6%) and an intermediate porosity limestone from Oland, Sweden (2-3%).10

The surface roughness values are based on ISO 4287 (1997) standards. Briefly, Sa is the arithmetic mean, Sq is the root mean square deviation, Sp is the maximum profile peak height, and Sz is the maximum height of the profile within a sample length. Figure 1 shows the surface profiles for each stone type. The deviation from zero represents the "flatness" of the stone resulting from milling and polishing, while the heavy dark line is the average value of the profile. One can easily see that the Absolute Black and the Bianco Carrara have very shallow surface profiles (<10 mm) whereas the Giallo Veneziano and Crema Europa have deep profiles where pores are about 50 mm deep.

The average pore diameter for the porous granite (Giallo and Juparana) is about 5 mm, but varies over an order of magnitude for the limestone. The Crema Europa had the largest pore diameter (again about 5 mm), while pore diameter for the Jerusalem Gold and Bianco Carrara is about 0.01 mm. Some larger pores are also evident in the granite (>100 mm), but near the outer range of the instrument.

Discussion

When a protecting liquid is placed in contact with a surface of a porous solid, the question arises as to whether it will penetrate into the pores. The answer is governed by the physical and chemical forces of the liquid-solid interface and the angle of contact between the liquid and the pore wall. Further insight into this interaction can be derived from surface area, porosimetry, and profilometry data. Analysis of the data reveal apparent correlations between the water absorption and mean surface roughness (Sa) (Figure 3), and between surface area and porosity (Figure 4).

It is fairly well know that roughness increases the apparent wetting ability of a fluid and is governed by the Wenzel Equation15. From this equation, it can be shown that

cosqobs = r cosq0

where qobs is the observed contact angle, r is a roughness factor (known as the "rugosity factor"), and qo is the contact angle for an ideal surface. This relation shows that as the r increases, qobs is reduced which results in a higher degree of wetting. This relationship -and the reasonably good agreement obtained between mean roughness and water absorption - suggests that the ASTM absorption test is to some degree a surface effect and may not represent actual internal pore space and volume. A correlation between water absorption and porosity/surface area was not observed. In view of water's relatively high surface tension value (about 72 dynes per centimeter), it may be that a threshold exists that governs the water absorption or that roughness plays a secondary role in influencing the actual water absorption. Perhaps the ideal test for "true" absorption might use a low surface tension liquid to overcome roughness influenced wettability; however, this would not represent real world conditions and therefore may not offer valuable information.

Conclusion

Three siliceous and three calcareous stones that have vastly different physical attributes were analyzed by mercury porosimetry, BET surface area analysis, and profilometry. Since the effectiveness of stone protection is directly affected by the degree of wetting and penetration of the product, a better understanding of these relationships will help facilitate better and improved products. Architects and building designers have an enormous number of natural stone types to choose from when selecting material. Besides basic color and aesthetics, criteria for specifying one stone over another can be found in the Marble Institute of America's "Dimensional Stone of the World," which typically provides absorption, density, compressive and flexural strength and abrasion resistance for a few thousand different types of stone. Besides water absorption, the surface roughness offers a clue in the long-term durability of the stone.

References

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