Abstract

A comparison has been made of various pore-related parameters of some 50 specimens of almost pure Jordanian limestone by employing the immersion technique with and without the use of a vacuum pump. Simple analysis of the results allows most pore-related parameters to be estimated with a fair degree of accuracy from these results. It is believed that the results of this work can be applied to similar pure limestones elsewhere in the world both in the field and also in laboratories where the necessary equipment, or time, is unavailable.

Introduction

The porosity of rocks is one of the prime determining factors with respect to their potential economic value in oil, mineral, and water exploration, as well as their use and durability as construction materials. Although optical and electron microscopy can provide useful information on the distribution, size, and shape of pores, these techniques do not provide a sufficiently accurate estimate of porosity, and the use of other experimental techniques is required (Brown, 1981; Halley, 1978).

Accordingly, in order to obtain the data necessary, water absorption and bulk density of some 50 specimens (representing 12 Jordanian limestones), other properties including grain density, apparent porosity, effective porosity, and degree of saturation were determined. Most of these limestones are chemically pure (with very low levels of dolomite, quartz, and clay). These data are presented here in a simple and direct way as a reminder to workers from different fields, who are interested in the porosity of rocks but do not have access to equipment or similar data.

It is important to note that in order to determine the specific gravity and water absorption, the ASTM standard (C 97-83) requires that the test specimens be immersed in water for 48 hours. However, as most of the water absorption takes place during the first few hours of immersion (Moh'd, 1996), the 24 hours immersion period used by BRE was considered sufficient to enable water absorption, density and effective porosity to be determined. The following definitions of terms are used:

*Bulk density*(g/cm^{3}): the weight of the oven-dried rock divided by its total volume (including pore-space), with volume being determined by normal immersion (without the use of a vacuum pump).

*Dry density*(g/cm^{3}): the weight of the oven-dried rock divided by its total volume, with volume being determined by immersion using a vacuum pump.

*Grain density* (g/cm^{3}): the weight of oven-dried rock divided by its volume (excluding pore-space).

*Water absorption (%)*: the weight of water absorbed by the rock after 24 hours of immersion in water divided by its oven-dried weight expressed as a percentage of its oven-dried weight.

*Apparent porosity* (%): the percentage of volume of voids over the total volume of rock.

*Effective porosity*(%):indicates interconnected pores and is the product of water absorption and bulk density.

*Saturation*(%): the percentage of pore volume, which can be filled with water after immersion in water for 24 hours.

In this study, comparisons were made between porosity parameters measured by free immersion in water with and without using a vacuum pump. The term 'total porosity' was not used in this work, as the only true total porosity is that obtained by pulverising the sample (Brown, 1981).

Formulae Used

The following measurements need to be made in order to determine the different pore-related properties

*W*_{0}: weight of oven-dried sample,

*W*_{1}: weight of sample soaked in water,

*W*_{2}: weight of sample (vacuum-pumped and then soaked) in air,

*W*_{3}: weight of normally immersed sample in air.

Assuming that the weight of the sample soaked in water (*W*_{1}) is equal to the weight of the sample normally immersed in water, then the following relationships exist:

Porosity = [( | (1) |

Normal water absorption = [( | (2) |

Vacuum-pumped water absorption = [(W | (3) |

Bulk density = | (4) |

Dry density = | (5) |

Grain density = | (6) |

Saturation = ( | (7) |

It is important to remember that the total volume of rock measured using a vacuum pump (Brown, 1981; RILEM, 1980; Price, 1975; Ross and Butlin, 1989) is higher than that measured by normal immersion, because air filling the pore space is removed in the former and hence water has better access to the pores. Consequently, bulk density, as prescribed in the ASTM standard (C 97-83), is higher than dry density.

If there is no access to a vacuum pump, so W_{2}can not be measured (which might be the normal case in the field and in some laboratories), from the 3 parameters W_{0}, W_{1}, and W_{3} it is possible to determine the normal water absorption (2), bulk density (4), and grain density (6). An approximate value of the apparent porosity may be gained (which even under the worst conditions is believed to be more accurate than is possible using a polarising microscope), if it is assumed that dry and bulk densities are equal, from the following relationship:

apparent porosity = [1 - (dry density/grain density)] * 100

therefore

Estimated apparent porosity = |
(8) |

The accuracy of this estimate is greater where the porosity is relatively low (i.e. estimated to be below 10%).

**Figure 1.**Porosity values with or without the use of a vacuum pump

If normal water absorption is known then a more accurate value of porosity can be obtained from Figure 2. Alternatively, dry density may be obtained from Figure 3 once bulk density is calculated using equation (4), then it is possible to determine apparent porosity using the equation (8).

**Figure 2.** Estimating porosity from water absorption values

**Figure 3.** Dry density versus bulk density

From the product of normal water absorption and dry density, it is possible to determine Hirschwald (effective) porosity. Finally, the degree of saturation of the rock can be calculated by dividing the effective porosity by the apparent porosity. Figure 4 shows the relationship between water absorption measured by both normal immersion and using a vacuum pump.

**Figure 4. **Values of water absorption with and without the use of a vacuum pump

Example

Assume that a pure limestone rock with an oven dry weight (*W*_{0}) 161.31 g is immersed in water for 24 hours. Its saturated surface dried weight (*W*_{3}) is 174.98 g, and saturated weight in water (*W*_{1}) 101.82 g. The following procedure is used to determine its bulk density, water absorption, grain density, approximate and accurate apparent porosity, effective porosity, and degree of saturation.

Bulk density = *W*_{0}/(*W*_{3} - *W*_{1})= 161.31/(174.98 - 101.82) = 2.20 g/cm^{3}

Grain density = *W*_{0}/(*W*_{0} - *W*_{1}) = 161.31/(161.31 - 101.82) = 2.71 g/cm^{3}

Water Absorption (normal) = {(*W*_{3} - * W*_{1})/W0}*100 = {(174.98 - 101.82)/161.31}*100 = 8.47%

Water Absorption (vacuum pumped) = 11.2% (from Figure 4)

Approximate apparent porosity = {1 - (bulk density/grain density)}*100

{1 - (2.2/2.71)}*100 = 18.82%

Accurate apparent porosity = 23.8% (from Figure 1 and/or 2)

(1 - dry density/2.71)*100; dry density = 2.065 g/cm^{3}

Effective porosity = (8.47)(2.065) = 17.49 %

Degree of saturation = effective porosity/apparent porosity = 17.49/23.8 = 0.74

Recommendations

The data presented in this note cover relatively pure limestones, which have very small amounts of clay minerals, silica, and dolomite present. The results of this work should not be generalised to apply to impure limestones and other lithologies without further study. Carrying out similar work on impure limestone lithologies (marl, marly limestones, dolomite, dolomitic limestones, and sandy limestones) and other rock types is highly recommended. A prerequisite for using immersion methods is that the tested rock should not swell appreciably or disintegrate when oven-dried and immersed in water.

Acknowledgments

The author is greatly indebted to Dr Tim Yates, and Bell Ferrier, of the Building Research Establishment, for access to equipment and for technical assistance.

References

- American Society for Testing and Materials. 1983. Absorption and bulk specific gravity of natural building stone. ASTM Designation C 97-83
- Brown, E. T. ( ed.) 1981. Rock characterisation, testing and monitoring, Pergamon, Oxford.
- Halley, R. B. 1978. Estimating pore and cement volumes in thin section. J. Sedm. Petrol. 48: pp.642-50.
- Moh'd, B. K., 1996. Evaluation of some Jordanian limestones as building stones. PhD thesis, University of London.
- Price, C. A. 1975.Testing porous building stone. Specification technical study 1. The Architects Journal 13:p.3
- RILEM, 1980.Recommended tests to measure the deterioration of stone and to assess the effectiveness of treatment methods. Mater. et Const. 13: pp. 175-253.
- Ross, K. D. and Butlin, R. N. 1989.Durability tests for building stone. Building Research Establishment, Garston.

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