The Weathering of Natural Building Stones by Schaffer R. J.;

Author:Schaffer, R. J.;
Language: eng
Format: epub
ISBN: 4354176
Publisher: Routledge

Crystallisation Forces

The origin of the forces which cause the disintegration of building materials when soluble salts are present is a problem of some interest.

When an anhydrous salt changes to the hydrated form with increase in volume, the bulk expansion is an adequate explanation of the origin of the disrupting forces. For instance, sodium sulphate is often found in building materials (pp. 58 et seq.), and its presence is usually associated with excessive decay. Sodium sulphate can exist in a hydrated form (Na2SO410H2O), which has a specific gravity of 1·49, and also in an anhydrous form (Na2SO4) with a specific gravity of 2·66 : it is obvious that the change of the anhydrous salt into the hydrated form must take place with a considerable increase in volume. Obermiller187. has shown that the changes in temperature and relative humidity which occur in the atmosphere are sufficient to cause repeated changes in the state of hydration of sodium sulphate exposed to it, and it is, no doubt, largely on account of repetition of these volume changes that the presence of sodium sulphate exercises so damaging an effect on building materials.

Most salts dissolve in water with a decrease in the total volume ; a few dissolve with increase in volume. But such changes in volume can hardly be adduced to explain disruption, although the direction of the volume change may be of some importance in determining whether the material will be more or less soluble in the stressed or unstressed condition. In accordance with Le Chatelier’s Law, a substance which dissolves with decrease in volume is more soluble in the stressed than in the unstressed condition and vice versa.

When a salt does not undergo changes in phase with consequent changes in volume, any thrust developed by its growth might be attributed to axial growth of the crystals as they separate from solution. There is, however, considerable doubt as to whether or not the axial growth is actually associated with a force which, as Cobb56. expresses it, is “perhaps somewhat akin to surface tension, which, by tending to secure continuity in the crystal form, enables the growing crystal to displace adjacent matter and so to exercise a disrupting action on any material enclosing it.” There is a certain amount of evidence which superficially appears to indicate that such crystallisation forces exist, but experimental work on the problem has not yet succeeded in demonstrating their existence with certainty.

In metamorphic rocks, minerals such as pyrite are often observed to be perfect in form in spite of their having developed under enormous pressures ; the laminations of slate often appear to have been forced apart by the growth of the crystals. As long ago as 1853, Lavalle155. drew attention to the fact that crystals in a saturated solution are able to grow on the face in contact with the bottom of the vessel : in fact, Lavalle’s observations led him to the conclusion that this face grows more quickly than the others. In 1905, Becker and Day38. showed

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