Engineering Geology

Chapter 6
drainage and escape of moisture from the frontal advance of the ice line and, therefore, are
less frost susceptible. Fine-grained rocks that have over 5% sorbed water are often very
susceptible to frost damage, whereas those containing less than 1% are very durable.
Freezing tests have proved an unsatisfactory method of assessing frost resistance. Capillary
tests have been used in France and Belgium to assess the frost susceptibility of building
stone, whereas in Britain, a crystallization test has been used (Anon, 1983).
Deleterious salts, when present in a building stone, are generally derived from the ground
or the atmosphere, although soluble salts may occur in the pores of the parental rock.
Their presence in a stone gives rise to different effects. They may cause efflorescence by
crystallizing on the surface of a stone. In subflorescence, crystallization takes place just
below the surface and may be responsible for surface scabbing. The pressures produced
by crystallization of salts in small pores are appreciable, for instance, halite (NaCl) exerts a
pressure of 200 MPa; gypsum (CaSO 4 ◊nH 2 O), 100 MPa; anhydrite (CaSO 4 ), 120 MPa and
kieserite (MgSO 4 ◊nH 2 O), 100 MPa; and are often sufficient to cause disruption. Crystallization
caused by freely soluble salts such as sodium chloride, sodium sulphate or sodium hydroxide
can lead to the surface of a stone crumbling or powdering. Deep cavities may be formed
in magnesian limestone when it is attacked by magnesium sulphate (Fig. 6.1). Salt action can
Figure 6.1
A cavity formed in magnesian limestone, parish church, Retford, Nottinghamshire, England.
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