『Abstract
Micro- and macroscale experiments which document the dynamics
of salt damage to porous stone have yielded data which expose
weaknesses in earlier interpretations. Previously unexplained
differences are found in crystal morphology, crystallization patterns,
kinetics and substrate damage when comparing the growth of mirabilite
(Na2SO4・10H2O)
and thenardite (Na2SO4)
versus halite (NaCl). the crystallization pattern of sodium sulphate
was strongly affected by relative humidity (RH), while a lesser
RH effect was observed for sodium chloride. Macroscale experiments
confirmed that mirabilite (crystallizing at RH＞50 per cent) and
thenardite (crystallizing at RH＜50 percent) tend to form subflorescence
in highly localized areas under conditions of constant RH and
temperature. This crystallization pattern was more damaging than
that of halite, since halite tended to grow as efflorescence or
by filling the smallest pores of the stone in a homogeneous fashion,
a result which contradicts Wellman and Wilson's theoretical model
of salt damage. Low RH promoted rapid evaporation of saline solutions
and higher supersaturation levels, resulting in the greatest damage
to the stone in the case of both sodium sulphate and sodium chloride
crystallization. At any particular crystallization condition,
sodium chloride tended to reach lower supersaturation levels (resulting
in the crystallization of isometric crystals) and created negligible
damage, while sodium sulphate reached higher supersaturation ratios
(resulting in non-equilibrium crystal shapes), resulting in significant
damage. ESEM showed no damage from sodium sulphate due to hydration.
Instead, after water condensation on thenardite crystals, rapid
dissolution followed by precipitation of mirabilite took place,
resulting in stone damage by means of crystallization pressure
generation.
It is concluded that salt damage due to crystallization pressure
appears to be largely a function of solution supersaturation ratio
and location of crystallization. These key factors are related
to solution properties and evaporation rates, which are constrained
by solution composition, environmental conditions, substrate properties,
and salt crystallization growth patterns.
When combined with a critical review of salt damage literature,
these experiments allow the development of a model which explains
variations in damage related to combinations of different salts,
substrates and environmental conditions.
Keywords: salt weathering; environmental conditions; crystallization
pressure; stone decay; sodium sulphate; sodium chloride』
Introduction
Salt weathering: Theories and models
Materials and methods
Stone type and properties
Microscale experiments
Macroscale experiments
Results
Microscale experiments
Macroscale experiments
Discussion
Sodium chloride versus sodium sulphate
Influence of crystallization pattern on salt weathering
Effect of relative humidity and physical properties of the saline
solutions
Influence of the stone pore system
Conclusions
Acknowledgement
References