Mechanisms of damage by salt
R. M. Espinosa-Marzal, G. W. Scherer, 2010. "Mechanisms of damage by salt", Limestone in the Built Environment: Present-Day Challenges for the Preservation of the Past, B. J. Smith, M. Gomez-Heras, H. A. Viles, J. Cassar
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Limestone is very susceptible to the aggressive action of salts. This paper describes the current understanding of the mechanisms by which salt crystallization causes damage to limestone. Crystallization pressure increases with the supersaturation of the solution, which may result from rapid drying and/or decrease in temperature. Salts with a tendency to achieve higher supersaturation owing to a high nucleation barrier are potentially able to induce more severe damage. In the presence of small pores (<100 nm), equilibrium thermodynamics indicates that crystallization pressure can result from the curvature dependence of the solubility of a salt crystal. Under non-equilibrium conditions, high transient stresses can occur even in larger pores. In the field, the complexity of salt weathering results from heat, moisture and ion transport coupled with in-pore crystallization during changing climatic conditions. This paper describes how progress in the modelling and numerical simulation of these coupled processes can contribute to a better understanding of the influencing factors and assessment of critical conditions.
Classically, tests such as the bursting test and the capillary rise experiment with simultaneous evaporation have been applied to evaluate qualitatively stone deterioration induced by salt crystallization. More recently our group has introduced other experimental methods to the field of salt weathering that provide quantitative information about nucleation and crystallization kinetics in porous materials (by differential scanning calorimetry), induced deformation and stress (by dynamic mechanical analysis and a novel warping test), and pore clogging caused by in-pore crystallization.
The final part of this paper is dedicated to a discussion of methods to prevent damage that may alter one of the crystallization steps, such as nucleation, crystal growth, disjoining pressure between mineral and crystal surfaces, or solution properties. Indeed, efficient treatments have been found for particular scenarios in the laboratory; however, the consequences of these treatments in the field, such as the behaviour at other temperatures and concentrations as well as the durability of the treatments, are not known yet.
Indeed, a lack of knowledge still exists in understanding the pore-level crystallization, such as the processes in the thin film between mineral surfaces and salt crystals that determine the disjoining pressure, or the dynamics of crystallization within the pore network that influence the salt distribution and stress in the stone. Atomic force microscopy, surface force measurements, nuclear magnetic resonance and simulations using molecular dynamics are promising methods to elucidate these points. By understanding these remaining questions a more reliable protection of stone against salt weathering will be achieved.
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Limestone is a highly successful and widely used building material, found in many important historic buildings and new monuments around the world. Whilst its success reflects its durability under a wide range of environmental conditions, there are still important questions surrounding the selection, use and conservation of building limestones. In order to make best use of new limestone today, and to conserve old limestone most effectively, we need to bring modern research methods to bear on understanding the characteristics of different limestones, what mortars to use, and how key limestones have responded to polluted atmospheres. This volume brings together recent inter-disciplinary research on these issues, illustrating the diversity of innovative techniques that are now being applied to furthering our understanding of building limestones.