The role of saline solution properties on porous limestone salt weathering by magnesium and sodium sulfates (original) (raw)

The Damage Mechanism of Sodium Sulfate in Porous Stone

Why is sodium sulfate so damaging to porous building materials? This question has remained unanswered for at least 170 years, since sodium sulfate began to be used to test the relative durability of different stones (de Thury 1828; Luquer 1895). Two important areas of recent research on this topic are 1) the non-equilibrium crystallization of thenardite (Na 2 SO 4 ) and 2) the generation of high supersaturation ratios and rapid mirabilite (Na 2 SO 4 •10H 2 O) crystallization by placing a saturated solution of sodium sulfate in contact with fine-grained thenardite crystals.

New insights on the consolidation of salt weathered limestone: the case study of Modica stone

Bulletin of Engineering Geology and the Environment, 2015

The deterioration of a stone material is related to its pore structure, which affects the interaction between surface and environmental agents. Indeed, salt crystallization is one of the most dangerous weathering agents in porous building materials. The crystallization pressure of salt crystals, growing in confined pores, is found to be the main cause for damage. The consolidation of such degraded stone materials represents a crucial issue in the field of restoration of cultural heritage. This paper presents the results of a laboratory experimentation carried out on Modica stone, a limestone largely used in the Baroque architecture of eastern Sicily. Several specimens, collected from a historical quarry near the city of Modica, were artificially degraded by salt crystallization tests. Then, degraded samples were treated with three different consolidating products: a suspension of nanolime in alcohol, a suspension of nanosilica in water, and ethyl silicate dispersed in white spirit. A systematic approach, including mercury intrusion porosimetry, peeling tests and point load test, was used to evaluate the correlation between the salt crystallization and the micro-structural features of the limestone, as well as the efficacy of treatments. The consolidating behavior of the tested products was also appraised by repeating salt crystallization tests after consolidation, in order to assess the resistance of treated stone to further salt crystallization phenomena. Results showed that nanolime provides a good resistance to the stone; conversely, ethyl silicate, although inducing an enhancement of stone cohesion, leads to an increase of the crystallization pressure, which generates dangerous susceptibility to weathering.

The crystallization behavior of sodium magnesium sulfate in limestone

The deterioration of porous building materials caused by single salts has been investigated extensively. Recently, more emphasis is given to the assessment of salt mixtures. Since a few years the ECOS/RUNSALT model is being used for the interpretation of the crystallization behavior of detected ion mixtures. The double salt bloedite or Na2Mg(SO4)2.4H2O is frequently found in the output of the predictive model. However, the crystallization behavior and the destructive effects of double salts such as bloedite are generally not documented. This paper presents the results of a research project carried out to assess the behavior of sodium magnesium sulfate in limestone related to that of the respective single salts. Limestone samples were contaminated with aqueous solutions of equimolar mixtures of sodium and magnesium sulfate at different concentrations and conditioned at different environmental conditions. The drying behavior as well as the crystallization behavior upon repeated drying...

Study of the effects of salt crystallization on degradation of limestone rocks

Periodico di Mineralogia

Salt crystallization is widely recognized as a cause of deterioration of porous building materials. In particular, the crystallization pressure of salt crystals growing in confined pores is found to be the main cause for damage. The aim of this study is to better understand the degradation of porous rocks induced by salt crystallization and correlate such processes with the intrinsic characteristics of materials. With this intent, an experimental salt weathering simulation was carried out on two limestones widely used in the Baroque architecture of eastern Sicily. A systematic approach, including petrographic, porosimetric and colorimetric analyses, was used to evaluate the correlation among salt crystallization, microstructural and chromatic variations of limestones. Results showed a quite different resistance of the two limestones to salt damage, and this was found to be strongly dependent on their pore structure and textural characteristics.

Salt Weathering of Natural Stone: A Review of Comparative Laboratory Studies

Heritage

Natural stone is an important component of historical heritage (buildings and art objects such as sculptures or rock engravings), and it is still widely used in contemporary works. Soluble salts are the main erosive agent in the built environment, and we review here comparative studies that subject the same rock type to testing with different salt solutions. The results mostly support the accepted notion of the major impact of sodium sulphate, although there are some exceptions. The effects of sodium chloride and calcium sulphate deserve specific discussion given field information on the relevance of these specific salts in the built environment. We relate the information collected to the issues of risk assessment (considering both geochemical conditions and salt effects) and conservation interventions (highlighting the interest of tests that do not produce damage to susceptible materials) and present some methodological suggestions to avoid a case study culture.

Salt Weathering: Influence of Evaporation Rate, Supersaturation and Crystallization Pattern

Earth Surface Processes and Landforms, 1999

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 per cent) 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.

The evaluation of crystallization modifiers for controlling salt damage to limestone

Journal of Cultural Heritage, 2002

Crystallization modifiers can significantly affect the capillary passage of dilute and concentrated solutions of sodium chloride and sodium sulfate through columns of limestone. In the absence of modifiers, sodium chloride passage through Monks Park limestone gave predominantly subflorescence with mild edge erosion while sodium sulfate mainly effloresced and severely damaged the stone column. With Texas Creme limestone, a stone of moderately higher porosity, essentially only efflorescence occurred with either salt and there was little or no stone damage. Uniquely, alkali ferrocyanides were found to impact significantly on the interaction of these solutions as they moved through the limestone. The addition of 0.10–1.00% of K4Fe(CN)6 to sodium chloride in Monks Park limestone experiments increased the flow rate of solutions through the stone, resulting in efflorescence in place of subflorescence, and yielded a massive formation of extended dendritic filaments without damaging the stone. This protection by additive was extended to sodium sulfate solutions, but only at lower salt concentrations. Results comparable to the effect of adding K4Fe(CN)6 to concentrated sodium chloride Monks Park limestone experiments were obtained with saturated sodium sulfate solutions without additives by conducting the experiments in a draft-free, high humidity environment—suggesting a potentially useful strategy for the conservation of fragile, salt-laden objects. These results are explained by factors causing evaporation of solution to occur either below or at the surface of the stone, and by the effect of modifiers on the crystal habit of the salts forming during evaporation in this region.

Role of pore structure in salt crystallisation in unsaturated porous stone

Journal of Crystal Growth, 2004

Laboratory driven crystallisation of sodium sulphate and sodium chloride from concentrated solution in unsaturated porous stones has been performed. This contributes to a better understanding of the mechanisms by which salts crystallise and as a consequence limit the durability of porous materials which has an impact on buildings, civil constructions, and historical monuments. The identification of minerals in porous materials has been performed by scanning electron microscopy (SEM), Environmental Scanning Electron Microscopy (ESEM) and sequential profiles of X-ray diffraction (XRD) under temperature control of sample. The study of porous stones has been combined with experiments in capillary tubes. Data from SEM show that halite tends to precipitate on the surface of the stone with a similar distribution in all samples. However, the mirabilite-thenardite precipitation takes place preferably inside the stone and its depth from the surface and its relative concentration depends on the pore size distribution. In addition, mirabilite (Na 2 SO 4 Á 10H 2 O) crystallises homogeneously, whereas thenardite (Na 2 SO 4 ) and halite (NaCl) tend to nucleate heterogeneously. To explain the precipitation sequence from concentrated solutions in unsaturated porous materials, a detailed analysis of the thermodynamic equations has been carried out by establishing a simple model. The proposed model shows the influence of the pore structure both on the water activity and saturation degree of involved salts. r