Damage Mechanisms of Porous Materials due to In-Pore Salt Crystallization

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Damage Mechanisms of Porous Materials due to In-Pore Salt Crystallization

Mara Schiro, Encarnacion Ruiz-Agudo, and Carlos Rodriguez-Navarro

Phys. Rev. Lett. 109, 265503 – Published 26 December 2012

Abstract

Pressure exerted by crystallization of salts within porous materials contributes to damage in historic and modern construction. By unequivocally identifying the precipitating phase(s) while simultaneously determining solution supersaturation and associated crystallization pressure in subsurface pores, we show that the formation of a thermodynamically metastable salt phase (heptahydrate; ${Na}_{2} {SO}_{4} \cdot 7 H_{2} O$ ) and the resulting transition to a less soluble stable phase (mirabilite; ${Na}_{2} {SO}_{4} \cdot 10 H_{2} O$ ) is largely responsible for the high supersaturation and crystallization pressure developed during evaporative crystallization of sodium sulfate, the most damaging salt known. These results help to explain why salts with various (stable and metastable) hydrated phases are the most damaging. We also show that damage associated with metastable-stable phase transitions can be suppressed by the use of crystallization promoters. These results open new ways for the prevention of salt damage to building materials.

Received 24 July 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.265503

Authors & Affiliations

Mara Schiro, Encarnacion Ruiz-Agudo, and Carlos Rodriguez-Navarro^*

Department of Mineralogy and Petrology, University of Granada, Fuentenueva s/n, 18002 Granada, Spain

^*Corresponding author: carlosrn@ugr.es

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Vol. 109, Iss. 26 — 28 December 2012

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Images

Figure 1
Salt damage of porous building materials. (a) Limestone block showing extensive damage due to sodium sulfate crystallization. Capillary rise and continuous evaporation over 1 week led to massive efflorescence in the bottom portion of the block and subflorescence (in-pore salt crystallization) and associated damage near the top of the block. (b) Detail of damage on the top part of the limestone block. (c) Example of salt damage (sodium sulfate) in the interior of the Granada Cathedral complex (Sagrario Chapel).Reuse & Permissions
Figure 2
Crystallization of sodium sulfate within porous media. (a) Evolution of the solution concentration (TG) and heat evolved (DSC) during evaporative crystallization. The onset of crystallization is indicated by the vertical dashed (blue) line. Dashed (black) line in DSC trace shows modeled heat evolution associated with solution drying assuming no sodium sulfate crystallization or dehydration. (b) Intensity map of sequential 2D-XRD patterns showing the initial crystallization of heptahydrate [marked by the horizontal dashed (1) red line], which starts to disappear [marked by the dashed (2) blue line] as shown by the reduction in intensity of its main Bragg peaks concurrently with the appearance of the main mirabilite Bragg peaks; finally, mirabilite starts to disappear (dehydrates) as thenardite Bragg peaks become most intense [marked by the dashed (3) green line]. (c) XRD patterns corresponding to the sequential formation of heptahydrate, mirabilite, and thenardite. The XRD line patterns of each phase are shown as a reference (red: heptahydrate [31]; blue: mirabilite, JCPDS card 11-0647; green: thenardite, JCPDS card 37-1465). (d) Same as (a) but following evaporative crystallization of sodium sulfate in the presence of 0.01 M borax. (e) Same as (b) but showing the initial precipitation of mirabilite in the presence of borax, followed by the formation of thenardite after mirabilite dehydration. (f) XRD patterns corresponding to mirabilite formation in the presence of borax and its dehydration to form thenardite.Reuse & Permissions
Figure 3
The $T$ -dependent solubility diagram of the system ${Na}_{2} {SO}_{4} − H_{2} O$ . This diagram shows the solubility curves of mirabilite [curve (a)], heptahydrate (values from Refs. 18 curve (b); and ICT [32], curve (c)), and thenardite [phase $V$ ; curve (d)], as well as the metastability curve of the latter from Ref. 18 [curve (e)]. Concentration values at the onset of crystallization in glass frits (TG-DSC results) are plotted [green solid diamond: initial 1 M ${Na}_{2} {SO}_{4}$ ; red solid square: onset of heptahydrate crystallization; blue solid circles and yellow solid circle (empty circle in printed version): onset of mirabilite crystallization in the presence of borax and DTPMP, respectively]. The dashed black arrow (1) shows the solution concentration evolution during evaporative crystallization. Dashed arrows (2) and (3) (brown) show the $T$ and concentration evolution during ESEM crystallization cycles where only hydrated phases were formed. Dashed arrow (4) (black) shows the concentration path during ESEM crystallization cycles involving anhydrous and hydrated phases.Reuse & Permissions
Figure 4
ESEM images of sodium sulfate crystallization in porous glass. (a) Hydrated sodium sulfate crystals formed within and on the porous glass following drying at $R H > 64 %$ (no dehydration of hydrated phases was allowed to occur). The large prism-shaped crystals show a morphology consistent with heptahydrate (Hept) [12]. (b) Mirabilite (Mir) crystals formed after deliquescence and reprecipitation of hydrated crystals shown in (a). Loss of substrate grains is observed, which can only be attributed to the heptahydrate-mirabilite phase transition. (c) Surface features of the porous glass after the first crystallization event in which all of the crystals are beneath the surface and undetectable in ESEM. (d) Mirabilite crystals, identified by their equilibrium morphology, formed upon dissolution of thenardite and recrystallization. Changes in the surface topography of the porous glass substrate reflect significant damage associated with this second crystallization event (i.e., thenardite-mirabilite phase transition). White circles mark unchanged reference points on the substrate.Reuse & Permissions

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