Damage due to the crystallization of salts is a common problem in porous building materials. Also mortars used in the restoration of historic buildings are often subjected to a high salt load, resulting in a rapid degradation and high maintenance costs. Especially lime-based mortars, used for example as render, plaster, bedding mortar or pointing mortar, are vulnerable to this type of damage. Despite the extensive research efforts, no definitive solution yet exists to tackle the problem of salt decay in building materials. Existing solutions to improve mortar resistance with respect to salt decay such as modifying the moisture transport properties of the mortar (e.g. plaster with water-repellent properties) or increasing its mechanical strength of the plaster mortar (e.g. by using a different binder such as cement), often show compatibility problems and might cause even more damage to the to be restored (historic) fabric. Recently a new approach, based on the use of crystallization modifiers to alter the salt crystallization process, has been proposed. The aim of modifiers is to prevent or mitigate salt crystallization damage in building materials. Modifiers are ions or molecules that can keep the salts longer in solution (inhibitors), facilitate the precipitation of a certain crystal phase (promoters) and/or change the shape and size of the grown crystals (habit modifiers). These effects are often present in combination, i.e. a promoter or inhibitor can at the same time act as habit modifier. @en

Salt crystallization damage in porous building materials is a widespread phenomenon. Several solutions to prevent, or mitigate, salt damage in building materials, prolonging thereby their service-life have been proposed. One of the latest approaches is the use of crystallization modifiers, aiming at reducing the crystallization pressure and/or favouring the crystallization of salts at the surface (efflorescence) instead of in the pores of the materials (crypto-florescence). This paper summarizes the working mechanisms of crystallization modifiers and critically reviews the available literature on their use in building materials; finally, it provides an outlook on the potentials of modifiers for the mitigation of salt crystallization damage in building materials.

@en

Weathering of porous building materials caused by the crystallization of soluble salts is a ubiquitous problem in the built cultural heritage. Especially lime-based mortars are susceptible to salt decay, due to both their bimodal pore size distribution and low mechanical strength. The addition of crystallization modifiers to mortars during mixing may confer them an improved resistance to salt decay. In this research, lime-based mortars additivated with ferrocyanide or borax (modifiers for sodium chloride and sodium sulfate, respectively) were prepared. An accelerated salt crystallization test was carried out to assess the effect of the modifiers on the salt resistance of the mortars. The development of damage was assessed by visual and photographical observations and by quantifying the salt and material loss. At the end of the test, SEM observations were performed on the surface and cross-section of the specimens, to study the effect of the modifiers on the crystallization habit of the salts. The ferrocyanide and borax additivated mortars showed a considerably improved durability with respect to salt crystallization damage. Both modifiers altered the growth morphology of the salt crystals inside the pores of the mortars.

@en

Additivating mortars with crystallization modifiers is a novel approach to mitigate salt crystallization damage in historic masonry. Once verified the effectiveness of crystallization modifiers in bulk solution, the next step consists in verifying whether: (i) modifiers are still effective when mixed in mortar and going through the carbonation process and (ii) modifiers alter any mortar properties which might limit their application. This research addresses these issues for sodium ferrocyanide and borax, modifiers for sodium chloride, and sodium sulfate, respectively. Several experimental techniques have been applied to elucidate these questions. The results show that the selected modifiers are still able to alter the salt crystallization after going through the carbonation process of the mortar. Besides, no major effects of the modifiers on the fresh and hardened mortar properties were observed. It can therefore be concluded that there are no restraints for the future use of these crystallization modifiers in restoration mortars.@en

In deze bijdrage worden de belangrijkste bevindingen gerelateerd aan het effect van toevoeging van de modificatoren op morteleigenschappen en op de duurzaamheid van de mortels gerapporteerd. Allereerst wordt er een experiment beschreven waarin wordt getest of de effectiviteit van borax wordt beïnvloed door het carbonatatieproces. Vervolgens worden de effecten van de modificatoren op morteleigenschappen zoals verwerkbaarheid, waterabsorptie en droging, en buig- en druksterkte gerapporteerd. Als laatste wordt de weerstand van de mortels met modificatoren tegen zoutschade bediscussieerd aan de hand van resultaten van een versnelde zoutkristallisatietest.@en

This paper presents the most important results of a research project which
focused on the use of crystallization modifiers mixed in lime mortar to mitigate
salt crystallization damage. The research focused on two of the most damaging
salts, sodium chloride and sodium sulfate, and suitable crystallization modifiers (sodium ferrocyanide and borax). We report the major findings related to the effectiveness of the modifiers when mixed in the mortar, and the results of characterization of the additivated mortars in comparison with reference mortars. Moreover, the durability of the developed mortars to salt decay is discussed, based on the results of an accelerated salt weathering test carried out in laboratory. No major effects of the modifiers on the fresh and hardened mortar properties were observed which might restrain the application of crystallization modifiers in restoration mortars. Additionally, the mortars with mixed-in modifiers showed a considerable improvement of the salt resistance when compared to reference mortars. Considering these results an outlook for future research pathways is given.@en

This paper describes some of the most important results of a four year PhD research on the use of crystallization modifiers mixed in lime mortar to mitigate salt crystallization damage. The research focused on two of the most damaging salts, sodium chloride and sodium sulfate, and suitable crystallization modifiers (sodium ferrocyanide and borax). We report the major findings related to the effectiveness of the modifiers when mixed in the mortar, and the results of characterization of the additivated mortars in comparison with reference mortars. Moreover, the durability of the developed mortars to salt decay is discussed, based on the results of an accelerated salt weathering test carried out in laboratory. No major effects of the modifiers on the fresh and hardened mortar properties were observed which might restrain the application of crystallization modifiers in restoration mortars. Additionally, the mortars with mixed-in modifiers showed a considerable improvement of the salt resistance when compared to reference mortars. Considering these results an outlook for future research pathways is given.@en

Borax has been identified as a possible crystallization modifier for sodium sulfate. Understanding the effect of borax on factors influencing transport and crystallization kinetics of sodium sulfate helps to clarify how this modifier might limit crystallization damage. It has been observed that the addition of borax to sodium sulfate solutions has no influence on the wetting properties (contact angle on glass, surface tension, or evaporation rate) and therefore will not influence solution transport. Additionally, the influence of borax on the crystallization kinetics of sodium sulfate was studied under controlled environmental conditions. This was carried out in mixtures in glass microcapillaries, and sequentially in droplets on glass plates. Under the here studied precipitation conditions, the addition of borax has no influence on the supersaturation ratio at the onset of crystallization, but it significantly affects the crystallization pattern of anhydrous sodium sulfate crystals (thenardite). Using RAMAN spectroscopy, two different hydrates of borax were identified after precipitation, depending on the initial concentration of the solution. Each hydrate has a different effect on the subsequent crystallization of sodium sulfate. The decahydrate polymorph of borax leads to the precipitation of hydrated sodium sulfate crystals (mirabilite) and the pentahydrate form favors the precipitation of the anhydrous sodium sulfate crystals (thenardite) with an altered crystal habit. Using X-ray diffraction, overdevelopment of the (111), (131), (222) and (153) faces of thenardite was identified. Additionally, the ratios between several peaks are reversed. These results confirm the deviation of the grown crystals of the equilibrium crystal shape of thenardite as observed with optical microscopy.@en

Although salt crystallization damage is a widespread damage process in the porous materials of our built cultural heritage, no definite solution yet exists to improve the durability of materials with respect to salt crystallization. Most research focuses on improving material properties, whereas only few studies concentrate on changing the crystallization process in order to make it less harmful.Within this last trend, recently the use of salt crystallization modifiers has been considered. Crystallization modifiers are ions or molecules that promote or inhibit crystal growth and/or change the crystal habit, thereby possibly reducing salt damage.We give in this paper experimental evidence that borax is a promising crystallization modifier for sodium sulfate and that it might mitigate salt crystallization damage when mixed in lime-based mortars. Lime-based mortars are chosen as a model system, since they are especially prone to salt crystallization damage due to their limited mechanical strength and bi-modal pore size distribution.@en