Effect of borax on the wetting properties and crystallization behavior of sodium sulfate

Abstract

Borax has been identified as 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 done in mixtures in glass micro capillaries, 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 is reversed. These results confirm the deviation of the grown crystals of the equilibrium crystal shape of thenardite as observed with optical microscopy.

Effect of mixed in crystallization modifiers on the resistance of lime mortar against NaCl and Na2SO4 crystallization

“…In fact, dehydrated mirabilite crystals usually show a much more porous and irregular structure than the 'bulky' structures observed here [29,30]. It is therefore more likely that in both cases anhydrous sodium sulfate crystals are grown, in spite of the fact that, at the drying conditions used, mirabilite would be expected; a similar behaviour has been reported also for crystallization experiments in bulk solution [13,31].…”

“…Additionally, ferrocyanide alters the habit of the normally cubic sodium chloride crystals to dendrites: the larger evaporation surface enhances the transport of salts to the surface. Borax acts on the crystallization of sodium sulfate, and its working mechanism is reported to depend on which phase of borax is actually present [13]. One phase (borax, sodium tetraborate decahydrate) favours the crystallization of hydrated sodium sulfate (mirabilite) at or near saturation, resulting in no or low crystallization pressure.…”

Characterization of lime mortar additivated with crystallization modifiers

“…and borax (sodium tetraborate decahydrate, Sigma-Aldrich, puriss.). The first is a crystallization inhibitor and habit modifier of sodium chloride crystallization (Van Damme Van Weele 1965), the second is a modifier of sodium sulfate crystallization (Granneman et al 2017; Ruiz-Agudo and Rodriguez-Navarro 2010). The effect of the carbonation process of the mortar on the effectiveness of the modifier was only studied for borax and sodium sulfate, as this was already studied for ferrocyanide and sodium chloride in previous research (Lubelli et al 2010).…”

“…The amount of borax to be added to the mortar was defined based on a previous research by the authors on the interaction between the modifier and the salt in bulk solution (Granneman et al 2017). Based on these results, two concentrations of borax were used to be mixed in the lime paste specimens (see section SEM study): 1.6 and 3.2 wt% with respect to lime (corresponding to 0.05 and 0.1 mol•L −1 borax concentration in the water used to make the pastes).…”

“…When borax crystallizes as the decahydrate, the precipitating sodium sulfate phase is the decahydrated mirabilite. However, when borax crystallizes as the pentahydrated tincalconite, the sodium sulfate crystallizes as anhydrous thenardite with {153} as the dominant crystal form (Granneman et al 2017).…”