Template-assisted crystallization of sulfates onto calcite: implications for the prevention of salt damage

In situ X-ray diffraction (XRD) and environmental scanning electron microscope (ESEM) crystallization experiments show oriented growth of magnesium sulfate crystals on a diethylenetriaminepenta- methylene phosphonic acid (DTPMP) template adsorbed onto Iceland spar (calcite, CaCO3) cleavage surfaces. Epsomite (MgSO4·7H2O) and hexahydrite (MgSO4·6H2O) crystallize (depending on the ambient conditions) in the presence of DTPMP with (010)epsomite//(101¯4)calcite and (1¯11)hexahydrite//(101¯4)calcite, whereas in the absence of DTPMP they show no preferred orientation. On the other hand, sodium sulfate (mirabilite, Na2SO4·10H2O) nucleates onto a Ca- DTPMP precipitate with (001)mirabilite//(101¯4)calcite. In contrast, different sodium sulfate phases crystallize and grow with no preferred crystallographic orientation in the absence of an organic additive. These results allow us to propose a model for the interaction calcite-DTPMP-Na and Mg sulfates based on the template-assisted nucleation and oriented heterogeneous crystallization, mediated by a Ca-precipitate, of inorganic salts on calcitic substrates. This effect results in a (measured) reduction in the critical supersaturation reached by these salts when crystallizing in confined geometries, i.e., a pore, thus resulting in a reduction in crystallization pressure and damage to porous substrates such as building stones. These results have implications in fields where in-pore crystallization of salts results in damage or plugging of the porous network, e.g., cultural heritage conservation and in the oil industry, where phosphonates are used as crystallization inhibitors.