Electroprecipitation of Magnesium and Calcium Compounds for Weathering Protection of Ornamental Rocks

摘要

Electroprecipitation can be considered as a novel consolidation technique that can compete with commercial consolidation treatments, since it allows reduction of its accessible porosity, both superficially and in-depth, increasing considerably its mechanical properties and precipitating a compound chemically compatible with the material to be treated. The purpose of this study is to assess the effectiveness of the electroprecipitation of four different inorganic compounds constituted by magnesium and calcium carbonate, calcium phosphate, and calcium oxalate, in 6 cm thick specimens of quartz-arenite. The suitability of the treatment was assessed in samples treated and aged for 4 years under environmental conditions, comparing the changes caused in structural properties (measuring the ultrasonic pulse velocity, anisotropy, tortuosity, pore size distribution, and mercury accessible porosity), mechanical properties (superficial hardness, uniaxial compressive strength, and drilling resistance) and aesthetic properties (color). In addition, the morphology and composition of the compound precipitated were assessed by XRD and SEM-EDX. This study found that the precipitation of the insoluble compounds by this technique (I) takes place in a homogeneous way, reducing the anisotropy of the material and its tortuosity, which increases the durability of the material against the action of external agents, such as water, soluble salts, and pollutants; (II) increases the mechanical properties and causes a considerable increase in the ultrasonic wave transmission; (III) allows filling almost completely the pores where it precipitates, which reduces the damage risk caused by crystallization pressures; (IV) produced, under these treatment conditions (environmental conditions, confined space and forced precipitation by an electric field), the precipitated phases magnesite, hydromagnesite, calcite, hydroxyapatite, and wewhellite. However, the color changes detected indicate that it is necessary to carry out previous studies in the laboratory to reduce this aesthetic problem, before applying it on-site on a certain material, especially in heritage constructions.