Characterization of limestone and its behavior in different conditions after treatment with CaZn2(OH)6·2H2O nanoparticles

Nowadays, extreme environmental conditions are one of the main factors causing the deterioration of monuments built with limestone. Therefore, this study proposed the evaluation of the consolidating effect of calcium zinc hydroxide dihydrate nanoparticles, with the molecular formula CaZn₂(OH)₆·2H₂O (CZ), applied on Calcehtok limestone. The consolidating effect of nanoparticles was characterized through mineralogical analysis by X-ray diffraction (XRD), and the morphological and elemental composition by scanning electron microscopy (SEM) on 2.2 cm × 1 cm × 1.5 cm limestone samples. The petrophysical properties as spectrophotometry, Leeb hardness test and contact angle, were measured on 5 cm × 5 cm × 3.5 cm limestone samples. In addition, XRD studies were carried out on the CZ-treated and -untreated stones under laboratory conditions and exposed to the natural environment. The results obtained through an accelerated chamber of relative humidity (RH, 80%) and temperature (25 °C) showed an improvement in Leeb hardness and contact angle, without affecting the color of the stone. Therefore, it is proposed that the warm conditions of relative humidity and temperature favor the consolidation, as the pores are filled by the binding of calcite grains as seen by SEM. After 15 days of exposure, a rapid transformation of portlandite into calcite was observed, while at 30 days, calcite and zincite can still be detected by XRD. The combination of SEM and XRD techniques, together with the measurement of petrophysical properties, corroborated these results, showing the homogenization of the surfaces of the samples due to the decrease of cracks, the filling of cavities, and pores; therefore, this treatment favored its resistance. The XRD results further confirmed the presence of CZ after 30 days under the both laboratory and natural conditions, and its degradation into calcite and zincite was still ongoing after 90 days of exposure.