Abstract
Ca(OH)2 nanoparticles in hydro-alcoholic dispersion (nanolime) were successfully employed in Cultural Heritage conservation, thanks to the ability to overcome the limiting aspects of traditional lime treatments. Nanolime were currently produced by chemical precipitation process, at high temperature, with long times of synthesis, and after several purification steps to remove undesired secondary phases. In this paper, an innovative, simple and original method for nanolime production was described. The method was based on an ion exchange process between an anionic resin and a calcium chloride aqueous solution, operating at room temperature. A pure Ca(OH)2 nanoparticles suspension can be rapidly obtained after separating the resin from suspension, and any purification step was necessary. The exhausted resins can be regenerated and reused for a cyclic nanolime production. Structural and morphological features of the produced nanolime were preliminarily characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, XRD measurements allowed estimating nanoparticles reactivity by following their carbonatation process in air, in relation to different water/alcohol ratios and medium or high relative humidity conditions. The produced Ca(OH)2 nanoparticles appeared hexagonally plated, with dimension less than 100 nm and, compared with those obtained by typical wet precipitation method, they proved to be more reactive.
Highlights
Consolidation and conservation of calcareous materials required products that were compatible with components originally used
As reported in literature [16] [18], nanolime was obtained by a chemical precipitation process, in supersaturated conditions, by mixing drop by drop equal volumes of sodium hydroxide (NaOH) and calcium chloride (CaCl2) aqueous solutions, both maintained at high temperature (90 ̊C)
Suspension synthesized by the “Two steps” process (WR2b suspension) allowed obtaining, after 5’ of stirring with the second resin, a nanolime suspension containing a reduction of chlorides of more than 99.8%, with a limit chlorides concentration of 12 mg/l
Summary
Consolidation and conservation of calcareous materials required products that were compatible with components originally used. Commercial lime products suffered of some drawbacks: incompleteness of carbonatation process, reduced penetration depth and formation of a white film on treated surfaces [4]-[7] These drawbacks were overcome by nanolime, i.e. Ca(OH) nanoparticles hydro-alcoholic suspensions, successfully employed in Cultural Heritage conservation and offering advantages in stone, mortar and plaster consolidation [8]-[14]. As reported in literature [16] [18], nanolime was obtained by a chemical precipitation process, in supersaturated conditions, by mixing drop by drop equal volumes of sodium hydroxide (NaOH) and calcium chloride (CaCl2) aqueous solutions, both maintained at high temperature (90 ̊C). In order to reduce time of synthesis, an alternative method was proposed by adding a surfactant agent in the initial aqueous solutions [19] This method presented same drawbacks in relation to washings procedures to remove NaCl and surfactant too. The results were compared with those obtained on nanolime particles produced by a typical drop by drop method, performed in aqueous solutions
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