Synthesis and morphological examination of high-purity Ca(OH)₂ nanoparticles suitable to consolidate porous surfaces

Highlights: • Pure Ca(OH)₂ nanoparticles (NP's) are synthesised to be used in coatings. • The effective yield of the method is, at least, 70% in active material (NP's). • Stable suspensions made of NP's dispersed in 2-propanol are produced by sonication. • The physical stability of the suspensions is negatively affected by water. • The reaction temperature plays an important role in the size and shape of the NP's. - Abstract: Adequate synthetic methods to obtain pure Ca(OH)₂ nanoparticles are scarcely documented in the literature. This paper presents a complete methodology to obtain highly-pure Ca(OH)₂ nanoparticles that are appropriate for strengthening heritage materials. The precipitation synthesis was operated in controlled atmosphere to avoid carbonation by atmospheric CO₂. A complete purification method was developed to eliminate the sodium chloride generated in the reaction. Several analytical techniques, such as electrical conductivity, pH, ion chromatography, X-ray diffraction (XRD) and thermogravimetric analysis coupled to mass spectrometry (TGA-MS) were used to analyse both the aqueous medium and solid phase. The amount of material obtained in the synthesis (yield) was quantified throughout the purification procedure. The influence of temperature on the nanoparticles' size and stability was studied by transmission electron microscopy (HRTEM) and sedimentation tests (light scattering). It was found that the synthesis yielded high-purity nanoparticles, whose morphological features were greatly affected by the reaction temperature.