Lanthanum-Modified LDH for Stone Masonry Consolidation

In this study, a lanthanum-modified MgAl double-layer hydroxide (La-LDH) was synthesized and investigated as a novel inorganic consolidant for gypsum, lime and cement-based mortars. The material was characterized by X-ray diffraction, WDXRF, SEM, dispersion stability analyses and DLS. XRD confirmed the formation of a hydrotalcite-like structure with secondary phases of lanthanum carbonate and hydroxylcarbonate, while SEM observations revealed a platelet-like morphology, typical of LDH materials. DLS analysis revealed a polydisperse granular distribution, with particle populations centered at approximately 99 and 383 nm, indicating the coexistence of nanometer sized LDH particles and larger aggregates within the consolidant dispersion. The prepared material (0.5 g/L) was dispersed in ethanol–water solution (40%:60%) and applied by brushing onto the specimens made for this purpose. The treatment resulted in minor chromatic variations (ΔE* < 1.5 for all substrates). No surface crusting, visible deposits or adverse aesthetic changes after treatment or accelerated ageing were observed. Of the substrates investigated, lime mortar showed the most pronounced response, indicating a 26.4% increase in surface cohesion, together with a substantial improvement in water permeability (up to 87%). In contrast, gypsum and cement mortars showed only limited improvements. Artificial ageing tests demonstrated good visual stability of the treated surfaces, while freeze–thaw tests indicated only minor changes in frost resistance. Salt crystallization tests showed that the treatment did not negatively affect the strength of any of the substrates investigated and, in several cases, delayed crack propagation and material loss during cyclic exposure to sodium sulphate solution.