Thin-film Al₂O₃ coatings on hydraulic lime mortar: Microstructural behavior and performance

Abstract

Hydraulic lime mortars (HLM) are widely used in heritage conservation due to their chemical compatibility and sustainability. However, their high porosity and, in some cases, limited durability may motivate the use of protective surface treatments in specific exposure conditions. This study explored the innovative application of Atomic Layer Deposition (ALD) of aluminum oxide (Al₂O₃) as a surface treatment for Hydraulic Lime Mortars (HLM). ALD-grown Al₂O₃, known for its high mechanical strength, wear resistance, and affinity for mineral substrates, was investigated as a means of reinforcing the surface of mortars without compromising their intrinsic properties. Mortar specimens cured for 28 and 60 days were coated with Al₂O₃ via ALD, and their surface characteristics were systematically evaluated. The bulk composition and mineralogical evolution were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD), whereas the coating morphology, surface wettability, visual impact, mechanical properties, and chemical durability were assessed by Scanning Electron Microscopy (SEM), X-ray Reflectivity (XRR), contact angle measurements, colorimetry, nanoindentation and acid attack tests. Uniform Al₂O₃ films were successfully deposited, with properties influenced by both substrate maturity and the number of ALD cycles. Colorimetric analysis showed minimal visual impact (ΔE < 3), although thicker coatings on well-cured mortars occasionally reached ΔE ≈ 5. Wettability tests indicated increased surface hydrophilicity, and nanoindentation revealed a positive correlation between ALD cycle number and surface reinforcement. Enhanced acid resistance further confirmed the protective role of the coating. Overall, the findings highlight the potential of ALD as a controllable and compatible surface modification technique for enhancing the mechanical performance and durability of lime mortars in heritage conservation.