Characterisation of earthen walls in the Generalife (Alhambra): Microstructural and physical changes induced by deposition of Ca(OH)2 nanoparticles in original and reconstructed samples

The purpose of this paper is to perform complete characterisation of Alhambra wall fragments using X-ray diffraction (XRD), X-ray fluorescence (XRF) and thermogravimetric analysis (TGA) coupled with mass spectrometry (MS). The analyses revealed that the binding phase of the wall was mainly composed of clay minerals (e.g. nontronite), carbonates and common soil constituents. Moreover, the external surface had calcareous nature, which suggests the use of protective aerial-lime mortars in the monument. In light of this evidence, Ca(OH)2 nanoparticles coatings were studied in both original and reconstructed earthen samples as they are fully compatible with Tapial. The coatings were examined by transmission electron microscopy (TEM), optical microscopy (OM) and scanning electron microscopy (SEM). It was found that Ca(OH)2 nanoparticles were preferentially deposited on certain zones of the surface. In addition, hardness measurements, erosion experiments and permeability tests confirmed that Ca(OH)2-coated samples had better physical performance than non-coated ones. In conclusion, Ca(OH)2 nanoparticles coatings are suitable treatments for reinforcing earthen walls in the Alhambra.