Hybrid nano-composites for the consolidation of earthen masonry

Earth is one of the oldest silicate-based materials in stone heritage, still largely used in architecture worldwide. Earthen materials are highly susceptible to wind and water erosion, leading to loss of cohesion and crumbling. Conventional consolidants (alkoxysilanes, synthetic or natural polymers) lack physico-chemical compatibility or effectiveness, and can promote degradation. We propose for the first time nano-composites for the surface consolidation of adobe, i.e. unbaked earth bricks often containing organic fibers and lime. We investigated, mimicking the setting of portland cement, the formation of calcium silicate hydrate (CSH) within adobe porosities, owing to the pozzolanic reaction between nanoparticles of silica and calcium hydroxide, to consolidate a powdery substrate. Different formulations were characterized by Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), dynamic light scattering (DLS), and turbidimetry (UV–Vis spectroscopy). A ternary composite made of SiO2 nanoparticles, Ca(OH)2 nanoparticles, and hydroxypropyl cellulose, dispersed in a (4:1) ethanol:water blend, was formulated. Each component is compatible with adobe, and plays a role in its consolidation. The treatment of adobe samples with the composite leads to the in situ formation of CSH, providing resistance to peeling, abrasion, and wet-dry cycles, with no aesthetic alteration. This opens new perpectives in the preservation of one of the most widely used construction materials.