Quantitative evaluation of nanosilica for consolidating sandstones in Nankan Grottoes: insights from X-ray computed tomography analysis

Nanomaterials show promise for heritage conservation, but their penetration in porous substrates remains poorly understood. This study quantitatively evaluated the consolidating effectiveness of nanosilica for Nankan Grottoes sandstones using X-ray computed tomography (CT). The physical and mechanical properties, chromatic alteration, and microscopic observations were analyzed. Higher concentration (5%) nanosilica exhibited greater absorptivity, color change, and coating thickness than 1% dispersion. For artificially weathered sandstone, 1% nanosilica more effectively improved overall mechanical strength (P-wave velocity, tensile strength), while 5% dispersion better enhanced surface hardness due to larger SiO₂ agglomerations and reduced penetration depth. CT revealed porosity decreased from 12.5% to 8.5% after 1% nanosilica consolidation, and from 14.3% to 9.3% after 5% treatment. Nanosilica primarily filled pores of 0-40 μm and >180 μm, reducing connected pores while increasing isolated pores, thereby decreasing pore connectivity and complexity. This study provides novel methodological insights for studying the consolidation effect of nanomaterials-based consolidants.