Solvent-based ethyl silicate for stone consolidation: influence of the application technique on penetration depth, efficacy and pore occlusion

The efficacy of stone consolidating treatments is significantly affected by the application procedure, including the technique used and the amount of product applied. Indeed, in the common on-site practice, consolidants are not applied until ‘apparent refusal’ (i.e. a drastic slowing down of the liquid absorption by the substrate), as usually recommended in commercial products’ technical data sheets and performed in laboratory tests, but a lower number of applications is performed for practical and economic reasons. Nevertheless, the influence of the application procedure on the absorption mechanism and distribution of the consolidant has not been fully elucidated yet, especially for solvent-based products, for which a competition between capillary uptake and solvent evaporation might arise. In this study, the influence of the treatment conditions on the consolidant efficacy was investigated with the aim of determining whether increasing the number of applications results in a deeper penetration depth and remarkably higher mechanical properties or in a higher saturation of the surface layer. For the experimental tests, an organic solvent-based ethyl silicate (or tetra-ethyl-ortho-silicate, TEOS) was selected, as it is the most commonly-used product in stone consolidation. Brushing was selected as the application method, because it is widely used in current practice and because it involves a discontinuous fluid supply to the stone, possibly causing an over-accumulation of product near the surface. Two different treatments were compared: brushing application by 5 strokes (T1) and 10 strokes (T2). The effect of the two treatments, applied on a porous limestone, was evaluated in terms of increase in mechanical properties (tensile strength, resistance to abrasion), alteration in microstructural features (determined by mercury intrusion porosimetry) and penetration depth (assessed by visual inspection of fractured samples, water sorptivity test and dynamic X-ray radiography). The results of the study indicate that increasing the number of TEOS applications from 5 to 10 brush strokes leads to a higher (but less than proportional) increase in mechanical properties (+33 % for T1, +47 % for T2) and a higher penetration depth (7–8 mm for T1, 12–13 mm for T2), while no accumulation of the additional product near the surface and no hard crust formation were found.