A comparative study of porous limestones treated with silicon-based strengthening agents

Porous bioclastic limestones of small pore radii and sandy limestones of large pore radii were treated with silicon-based strengthening products, such as tetraethoxysilane, elastified silicic acid ethyl ester (PP), an aqueous colloidal dispersion of silica particles (LD), as well as an oligomeric siloxane water repellent (HP). Changes in color, water vapor permeability, porosity, pore size distribution and tensile strength were evaluated after treatment. Energy Dispersive X-ray Fluorescence (EDXRF) indicated that the products reached a depth from the surface of 30 mm and were evenly distributed inside the stone, except for LD, which accumulated up to the first 10 mm from the surface. Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) showed that linkages created between limestone and gels, due to the hydroxyl groups of the quartz and aluminosilicate minerals included in the limestones. PP and LD reduced the water vapor permeability coefficient to acceptable limits and induced the least chromatic variation. Mercury intrusion porosimetry (MIP) results indicated that the precipitation of the formed gel increased the specific surface area. The porosity and pore size decreased in the treated bioclastic limestones and in the KS and LD treated sandy limestones. In contrast, the PP and PP + HP treated sandy limestones showed an increase in porosity and average pore radius, as a result of the polymer cracking. When polymers were applied to the sandy limestones of large pore radii, the formed gels underwent cracking from drying shrinkage and the mechanical resistance of the stone was reduced. The gels when deposited in the small pore radii of bioclastic limestones behaved as thin films, thereby no cracking was induced by the consolidant from drying shrinkage. This study demonstrated that the performance of a consolidant is greatly dependent on the pore size of the stone. None of the above studied products can be recommended for consolidation of the sandy limestones, while the elastified silicic acid ethyl ester (PP) is only recommended for consolidating the bioclastic limestones.