The impact of consolidants on the properties of intact and damaged porous limestones

Natural stones used in most buildings undergo degradation due to their exposure to outdoor environments, particularly to sometimes extreme variations in temperature and humidity conditions. In historical buildings, consolidants are often used to restore the strength of the damaged building materials and prevent further deterioration. The most commonly used consolidant in this context is ethyl silicate but it is incompatible with salt-contaminated stone. A salt-compatible alternative, lithium silicate, has been developed but remains rarely used. This is partly due to the limited number of available scientific studies and the difficulty of detecting lithium using conventional analytical methods. The aim of this study was to compare the effectiveness of these consolidants on two porous limestones subjected to thermal and mechanical stresses. Saint-Maximin and Leitha limestones were selected for this study because both rocks were widely used as building stones, in France and Austria, respectively. Cylindrical samples of 40 mm in length and 20 mm in diameter were prepared from large blocks. The average porosities of our Saint-Maximin and Leitha limestones were 38% and 41%, respectively. The permeability of both limestones was greater than 1 Darcy. Three groups of samples were tested: intact samples, samples thermally treated up to 400°C and samples deformed uniaxially to the peak stress. Consolidants were introduced in these samples through imbibition experiments that lasted a minimum of 48 h. Within the limestones, the consolidants underwent hydrolysis and condensation reactions to first form a gel on the pore surface. This gel progressively polymerized to form a thin solid layer on the pore surface. Treated samples were typically left to cure in a dry environment for at least a month. Minor variations of the porosity and permeability were observed in all the consolidated samples. The Uniaxal Compressive Stress (UCS) of the intact samples predictably increased by a factor of two for both rocks and both consolidants. After thermal treatment up to 400°C, the UCS of samples of Saint-Maximin and Leitha typically decreased by about 25% due to the thermal expansion of the grains and thermal microcracking. We found that consolidation with ethyl silicate erased this weakening effect. For lithium silicate, the samples also recovered part of their strength, but the effect was less pronounced. When damage was introduced into the rocks through uniaxial compression, ethyl silicate produced a more significant strengthening effect than lithium silicate. In the context of cultural heritage conservation, it is essential that consolidated stones present petrophysical properties similar to the original material in order to prevent further mechanical alterations. Both products exhibit a consolidating effect, but stones consolidated with lithium silicate display properties closer to those of the original rock.