Consolidation of a Tunisian bioclastic calcarenite: From conventional ethyl silicate products to nanostructured and nanoparticle based consolidants

Abstract

Mediterranean calcarenite stones are exposed to several weathering processes related to the climate and environment, producing alteration features conducing to loss of cohesion in their structures. Samples of these building materials were collected from the Spanish Fort of Bizerte (Tunisia) to carry out laboratory tests in order to assess the consolidation effect of the nowadays most frequently used products and most innovative consolidation materials based in nanoparticles. Stone specimens were consolidated with the most employed alkoxysilane product (ethyl silicate), with a surfactant-templated novel sol-gel product (that avoids the tendency to crack) and with inorganic products based on calcium hydroxide nanoparticles (Ca(OH)2) and silica nanoparticles (SiO2) under very humid and dry environmental conditions. Samples were characterized by scanning electron microscopy, peeling test, superficial hardness, drilling resistance, water absorption by capillarity and under vacuum, water desorption and spectrophotometry, before and after one month of the application of the products to evaluate their consolidation effect under different environmental conditions. The results show great differences in the consolidation effect and in the changes produced in the physical properties of the substrate after using alkoxysilane products (even nanostructured) or products based in nanoparticles. In the case of alkoxysilane and nanostructured products, especially exposed to high RH conditions, the mechanical properties of the substrate, internal and surface, increase. In both cases a layer on the substrate which occludes the pores is generated maintaining a hydrophobic behavior after one month causing drastic changes in the hydric behavior with visually detectable aesthetic changes. In the case of inorganic nanoparticles, changes in the porosity of the substrate caused by the creation of micropores occur in both cases. In the case of SiO2 nanoparticles, moderate physical changes occur in dry conditions resulting in less shrinkage and color changes. Finally, Ca(OH)2 nanoparticles are the products with the lowest surface and internal consolidation effectiveness which barely change the physical properties of the stone.