Three innovative strengthening, protective and self-cleaning agents for marble and calcareous stones were synthesized by using a simple sol-gel route at ambient conditions. TiO2 nanoparticles and hydroxyl-terminated polydimethylsiloxane (PDMS) were incorporated in a tetraethoxysilane (TEOS) matrix in the presence of oxalic acid (Ox). Both the Ox concentration and the addition sequence of the reagent agents were assessed through the evaluation of the microstructure and of the physicochemical and self-cleaning properties of all the synthesized nanocomposites. The presence of Ox contributed to the production of both homogeneous materials and photoactive independent domains of anatase TiO2 crystals at ambient conditions. In addition, the role played by Ox as a hole-scavenger increased the photocatalytic activity of the nanocomposites. The synthesized crack-free, homogeneous, transparent and photoactive nanocomposites provide self-cleaning, water repellency and consolidation properties to building substrates, while respecting their aesthetic qualities.