A nontoxic strengthening and protective agent for porous calcareous' stones and cement mortars has been ;synthesized in a one-Tot synthesis via the sol-gel method, incorporating nanoparticles of synthesized amorphous calcium oxalate monohydrate (ACO) in tetraethoxysilane (TEOS). Calcium hydroxide and oxalic acid added into TEOS produce ACO, which is then incorporated into the silica Matrix, while oxalic acid also acts as catalyst for TEOS hydrolysis. The crack-free nanocomposite derived possesses a uniform microstructure with average pore diameter of 2.73 nm and particles of approximately 7-15 nm in size. The ACO incorporated into the silica matrix gives good interfacial compatibility between the nanocomposite and building materials, such as calcareous stones and cement mortars, and improves their mechanical properties: The hybrid, nanocomposite can penetrate inside the lithic substrate and acts as a strengthening agent with protective effect against environmental loading. This one-pot synthesis allows an easy and cost-effective industrial scaling up.