The Tabularium, old Town Council of the Romans, burned down in 83 BC, was rebuilt using a volcanic tuff named Lapis Gabinus. The results of a detailed study of the mineral and petrographic characteristics and the physical and mechanical properties of this tuff are reported. The use of the Tabularium as a salt deposit in medieval times induced a marked decay of the tuff blocks and columns, with extensive phenomena of scaling, crumbling, and alveolar weathering. Chemical consolidating products were used 20 years ago and the result of tests with acrylic resins and silicic compounds are discussed together with their possible interaction with the abundant zeolitic component of the tuff.