CHEMICAL AND PHYSICAL WEATHERING OF THE COMTE DE ROCHAMBEAU MONUMENT, LAFAYETTE SQUARE PARK, NATIONAL HISTORIC LANDMARK DISTRICT, WASHINGTON D.C.: MINERALOGY, SURFACE REACTIONS, AND POROSITY

The Comte de Rochambeau monument displays weathering of the pedestal, including mass loss of stone material on exposed surfaces, discoloration and staining, deposition of mineral salts, and detachment of stone pieces as the result of chipping and disaggregation. Petrography and X-ray analyses establish that the pedestal is a limestone—a packstone consisting of blocky calcite, shell fragments, and biomicrite, with minor amounts of quartz, clay, and iron-sulfide minerals. Surface area measurements by BET method range from 1.0 to 1.5 m2/g for partially weathered fragments. The pore structure was characterized by N2 absorption; average pore widths range from 90-120 Å. Blue-green staining on the pedestal is produced by oxidation of copper sourced in the bronze statue; some brown discoloration results from alteration of iron-sulfide and iron-oxide minerals inherent in the original limestone. Orange and black carbonaceous matter, in the form of soot, pine pollens and filaments, covers crystal surfaces and fills micro-cracks and crevices.

Decay of the limestone results from characteristics inherent both in the rock and in the urban environmental setting. The original limestone (calcite, bioclasts, biomicritic matrix) is reacting with fluids in the environment, which may include, for example, rainwater, automobile exhaust, aerosols, and road spray. SEM and X-ray analyses show the presence of secondary calcite, gypsum, and Na- and K-salts on exterior crusts (specific surface area = 1.24-1.46 m2/g), which indicates dissolution of carbonate material by sulfuric and nitric acid components when exposed to the Washington D.C. environment. The porosity of the original limestone allows fluids to infiltrate and contributes to local dissolution of carbonate bioclasts and calcite. Slow drainage and surface tension permit prolonged reaction with carbonate in the interior of the limestone (specific surface area = 1.01 to 1.41 m2/g), which causes a decrease in surface area and porosity by infilling with secondary salts and leads to local disaggregation. Efforts to preserve urban monuments may benefit from an initial characterization of the industrial material (original rock composition, surface area and porosity) and the identification of key minerals, fluids, and weathering reactions, prior to employing remedial and preventative measures.