Treatment of Salt Weathering

One of the more devastating effects of climate change is increased rainfall and flooding, as seen in Prague in 2001, in New Orleans in 2005 with the catastrophic levee failures following Hurricane Katrina, and in Iowa in 2008, which experienced devastating five-hundred-year-level flooding.

Historic buildings located in coastal areas and along major rivers are especially vulnerable to flooding and its long-term effects. Flooding increases salt loads in structures by mobilizing or bringing salts into porous building materials, resulting in cycles of crystallization, surface salt efflorescence, and damage to masonry. While flooding is a short-term event, the resulting wetting and drying cycle of thick stone walls often takes years to complete and may activate salts from deep within the stone that have accumulated from air pollution, deicing salts, or encroachment of brackish groundwater.

Fifty years after sea flooding in parts of the Netherlands, salt weathering continues to be problematic in several churches in formerly flooded areas. In Prague, buildings never previously threatened by groundwater, rising damp, or salts are now seriously affected because of the higher groundwater table created by episodic flooding.

Treatment of salt weathering on historic structures and monuments often requires architectural and engineering interventions to control moisture—installation of a damp-proof course in the foundation, redesign of roof drainage systems to accommodate larger storms, or maintenance of architectural details that shed rainwater away from the foundation, doors, windows, and walls. A leaky roof, overflowing drain pipes, or condensation can contribute to the mobilization of salts.

Once these moisture issues have been addressed, a common treatment option involves the application of a desalination poultice to reduce the load of accumulated salts on these structures. The ones used have been based on a typical cleaning poultice and often contain a substantial amount of water. However, architectural conservators often experience unexpected outcomes when large-scale poulticing is undertaken—salts move in unexpected ways, come back quickly, are pushed deep into the stone, or are not removed by the treatment.