Natural Weathering of Granite: A Case Study

Over the past few decades, natural stone granite claddings have become very widely used. The long-term weathering behavior of granite has been less studied than that of other natural stones, such as marble, that were more extensively used in the past and have experienced notable weathering related problems when used as an exterior veneer cladding in certain climates. A white domestic granite, in place for more than 80 years and exposed to a northern midwestern climate, was studied to determine the cause of weather-related surface deterioration. The exterior walls of this monumental building, constructed of granite blocks mostly more than 200 mm (8 inches) thick experienced weathering distress including surface sugaring (saccarification), exfoliation, spalling, and discoloration. A loss of as much as 3 mm (1/8 inch) of granite from the bush-hammer surface of units was documented. The surface deterioration has had no apparent structural effect on the 200 mm thick units. However, similar deterioration, if it were to occur on 20 to 30 mm-thick veneer cladding, made of this material, would have a significant structural, as well as aesthetic impact. The major impact on the stone in this study was aesthetic. The weathering deterioration caused the stone surface color to alter and become irregular. The stone surface, which was originally uniformly light gray to white, became blotchy, ranging in color from white to dark gray. Studies of the deteriorated granite included documentation of the extent and various types of distress present, as well as laboratory studies of removed distressed and intact samples. The effects that weathering exposure have on the degree of deterioration and overall aesthetic impact were explored. Scanning electron microscope studies, petrographic analysis, and x-ray diffraction analysis were also performed. This paper presents the results of the studies performed to determine the chemical cause of the deterioration observed and provides a basis for understanding the fundamental causes of natural weathering of this granite. Also presented are the results of various treatment methods that were evaluated to determine if there is an effective means of restoring or providing a more uniform granite finish.