SURFACE RECESSION BY SALT-WEATHERING-INDUCED GRANULAR DISINTEGRATION OF POROUS BUILDING STONE IN CENTRAL MICHIGAN

The building presently known as Morrill Hall on the campus of Michigan State University was dedicated in 1900. Lake Superior Red Sandstone (LSRS; Jacobsville Sandstone) was used for multiple courses of exterior stone at and above ground level, and for decorative elements at entrances, including Tuscan columns atop an elevated porch at the head of the steps leading to the main entrance. Photographs from the 1950s show intact stone features at the main entrance. By 1986, surface recession had reduced the dimensions of all surfaces of the column bases (including the vertical surfaces of the plinths and much of the molding on the bases) and their supporting stone blocks, all consisting of LSRS. Up to centimeters of surface recession had occurred since the most recent previous tuck-pointing of the joints between the various elements of the column bases. Red sand was noted on the concrete steps immediately beneath the bases of the columns. During prolonged dry spells after wet episodes (e.g., snowmelt, rain), white efflorescences were widespread on affected surfaces and in LSRS blocks elsewhere near the steps leading to the entry porch. The efflorescences consisted of halite apparently derived from deicing salt by leaching of brackish slush into pores, capillary rise, and evaporation from rock surfaces, precipitating halite near the stone surface and disaggregating the stone by crystal (salt) wedging. The poorly cemented, friable nature of the LSRS made this stone particularly vulnerable to granular disintegration, especially at entrances where heavy foot traffic encouraged application of large quantities of deicing salt. In the early 1990s the severely degraded column bases, plinths, and supporting stones were replaced. As LSRS was no longer available, Indiana Limestone was used and painted to match the color of the remaining LSRS. Within two years, much of the paint had separated from the stone, and by the early years of this decade, limestone disaggregation had completely removed the paint. Considerable surface relief has developed on formerly finished surfaces due to differential disaggregation of cross-beds in the limestone, and gray calcareous sand occurs on the concrete steps adjacent to the column bases. The porous Indiana Limestone appears to disaggregate by the same process that disaggregated the porous LSRS.