This study characterized mortars from a masonry fortification in Charleston, South Carolina (USA), harbor where construction was during the period 1839-1860. This location for analysis was interesting because of the sea water impingement on the structure. The study was included as part of an overall structural assessment with restoration as an objective. The mortars were found to be cement, lime, and sand mixtures in proportions similar to ones expected from the historic literature, that is, one part binder to two parts of sand. The binder was found to be American natural cement, a substance analogous to the European Roman cement. The results suggest that the thermal history of the cement during manufacturing affected setting rate explaining why the cements were considered as variable during the mid-to-late 1800s. Fine pores were found in mortars exposed to sea water resulting from corrosion. Contemporary natural cement was shown to release calcium in aqueous solution. While this release of calcium is necessary for setting in natural and Portland cements, excessive calcium solution, as exacerbated by sea water contact and repointing with Portland cement mortars, was shown to result in brick scaling or decay through cryptoflorescence.
A single test method that is capable of determining the efflorescence potential for all materials in a masonry system is needed. The need for preconstruction testing is referenced in ASTM C1400-1—“Standard Guide for Reduction of Efflorescence Potential in New Masonry Walls”—but the standard acknowledges that there is not a suitable test method for all masonry materials. In the past, soluble salt measurements have been used to quantitatively access the efflorescence potential of masonry materials. To determine the water-soluble salt content of masonry materials, a simple leaching procedure was developed to remove the salts followed by quantification of the water-soluble salt content by ion chromatography. A series of modified efflorescence tests using salt solutions and real masonry materials has been used to determine the significance of water-soluble salt measurements. A clear threshold for soluble sulfate content that indicates a high potential for the development of visible efflorescence has been identified. Sulfate salts are the most common water-soluble compounds associated with efflorescence complaints.
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