Application of thermal analysis in preservation and restoration of historic masonry materials

Abstract

Thermal analysis techniques have been used in characterizing building materials from significant historic properties in the Charleston, South Carolina area. Determining the chemical and physical effects of deterioration resulting from long periods of exposure is a first step in formulating preservation strategies. In this regard, simultaneous thermal analysis coupled with evolved gas analysis has been used to study reactions between air, seawater, and masonry materials. Further, the traditional petrographic identification of mortar composition is greatly facilitated through use of thermal analysis. Simultaneous thermal analysis allows for an exact determination of the calcium carbonate content in mortars as an alternative to the use of an inferred value based on chemical analysis data. The partial dissolution of calcium carbonate in the presence of sea salt is a major deterioration process. Further, natural cements manufactured in the United States are identified, in part, based on their thermogravimetric (TG) traces and their evolved gases. The data indicates that natural cements form some carbonate phases in addition to the major hydrate phases. Clay bricks are found to exhibit interaction with sea water, with uptake of bicarbonate suggested. Additionally, there is evidence of re-hydroxylation in the 160 year old bricks. The bricks made in coastal zones contain a considerable free silica fraction that is composed of a small percentage of cristobalite. The silica content of the clay bricks is seen to result in very high thermal expansion coefficients in the area of 10 × 10⁻⁶ to 12 × 10⁻⁶ K⁻¹. These studies provide guidance in restoration efforts where authenticity of cements is important. In the event that replacement bricks are required, matching the thermal expansion coefficient of the original bricks is a requirement for preservation of the masonry structure.