ANALYSIS OF GYPSUM-CONTAINING LIME MORTARS: POSSIBLE ERRORS DUE TO THE USE OF DIFFERENT DRYING CONDITIONS
A. ELENA CHAROLA, & SILVIA A. CENTENO
Chemical analysis usually requires that samples be dried to eliminate sorbed moisture so that the results can be reported as a function of the dry weight of the sample. This requirement also applies to mortar analysis, especially since a high moisture content can be indicative of the presence of contaminating soluble salts. In traditional chemical analysis, samples are dried at 105șC, this temperature having been chosen on the basis of the 100șC evaporation point of water at standard pressure conditions.
Some procedures for the analysis of historic mortars use this traditional approach (Callebaut et al. 1999). However, in the case of historic materials, which might presumably have received a conservation treatment in the past, it has been suggested that a drying temperature of 60șC is more appropriate since it would prevent decomposition of these organic compounds (NORMAL 1988, NORMAL 1989b). Even this temperature was considered inappropriate for the special case of historic gypsum mortars, given the decomposition that gypsum suffers at temperatures above 40șC (Mehlmann 1993). Nonetheless, practical experience found that drying gypsum-containing mortar samples at ~55șC for only 16 hours prevented dehydration of gypsum, provided the samples were not damp and only three samples were dried at a time (Dupas 2001). Furthermore, some procedures do not specify a drying temperature (NORMAL 1989a), while in other cases both temperature and drying time are specified—e.g., sample is dried at 110șC for 24 hours for a simple acid attack and determination of sand and fines; or at 60șC for 48 hours for the calcimetric determination of CO2(Teutonico 1988).
This article describes the results obtained during an investigation of the drying conditions commonly used in the analysis of mortars following standard laboratory procedures, using simple mixtures of fine silica, calcite, and gypsum powder, to determine the magnitude of the errors introduced into calculations from the incomplete drying of the sample or the partial decomposition of gypsum.