The effect of barium hydroxide on the physicochemical properties of lime-based conservation mortars

Abstract

The research undertaken concerns the analysis of original Byzantine mortars and the study of new compatible lime-based mixtures, for conservation purposes, which present enhanced resistance to sulphates. The thesis focuses on the effect of barium hydroxide, as an additive, to both the physicochemical properties and the durability of conservation mortars. Both binary pastes and mortar mixtures, containing varying amounts of barium hydroxide, were prepared and their physicochemical properties and durability determined. Comparisons were made with a reference mixture containing no barium hydroxide. The laboratory mixtures were synthesised according to the analysis of original mortars, collected from six different Byzantine mosaic pavements. The durability of the mortar mixtures against sulphate action was studied through the use of accelerated aging tests: crystallisation of soluble salts; electrochemical degradation; sulphate fixation; and resistance to leaching of cementing material. The results of these tests were considered together with the physical properties of the mortars, since these also influence their durability. From the interpretation of the results it was shown that the setting of lime-based barium mixtures can be described by two mechanisms; the carbonation process of calcium hydroxide and barium hydroxide; and the precipitation of a barium calcium carbonate [BaCa(CO3)2] solid solution. It has been shown that the presence of barium hydroxide in lime-based mortars has a physical effect on the pore space characteristics of the mortar. It has also been shown that its presence improves the durability of the mortar, playing a protective role against sulphate attack