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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.

Lime has been used in construction for millennia, and its value, especially in the field of conservation architecture, has only recently been rediscovered. Lime mortars harden through carbonation, and this thesis is a study of that process. The research conducted has resulted in the development of two novel techniques for the measurement and detection of carbonation. The first technique is a method of thermogravimetric analysis which allows the carbonation profile to be measured within an acceptable time-frame. The second technique is the use of drilling resistance measurement to visualise the carbonation profile. The potential of elemental analysis to measure the carbonation profile has also been identified. It has been demonstrated that the lime/water ratio has less impact on the compressive strength of air lime mortars than had previously been supposed. The change in the pore size distribution of air lime mortars caused by carbonation has been studied, and a theory has been proposed to explain this phenomenon. Five different forms of air lime binder were studied. The impact of these on the structural performance of the resultant mortars has been assessed. It was concluded that mortars made with lime putties perform better than mortars made with dry lime hydrate. Mortars made with dispersed hydrated lime appear to perform as well as mortars made with lime putties, but at a slower rate of strength growth. The use of extra mature lime putty does not appear to confer structural performance benefits when compared with ordinary lime putty. It has been shown that the use of calcitic aggregates can produce air lime mortars which perform as well as moderately hydraulic lime mortars. It is theorised that this phenomenon is not directly related to carbonation, but rather to a complex interaction of the granulometry, mineralogy, chemistry and porosity of the aggregate with the binder.