Historic gypsum mortars from Cyprus characterization and reinvention for conservation purposes

Abstract eng:
Gypsum was widely used in the production of composite materials in Cyprus in the past, due to its abundance on the island. However, the study of local historic gypsum mortars is not extensive. In this paper, 25 samples of gypsum mortars have been collected from various archaeological sites and studied systematically, aiming not only at gathering important details about their original production technology but also at reproducing compatible mortars for conservation purposes. The samples were subjected to macroscopic and microscopic observations, sieve analyses and a series of analytical tests. The results confirm the predominant presence of hydrated calcium sulphate in all composites; calcite was also detected in both the binder and aggregate fractions of the samples. The most common ratio of binder to aggregates was found to be 1:2.5. MIP showed porosity values between 14-48% and real densities between 1-1.8 g/cm3. The average pore diameters were found smaller in the case of mortars with lower porosity. Drilling resistance measurements indicated lower forces for bedding mortars, than for plasters; this confirms the occurrence of crystallisation and re-crystallisation close to the exposed surfaces of plasters due to long-term weathering. The results of the study on historic mortars have been used in the reinvention of gypsum-based composites, using widely available local raw materials, exploiting the production technology of the past. In the laboratory produced mortars, gypsiferous and calcitic materials were used in different proportions, both as binder and as aggregates. The new mixtures were tested in fresh and hardened state, at several time intervals after the day of production. The results showed higher compressive strengths (7.6-9.7 MPa) when pure gypsum was used both as binder and as aggregates. Porosity and drilling resistance values were found compatible with the original samples and local building stones. The range of results achieved with the various laboratory mixtures, enhances the possibility of selecting the appropriate repair mortar mix design, depending on the required application and project under study.

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