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Lime Mortars for the Conservation of Historic Buildings

Abstract Awareness of the need for compatible materials for the preservation of the architectural heritage has resulted in the revival of lime-based mortar technology and applications. However, knowledge of the preparation process and procedure influencing the final quality of lime mortars is limited, and controversy persists in the conservation community regarding the most appropriate material for conservation treatments (for example, hydrated lime versus aged lime putty). This paper reviews current knowledge on lime mortar technology, including burning, slaking, aging and carbonation of lime. Special emphasis is given to the effects of aging on the morphological evolution of hydrated lime and on the carbonation process, since these aspects have not been discussed thoroughly in the technical and conservation literature. The improvements observed in the physical properties of hydrated lime after prolonged storage under water can be attributed to particle size reduction (<1μm) and morphology changes (from prism to plate-like crystals). Studies on the carbonation of non-aged commercial hydrated lime and traditionally aged slaked lime revealed higher carbonation rates in the case of aged lime. Some recommendations are given for the processing of lime and the preparation of lime mortar for conservation treatments. The use of aged lime putty is recommended because this material, with higher plasticity and water-retention capacity, results in mortars of higher strength that carbonate faster.