Functional mortars for conservation of cultural heritage structures

Mortar aging and deterioration are serious problem for architectural heritage conservation. The solution might be sought in advanced functional materials which could provide repair and lasting surface protection from atmospheric pollution and microbiological corrosion. In recent years, extensive studies have been conducted on the use of bacteria with biocalcification potential for self-healing effect in cements materials, but only a few publications deal with self-healing capacity of historical lime-based mortars.

The main focus of our research was development of new bio-activated self-healing system and its application in laboratory conditions. The objects of the work were historical mortar samples from medieval Bač Fortress in Serbia and laboratory prepared and aged mortar models. Aiming to achieve high compatibility, laboratory models were prepared based on our previous results of historical mortars characterization. The bio-activated self-healing agent was made as two-component liquid system using bacterial cells of Sporosarcina pasteurii DSM 33 and nutrients. The components of the models were hydraulic lime, milled limestone, river sand, and crashed brick as aggregates, and water. Comparative characterization of historic mortars and aged models was performed by mechanical and colorimetric testing as well as examination of mutual interaction and cohesion between old and new material.

The next step was efficiency evaluation of the external bacteria-based repair healing method in/on the laboratory samples. The detailed study of the cracks of the historical samples and the prepared models, and the bacterial suspension diffusion assessment were done by comparison of the results obtained by different complementary imaging techniques (optical and scanning electron microscopy). The experiments were performed on both samples of old and new materials treated with and without bio-activated self-healing agent.

The obtained results are promising and support the development of the external bio-activated self-healing method. This solution represents functional system which could allow historical mortars and modern structures to heal themselves in the long-term, preserving their functional and aesthetic properties.