Functional mortars for conservation of cultural heritage structures

Abstract

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.

Bio-Stimulated Surface Healing of Historical and Compatible Conservation Mortars

“…We chose this object based on several reasons. First of all, our team already had significant experience in the development of and recommendations of instructions for the conservation of the finishing floor mortars of the inner part of the dungeon tower of Bač Fortress [ 6 ]. In addition, at the time of study, conservation works were ongoing, and a large number of render mortar samples were available for our experiment.…”

“…The use of bacteria to induce the precipitation of calcium carbonate has great potential for fast and effective crack repair. This process is not only environmentally friendly but also provides compatibility and strong bonding between the matrix and the repair material [ 6 , 7 ]. When bacteria with bio-calcifying potential are incorporated into the matrix or onto the surface, the viable bacterial cells can activate the healing process by the precipitation of inorganic minerals.…”

Relationship between Bacterial Contribution and Self-Healing Effect of Cement-Based Materials

“…After carbonation, calcium ions are in a less soluble form, and bacteria cannot utilize targeted ions [ 116 ]. Some evidence, especially for historical samples where the carbonation rate is high [ 117 ], indicates that the bacterial SH effect may not occur if optimal amounts of nutrients (with calcium ions) are not directly added to the system or on the surface of the cracks.…”

Comparison of Microbially Induced Healing Solutions for Crack Repairs of Cement-Based Infrastructure

“…The bacteria used for application in a concrete environment need to be alkalophilic or alkalitolerant and, ideally, spore-forming, in order to survive unfavourable conditions that may occur during the service life of the structure. The healing processes based on bacterial activity have been variously described as bio-stimulation, bio-augmentation [23], bio-activation [24], bio-induction, and (bio-)remediation [25]. Bio-stimulation is in fact the process of adjustment of environmental conditions such as nutrients, substrate, water, etc., to enhance favourable metabolic processes of naturally present microflora.…”

“…Bioaugmentation, on the other hand, relies on introduction of previously absent bacteria on the material to be treated, usually employing specialised strains with selected bestsuited characteristics [26], although locally isolated strains have also been studied in this regard [23,27,28]. (Bio-)activation is a somewhat ambiguous term used to describe both the bio-augmentation approach [24] and the bio-stimulation one [29,30], and it would be beneficial to consider standardising this term for future reference to avoid possible misunderstandings. Bio-induction has been used synonymously with the term microbiologically induced calcium-carbonate precipitation (MICP), coined by Stocks-Fischer et al (1999) [31], employing various types of bacteria that can be classified by their metabolism into three main groups: ureolytic, denitrifying and aerobic heterotrophic.…”