Kerstin Elert

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.

Abstract Enhanced salt weathering resulting from global warming and increasing environmental pollution is endangering the survival of stone monuments and artworks. To mitigate the effects of these deleterious processes, numerous conservation treatments have been applied that, however, show limited efficacy. Here we present a novel, environmentally friendly, bacterial self-inoculation approach for the conservation of stone, based on the isolation of an indigenous community of carbonatogenic bacteria from salt damaged stone, followed by their culture and re-application back onto the same stone. This method results in an effective consolidation and protection due to the formation of an abundant and exceptionally strong hybrid cement consisting of nanostructured bacterial CaCO 3 and bacterially derived organics, and the passivating effect of bacterial exopolymeric substances (EPS) covering the substrate. The fact that the isolated and identified bacterial community is common to many stone artworks may enable worldwide application of this novel conservation methodology.

The formation of calcite (CaCO3), the most abundant carbonate mineral on Earth and a common biomineral, has been the focus of numerous studies. While recent research underlines the importance of nonclassical crystallization pathways involving amorphous precursors, direct evidence is lacking regarding the actual mechanism of calcite growth via an amorphous phase. Here we show, using in situ atomic force microscopy and complementary techniques, that faceted calcite can grow via a nonclassical particle-mediated colloidal crystal growth mechanism that at the nanoscale mirrors classical ion-mediated growth, and involves a layer-by-layer attachment of amorphous calcium carbonate (ACC) nanoparticles, followed by their restructuring and fusion with the calcite substrate in perfect crystallographic registry. The ACC-to-calcite transformation occurs by an interface-coupled dissolution–reprecipitation mechanism and obliterates or preserves the nanogranular texture of the colloidal growth layer in the absence or presence of organic (macro)molecules, respectively. These results show that, in addition to classical ion-mediated crystal growth, a particle-mediated growth mechanism involving colloidal epitaxy may operate in the case of an inorganic crystal such as calcite. The gained knowledge may shed light on the mechanism of CaCO3 biomineralization, and should open new ways for the rational design of novel biomimetic functional nanomaterials.