Black-crust growth and interaction with underlying limestone microfacies
Gilles Fronteau, Céline Schneider-Thomachot, Edith Chopin, Vincent Barbin, Dominique Mouze, André Pascal, 2010. "Black-crust growth and interaction with underlying limestone microfacies", Natural Stone Resources for Historical Monuments, R Přikryl, Á Török
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Black crust growth mechanisms on three French building stones are described using diagenetic models that reveal the close links between the crust–stone interfaces and the microfacies of the host limestone. Each limestone is representative of a specific sedimentary facies and displays mixed pore structure: crinoidal limestone (Euville limestone), oolitic limestone (Savonnières limestone) and bioclastic matrix-supported limestone (Courville limestone). The crinoidal limestone is mainly made of well-developed calcitic cement (spar syntaxial calcite) with low macrocroporosity (15–20 vol. %). The oolitic limestone is macroporous (30–40 vol. %), oolite nucleus being partially or completely dissolved. The third building stone studied is less porous (14 vol. %) but presents a significant microporosity.
Weathering of the Euville limestone proceeds primarily through preferential exploitation of cleavages and microcracks and secondly by progressive recrystallization in the areas separated by previous gypsum fill-in (micro-box work). In the Savonnières limestone (oolitic limestone), gypsum recrystallization could occur without microcracks: elements are sometimes nearly totally weathered, while the palisadic calcitic cement surrounding the oolites was still preserved. In the matrix-supported limestone (Courville limestone), weathering could deeply affect the matrix while elements are not weathered. When a layer of microcrystalline calcite is observed on the surface of the limestone, however, the black crust growth seems to be limited to the external part of the stone.
Porous characteristics of limestones directly depend on sedimentary and diagenetic phases developed. The pore network controls moisture movement and also determines the reactivity of the stone to gypsum recrystallization.
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Natural stone is considered to be a versatile, durable and aesthetically pleasing building material. From the beginning of civilization, important structures and monuments have been built from, or based on, natural stone. Until the end of the nineteenth century, the use of local stone resources was mostly in balance with the local environment. Strict environmental legislation has resulted in the closing of many long-standing quarries in industrialized countries, which has led to a shortage of traditional stone varieties. This has caused problems for restoration practice. Cheap, imported stone from less industrialized countries has become more widely available in recent years.
Some of the issues related to built stone conservation and restoration covered by this volume are: the establishment of inventories of possible replacement stones; understanding the decay mechanism and use of preventive conservation methods for slowing down decay processes; evaluation of the properties of natural stone; and assessing the risks of using replacement stones of different qualities.