Black-crust growth and interaction with underlying limestone microfacies

Abstract

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.

Neutron radiography and X-ray computed tomography for quantifying weathering and water uptake processes inside porous limestone used as building material

“…Quantification of the changes in radial intersections demonstrate that the external crust almost completely shields the surface, thus preventing gasses and water to interact with the stone. Due to the syntaxial overgrowth of the crinoid fragments, the observed weathering processes appeared to be similar to the processes described on the Carrara marble [28,48].…”

“…The crystallization inside the stone resulted however in a decrease in porosity and a fragmentation of the open pore network. This complete conversion of the ooids was already observed in 2D [28,45]. In addition, also the oolitic limestones in Budapest (Hungary) were intensively investigated [12,46].…”

“…The thick gypsum crust significantly prevented the stone from secondary weathering. For both the weathering of the syntaxial cement as well as for the crinoid fragments, the direction of the weathering follows the cleavage planes in the calcite monocrystals [28]. Quantification of the changes in radial intersections demonstrate that the external crust almost completely shields the surface, thus preventing gasses and water to interact with the stone.…”

“…In the case of the Savonnières limestone, the cementation does not isolate the ooids from weathering fluids. The fluids can penetrate easily because of the residual intergranular, open porosity [28,47].…”

“…A subvolume of both limestones was analysed in order to try to quantify the pore clogging and pore coalescence due to gypsum formation. A large number of studies have been devoted to gypsum crystallization [17,[24][25][26][27][28]. At rain exposed places, the soluble gypsum dissolves in water and no formation of a thick gypsum crust occurs, resulting in a gradual erosion of the surface.…”