Abstract
This paper analyses the mineralogical composition, texture, and structure of a stalactite sampled from the city-wall storerooms of the Nueva Tabarca fortress (southeast Spain). This speleothem presents an uncommon mineral assemblage: aragonite, brucite, gypsum, silica, and halite. Internally, it shows complex structure: (1) a central soda-straw composed by aragonite; (2) an external puff-pastry cone-crust formed preferentially by aragonite and brucite; and (3) an internal branching of coralloids, showing a subtle layering between brucite and aragonite. Gypsum, halite, and silica locate in the outer coating of the cone-crust. The sequent mineral precipitation sequence has been established: aragonite > brucite > gypsum/silica > halite. Speleothem formation is directly related to the chemical weathering of the rocks and mortars used as building materials of the city-wall. Brucite precipitates has been always linked to the presence of MgO-based geomaterials. However, the lack of these compounds as building materials in Nueva Tabarca fortress makes this investigation a unique example of brucite precipitation. PHREEQC calculations showed that interaction between pore waters and the minerals of mortar aggregates (dolomite, pyroxene, and amphibole) leads to rich-magnesium solutions. Evaporation modelling of lixiviated waters describes the precipitation of the mineral assemblage of the brucite-aragonite speleothems.
Highlights
The presence of brucite (Mg(OH)2 ) as constitutive of building materials in cultural heritage is very scarce and it is almost exclusively limited to dolomitic limes [1,2]
The main mineral phases in dolomitic limes are portlandite (Ca(OH)2 ) and brucite that react during the carbonation process, out of which comes out calcite (CaCO3 ), hydromagnesite (Mg5 (CO3 )4 (OH)2 ·4H2 O), and a variety of hydroxycarbonates whose formation depends on the water content of the mortar as well as the carbon dioxide and moisture content of the air [3]
Mineralogical determination of speleothems using X-ray diffraction showed that they are mainly composed of brucite (49%), aragonite (24%), gypsum (19%), halite (8%), and amorphous silica
Summary
The presence of brucite (Mg(OH)2 ) as constitutive of building materials in cultural heritage is very scarce and it is almost exclusively limited to dolomitic limes [1,2]. Brucite is found as weathering product of cement-based materials, resulting from the dissolution and reprecipitation of ions from the cement past and pore waters, especially in marine exposure [4,5,6]. Brucite, in these cases, use to form layered precipitates, frequently intermixed with other mineral phases (i.e., calcite, aragonite, and ettringite) filling cracks in the concrete mass. In these cases, use to form layered precipitates, frequently intermixed with other mineral phases (i.e., calcite, aragonite, and ettringite) filling cracks in the concrete mass These fillings can close totally or partially the initial fracture, making difficult the ingress of aggressive substances and improving the durability of the structure. Self-healing mechanisms in cementitious materials can Minerals 2020, 10, 599; doi:10.3390/min10070599 www.mdpi.com/journal/minerals
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
