Performance of porous limestone in monuments; the role of fabric in assessing the durability of Hungarian porous limestones
|Location||International Geological Congress,oslo 2008|
|Holding Date||17 September 2008|
Porous limestones, although have been widely used as dimension stones are very often prone to weathering and show severe signs of decay soon after the construction of the stone structure. The paper aims to demonstrate the role of fabric in controlling the durability of porous limestones by using Hungarian examples. Three types of porous Miocene limestones; a fine-grained, a medium-grained oolitic and a coarse-grained bioclastic, were studied. Micro-fabric, physical properties of quarry stones were tested under laboratory conditions. Similar lithotypes were found in monuments and non-destructive tests such as Schmidt hammer rebound, Duroscope water absorption and water content were used to assess the physical properties of the weathered surfaces. The weathering features and forms were mapped on ashlars of public buildings. The surface alteration is characterized by the presence of white (thin and thick) and black (laminar and framboidal) weathering crusts. Flaking, scaling and blistering are common crust detachment forms. The exposed weaker surface below the crusts often shows granular disintegration. To assess the processes of crust formation and detachment, description of lithologies and associated weathering features were combined with micro-fabric and mineralogical analyses. These tests were compared to quarry blocks of the same lithologies. The analyses clearly document the presence of thin and thick weathering crusts and the degradation of underlying fine- and medium-grained limestones. A textural change is marked by pore occluding calcite and reduction of porosity in the crust zone. Crust detachment is initiated by opening up of micro-fissures that develop below the cemented crust zones. The most sensitive stone type is the fine-grained limestone which appears to be less durable than its coarse-grained counterpart. The analyses have demonstrated that the air pollution related gypsum crystallisation with combination of freeze/thaw weathering lead to crust detachment with rates strongly controlled by the micro-fabric of limestone substrate.