Surface strength and mineralogy of weathering crusts on limestone buildings in Budapest

The oolitic limestone blocks on the buildings of Budapest show extensive crust formation, which is related to atmospheric pollution and continental climate. Crusts can be grouped according to their colour and morphology. Two types of white crusts and two types of black crusts have been studied in detail. Physical properties (Schmidt hammer rebound, Duroscope rebound, water absorption) and mineralogical composition (XRD, DTA–DTG) were measured and compared. Most crusts have higher Duroscope and Schmidt hammer rebound values than their host rock. The strength of the crust is related to the thickness, to the mineral composition, to the crust morphology and to the water absorption capacity. Calcite-rich, impermeable, thick white crusts have the highest strength, which is followed by gypsum-rich laminar black crusts alongside with thin gypsum-rich white crusts. These differences in strength and mineralogy are attributed to differences in rain/wind exposure and conditions conducive to pollution entrapment.