Laboruntersuchungen zur Wechselwirkung zwischen Mauer· und Ausfugemörtel und den Mauersteinen / Laboratory Studies of the Interaction between Bedding and Pointing Mortars and Building Stones

The physical and chemical properties of bedding and pointing mortars have a major influence on the nature, rate and extent of decay of pointed stonework. Specially designed mortars can prevent physical damage and greatly reduce chemical and biological decay in both the mortars themselves and the adjacent stones. This study describes the physical and chemico-mineralogical changes that occur in a number of different stone-mortar combinations. Three types of mortar were designed and fabricated in the laboratory in accordance with [1,2]. These were examined in combination with Leinster granite, Portland limestone and Baumberger sandstone by exposing standard samples comprised of two pieces of stone 5 cm χ 5 cm χ 5 cm separated by a 5 mm mortar filled joint to artificial ageing tests in a chamber. Thin sections were examined to determine what changes had taken place in the stone and the mortar. The physical properties of the stones and mortars were determined separately. The lime mortar, though chemically well matched with the limestone and sandstone, proved to be unsuitable because of loss of cohesion. The resultant microcracking, detected by pétrographie microscope, appeared to be caused by shrinkage during drying rather than by carbonation or cycling-alteration processes. Granite-lime mortar combinations showed low adherence and decreased durability of the mortar. This effect may be the result of the greatly differing physical properties of the lime mix and granite. The ageing tests revealed a high durability in the cement mixes. The study demonstrated the suitability of cement content mortars with poorly sorted aggregate and plasticiser for bedding and pointing in granite stonework. They also illustrated the need for care with the design of lime mortar mixes to ensure adequate binder-aggregate adhesion.