In-situ micro-rheology - a method for the study of skin and crust formation at mortar surfaces (Part 1)

摘要

Once a mortar is applied as a thin layer to the substrate, the mortar surface is exposed to the environment and changes its properties over time. In the case of a tile adhesive, the surface of the applied mortar readily starts to form a thin skin (< 0.1 mm), which can continuously thicken to form a dry crust with a thickness of 0.5-1.0 mm within 30 minutes of exposure time. Such a crust is a barrier between the fresh mortar underneath and the tile above and prevents a proper wetting between the two materials. As a result, the adhesion strength is reduced, as can be measured by the open time test (EN 1346). This paper presents a new rheological in-situ method for the measurement of the crust build-up during the first 30 minutes with a spatial resolution of one micron. First measurements indicate two principal types of mortars. Some of which form a crust and others thicken homogeneously throughout the entire mortar layer. The method can be applied to any soft material with a viscosity gradient perpendicular to its surface.