Dynamics of skin formation and mechanisms of open time performance of tile adhesive mortars

R. Zurbriggen (1), Th. Bühler (2), U. Pieles (2), A. Wetzel (3), M. Herwegh (3)
1 Nouryon Chemicals AG, Sempach Station
2 Institute for Life Science, University of Applied Sciences Muttenz, Northwestern Switzerland
3 Institute of Geological Sciences, University of Berne, Berne, all Switzerland

When a mortar is applied as a thin layer it begins to change its surface properties due to exposure to the environment. In case of an adhesive mortar the exposure time has a dominant influence onto the adhesion properties to the ceramic tile, which is laid on top of this mortar layer.
The paper differentiates two stages of surface changes: Firstly, a thin skin (< 0.1 mm) forms within the first 5-10 minutes of exposure (time span before the tile is laid). Skinning causes a loss in wetting capability of c. 50 area %. However, in most cases this reduction is not yet critical with respect to the final adhesion strength, which is still above 0.5 MPa.
Secondly, a crust (c. 0.5 mm thick) forms within 10-30 minutes after application of the mortar. Crust formation reduces the wetting capability and resulting adhesion strength to critical values below 0.5 MPa. According to EN 1346, Open Time performance is gone.
Based on different analytical approaches, the obtained insights into the mechanisms of skinning and crusting provide a better understanding of surface mechanisms within the first 30 minutes of exposure and the resulting adhesion properties.