1. Introduction
Indoor plasters have the primary task of covering substrates (walls and ceilings) for thermal, hygienic and aesthetic purposes. Gypsum plasters are remarkable for a variety of distinct advantages. Their most significant advantage is that they are able to create a pleasant room atmosphere. Their physical properties allow for the regulation of room climate, and their low thermal conductivity leads to a sense of cosiness. They are quick and easy to process; they dry easily. Apart from these factors, gypsum plasters also contribute to the improvement of fire protection qualities. Gypsum plasters can be applied on almost all kinds of substrates (background surfaces):
Concrete
Walls made of brick, lime-sandstone or aerated concrete
Light excelsior boards (wood wool)
Hard foam boards
Since the 1960 s, their application has generally been based on dry mortars, and is carried out with the help of machines.
The adhesive mechanism of plasters on surfaces can be described as follows (cf. KHODA [23] and OHNEMULLER [11]):
Adhesion of fresh plaster based on the negative pressure effect during rough casting of plaster, air is dispelled and a zone of low pressure that enhances adhesion is created;
Adhesion of fresh and hardened plaster based on interlocking surface coarseness of the substrate leads to interlocking with the plaster the crystals emerging from the hydration process in the interface layer also continue to partly develop in the pores of the substrate, thus resulting in additional mechanical interlocking;
Adhesion of hardened plaster based on chemical bonding crystal in growth between plaster and substrate requires similar or chemically related plastering surfaces; this, however, is a seldom occurrence.