Process and additive to improve adhesion of building compositions to substrates

Process to adhere a mortar to a building substrate, the process comprising the steps of (A) mixing the mortar with at least one additive, (B) applying the mortar-additive mixture to a building substrate selected from the group consisting of polystyrene-containing substrates, polyolefin-containing substrates or polyvinyl chloride-containing substrates, and (C) allowing the mortar-additive mixture to dry, the additive comprising

• (i) a plasticizer that is liquid at 50° C. or lower, has a boiling point of 100° C. or higher, is capable of dissolving 1 wt % of the building substrate, and that has a solubility parameter δ at 25° C. between 22.5 MPa1/2 and 11 MPa1/2, the plasticizer being selected from esters,
• (ii) optionally, an inorganic filler that has a BET surface area of at least 40 m2/g;
• (iii) a polysaccharide selected from cold water-soluble polysaccharides, polysaccharide ethers, and synthetic anionic, nonionic and cationic heteropolysaccharides, the polysaccharide optionally being chemically modified with a substituent selected from the group consisting of carboxymethyl, carboxyethyl, hydroxyethyl, hydroxypropyl, methyl, ethyl, propyl, sulfate, phosphate and long chain alkyl groups;
• and/or
• (iv) a stabilizer selected from protective colloids;
• and
• wherein the additive is
• in the form of a liquid dispersion or powder obtained by mixing the plasticizer (i) with the polysaccharide (iii) and/or the stabilizer (iv) in pasty, swollen or dissolved form, optionally followed by drying said liquid dispersion, whereby the plasticizer (i) is not added to an aqueous dispersion containing a water-insoluble film-forming (co)polymer based on ethylenically unsaturated monomers, and
• wherein the additive is used in an amount of 0.005-5 percent by weight, based on the dry weight of the mortar, including the additive.