Ceramics-Silikáty 36, (3) 121 - 128 (1992)

If for the production of autoclaved porous concrete materials is employed as the main raw component quartz sand with a considerable content of sodium, potassium or sodium-potassium feldspars, under suitable conditions in the atmosphere efflorescence are set up in the civil engineering structure after their application. Their essence is thenardite (Na₂SO₄), the formation of which is caused by the decomposition of sodium feldspar with Ca(OH)₂ in the course of the autoclave process and through the subsequent reaction with the present sulphate ions. The degree of decompositon of potassium feldspar is under the same conditions pronouncedly lower. With regard to the lowest solubility of thenardite in water in comparison with the other alkalic sulphates there occurs the undesirable formation of efflorescence not only on edges, but also on the surfaces of porous concrete products.