DRY MIXTURES

[:ru]Бауэрэггер С.[:en]Baueregger, S.[:] [:ru]доктор, проф., Мюнхенский технический университет, Германия[:en]Prof. Dr., Technische Universitat München, Germany[:]

[:ru]Планк Й.[:en]Plank, J.[:] [:ru]проф., доктор техн. наук, зав. кафедрой строительной химии, Мюнхенский технический университет, Гархинг, Германия[:en]prof, Dr. Eng. Sci., Chair for Construction Chemistry, Technische Universität München, Garching, Germany[:]

Alitinform №4-5 (36) 2014 г. 92-105 p.

Abstract

In this article, new results on the performance, application and working mechanism of polycarboxylate superplasticizers, polyphosphate retarders and methyl cellulose water retention agents in CaSO₄ systems are presented. At first, the effectiveness of polycarboxylate (PCE) superplasticizers possessing different chemical compositions was investigated in anhydrite-based floor screeds. It was found that PCEs possessing high anionic charge density disperse anhydrite significantly better than those with low anionic charge. The reason behind the difference is competitive adsorption between the PCE molecules and sulfate ions. The latter hinder and reduce adsorption of PCE polymers possessing low anionic charge.
Next, the retarding effect of linear sodium polyphosphates exhibiting different chain lengths on α-calcium sulfate hemihydrate was investigated. The results confirmed that polyphosphates present highly effective retarders which work at low dosages and constitute a viable alternative to commonly used tartaric or citric acid. Polyphosphates exhibiting short chain lengths (e. g. n_PO3 = 4) work best.
In the third part, performance of the water retention agent methyl hydroxypropyl cellulose (MHPC) in gypsum plaster was studied. It was found that the high sulfate concentrations present in gypsum pore solution can affect the water retention capacity of MHPC significantly by hindering the formation of large hydrocolloidal MHPC associates which are responsible for the water retention effect. This result explains why higher dosages of MHPC are required to achieve comparable water retention than in cement based systems.

Key words:

gypsum; polycarboxylate; polyphosphate; methyl cellulose; admixture