Ceramics-Silikáty 40 (1) 36-40 (1996)

PROPERTIES OF GYPSUM-FREE PORTLAND CEMENT PASTES WITH A LOW WATER-TO-CEMENT RATIO

F. Škvára

Gypsum free Portland cement is a hydraulic binder based on ground Portland clinker in which the controlling effect of gypsum has been replaced by a synergistically acting mixture of an anion-active tenside (usually a sulphonated polyelectrolyte) and an inorganic salt (usually Na₂CO₃). The properties of pastes of the cements depend on concentrations of the additives replacing the effects of gypsum. In the dependence of paste viscosity (at a low water-to-cement ratio) on the concentration of additives (e.g. lignin sulphonate and soda) there exists an optimum where the suspension exhibits minimum viscosity. The presence of gypsum in the system ground clinker-sulphonated electrolyte-salt of an alkali metal-H₂O leads to a worsening of rheological properties where an initial liquefaction is followed by a rapid increase in paste viscosity. The rheological properties of the pastes are also affected by the particle size distribution where there holds a direct relationship between viscosity and deviation from the ideal Fuller curve. This effect is demonstrated by differences in workability (viscosity) of pastes of GF cements prepared in various types of grinding equipment. The time course of strength development of the hardened system ground clinker-sulphonated polyelectrolyte-alkali metal salt-H₂O depends on the type of the sulphonated electrolyte. Distinctly higher early strengths were achieved with the system sulphonated polyphenolate-Na₂CO₃ compared to the system lignin sulphonate-Na₂CO₃. The effect can be explained by a higher pH of the aqueous solutions of the former system.