Gypsum-Free Portland Cement Pastes of Low Water-to-Cement Rat10
- PDF / 2,729,744 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 22 Downloads / 240 Views
FRANTISEK SKVARA
Institute of Chemical Technology, Department of Glass and Ceramics Technology 5,16628 Prague 6, Czech Republic ABSTRACT Gypsum-free portland cement is a low porosity hydraulic binder based on finely ground portland cement clinker with addition of synergetic system containing an anion-active surface active agent (usually a sulphonated polyelectrolyte) and an inorganic salt (usually sodium carbonate) for regulation of the hardening process. The properties of GF cement are different from ordinary portland cement; they display, for example, higher strength, better corrosion resistance and thermal stability. These positive differences arise from the different mineralogy and microstructure of the hydration products, for example the absence of portlandite crystals. The main component of the binder product in hardened GF cement pastes is C-S-H (mean C/S ratio 2.7, based on EDAX analysis) intergrown with very fine Ca(OH) 2 and highly dispersed C-A-H phases (hexagonal and cubic). The absence of crystalline formations in the GF hardened pastes is responsible for higher mechanical strength. In the Czech Republic, GF cement is produced in the cement works of CEVA Prachovice Inc. ( Holderbank group) and is used for special works in the building industry. INTRODUCTION Research on gypsum-free portland cements was stimulated by Rebinder and coworkers1 who substituted calcium ligninsulphonate + K2CO 3 for gypsum. Their work was followed by that of Brunauer 2 . Our results (plus technological investigations) resulted in the commercial design of gypsum-free portland cements (GF cements). In the Czech Republic GF cement is produced 3 ,4 on an industrial level in the CEVA Prachovice Inc. cement works (Holderbank group, Switzerland) and has been used for special works in the building industry since 1989. GF cements may be described as a system of: ground Portland cement clinker (specific surface 300-700 m2/kg) + anion-active surface active agent with hydroxyl groups (ligninsulphonate. sulphonated lignin, sulphonated polyphenolate) + a hydrolyzable alkali metal salt (carbonate, bicarbonate, silicate). The difference between GF and common portland cement lies in the grinding (absence of gypsum, and/or greater grinding fineness, grinding admixture) and in the set regulator. A particular quality of the GF cements is their ability to set and harden at low and sub-zero temperatures 5 ; in addition, they show a rapid strength increase as a result of a small short-time rise in the external temperature. Apart from the above properties, hardened GF cements show a low absorption capacity and high resistance to aggressive media (salts, low and high pH) and recently also resistance to higher temperature (to 11500 C) was reported. The above-described GF cement properties depend, apart from their components, also on the specific surface. The chemical and mineralogical compositions of the clinker affect the properties of GF cements. However, these effects differ from these in the case of Portland cement. The effect of free CaO on initi
Data Loading...
