ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 15, 2007 Corresponding author’s e-mail: kubens.s@ibri.is Corresponding author’s fax: +354 5707311 Rheological and structural investigations on cement slurries Olafur H. Wallevik 1 , Stefan Kubens 1 , Halldór Pálsson 2 , Benedikt Helgason 2 and Sif Grétarsdóttir 2 1 The Icelandic Building Research Institute, Reykjavík, Iceland 2 University of Iceland, Reykjavík, Iceland ABSTRACT The cost of drilling wells for geothermal power plants are considerably affected by the slurry used for the cementing job. Present slurry used for geothermal wells has a relatively high water content which results in a low early strength. To shorten the production process for each well the slurry was modified by help of chemical admixtures and a light-weight aggregate. INTRODUCTION Geothermal power plants play an important part in the electric power production in Iceland. More geothermal power plants are both under construction and being planned, because of the rapid increase in demand for electricity. The driving force for this development is the growing aluminium industry in Iceland. If the trend continues, Iceland will be one of the major producers of aluminium worldwide by the year 2010. The wells are a fundamental part of a geothermal power plant. They are drilled vertically into the rock formations of former volcanoes in the earths crust. A geothermal well consists of 3-4 drilling sections, each with a different diameter. After each drilling section, a steel casing is placed into the well and the space between the rock and the casing is filled with cement slurry. When the annular space is filled completely with slurry the cementing work is finished. The time span between pumping the slurry and the hardening affects the drilling cost for each well greatly. The faster the slurry hardens, the more economical the drilling operation becomes because of the reduced waiting time for the drill. Present cement slurry has a relatively high water content which results in a low strength and a slow hydration progress. The objective of this study was to determine (1) if an acceleration of the hardening process is possible with slight modifications on the slurry presently used for the cement works, (2) while at the same time keeping rheological parameters and density constant or close to the slurry presently used. Several slurry types were tested and compared with respect to rheology and strength. MATERIALS AND METHODS Tests on materials used Three locally available cement types were incorporated in the study. At the present slurry expanded perlite is used as light- weight aggregate, Wyoming Bentonite is added to the slurry as a stabilizer and silica sand as a filler. In a first step of the present study the effect of w/c-ratio on rheology in blank mixes was evaluated. Thereafter, the strength development at different w/c-ratios was investigated. When the w/c-ratio is lowered, the specific gravity of the slurry will normally increase. One requirement for