ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 16, 2008 Corresponding author’s e-mail: kubens.s@ibri.is Corresponding author’s fax: +354 5707311 Some effects of silica fume on variations in rheology of mortar due to production date of cement Stefan Kubens 1 , Hong Peng 2 , Sonja Oesterheld 1 and Olafur H. Wallevik 1,3 1 Innovation Center Iceland, Reykjavík, Iceland 2 ELKEM Materials, Kristiansand, Norway 3 Reykjavik University, Iceland ABSTRACT Rheological properties of cementitious suspensions (such as mortar and concrete) can change severely when a new cement batch/production date 1,2 is used. Particularly mixes containing dispersing admixture can be affected. Such a cement-admixture interaction problem 3 can cause various difficulties on construction sites. Too stiff concrete needs more workforce to be compacted properly and too liquid concrete could segregate. Previous research has shown that silica fume can reduce this effect. This means that silica fume can provide a more stable concrete production. A study was carried out on mortar mixes to investigate the influence of silica fume on rheology. The objective of this study is twofold. Firstly, to determine if silica fume can reduce the variations in rheology when cement of different production dates is used, thus reducing variations in rheological properties in concrete. Secondly, to quantify the advantageous effect of the silica fume. INTRODUCTION Concrete plants and pre-cast factories with high daily consumption of cement sometimes experience differences in rheological parameters of fresh concrete when using a new cement delivery (e.g. a new cement truckload). Nevertheless, the cement is of same type and coming from the same plant and production line. It only differs in production date. Particularly, mixes with high cement content and high admixture dosage (such as Self Compacting Concrete) can be affected by such a cement- admixture interaction case. During a previous study 4 more than ten different production dates of a Portland composite cement 5 were tested on their rheological properties. All cement samples were of same type, a CEM II/A-M 42,5R already blended with 4% of silica fume. Due to the high reactivity of Icelandic aggregates (with respect to alkali silica reaction), all cements produced in Iceland should be blended with silica fume. The cement samples were produced from October 2006 to February 2008, representing more than a one year production period. The cement samples were coming from the same production line. It was found that the yield stress was relatively stable in the blank mixes, whereas the yield stress was fluctuating significantly in the mixes containing polycarboxylate. MATERIALS AND METHODS The objective of this study is to investigate the effect of silica fume addition on rheology of mortar produced from one low yield cement and one high yield cement. In a series of cement deliveries (production dates) those which generate yield stress below the mean minus one standard deviation of that series are named