ABSTRACT
Abstract The total and external chemical shrinkage were determined for 10 different portland cements with a wide range of mineral composition and fineness. The total chemical shrinkage was determined by a dilatometry technique monitoring the change in water level in a graded pipette. Measurement of true external chemical shrinkage is very difficult to carry out because of the disturbance of bleeding. To avoid this, some of the samples were placed in thin walled, elastic rubber containers and rotated continuously under water, while most were measured at low w/c. The external chemical shrinkage was equal to the total chemical shrinkage until it "flattened" out - demonstrating that a semi-rigid skeleton was established which led to empty contraction pores during further hydration (i.e. shrinkage). The fineness of the cements dominated the initial rate of chemical shrinkage (i.e. hydration rate) and the "flattening out level" (i.e. fraction of external chemical shrinkage) seems to be quite independent of fineness. The induction period prior to setting is shown to not be "dormant", but rather quite active in terms of volume changes the first hour. The influence of the different cement minerals on the chemical shrinkage is discussed as well. Keywords: Chemical shrinkage, cement characteristics
1 Introduction
There is currently wide interest in the early volume change of concrete since it is considered a main "driving force" to early age cracking. Early age cracking is a relevant practical problem, particularly in connection with high performance concrete. The present paper is a continuation of previous work [2, 3, 4] concerned with developing
methods to measure both the chemical shrinkage associated with cement hydration and the external manifestation of the phenomenon.
