Ceramics-Silikáty 44 (1) 35-38 (2000)

DETERMINATION OF MOISTURE CONTENT IN HYDRATING CEMENT PASTE USING THE CALCIUM CARBIDE METHOD

J. Poděbradská, J. Maděra, V. Tydlitát, P. Rovnaníková, R. Černý

The calcium carbide method is used for measuring the moisture content in hydrating cement paste which could be considered as a measure of the course of the hydration process. The experimental measurements show that the readings on the acetylene manometer used for calculating the residual moisture content in the material increase with time significantly up to approximately six hours which is remarkably different from the measurements on the most of other materials such as bricks, gypsum, etc., where the final constant readings are achieved already after ten minutes. Two hypotheses for the explanation of this fact are proposed. The first consists in the assumption that calcium carbide reacts not only with free water but also with a part of water already bonded in the cement binder. The decomposition of ettringite in the reaction bottle accompanied by partial loss of bonded water, which becomes available for the reaction with calcium carbide should be the most probable mechanism in this case. The second hypothesis is based on the assumption that the delay in determining the final values of residual moisture content is caused by a slow release of water from cement gel due to the high bonding forces of water to the porous matrix in small gel pores. In any case, the relatively long time necessary for the achievement of final readings makes the application of calcium carbide method for monitoring the hydration process questionable because its main advantage, fast determination of moisture content, cannot be effectively utilized.
 

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