Determination of moisture content in hydrating cement paste using the calcium carbide method

The calcium carbide method is used for measuring the moisture content in hy drating cement paste which could be considered as a measure of the course o f the hydration process. The experimental measurements show that the readin gs on the acetylene manometer used for calculating the residual moisture co ntent in the material increase with time significantly up to approximately sir hours which is remarkably different from the measurements on the most o f other materials such as bricks, gypsum, etc., where the final constant re adings are achieved already after ten minutes. Two hypotheses for the expla nation 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 wa ter already bonded in the cement binder: The decomposition of ettringite in the reaction bottle accompanied by partial loss of bonded water, which bec omes available for the reaction with calcium carbide should be the most pro bable mechanism in this case. The second hypothesis is based on the assumpt ion that the delay in determining the final values of residual moisture con tent is caused by a slow release of water from cement gel due to the high b onding 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 hydratio n process questionable because its main advantage, fast determination of mo isture content, cannot be effectively utilized.