284565 A Continuum-Based Dissolution-Precipitation Model for Very Early Age Hydration of Alite

Monday, October 29, 2012
Hall B (Convention Center )
Manohar Gottapu, Department of Chemical Engineering, Tennessee Tech University, Cookeville, TN and Joseph J. Biernacki, Department of Chemical Engineering, TennesseeTechnological University, Cookeville, TN

Alite (impure iron and aluminum bearing monoclinic tricalcium silicate (C3S(m))), the major constituent of ordinary portland cement (OPC), hydrates upon addition of water through the concurrent reactions of dissolution and precipitation.  The process of C3S(m) hydration is in general divided into five stages: (I) dissolution; (II) induction; (III) acceleration; (IV) deceleration; and (V) slow prolonged hydration (steady-state) as  observed, for example, by isothermal calorimetry.  The present work mainly focuses on the first ten minutes of hydration referred as very early age hydration which includes fast dissolution (i.e. Stage (I)) and its immediate inhibition (i.e. the beginning of Stage (II)). There exist a number of unresolved questions: Is alite dissolution congruent?  Why does alite dissolution slow so abruptly?  Why does the rate of early hydration depend on water to cement (water to C3S) ratio?  What determines the length of the induction period?  Accurate modeling of these early hydration events is so vital as it is responsible for several application important characteristics of portland cement-based concrete including workability and set time.  A systematic analysis was performed and a continuum based dissolution-precipitation model was developed. The model combines solution phase chemistry together with a recent hypothesis that suggests that dissolution inhibition is due to a sudden change in mechanism triggered by a critical undersaturation limit.  The model qualitatively agrees with the available experimental results and predicts some of the important characteristics such as the existence of solubility relationship between the solution and hydrated solid phase and a strong dependence of the rate of early hydration on water to cement ratio.

 


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