(180br) A Continuum-Based Dissolution-Precipitation Model for Very Early Age Hydration of Alite

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.