Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 1 INTRODUCTION
assemblages to be predicted under different conditions (e.g., initial clinker composition,
water-to-cement (w/c) ratios, etc.) and for longer time scales.
Hydration models have been developed in the past years to quantify the composition of
the solid phases and liquid phases in cementitious systems during hydration [3-6]. For
OPC systems, thermodynamic modeling in combination with calculated hydration rates
correctly predicts the depletion of gypsum within the first day of hydration (Fig. 1). In
this phase a strong decrease of the sulfate concentration in the pore solution is observed
as ettringite continues to precipitate (Fig. 2). The depletion of aqueous sulfate during the
first day is compensated by the release of OH - to fulfill conditions of electroneutrality in
solution, which gives rise to a significant increase in pH. This increase in pH decreases
the Ca concentration constrained by the portlandite solubility (Fig. 2). After the first day
the precipitation of ettringite (6Ca(OH)2·(AlxFe1-x)2(SO4)3·26H2O(s)) ends as gypsum is
exhausted. Subsequently, calcium monocarbonate (3CaO·(AlxFe1-
x)2O3·CaCO3·11H2O(s)) and hydrotalcite (Mg4Al2(OH)14·3H2O) start forming. Calcite is
slowly consumed due to the formation of monocarbonate. After hydration time of one
month and longer, the solid paste is mainly composed of C-S-H, portlandite, ettringite
and monocarbonate. With the exception of ettringite, the amount of hydration products
continues to slowly increase with time.
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