Karen Scrivener and Patrick Juilland
Ecole Polytechnique Federale de Lausanne, Switzerland
Early hydration of alite, in particular the reason for the onset of the induction period, have been a subject of controversy for many decades. Several theories have been proposed, principally the formation of a protective membrane or delayed nucleation and growth, but none have successfully taken into account all the experimental data available. This presentation proposes a new mechanism, based on a geochemical approach to crystal dissolution, that fully explains the origin of the induction period. It implies that during cement hydration, dissolution is initially dominated by the formation of etch pits on surfaces and later becomes limited to step retreat from such pits. This change in mechanism alone can account for the rapid decrease in reaction after fist contact with water, without the need to invoke the formation of a protective membrane. Furthermore it can explain all the experimental findings in the literature. While this geochemical view of dissolution explains many features of the induction period it does not account for its end. For this it is found that the most probable explanation appears to be the onset of the nucleation of CH or the transformation of C-S-H facilitating its growth.