Dissolution theory applied to the induction period in alite hydration

The early hydration of alite, in particular the reason for the onset of the induction period, has been a subject of controversy for many decades. Several theories have been proposed, principally the formation of a protective phase inhibiting dissolution or delayed nucleation and growth, but none have successfully taken into account all the experimental data available. This paper 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 first contact with water, without the need to invoke the formation of a protective phase. 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. This remains a question for further research, but the most probable explanation appears to be the onset of rapid growth of C–S–H.