Upon contact of CAC with water, a small amount of monocalcium monoaluminate (CA) dissolves, which is followed by a dormant period . Several models were proposed to explain the occurrence of a dormant phase. All models propose a change in the surface chemistry of the cement particles . According to Fujii et al. , a superficially hydrated layer grows on the CA particle surface and prevents further dissolution. After a critical thickness has been reached, the stress of intruding water molecules destroys the layer. In the model of Goetz-Neunhoeffner , a dense impermeable Al x (H 2 O) y layer is proposed which prevents the calcium ion diffusion into the pore solution. Bertrandie and Barret postulated the transformation of AlO - 2 sites on the CA particle surface into [Al(OH) 4 ] -ions. This process would form an inert, superficially hydroxylated layer with the composition Ca[Al(OH) 4 ] 2 immediately after water addition around the CA grain (CA sh ). The renewed CA-dissolution and precipitation of hydrate phases mark the end of the dormant period and the beginning of the CAC main reaction. After reaching its maximum, the deceleration period sets in, mainly characterized by the hydration of dicalcium monoaluminate (CA 2 ), the second hydraulic CAC phase. Using synthetic CA/CA 2 -mixes, Klaus et al. and Hüller et al. demonstrated that the hydration of CA 2 begins once the hydration rate of CA has reached its maximum. Even very low CA-contents (5 wt.%) were sufficient to retard the hydration of CA 2 . This retarding effect of CA was also reported for C 3 S and C 6 AF 2 in synthetic mixes of the pure phases. The authors attributed this retarding effect to high calcium and aluminum concentrations in the pore solution resulting from initial CA dissolution without precipitation of hydrate phases.