Livingston – The Transition to Diffusion-Limited Kinetics in Hydrating Cement Pastes
Richard A. Livingston1 (rliving1@umd.edu) with , Norbert Nemes2 and Dan Neumann3
1 Dept. of Materials Science & Engineering, University of Maryland, College Park, MD USA; 2 GFMC, Dpto. Fisica Aplicada III, Universidad Complutense de Madrid, Madrid, Spain; 3 Center for Neutron Research, NIST, Gaithersburg, MD, USA
The hydration kinetics of Portland cement paste shows a characteristic change after few tens of hours from rapid reaction progress in the nucleation and growth period to a slower rate that is proportional to the square root of time. Recent quasi-elastic neutron scattering measurements on hydrating tricalcium silicate pastes made with different water/cement ratios have shown that the transition occurs at the same point in time, but at different values of the reaction progress variable, i.e. the boundwater index. However, these curves can be collapsed into a single master curve by a simple vertical scaling. Renormalization to a reaction progress variable consisting of the boundwater/cement ratio indicates that there is an invariant at which the transition occurs. This is consistent with a free-volume model. During the nucleation and growth period the C-S-H gel growth is dominated by the formation of outer product in the through-solution reaction in the available pore-space. Once a critical gel packing factor has been reached, this reaction is severely restricted, and further growth of the gel occurs as inner product formation due to the topochemical reaction within the cement grain. The rate-limiting step in this type of kinetics is diffusion. However, the exact form of the diffusion model remains uncertain. The concept of a critical packing factor, or contact area, is supported by numerical simulations performed at the Technical University of Delft.