Schweitzer – The Mechanism of the Induction Period in Cement Hydration Inferred from Nuclear Resonance Reaction Analysis
J.S. Schweitzer1(jeff.schweitzer@uconn.edu) with R. A. Livingston2, J. Cheung3, C. Rolfs4, H.-W. Becker4, S. Kubsky5, T. Spillane1, J. Zickefoose1, S. Brendle6
1 University of Connecticut, Storrs, CT, USA; 2 University of Maryland, College Park, MD, USA; 3 WR GRACE, Cambridge, MA, USA; 4 Ruhr-Universität Bochum, Bochum, Germany; 5 Synchrotron SOLEIL, Saint-Aubin, Gif-sur-Yvette CEDEX, France; 6 Delft University of Technology, Delft, The Netherlands
A better understanding of the mechanisms and kinetics of cement hydration during the induction period is critical to improved concrete technology. The hydration reactions on cementitious materials proceed from the grain surfaces into the grain. This hydration profile can be studied nondestructively with nuclear resonance reaction analysis (NRRA) by using the 15N(p,α,γ)12C reaction to measure the distribution of hydrogen with depth with a spatial resolution of a few nanometers. Time-resolved measurement of the hydration profile is achieved by stopping the chemical reactions at specific times. The diffusion gradient of the hydrogen cannot be represented by a simple Fickian profile, indicating that multiple diffusion and reaction processes are occurring. The end of the induction period is indicated by a major change in the shape of the hydrogen depth profile. This allows the time for the end of the induction period to be determined to better than a few percent. The mechanism controlling the induction period of tricalcium silicate is the rapid development of a semi-permeable layer that acts as a barrier on the grain surface. This allows Ca2+ ions to exchange with water in the pore solution, but blocks the transport of silicate ion. Consequently, a layer of silicate gel accumulates beneath the surface barrier. The swelling of this silicate layer eventually leads to the mechanical breakdown of the surface barrier, which marks the end of the induction period. The alternate mechanism for the end of the induction period, the precipitation of solid Ca(OH)2, cannot operate in this case because the hydration is carried out in a solution with an excess of solid Ca(OH)2 from the start. The effects of additives, such as accelerators and retarders on the chemical reactions of cement components have been studied.