Characterization of the induction period in tricalcium silicate hydration by nuclear resonance reaction analysis
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J.S. Schweitzer Department of Physics, University of Connecticut, Storrs, Connecticut 06269-3046
C. Rolfs, H-W. Becker, and S. Kubsky Institut fu¨r Physik mit Ionenstrahlen, Ruhr-Universita¨t Bochum, Bochum, Germany (Received 16 June 2000; accepted 11 December 2000)
Nuclear resonance reaction analysis has been applied for the first time to measure the development of the hydrogen depth profile in the early stages of hydration of tricalcium silicate using the 1H(15N,␣␥)12C reaction. The surface layer had an H concentration and thickness consistent with a few unit cells (1.1 nm) of tobermorite-like material. The inner regions exhibited diffusion-controlled growth with time until the hydrogen concentration approaches that of the surface layer at 4.25 ± 0.07 h. This event marked the end of the induction period and the onset of the rapid hydration reaction period.
I. INTRODUCTION
Although Le Chatelier discovered the basic process of Portland cement hydration over a century ago,1 many details, particularly those about the kinetics of the reaction, remain uncertain. We report here the first use of nuclear resonance reaction analysis (NRRA) to measure the hydration depth profile and its development over the induction period. The cementitious properties result from the reaction between water and Portland cement powder, which yields the products of calcium–silicate–hydrate (C–S–H) gel, the binder in concrete, along with calcium hydroxide.2 [In cement nomenclature, C ⳱ CaO, S ⳱ SiO2, and H ⳱ H2O. Therefore, CH ⳱ calcium hydroxide, C3S ⳱ tricalcium silicate, etc. The hyphens in C–S–H indicate variable stoichiometry.] The equation for the hydration reaction is usually given in terms of tricalcium silicate, the main reactive constituent: Ca3SiO5 + zH2O → CaxSi共OH兲y ⭈ nH2O + 共3 − x兲Ca共OH兲2 .
(1)
The calcium/silicate ratio in the C–S–H gel, x, the hydroxyl content, y, and the interlayer water content, n, all vary over the course of the reaction. The values of y and n remain uncertain. Thermal analysis data show that the kinetics of this reaction are very nonlinear. After a brief period of rapid reaction on the order of minutes, the rate slows down significantly for an interval of several hours, reJ. Mater. Res., Vol. 16, No. 3, Mar 2001
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ferred to as the induction period. During this time, the cement/water mixture remains relatively fluid. At the end of the induction period, the reaction rate accelerates rapidly. By this time, the cement has hardened and can no longer be worked. Portland cement is actually a mixture of several calcium silicate and calcium aluminate phases, each having different hydration reaction kinetics. However, the main constituent in ordinary Portland cement is tricalcium silicate (C3S). Moreover, the reaction between pure C3S and water shows a distinct induction period. Consequently, in cement chemistry research C3S is often used as a simplified model for the more complex Portland cement. Recently the application of quasi-elastic neutron scattering
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