²⁹Si Enrichment and Selective Enrichment for Study of the Hydration of Model Cements and Blended Cements

Abstract

²⁹Si enrichment has been used to enable acquisition of multiple ²⁹Si MAS and CP-MAS NMR spectra from samples of C₃S hydrating in situ in a MAS NMR probe. Data with excellent signal-to noise ratios were obtained at 20, 50, and 75°C, with minimal use of spectrometer time, and without the need for quenching of multiple samples; the data were however consistent with those obtained conventionally. The bulk hydrate initially produced was dimeric; at later stages of reaction, polymerization occurred. Arrhenius energies of 35 and 100 kJmol⁻¹ were calculated for formation of the two products. A model of hydration in which dimeric silicate units are linked together by monomers was found to be consistent with the observed results. The pozzolanic reaction with C₃S and SiO₂ has also been studied with enrichment of both starting materials — the hydrate produced is more ordered and of longer chain length than that produced by hydration of C₃S alone. By enrichment of the SiO₂ only it was possible selectively to observe hydrate which contains silicate species which originate from the SiO₂; this material had a much higher degree of order, and a much longer chain length than the remainder of the hydrate. Thus isotopic enrichment and especially selective isotopic enrichment are valuable tools for the study of cementitious systems.