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The physicochemical and engineering performance properties of several API class G and H ordinary Portland cements (OPCs) from various foreign and domestic sources have been investigated in comparison with the tricalcium silicate/dicalcium silicate ratio (C 3S/C 2S) as determined by magic angle spinning (MAS) 29Si nuclear magnetic resonance (NMR) experiments. XRF-derived oxide analysis appears to provide a lower C 3S/C 2S ratio than determined by NMR analysis. Furthermore, oxide analysis suggests that all the cements have a C 3S/C 2S ratio of 2−5, while our NMR method suggests the actual range is significantly broader. Determination of C 3S/C 2S ratios by NMR provides an effective method of analysis for cements, owing to NMR’s direct measurement of the minerals in question. NMR C 3S/C 2S ratios demonstrate predictive ability for the determination of engineering performance properties. This is especially the case for prediction of strength development; in keeping with generally accepted understanding of cement hydration behavior, the strength development correlates with increasing C 3S/C 2S ratio, i.e., C 3S content. The observed correlation between NMR-derived silicate ratio and strength development holds for cements in the presence of either a retarder (lignosulfonate) or a fluid loss additive ( N,N-dimethylformamide/2-acrylamido-2-methylpropanesulfonic acid copolymer). No significant correlation is observed between C 3S/C 2S ratio and the 72 h crush strength. The lack of dependency of either thickening time or Young’s modulus to the C 3S/C 2S ratio as determined by MAS 29Si NMR measurements suggests that these physical properties are independent of the relative silicate composition. No correlations are observed between any physical property and the silicate ratio derived from XRF data. [ABSTRACT FROM AUTHOR]

The determination of the relative concentrations of tricalcium silicate (C3S) and dicalcium silicate (C2S) within a cement sample is potentially important in the prediction of the setting properties of the cement. For model systems, we have demonstrated that the presence of a paramagnetic species in intimate (physical) contact with an otherwise diamagnetic silicon-containing mineral can dramatically decrease the longitudinal relaxation time T1and consequently allow for the rapid data collection of the 29Si MAS NMR spectrum. Thus, it appears that, for cements, the T1values of C3S and C2S will be significantly altered because of the paramagnetic Fe3present in the C4AF matrix that makes intimate contact with each of the silicate phases. The presence of the paramagnetic C4AF matrix thus allows for the rapid collection of spectra of cements. Experiments that might otherwise take weeks or even months now take mere hours. Three methods of NMR relaxation have been examined and compared:  inversion recovery, variable-delay Bloch decay, and saturation recovery techniques. Analysis of an API class H cement demonstrated that the saturation recovery method for analysis of 29Si NMR data offers the best combination of accuracy, speed, and ease of processing. The saturation recovery method was used to determine the C3S/C2S ratios for eight different cements:  three API class G cements and five API class H samples. Determination of C3S/C2S ratios by NMR spectroscopy provides an effective method of analysis for cements, owing to NMR spectroscopy's direct measurement of the minerals in question without the assumptions of composition that are required with oxide analysis from X-ray fluorescence and application of the Taylor-modified Bogue equations. Given the importance of C3S and C2S (and hence their ratio) in defining the performance of cements, we propose that 29Si MAS NMR spectroscopy represents a viable method for cement characterization. [ABSTRACT FROM AUTHOR]