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Ind. Eng. Chem. Res. 2008, 47, 5456–5463
Correlation of Cement Performance Property Measurements with C3S/C2S Ratio Determined by Solid State 29Si NMR Measurements Christopher L. Edwards,†,‡ Rickey Morgan,‡ Lewis Norman,‡ Gary P. Funkhouser,‡ and Andrew R. Barron*,†,§,| Department of Chemistry, Rice UniVersity, Houston, Texas 77005; Halliburton Energy SerVices, Duncan, Oklahoma 73536; Department of Mechanical Engineering and Materials Science, Rice UniVersity, Houston, Texas 77005; and Energy and EnVironmental Systems Institute, Rice UniVersity, Houston, Texas 77005
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 (C3S/C2S) as determined by magic angle spinning (MAS) 29Si nuclear magnetic resonance (NMR) experiments. XRF-derived oxide analysis appears to provide a lower C3S/C2S ratio than determined by NMR analysis. Furthermore, oxide analysis suggests that all the cements have a C3S/C2S ratio of 2-5, while our NMR method suggests the actual range is significantly broader. Determination of C3S/C2S ratios by NMR provides an effective method of analysis for cements, owing to NMR’s direct measurement of the minerals in question. NMR C3S/C2S 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 C3S/C2S ratio, i.e., C3S 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 C3S/C2S ratio and the 72 h crush strength. The lack of dependency of either thickening time or Young’s modulus to the C3S/C2S 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. Introduction Cement hydration is a very complex process, not solely dictated by simple solvation and reprecipitation. The prediction of the set kinetics, reaction to inhibitors, and the properties of the set cement is presently not available. Instead of prediction, a series of experiments must be performed on each sample to obtain the property information required. It would be desirable to have a simple analytical method (or short series of methods) that would allow for the prediction of the chemistry of individual samples of cement from various sources. In this regard an attempt was made to correlate data from a wide range of spectroscopic and analytical techniques with experimental data obtained by traditional methods. The ultimate goal of this project is the development of a tool for the prediction of the hydration chemistry of various cements based upon their chemistry. Various attempts in the past have been made to correlate engineering performance properties with more conventional spectroscopic analyses. Barnes and co-workers were able to identify several key factors to the role of gypsum in preventing “flash set” by means of time-resolved in situ X-ray diffraction.1 Parrott and co-workers applied a series of different tests, including quantitative X-ray diffraction and conduction calorimetry, to attempt correlations with degree of hydration.2 Vlachou and Piau conducted a study of additive response and * To whom correspondence should be addressed. E-mail: arb@ rice.edu. † Department of Chemistry, Rice University. ‡ Halliburton Energy Services. § Department of Mechanical Engineering and Materials Science, Rice University. | Energy and Environmental Systems Institute, Rice University.
its correlations with SEM.3 Ghosh and Handoo reviewed the progress in using infrared and Raman spectroscopies to monitor cement hydration behavior.4 Coveney, Fletcher, and co-workers were able to build on the existing infrared knowledge base by applying artificial neural net (ANN) analysis to diffuse reflectance (DRIFTS) spectra of cements.5,6 We have recently reported that for the MAS 29Si NMR of cements the longitudinal relaxation time T1 of C3S and C2S is significantly altered because of the paramagnetic Fe3+ present in the C4AF matrix that makes intimate contact with each of the silicate phases.7 The presence of the paramagnetic C4AF matrix thus allows for the rapid collection of spectra of cements. Furthermore, the relative concentration of tricalcium silicate (C3S) and dicalcium silicate (C2S) within a cement sample (i.e., C3S/C2S ratio) can be determined using the saturation recovery method, offering the best combination of accuracy, speed, and ease of processing. Our studies showed that the peak intensities are dependent on the relaxation times (τ). As the relaxation time is increased during the 29Si NMR saturation recovery experiment, the volume fraction of the silicate particle (crystallite) being “observed” increases until the entire particle is being sampled (Figure 1). Unfortunately, deconvolution analysis of the resulting spectra does not provide an absolute concentration or weight percentage. However, the processed peak areas attributed to the C2S and C3S content do allow for an accurate derivation of the C3S/C2S ratio.7 Using the data from the longest relaxation times (i.e., after the intensity curve in Figure 1 plateaus) will provide the relative concentrations of C3S and C2S. Thus, the C3S/C2S ratio can be calculated by a consideration of the intensity ratio at τ values above which the slope in the plot of intensity of the various saturation recovery parameters
10.1021/ie8000925 CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5457 Table 1. Mineral Composition of Portland Cements from XRF and NMR Analysis from XRF cement BRZ CAP ELT RUS SDN THA TXI
from NMR
C3S
C2S
C3A
C4AF
C3S/C2S
C3S/C2S
57.77 54.90 61.40 54.49 52.13 57.46 55.99
16.23 22.40 13.14 22.04 24.89 21.78 22.13
5.31 0.00 0.00 2.65 2.68 1.37 5.72
9.87 18.10 18.93 12.45 12.55 13.62 9.74
3.55 2.45 4.67 2.47 2.09 2.63 2.53
8.82 4.95 5.85 6.34 5.00 5.37 3.07
Table 2. Consistometry Data for Cements without Additives, with Retarder, and with Fluid Loss Additive
Figure 1. Scaled signal intensity for C3S (9) and C2S (0) in a typical Portland cement (CAP) as a function of NMR relaxation (τ).
plotted versus τ is essentially 0 (e.g., τ g 1 s in Figure 1). Determination of C3S/C2S ratios by NMR provides an effective method of analysis for cements, owing to NMR’s direct measurement of the minerals in question without the assumptions of composition that are required with oxide analysis from X-ray fluorescence (XRF) and application of the Taylormodified Bogue equations.8 In a typical series of experiments that must be performed on a given cement sample to match it to the specifications of a particular cementing job, there are four key measurements: thickening time determination in a consistometer, ultrasonic analysis of compressive strength development, Young’s modulus, and crush strength (the latter two obtained from destructive mechanical testing). However, irrespective of the final analytical process, the Portland cement samples must be hydrated and set while measurements are performed. If there is to be an application of NMR as a tool for guiding downhole applications of cement, then it is necessary to demonstrate whether there exist predictive correlations and for which parameters. Herein we have investigated the existence of correlations between the C3S/C2S ratio, as obtained by the MAS 29Si NMR saturation recovery method, with a range of downhole simulation performance parameters typically measured for cements. Experimental Section In the present study seven different cements were studied. Each sample was stored in a drybox under argon prior to NMR analysis. Three of the samples are API Class G cements from foreign countries: Brazil (BRZ), Russia (RUS), and Thailand (THA). The remaining four samples are API Class H cements from domestic cement manufacturers: Texas Industries (TXI), El Toro (ELT), Southdown (SDN), and Capitol (CAP). Class G and H cements both use the same specifications for elemental composition and restrict grinding aids to gypsum but differ with respect to fineness, with class G cements being the more finely ground of the two. All cements were obtained from Halliburton Energy Services, Duncan, OK. All materials were used as received. XRF mineral analysis for each cement sample is summarized in Table 1. Physical Measurements. Cement slurries are mixed by independently weighing each component, dry blending the dry additives into the cement powder, and mixing the water and wet additives in a blender at low shear. The cement is then added to the liquid under low to moderate shear within 10-15 s. The mixture is then blended under maximum shear conditions for 35 s, with the operator scraping the sides of the blender to incorporate caked cement powder on the sides of the vessel.
sample
retardera
fluid loss additiveb
BRZ
no yes no no yes no no yes no no yes no no yes no no yes no no yes no
no no yes no no yes no no yes no no yes no no yes no no yes no no yes
CAP ELT RUS SDN THA TXI
time to 50 Bc (min)c
time to 70 Bc (min)c
time to 100 Bc (min)c
53 66 51 78 90 73 90 155 94 69 78 75 80 105 78 54 78 45 78 124 91
56 72 61 95 98 74 120 168 98 71 87 81 96 128 82 65 83 70 87 137 92
63 78 70 108 107 75 134 180 102 79 93 85 110 139 84 73 92 78 98 147 94
a Lignosulfonate retarder at a loading of 0.5% bwoc. b Copolymer of N,N-dimethylformamide and 2-acrylamido-2-methylpropanesulfonic acid. c Bearden consistency units, proportional to the torque on the paddle in the rotating cup of the consistometer.
For the purpose of evaluating a manageable data set, three different cement formulations were prepared with each cement sample. All cement formulations are based on a nominal 16.4 lb/gal slurry achieved by mixing cement with 39.4% water by weight of cement (bwoc). The first slurry considered was prepared without additives. The second formulation added a lignosulfonate retarder at a loading of 0.5% bwoc. The third and final formulation added to the base cement a copolymer of N,N-dimethylformamide and 2-acrylamido-2-methylpropanesulfonic acid, a fluid loss additive used in oil well cement formulations, at a loading of 0.15% bwoc. Thickening time, i.e., the time between initial mixing and an apparent viscosity of 70 Bearden consistency units (Bc) at a given temperature (
