Quantification of amorphous siliceous fly ash in hydrated blended cement pastes by X-ray powder diffraction

Abstract

X‐ray powder diffraction (XRPD)‐based quantitative phase analysis is a common technique for studying the hydration of cementitious systems. Hydrated cements often comprise several amorphous or nanocrystalline phases. This paper presents a protocol for the quantification of amorphous siliceous fly ash in hydrated cement using XRPD based on the Rietveld PONKCS (partial or no known crystal structure) method. The protocol is validated by comparison against independent measurements, such as Ca(OH)2 content by thermogravimetry and isothermal calorimetry to evaluate the fly ash degree of reaction. A sensitivity analysis of the protocol was carried out to test the robustness of the results with regard to sample preparation, data collection strategies and refinement model parameters. The key sensitive aspects of the protocol are (i) the preservation and preparation of the hydrated cement sample for XRPD measurement, (ii) the selection of a 2gθ angular range for the Rietveld analysis that avoids low‐angle scattering interferences, and (iii) the use of the peak profile to account for the contribution of other amorphous phases such as the diffraction pattern of nanocrystalline calcium silicate hydrate (C–S–H). The results show good accuracy in terms of quantification if the initial fly ash content is more than 10 wt%. Example TOPAS fly ash and C–S–H codes, as well as the raw XRPD data set, are provided.

Elucidation of the Hydration Reaction of UHPC Using the PONKCS Method

“…Based on this method, the scale factor, virtual Miller constant, and corresponding intensity of silica fume were probabilistically calculated to generate the hypothetical space group for the diffraction pattern of silica fume, and a similar method was used to generate the C-S-H pattern. The XRD pattern obtained from the OPC cured for 7 years was used after excluding peak contributions from the known crystalline phases [ 36 ]. Then, the fitted patterns of the silica fume and C-S-H were used for the quantitative phase analysis of the UHPC to evaluate the weight variation of the two amorphous phases.…”

“…For the PONKCS method and subsequent refinement processes, the background was excluded by using a polynomial function of an order less than 5. Previous studies indicated that inaccurate fitting may occur when a low angle range is included in the PONKCS method or Rietveld analysis [ 36 ]. In the present study, the refinement was performed from 8˚ 2θ degrees, considering the presence of ettringite.…”

“…However, it also may have suggested that the crystallinity between C-S-H due to OPC hydration and pozzolanic reaction can be different. This potential difference could impact the accuracy of fitting C-S-H with the PONKCS method, which only relied on the C-S-H pattern subtracted from OPC hydration at 7 years [ 36 ].…”

In-situ laboratory X-ray diffraction applied to assess cement hydration

“…This would require either the use of a high-order polynomial or a high starting angle. For example, Li et al [109] quantified the presence of fly ash in hydrated pastes using powder and slice samples. The authors observed strong low-angle scattering, so the best XRD pattern fitting was obtained for the starting angle of 13.5° 2θ (CuKα).…”

“…The fact that the amorphous/nanocrystalline structure of C-(A)-S-H yields a diffuse scattering signal in XRD is well known in the literature, and different approaches were used to describe its contribution in powder, slice, and fresh samples. Snellings and co-authors [103,109] created a C-S-H model based on a set of four pseudo-Voigt (PV) peaks obtained from a seven-year-old hydrated white cement paste. Bergold et al [24] created a hkl phase using a Pawley range to fit a fully-hydrated C3S sample (containing only portlandite and C-S-H) with the crystal structure of 14 Å tobermorite (F2dd space group) from [112] as a starting point, obtaining the refined lattice parameters a = 11.81906 Å; b = 7.07097 Å; c = 58.92196 Å.…”