Angeles G. De la Torre

The superstructure of C3S from synchrotron and neutron powder diffraction and its role in quantitative phase analyses

We have synthesised the room temperature MIII form of alite stabilised by doping with Mg and Al. The complex disordered superstructure of this tricalcium silicate [Ca3SiO5 (C3S)] sample has been studied by a joint Rietveld refinement of ultra-high-resolution synchrotron X-ray powder diffraction data, medium-resolution neutron powder diffraction data and soft constraints of interatomic distances. Alite crystallises in a monoclinic cell with dimensions a=33.1078(6) A u , b=7.0355(1) A u , c=18.5211(4) A u , b=94.137(1)� and V=4302.9(2) Au 3 . The final R factors were RWP=8.76% and RF(C3S)=3.45% for the synchrotron data and RWP=6.09% and RF(C3S)=5.10% for the neutron data. The reported superstructure is simpler than those previously reported, and it fits properly to a variety of Portland clinker and cement patterns. The Rietveld analyses of four clinkers with variable Mg contents have shown that the refinements are good. The Bogue approach gave quite poor results when compared to these state-of-the-art powder diffraction analyses. Bogue method slightly underestimates the C3S+C2S content, overestimates the C3A fraction and underestimates the C4AF content. Similar analyses of Portland cements with nine crystalline phases are shown to be feasible. D 2002 Elsevier Science Ltd. All rights reserved.

Structure and microstructure of gypsum and its relevance to Rietveld quantitative phase analyses

~Received 2 December 2003; accepted 8 March 2004!Single crystals of gypsum were studied in a diffractometer equipped with a CCD two-dimensionaldetector. The microstructure of the crystal gave wide poorly shaped spots showing sometimescurved streaks around the spots, which made the integration process very difficult, yielding a lowquality structure. The crystal structure and microstructure of gypsum has been studied byhigh-resolution synchrotron powder diffraction of a ground single crystal. The intensities in thesynchrotron powder pattern can be reliably fitted although the peak shape displays anisotropic peakbroadening. The Rietveld results for gypsum were a56.52291(3) A, b515.19763(9) A, c56.52291(3) A, b5118.479(1)°,V5494.536(5) A

Quantitative analysis of mineralized white Portland clinkers: The structure of Fluorellestadite

Fluorellestadite, Ca 10 (SiO 4 ) 3 (SO 4 ) 3 F 2 , has been synthesized as single phase. This compound crystallizes in the apatite type structure, s.g. P6 3 /m, with parameters a =9.4417(1) A, c =6.9396(1) A and V =535.76(1) A 3 . The refinement of its crystal structure converged to R WP =12.33% and R F =4.58%. The atomic parameters have been used to analyze the phase content of mineralized white Portland clinkers. These clinkers contain Ca 3 SiO 5 , Ca 2 SiO 4 , Ca 12 Al 14 O 32 F 2 and Ca 10 (SiO 4 ) 3 (SO 4 ) 3 F 2 . The agreement between the elemental composition inferred from the Rietveld phase analysis and that measured by XRF is noteworthy. This comparison does not take into account the presence of amorphous phases and unmodeled elemental substitutions in crystalline phases. Similar Rietveld studies on commercial white Portland clinkers are also shown to be feasible.

A new family of oxide ion conductors based on tricalcium oxy-silicate

Tricalcium oxy-silicates, Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01, have been prepared as crystalline single phases. Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01 have triclinic and monoclinic structures, respectively. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. At 1023 K, the oxide transport numbers range between 0.97 and 0.85 from reducing (dry 5%-H2-Ar/air gradient) to oxidizing (O2/air gradient) conditions in the 1023-1173 K interval. The thermal analyses showed a large weight loss on heating due to the presence of water in the materials. The monoclinic compound has ionic conductivities higher than those of the triclinic stoichiometric oxy-silicate, as expected due to the introduction of oxide vacancies. Typical total conductivities for these un-optimised solids are 10(-5)-10(-4) S cm(-1) at 1100 K. These compounds may contain a small amount of water, approximately 0.05 H2O moles per chemical formula, and they display an important proton contribution under a humidified atmosphere.

Rietveld quantitative phase analysis with molybdenum radiation

Building materials are very complex samples of worldwide importance; hence quantitative knowledge of their mineralogical composition is necessary to predict performances. Rietveld quantitative phase analysis (RQPA) allows a direct measurement of the crystalline phase contents of cements. We highlight in this paper the use of laboratory X-ray powder diffraction (LXRPD) employing high-energy radiation, molybdenum (Mo), for attaining the RQPA of cements. Firstly, we evaluate the accuracy of RQPA employing a commercial calcium sulfoaluminate clinker with gypsum. In addition to Mo Kα 1 and Mo Kα 1,2 radiations, Cu and synchrotron patterns are also analyzed for the sake of comparison. Secondly, the assessment of the accuracy of RQPA results obtained using different radiations (synchrotron, Mo, and Cu) and geometries (reflection and transmission) is performed by analyzing two well-known commercial samples. As expected, for LXRPD data, accuracy in the RQPA results improves as the irradiated volume increases. Finally, three very complex aged hydrated cements have been analyzed using Mo K α 1 -LXRPD and Synchrotron-XRPD. The main overall outcome of this work is the benefit for RQPA of using strictly monochromatic Mo Kα 1 radiation. Best laboratory results arise from Mo Kα 1 data as the effective tested volume is much increased but peak overlapping is not swelled.

Powder diffraction analysis of gemstone inclusions

Gemstones are pieces of materials that once cut and polished are used as jewels or adornments. Gemstones may be single crystal such as diamonds , polycrystalline such as lapis lazuli , or amorphous such as amber . In any case, gems may have inclusions that may yield a variety of optic effects. It is also important to unravel the crystal structure of the inclusion s in order to determine the origin of the gem and to help to understand their formation mechanism. Here, we expand the use of powder diffraction to identify crystalline inclusions in bulk gemstones highlighting Mo K radiation to penetrate within compact gems. Initially, rock crystal quartz with rutile needles was investigated and rutile diffraction peaks were more conspicuous in the Mo pattern than in the Cu pattern. Next, rock crystal quartz with beetle legs was characterized and the red iron oxide inclusion was identified as hematite. The study of a fake gem, glass showing aventurine effect, gave the diffraction peaks of metallic copper. Later, polycrystalline gems, moss agate, and aventurine quartz were also studied. The powder patterns of these compact gemstones could be successfully fitted using the Rietveld method. Finally, we discuss opportunities for further improvements in laboratory powder diffraction to characterize inclusions in compact gems.

Multiscale understanding of tricalcium silicate hydration reactions

Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore, PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched as recently predicted by first principles calculations. These outcomes, plus additional laboratory characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed densities and Ca/Si atomic ratios at the nano- and meso- scales.

Understanding cement hydration by pair distribution function and Rietveld analyses

There are many commercially important multiphase materials which contain amorphous/nanocrystalline phases, such as cement pastes, porcelains, or pharmaceutical compounds. The analysis of amorphous/nanocrystalline phase(s) within cement matrices that contain high amounts of crystalline phase(s) is very challenging. Synchrotron techniques can be very useful to characterize such complex samples [1]. Here, we report measurements of total scattering data quantitatively analyzed by Pair Distribution Function (PDF) and Rietveld methodologies to determine nanocrystalline and microcrystalline phase contents. Furthermore, laboratory techniques (laboratory powder diffraction using internal standard, thermal analysis, and magic-angle-spinning nuclear magnetic resonance) were also used to complement the sample characterization.

Hydration Study of Synthetic Yeelimite using LXRPD and SXRPD

XRPD is a powerful tool for material characterization in general, and for in-situ studies of chemical processes in particular. The use of an intense X-ray source, .i.e. synchrotron X-rays, coupled with fast X-ray detection permits time-resolved diffraction experiments allowing in-situ quantitative phase analysis during the early ages of cement hydration. Calcium sulfoaluminate, CSA, cements may have variable compositions, but all of them contain high amounts of ye’elimite, Ca4Al6O12SO4. Commercial CSA cements have special applications such as high strength developments at early-ages. Ye’elimite is very reactive and most of its hydration heat is released during the first eight hours of hydration . The aim of this work is to better understand the early age hydration of stoichiometric (orthorhombic) and doped (pseudo-cubic) ye’elimite samples. The parameters studied by SXRPD, LXRPD and calorimetry have been: polymorphism; water/ye’elimite ratio; and sulfate (gypsum and anhydrite) contents. This work has allowed establishing mechanisms and kinetics for hydration of ye’elimite samples by in-situ SXRPD with internal standard methodology. Moreover, pastes were also studied by ex-situ LXRPD with the external standard method, G-factor, at 2 and 7 days. Both strategies were able to quantify the amorphous contents, including free water. It is important to highlight that the results obtained at early ages, by the internal standard method, are in agreement with those obtained at later ages, G-method, showing the consistence and complementarity of both methodologies. The hydration of stoichiometric ye’elimite in the presence of gypsum is strongly hastened, when compared to the hydration process without gypsum. However, the presence of gypsum has a little effect in the hydration of doped ye’elimite. Moreover, anhydrite has also accelerated the hydration of stoichiometric ye’elimite, although its lower solubility has provoked the formation of an intermediate phase in the first hours.