Peter Stemmermann

Characterisation of crystalline C-S-H phases by X-ray photoelectron spectroscopy

Abstract We have prepared a number of crystalline calcium-silicate-hydrate (C-S-H) phases hydrothermally, with calcium–silicon ratios varying from approximately 0.5 (K-phase) to 2.0 (hillebrandite and α-dicalcium silicate hydrate). The phases were then analysed using X-ray photoelectron spectroscopy (XPS). Increasing calcium–silicon ratios resulted in decreased silicon binding energies. Additionally, changes in the O 1s spectra could be explained in terms of bridging (BO) and nonbridging oxygen (NBO) moieties. Finally, the modified Auger parameter has proved particularly useful in determining the extent of silicate anion polymerisation. Of note also are the apparently unusual spectra for 11 A tobermorite. The silicon and oxygen photoelectron spectra indicate a phase with a lower degree of silicate polymerisation than predicted from its composition. The main contributing factor is the intrinsic disorder within the tobermorite structure. This study has shown how XPS may be used to obtain valuable structural information from C-S-H phases, and our analysis of the crystalline phases is the first step towards the analysis of real C-S-H-based cement systems.

X-ray photoelectron spectroscopy of the cement clinker phases tricalcium silicate and β-dicalcium silicate

Abstract We have used X-ray photoelectron spectroscopy (XPS) to investigate both tricalcium silicate (Ca3SiO5, C3S) and β-dicalcium silicate (Ca2SiO4, β-C2S), the principal components of cement clinkers. In addition to showing how the two phases may be characterised and differentiated, we show how the sensitivity of these phases to atmospheric carbon dioxide and moisture may, as a result of improper sample preparation, lead to erroneous results. The observed alteration processes of the clinker minerals shed light upon the aging process of cement clinker during storage.

An unusual phase transition Ca2[SiO3(OH)](OH) – Ca6[Si2O7][SiO4](OH)2 (dellaite) as revealed by single crystal IR and X-ray powder diffraction

Cement is an inorganic hydraulic binder widely used in civil engineering. By the reaction of cement with water calcium-silicate-hydrate (C-S-H) gel is the principal hydration product [1]. For example, buildingmaterials based on Portland cement could contain up to 70 wt% C-S-H gel. Therefore, the structure of the C-S-H gel is responsible for the mechanical properties of the hardened cement paste. The chemical compostion of C-S-H gel varies between the molar CaO/SiO2 (C/S) ratio of 0.5 in older and partly carbonated hardend cement paste and 2.2 in fresh ones. Reaction products (alkali containing C-S-H gels) of the deteriorative alkali-silica-reaction (ASR) show an even lower Ca content. Additionally, these gels have a significant amount of alkalis. Their formation processes and the reaction mechanism of ASR are still unknown and a subject of investigation. To get insights into the structure and formation processes of alkali-C-S-H gels, samples with differerent chemical compositions have been synthesized with the agate-ball milling technique. A part of the gels was converted into their crystalline analogues by hydrothermal treatment. Due to their low crystallinity the gels were investigated with complementary spectroscopic and diffraction methods. Synchrotron based measurements at the diffraction-, XAFSand IR-beamlines at ANKA (Angstromquelle Karlsruhe) were performed. By comparison of the gels and crystalline alkali-C-S-H with XAFS measurements at the Ca absorption edge, it turned out that the Ca environment is stronger affected upon hydrothermal treatment in Ca poor samples than in samples with a higher C/S ratio. Similar results for theK environment were achieved fromXAFS measurements at the K absorption edge. Data from IR spectroscopy show distinct changes in the region of Ca polyhedra vibrations around 240 cm [2]. This result is in good agreement with the XAFS measurements. To point out the role of water in the gels structure the dehydration of C-S-H and alkali-C-S-H was followed in situ with IR-microscopy and the total water loss was analyzed with DTA/TG. The IR data reveal a structural reorganization of the gel. Especially the bands in the Si-O-Si stretching region show distinct changes during the drying process whereas bands attributet to Ca polyhedra vibrations remain largely unchanged.

Raman and IR spectroscopic study of nano-crystalline calcium silicate hydrates of type C-S-H(I)

Nano-crystalline calcium silicate hydrates (C-S-H gels) are formed upon hydration of Ca3SiO5 and β-Ca2SiO4, and are therefore the primary hydration products of Portland cements. Raman and IR spectra of a series of mechanochemically prepared C-S-H samples of type C-S-H(1) with Ca/Si ratios ranging from 0.4 to 1.5 reveal changes in structure dependent upon Ca/Si ratio. Finite silicate chains (Q) dominate the structures of the samples at Ca/Si ratios 0.4-1.0, the spectra showing characteristic symmetrical stretching bands between 1010 and 1020cm (R) and asymmetrical stretching around 965cm (IR), with Q units present at Ca/Si 1.00, dimers (Q) are the main building unit of the silicate anionic structure (Raman bands at 890cm, IR band at 945cm). In these samples portlandite is also observed. The similarity of the silicate contribution to the spectra (R and IR) of the samples with Ca/Si = 1.33 and 1.50, and their differing portlandite contents, suggests a similar limit of incorporation of Ca into the C-S-H(I) structure in both phases, which lies under Ca/Si=1.33. In these phases we observe increased ordering of Ca-bonded OH groups. The same trend of increased ordering of the Ca environment is also seen in the development of the intensity and FWHM of the Ca-O band at 332cm (R), 255cm (IR) which is consistent with observations in the Ca2p XPS spectra [2].

Calcium silicate hydrates: importance in disposal of nuclear wastes

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s58 Acta Cryst. (2007). A63, s58 MS23 O1 Calcium silicate hydrates: Importance in disposal of nuclear wastes Peter Stemmermann, Krassimir Garbev, Biliana Gasharova Institut für Techische Chemie (ITCTAB), Forschungszentrum Karlsruhe. b Institut für Synchrotronstrahlung (ISS), Forschungszentrum Karlsruhe. E-mail: peter.stemmermann@itc-tab.fzk.de

Examinations of the structure of highly polymerized alkali containing C-S-H phases

Cement is an inorganic hydraulic binder widely used in civil engineering. By the reaction of cement with water calcium-silicate-hydrate (C-S-H) gel is the principal hydration product [1]. For example, buildingmaterials based on Portland cement could contain up to 70 wt% C-S-H gel. Therefore, the structure of the C-S-H gel is responsible for the mechanical properties of the hardened cement paste. The chemical compostion of C-S-H gel varies between the molar CaO/SiO2 (C/S) ratio of 0.5 in older and partly carbonated hardend cement paste and 2.2 in fresh ones. Reaction products (alkali containing C-S-H gels) of the deteriorative alkali-silica-reaction (ASR) show an even lower Ca content. Additionally, these gels have a significant amount of alkalis. Their formation processes and the reaction mechanism of ASR are still unknown and a subject of investigation. To get insights into the structure and formation processes of alkali-C-S-H gels, samples with differerent chemical compositions have been synthesized with the agate-ball milling technique. A part of the gels was converted into their crystalline analogues by hydrothermal treatment. Due to their low crystallinity the gels were investigated with complementary spectroscopic and diffraction methods. Synchrotron based measurements at the diffraction-, XAFSand IR-beamlines at ANKA (Angstromquelle Karlsruhe) were performed. By comparison of the gels and crystalline alkali-C-S-H with XAFS measurements at the Ca absorption edge, it turned out that the Ca environment is stronger affected upon hydrothermal treatment in Ca poor samples than in samples with a higher C/S ratio. Similar results for theK environment were achieved fromXAFS measurements at the K absorption edge. Data from IR spectroscopy show distinct changes in the region of Ca polyhedra vibrations around 240 cm [2]. This result is in good agreement with the XAFS measurements. To point out the role of water in the gels structure the dehydration of C-S-H and alkali-C-S-H was followed in situ with IR-microscopy and the total water loss was analyzed with DTA/TG. The IR data reveal a structural reorganization of the gel. Especially the bands in the Si-O-Si stretching region show distinct changes during the drying process whereas bands attributet to Ca polyhedra vibrations remain largely unchanged.