C-S-H structures

The sorption of Pb(II) to C-S-H has been investigated by Moulin (1999) and Pointeau (2000). In fact XAFS studies indicate that, as for Zn(II), Pb(II) is adsoibed to the silicate chains within the C-S-H structure (Rose et al. 2000). Figure 7 shows sorption isotherms for C-S-H of different Ca Si ratios. It appears that sorption is greater at a lower Ca Si ratio. 

According to Lieber [85], the zincate decomposes and zinc enters the C-S-H structure. Simultaneously the C/S ratio in C-S-H increases and the long fibres are formed. 

Parrott et al. [153] found an exact relationship between the rate of water replacement by methanol and the ratio of pores greater than 50 nm. With lowering of this ratio to the level of about 0.4, the diffusion becomes extremely low (Fig. 5.71) the authors attribute this to the filling of pore chaimels by some amount of hydration products. However, Feldman [138] remarks that there are some questions as the water replacement by methanol is concerned. The reactivity of methanol towards Ca(OH)2 and C-S-H, resulting in the formation of calcium methoxide or a methylated complex compounds was reported by some authors [138,156]. These reactions occur very quickly and a substantial amount of products appears, modifying the paste microstracture. The swelling of paste, observed by Feldman [138] was about 0.1 % after 14 days when the paste absorbed 85 % methanol as water replacement. This effect can be attributed to the intrasion of methanol between the layers in C-S-H structure and the calcium methoxide formatioa At extended distance between C-S-H layers the specific surface of paste markedly increases, as it has been proved by Winslow and Diamond [ 13 5]. In turn, the water replacement by propan-2-ol, re-... 

The main advantage of solid-state NMR spectroscopy in studies of cement-based materials is its ability to detect and quantify amorphous and crystalline phases in an equal manner. This property and the fact that the detected NMR parameters are most sensitive to the local structure, roughly the nearest one to three coordination spheres, makes it a valuable and important supplement to XRD techniques that probe long-range order and crystalline phases. Moreover, the nuclear-spin selectivity often implies that unambiguous structural or quantitative information can be achieved for a subsystem of a complex multicomponent material. For example, Al MAS NMR provides direct information about the aluminium coordination state and, for Portland cement systems, it allows identification of aluminium in tricalcium aluminate, as guest ions in the alite and belite phases, in the AFm and AFt hydration products and as a part of the C-S-H structure, despite a low bulk AI2O3 content of 2-5 wt.% for such systems. 

MAS NMR and MAS NMR have made very important contributions over the last 3 decades to our current knowledge about the structure and composition of C-S-H phases. In that research, the full potential of MAS NMR spectroscopy has not been utilised yet. It is expected that future applications of a range of rf pulse sequences for probing homonu-clear- or heteronuclear-spin connectivities and for distance measurements for dipolar coupled spins will bring new and more detailed information about the C-S-H structure. 

M20. Michelson, A. M Blake, C. C., Evans, S. T and Orkin, S. H., Structure of the human phosphoglycerate kinase gene and the intron-mediated evolution and dispersal of the nucleotidebinding domain. Proc. Natl. Acad. Sci. U.S.A. 82,6965-6969 (1985). 

The structure of C-S-H has been discussed in detail in various excellent publications (Glasser 1993 Richardson Groves 1993 Taylor 1997) and will only be discussed here in terms of its potential for binding metal and metalloid ions. 

From the above description, it becomes apparent that with such disorder, the sorption of both heavy metal cations and oxyanionic species is favoured. Owing to the lack of knowledge of the structure of the C-S-H gel itself, however, it is unlikely that an exact binding mechanism could be discerned at present... 

Incorporation in cement minerals will lead to a similar relationship, which may be described by a distribution ratio with the exception that uptake may be much greater than that of surface sorption ( ). Such mechanisms may apply to C-S-H, AFt, and AFm phases. The mechanism of incorporation may be by isomorphic substitution of a particular species within a crystal lattice. A good example here is the exchange of SO in ettringite for another anion. Another possibility is the adsorption to sites within a crystal structure, as may occur at silicate sites within C-S-H. 

In each case the C s are bonded to each other, but there are not enough H s to satisfy the tetravalence of C. With only two C s. ring structures are impossible multiple bonds must therefore occur. 

Chen, C. S. H., N. Colthup, W. Deichert and R. L. Webb Formation of ketene-imine structures during polymerization of acrylonitrile with X-rays. Paper submitted to J. Polymer Sci. 

Nugent, J.H.A. and Evans, M.C.S. 2004. Structure of biological solar energy converters -further revelations. Trends Plant Sci. 9, 368-370. 

Evidence considered in Sections 5.3.1 and 8.3 indicates that the C-S-H gel of calcium silicate or cement pastes has a layer structure, and that, together with a pore solution, it forms a rigid gel in which the pores range in size from macroscopic to enlarged interlayer spaces of nanometre dimensions. One can therefore define a water content only in relation to a specified drying condition. Three such conditions will be considered. 

The characteristic products of the late stage of hydration are Types 111 and IV C-S-H, and more CH. STEM examination of Type III material in ion-thinned section (JIO) shows that it, too, consists of interlocked and interleaved thin foils (Fig. 5.4), Type IV material is almost featureless even at the 100-nm level, though a fine pore structure has been observed (D13,G35,G36). 

Because C-S-H gel is nearly amorphous. X-ray diffraction has given only very general indications of its structure. The nature of the silicate anions has been determined from the kinetics of the reaction with molybdate (S43), and, in greater detail, by trimethylsilylation (TMS) and Si NMR. In TMS methods, the sample is treated with a reagent that converts the silicate anions into the corresponding silicic acids, which then react further with replacement of SiOH by SiOSi(CH3)3. The resulting TMS derivatives can be identified and semiquantitatively determined by various procedures, of which the most widely used have been differential evaporation to isolate the... 

Early eonelusions from TMS studies that the average size of the silieate anions in pastes inereases on drying (M46,B54) were not supported by later work (P21). An NMR study on the effeet of drying eondilions on the anion structures of C-S-H prepared in suspension (M47) (Seetion 5.4.4) has important implications for pastes. 

All data are for the pseudocells and compositions staled. The true cells have doubled values of b and can be in varying degrees disordered a and c must often be differently defined. The structure of C-S-H(I) has little more than two-dimensional order, and that of C-S H(II) has less than full three-dimensional order. 

The silicate anion structures of C-S-H(I) preparations appear to be affected by how long the material remains in contact with its mother liquor and by how strongly it is subsequently dried. Experiments using the molybdate method showed that the anions in precipitates made from CaCl, and sodium silicate solutions at 0 "C were mainly those present in the silicate solution, and thus monomeric if the latter is sufficiently dilute (S45). By allowing such products to stand in contact with their mother liquors at O C. and drying at — 10°C, preparations with Ca/Si ratios of 1.2-1.5 were obtained that contained only dimeric silicate anions. 

The XRD powder pattern of C-S-H gel (Fig. 5.6) suggests structural i similarities to 1.4-nm tobermorite and jennite. The band at 0.27-0.31 nm and the peak at 0.182nm could be derived from the sharper peaks in i corresponding parts of the patterns of either or both of those phases. The.I... 

From their studies on C-S-H(di,poly) , Stade and Wicker (S45) concluded that Ca and OH ions could both be present in the interlayer region of a tobermorite-like structure. Stade (S46) suggested that in the products containing both dimeric and polymeric ions, one surface of each tobermorite-type layer was composed of dimeric, and the other of polymeric, ions, thus accounting for the observed near-constancy of dimer/ polymer ratio. In the purely dimeric material isolated at — IO C. both surfaces were composed of dimeric ions. 

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