C-S-H surface

Fig. 3.33 Model of the double layer on the C-S-H surface loosely based on the model proposed...

It will be worthwhile to present a simplified model of double layer on the C-S-H surface in the paste, loosely based on Divet s et al. [77] model (Fig. 3.33). The adsorption phenomena consist in the exchange of anions in the Stem s external layer. The internal Stem s layer is composed mainly of Ca + Na+ and K+ cations and the external one— from the OH and hydrated cations. 

The modified C-S-H morphology in the presence of CaCl2 is attributed to the chemisorption of chloride on the C-S-H surface. Ramachandr [70] investigated various forms of chloride occurrence in C-S-H gel, applying the washing with alcohol. In Fig. 4.26 the summarized results of his studies are shown. At early age... 

However, the precise sorption measmements were not correlated with the data presented above. The differences associated with the use of different adsorbates can be caused by the mutual interactions between the adsorbate and adsorbent during the sorption process [38]. There are some premises to presmne that the energy of interaction of water molecules with the C-S-H surface is significantly higher than in case of the other molecules, for example methanol, and therefore water can intmde to the space inaccessible for these molecules [38]. The hypothesis of Jermings and Termis [50], as well as their model concerning this issue, is discussed in Sect. 3.2.3. 

The sulphate ions sorption ability of C-S-H is important for the Famy s Itypoth-esis [152]. The experiments linked with this problem were carried out by Barbulo et al. [174]. They studied the sorption of sulphate ions of C-S-H phase and found the dependence between sorption and Ca/Si ratio. According to these authors [174] the sulphate ions are adsorbed on the sites occupied by Ca ions, on the C-S-H surface. It can be expressed by the formula ... 

Collepardi, et al., studied the hydrates formed during the hydration of C3S in the presence of calcium chloride.In an autoclaved sample containing CaCl2, they detected an exothermic peak in the thermogram that was attributed to the chemisorption of chloride on the C-S-H surface and its penetration into the molecular layers of C-S-H. 

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. 

The recent detailed investigations on Zn(II) and Pb(II) have advanced our understanding of heavy metal cation binding to C-S-H. However there are still a number of important questions that require attention. We need to understand the relationship, if any between Ca Si ratio and metal uptake. We also need to understand how to interpret the sorption process, whether as a solid solution (Tommaseo Kersten 2002) or as precipitation within the C-S-H particles or as a sorption process suggested by Glasser (1993) to the surfaces of the crystalline domains. 

F Glass fragments Sj - Main shock wave H = Head of rarefaction wave C s Contact surface air/atr, initial inner pressure 350 psi, temp 300°K (1) = atm external to cylinder (4) interior region of cylinder... 

Chemically bound water is most reasonably defined as including that present in interlayer spaces, or more firmly bound, but not that present in pores larger than interlayer spaces. As will be seen in Chapter 8, the distinction between interlayer space and micropores is not sharp water adsorbed on surfaces of pores further blurs the definition. From the experimental standpoint, the determination is complicated by the fact that the amount of water retained at a given RH depends on the previous drying history of the sample and on the rate at which water is removed. An approximate estimate is obtained by equilibrating a sample, not previously dried below saturation, with an atmosphere of 11% RH (F12,F13,F14). Saturated aqueous LiCl HjO gives the required RH (partial pressure of water vapour = 2,7 torr at 25°C). To achieve apparent equilibrium in a reasonable time (several days), the sample must be crushed and the system evacuated the salt solution should be stirred, at least intermittently. Young and Hansen (Y5) found the composition of the C-S-H in C3S paste thus... 

Fig. 5.4 (A.B) Types I and II C-S-H, respectively (SEM of fracture surfaces courtesy K. L. Scrivener). (C,D) SEM/STEM pair of ion beam thinned section, showing Type III C-S-H (top, right) and Type IV C-S-H (top, left and bottom, right Jennings et al. (HO)). (A) is of an ordinary Portland cement paste, w/c = 0.5, aged 10 h. (B) is of a paste of an oil well cement, w/c = 0.44, with 2.4% of CaClj on the weight of cement, aged 1 day. (C) and (D) are of a CjS paste, w c = 0.47, aged 330 days.

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. 

Reaction of CaS to produce outer product C-S-H on AFt rod network leaving - jm between grain surface and hydrated sheM... 

Stubby rods of AFt phase are also seen (D25 D27). They are typically some 250 nm long and 100 nm thick. Studies using wet cells show them to occur both on the surfaces of the grains, and at some distance away (S41,S68) (Fig. 7.6b). They are probably more abundant near to the surfaces of the aluminate phase, and appear to nucleate in the solution and on the outer surface of a layer of gel. On drying, this layer shrinks, and the AFt crystals fall back onto the surfaces of the cement grains. The early products thus differ in morphology and composition from the exfoliating foils or honeycombs of C-S-H that have been observed in CjS pastes. 

Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7.6c). In this respect, the hydration of cement differs from that of C3S, in which the C-S-H grows directly over the C3S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. 

The evidence from microstructure, calorimetry and other sources suggests that the hydration processes of cement and C3S are essentially similar. There are important differences in the nature of the early product and in where the C-S-H formed in the middle stage of reaction begins to deposit, but in both cases it would appear that the early reaction slows down because of the deposition of a layer of product, which either isolates parts of the anhydrous surfaces from the main solution or allows the concentrations close to those surfaces to rise to values approaching the theoretical solubilities of the anhydrous compounds. In both cases, the initiation of the main reaction and the kinetics in its acceleratory phase appear to be controlled by the nucleation and growth of C-S-H. 

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

If two solid surfaces are in contaet and the attractive forces between them are outweighed by those existing with moleeules of the liquid, the latter may be drawn in between them, so forcing them apart a disjoining pressure is said to exist. The effect occurs with many clay minerals. The main attractive forces in the latter case are probably ion-dipole forces between ions in the solid surfaces and water molecules, and the same could apply to C -S-H. 

Tanaka et at. (T45) partially masked the surfaces of disks of slag glass with gold coatings the disks were then embedded in cement pastes for various periods, after which they were removed and examined by SEM. This showed that two layers of in situ product were formed. The innermost one had Si/Ca 0.64, Al/Ca 0.32 and Mg/Ca 0.34 the outer one had Si/Ca 0.61, Al/Ca 0.24, Mg/Ca 0.20. The product formed immediately outside the original boundary of the slag had Si/Ca 0.48, Al/Ca 0.09, Mg/Ca 0.17. The two layers of in situ product could have been mixtures in differing proportions of C-S-H. a hydrotalcite-type phase and possibly AFm phase. 

Calorimetric studies show that pfa retards the reaction of alite in the early stage of reaction (G70,J30,H51,W29), but with one exception (W29) studies on the middle stage show the alite reaction to be accelerated (K45,L47,H51,T44,D12). SEM shows that C-S-H and CH are deposited on the surfaces of pfa grains before these have started to react significantly (D28). The accelerating effect is probably due mainly or entirely to the provision of additional nucleation sites on the pfa, and occurs with other fine powders (K46). A QXDA study showed that the aluminate and ferrite react more rapidly in the presence of pfa, but with belite there was no detectable effect up to 28 days and a marked reduction in rate of consumption thereafter (D12). 

The broad conclusion indicated by the evidence considered in this section is that organic retarders are substances that are readily adsorbed on to the surfaces of growing particles of hydration products, especially C S H, and which do so in preference to complexing Ca ions in aqueous solution. The monodentate mode of attachment proposed by Thomas and Birchall (T49) explains the differences in behaviour among sugars, but its applicability in this respect to a wider range of substances has yet to be examined. It does not appear to account for the observation that the effective retarders are compounds in which two oxygen atoms are located on adjacent carbon atoms, or are otherwise able to approach each other closely. 

From an SEM study of mortars using both fracture surfaces and BEI of polished sections. Scrivener and Pratt (SI 12) concluded that a layer of C-S-H about 0.5 pm thick was formed over the surfaces of the sand grains during the first day. This layer resembled that formed on pfa particles. They confirmed the tendency to increased content of ettringite in the interfacial region. Large crystals of CH were sometimes formed adjacent to the sand... 

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