C-S-H product

DTA is a convenient method to follow the hydration of C3S as a function of time. In Fig. 16, the onset of hydration is evident from the small endothermal effect below 200°C.t ] This effect is caused by the removal of loosely bound water as well as firmly held water from the C-S-H gel. The increase in the intensity of this effect with time is indicative of increased formation of the C-S-H product with time. A very small endothermal effect at about 480°C appears within a few minutes, becomes more evident at one hour, and is attributed to the dehydration of Ca(OH)2. In the first eight hours, the amoimt of Ca(OH)2 produced is about 25% of that formed in thirty days. 

Addition of silica fume accelerates the hydration of C3S. The Ca0/Si02 ratio of the C-S-H product is also changed. For example, at a w/s ratio of 10, C3S yields a C-S-H product with CaO/Si02 of 0.8 at 24 hours, and this ratio decreases to 0.33 and 0.36 at C3S/Si02 ratios of 1 and 0.4, respectively. In the hydration of cement, the calcium hydroxide formed as a product is consumed by silica fume. Silica fume in cement increases the overall rate of reaction. Figure 13 gives the strength results of concrete... 

The prineiple hydration produets of slag cements are essentially similar to those found in pordand eement pastes. The microstructure of slag cement pastes is also similar to that of portland cement pastes. X-ray microanalysis has, however, shown that the C/S ratio of C-S-H product in hydrated slag cement is lower than that found in portland cement paste. 

Because RHA has a high surface area, it acts as a good pozzolan. A mixture of CH and RHA promotes the formation of a C-S-H product of composition Ca 5 Si03 5 XH20J The TG/DTA curves for the system RHA-CH mixtures are shown in Fig. 23. The evaporation of water is denoted by an endotherm at about 140°C. Thereafter, there is a gradual loss of water and at 827°C crystallization to wollastonite is indicated by an exothermic peak. 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

The C( lS) state lies 2.683 eV above the ground state C(3P) with a lifetime of 2 sec (32). The production of C( S) atoms is observed in the photolysis of carbon suboxidc in the vacuum ultraviolet. The C( S) atom production can be detected by absorption at 2479 or 1752 A. Rate constants of C( S) with molecules have been measured by Meaburn and Perner (687), Husain and Kirsch (505), and Braun et al (141). The rate constants are in general much smaller (collision efficiencies 10-2 to 10 h) than those for C( D), in analogy with the results for O( D) and O( S) atom quenching rates given in Table IV-3. 

Photolysis of [czs-Fe(H)2(dmpe)2] with simple thiophenes at 273 K gives C-H and C-S insertion products. Prolonged photolysis (15 h) of the thiophene product mixture at 273 K does not appear to affect the relative ratios of the C-H and C-S adducts suggesting that they are stable under the photochemical conditions of the experiment. 

Lee, S. Y., Wong, H. H., Choi, J., Lee, S. H., Lee, S. C., and Han, C. S. 2000. Production of medium-chain-length polyhydroxyalkanoates by high-cell-density cultivation of Pseudomonas putida under phosphorus limitation. Biotech. Bioeng., 68,466-410. 

This chapter deals primarily with the reactions and products of hydration of CjS and P-C2S in pastes at ordinary temperatures. Except where stated, a temperature of 15-25 C is assumed. Background information has been obtained from studies of C S or P-CjS hydration at higher w/s ratios, i.e. in aqueous suspensions, or on C-S -H prepared in other ways, and the results of such studies are also 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). 

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. 

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. 

Glasser el cil. (G50) calculated the activities of the principal ionic species in solution in equilibrium with C-S-H of various Ca/Si ratios, and solubility products defined in terms of those species. Using values of AGf for the ionic species (B62), values of AGf° for the solids were then calculated. The results depended somewhat on the set of solubility data used, but were broadly similar to those obtained by Fujii and Kondo (FI 8). Using one set of data, values ranged from — 1720 kJ mol at Ca/Si = 0.9.3 to — 2350 kJ mol at Ca/Si = 1.63, referred to formulae containing one silicon atom in each case. 

Fig. 5.13 shows the general form of the curve relating the fraction of C3S consumed (a) to time in a paste of w/s 0.5 at about 25°C and with moist curing. Such curves have been determined using QXDA for unreacted C3S (e.g. Refs K20,OI0), though the precision is low for values of a below about 0.1. At low values of a, other methods are available, such as conduction calorimetry (e.g. Ref. P22), aqueous phase analyses (e.g. Ref. B63) or determinations of CH content or of non-evaporable water. At very early ages, it may be necessary to allow for the fact that the property determined depends on the nature of the hydration products e.g. precipitation of C-S-H begins before that of CH. 

The induction period is shortened by adding prehydrated CjS (013), but additions of lime or CH, including that formed from CjS, are variously reported to be ineffective (013,B67) or to lengthen it, though shortening it with cement (Ull). In cement mixes, additions of pfa or some other finely divided materials accelerate hydration after the first day, apparently by acting as nucleation sites for C-S-H (Section 9.3.3). Additions of reactive silica markedly accelerate hydration (S53). Most of this evidence supports hypothesis 3 and tells against hypothesis 4. Hypothesis 3 does not exclude hypothesis 1, as the breakdown of a protective layer could be associated with formation of a new product. 

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