Silicate Hydration

The complexity of any individual binder reaction with water is well illustrated on the example of gypsum hemihydrate hydratioa The hydration process in the case of multiphase material as Portland cement is, must be much more complex. The reactions of individual cement phases with water occur simultaneously and interfere the presence of minor components, first of all alkalis and sulphates, modifies further the composition of the liquid phase. Therefore the rate of hydration of basic cement phases is strongly affected. 

It should be remember that the hydration of cementitious materials is a heterogeneous process therefore among the rate controlling agents are not only the state parameters, such as temperature, pressure and concentration, but also the other factors. The most important are the fineness of the solid, the surface constitution and the presence of crystals stmcmral defects. 

The following imit processes can be distinguished in any hydration reaction mechanism  

The composition of the liquid phase co-existing with the solid, as it has been mentioned above, is of special importance in the hydration process. This system is far from the equilibrium and at the usual water content on the level 33 % approximately (w/c=0.5), there are the micro-areas of different composition. Simultaneously, the diffusion becomes more and more difficult as the hydrates are formed. The gradients of concentrations appear, as well as the differences of temperature between the particular micro-areas. Therefore the image of the process becomes more sophisticated. To simplify this, we take into account the model, three-component systems CaO-Si02-H20 or Ca0-Al203-H20 which correspond to the calcium silicate or calcium aluminate hydration. However, the processes occurring in these simplified systems are complex, as one could conclude from the aforementioned considerations. 

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Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

Table 6. Calcium Silicate Hydrates Formed at Elevated Temperatures ...

AfwiUite can also be formed, and appears to be the thermodynamically stable calcium silicate hydrate in pure systems, at room temperature. 

Si aluminates Ca silicates Hydrated silica Anhydrous silica... 

Calcium selenite, 22 73t Calcium silicate(s), 12 63 hydration, 5 475-477 Calcium silicate hydrate, 5 477t hydration, 5 475... 

When anhydrous cement mix is added to water, the silicates react, forming hydrates and calcium hydroxide. Hardened Portland cement contains about 70% cross-linked calcium silicate hydrate and 20% crystalline calcium hydroxide. 

Concrete is made of cement aggregate and water mixed together to form a paste. The aggregate is usually a tiller material composed of inert ingredients such as sand and rocks. When water is added, the components of cement undergo a chemical reaction known as hydration. As hydration occurs, the silicates are transformed into silicate hydrates and calcium hydroxide (Ca OII 2), and the cement slowly forms a hardened paste. 

The C3A phase is subjected to competitive reaction between water and gypsum to form a small amount of the initial C3A hydrates covered by a protective coating of ettringite, and also a proportion of the C3S hydrates to form a calcium silicate hydrate of low calcium content in the region of 100 nm thick. 

During this phase the initial self-retarding calcium silicate hydrate layer appears to break down, allowing further hydration products from the C3S and C2S phases to be formed. These appear to be produced in three... 

In the presence of calcium lignosulfonate [58], the calcium silicate hydrate gel from the C3S and C2S... 

Addition of dampproofers based on caprylic, capric or stearic acids, stearates or wax emulsions do not have any effect on the setting characteristics of hydration products of Portland cement. However, the unsaturated fatty acid salts, such as oleates, although not affecting the tricalcium silicate hydration, have a marked effect on the ettringite and monosulfate reaction [12] and this is illustrated in the isothermal calorimetry results in Fig. 4.4. It is possible that a calcium oleoaluminate hydrate complex is formed involving the double bond of the oleic acid. 

F/g. The lime-silica ratio of hydration products of tricalcium silicate hydrating in the presence of 0 and 2% calcium chloride as a function of the degree of hydration (Odier). 

Comparing metal concentrations in the BA with the amount of the different types of waste (household, industrial, etc.) only one poor correlation (R = 0.6) was observed a higher amount of waste coming from the construction sector caused higher CaO contents in the BA. It is likely that in this case Ca-rich CSH phases (calcium-silicate-hydrate) and gypsum account for a higher CaO content in the BA. 

Tits, J., Stumpf, Th., Rabung, Th., Wieland, E. Fanghanel, Th. 2003. Uptake of trivalent actinides (Cm(III)) and lanthanides (Eu(III)) by Calcium silicate hydrates a wet chemistry and time-resolved laser fluorescence spectroscopy (TRLFS) study. Environmental Science and Technology, 37, 3568-3573. 

BONHOWE, I., WlELAND, E., SHEIDEGGER, A. M., Ochs, M. Kunz, D. 2003. EXAFS study of Sn(IV) immobilization by hardened cement paste and calcium silicate hydrates. Environmental Science and Technology, 37, 2184-2191. 

Johnson, C. A. Kersten, M. 1999. A possible mechanism of Zn binding to Ca silicate hydrates. Environmental Science and Technology, 33, 2296-2298. 

Richardson, I. G. Groves, G. W. 1993. The incorporation of minor and trace elements into calcium silicate hydrate (C-S-H) gel in hardened cement pastes. Cement and Concrete Research, 23, 131-138. 

Rose, J., Moulin, I. et al. 2000. X-ray absorption spectroscopy study of immobilization processes for heavy metals in calcium silicate hydrates 1. Case of lead. Langmuir, 16, 9900-9906. 

Tommaseo, C. E. Kersten, M. 2002. Aqueous solubility diagrams for cementitious waste stabilization systems. 4. Mechanism of zinc immobilization by calcium silicate hydrate. Environmental Science and Technology, 36, 2919-2925. 

Ziegler, F., Giere, R. Johnson, C. A. 2001a. The sorption mechanisms of zinc to calcium silicate hydrate Sorption studies and microscopic investigations. Environmental Science and Technology, 35, 4556-4561. 

CASH CBM CBO CBPC CC CCB CCM CCP CDB CEC CFBC CFC CFR CMM COP CSH CT Calcium aluminosilicate hydrate Coal bed methane Carbon burn-out Chemically-bonded phosphate ceramics Carbonate carbon Coal combustion byproducts Constant capacitance model Coal combustion product Citrate-dithionate-bicarbonate Cation exchange capacity Circulating fluidized bed combustion Chlorofluorocarbon Cumulative fraction Coal mine methane Coefficient of performance Calcium silicate hydrate Collision theory... 

The properties of cured Pordand cement are affected by these four constituents of the manufactured Pordand cement. Tricalcium silicate hydrates and hardens rapidly, giving rise to the initial set and eady strength. Increased concentrations of tricalcium silicate causes an increase in the early strength of Pordand cement concretes. Dicalcium silicate hydrates and hardens more slowly, giving the cured concrete its strength increases beyond one week. 

FURNACES, FUEL-FIRED] (Vol 12) a (A) Dicalcium silicate hydrate [15630-58-7]... 

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