Calcium Aluminate Hydrated

The reactions in the regulated-set cements containing Cjj A3CF2 (note mixed notation) as a principal phase resemble those in ordinary Portiand cements. Initial reaction rates are controlled by ettringite formation. Setting occurs with formation of the monosulfate, along with some transitory lower-limed calcium aluminate hydrates that convert to the monosulfate within a few hours. 

Turriziani, R. 1964. The calcium aluminate hydrates and related compounds. In Taylor, H. F. W. (ed) The Chemistry of Cements. Academic Press, London, Vol. 1, 233-286. 

An alternative to silicate-based Portland cement is the calcium aluminate cement, ciment fondu, which originated with the Lafarge company in France in 1908. Ciment fondu is typically made by heating limestone with bauxite, which is mainly AIO(OH) but contains much iron oxide (see Section 17.2). As noted above, calcium aluminate hydrates and hardens much more rapidly than alite, and so ciment fondu, either as such or mixed with Portland cement, can be used whenever a rapidly setting cement is required, for example, for construction at low temperatures. Concretes made from aluminate cements remain serviceable at higher temperatures than Portland cements and so are used to make cast refractories for pyrometal-lurgical applications. 

Mylius (M60) described the preparation of many calcium aluminate hydrates and related compounds. As with C-S-H, it is normally essential to exclude atmospheric CO2 and to avoid prolonged contact with glass apparatus. Dosch and Keller (D20) described methods for obtaining many AFm phases by anion exchange or other special procedures. 

The ettringite crystals on the surface of the cement particles are so fine that they cannot bridge the gap between the cement particles and therefore do not form a solid structure. This is the basis of the delay in solidification upon adding gypsum to cement. Without gypsum the tricalcium aluminate immediately reacts with water to calcium aluminate hydrate, which fills the space between the cement particles with its large crystals and leads to very rapid solidification of the cement slurry. 

The properties of cement mortar and concrete are closely related to the cement hydration. Polymers have good effect on the mechanical properties and durability of cement mortar and concrete. This improvement is probably come from the effect of polymer on the microstructures and the cement hydration. In this case, some reports about the effect of polymer on cement hydration are found in the references [1-3]. The authors have carried out some research on the influence of polymer dispersion and powder on cement hydration [4-7]. This paper will show some of the results focusing on the effect of SBR dispersion and powder on cement hydration, including the heat evolution, the calcium aluminate hydrates and the calcium silicate hydrates. 

Fig. 4 shows the morphologies of fractured cement pastes with and without SBR dispersion or powder observed by ESEM. Irregular hydrates is observed on the surface of cement particles in the control paste after 10 minutes, and AFt or calcium aluminate hydrates with relative regular shape are also found (Fig. 4 (a)) while at this time a polymer-particle layer is seen on the surface of cement particles in the pastes with SBR dispersion or powder, and AFt or calcium aluminate hydrates appear among the polymer particles (Fig. 4 (b) (c)). 

After 6 hours, C-S-H gel is more obvious in the control paste (Fig. 4 (d)) and also appears in the pastes with SBR dispersion or powder, meanwhile AFt or calcium aluminate hydrates become more and bigger (Fig. 4 (e) (f)). The shape of SBR particles changes and some coagulates together. 

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. 

There is a significant number of metastable calcium aluminate lydrates. Then-identification is difficult because of a laige number of polymorphs and high susceptibility to the formation of caiboaluminates under the influence of CO2, although they occur as well ciystallized hydrates. In cement paste they can also form the nanometric mixtures with the C-S-H phase. Therefore it will be convenient to begin the discussion of calcium aluminate hydration from the presentation of the CaO-Al203-H20 system (Fig. 3.34). 

In the C-A-H system, protected against the CO2 influence, there is a large number of so-called hexagonal hydrates, ciystalhzed in the form of hexagonal plates. These are the metastable phases, because cubic CjAHg is the only stable calcium aluminate hydrate [83, 84]. This phase is, however, formed in the reaction of calcium aluminates with water only at temperature higher than 45 °C [85]. At lower temperatures... 

Fig. 3.35 The structure of calcium aluminate hydrate CAHj. (According to [86])...

Table 3.2 Calcium aluminate hydrates with organic cations [113] A...

Pollmann [109, 110, 112-114], as well as StOber and POllmann [111] found the numerous calcium aluminate hydrates with organic cations in the interlayer positions. They can form sohd solutions ( 4) or compound of defined composition (5). The examples are given in Table 3.2. 

Spmng and Rechenberg [132] state that in the pastes of cements rich in alkalis C3A dissolves quickly because of the reduced Ca(OH)2 solubility. At low gypsum content the quick set can be observed as a result of the hexagonal calcium aluminate hydrates formation In the presence of gypsum excess the similar effect can be the consequence of ettringite formation. 

The orientation of CH crystals is disturbed when some irregularities occur, such as rough aggregate surface, as well as the formation of calcium aluminate hydrate... 

There is a cormnon opinion that corrosion of concrete needs the liquid environment or at least an atmosphere of high hmnidity. The transport of liquid through the concrete causes the sequence of processes, includiug at first the concrete components of the highest reactivity calcium hydroxide and calcium aluminate hydrates. One can thus conclude that the phase composition of cement has a great impact on the behavior of concrete in any aggressive environment. 

There is an opinion that the concrete should be matured for at least 24 h at the temperature of 20 °C or close to 20 °C. The calcium aluminate cement concrete shows the lowest strength at the temperature of 800-900 °C, because the calcium aluminate hydrates are decomposed and the ceramic bond has not sufficient strength. The CJ2A7 phase is detected as a first dehydration product, and at temperature of about 600 °C CA2, formed as a result of active AI2O3 (in statu nascendi) reaction this AI2O3 is the product of AH3 decomposition [12]. The sintering is greatly accelerated at temperatures above 800 °C. 

The calcium aluminate cements are not resistant to the attack of alkalis, because the aluminum hydroxide is soluble in the water solutions of sodium and potassium hydroxides, with which the calcium aluminates are also reacting with. The alkali metals carbonates react with calcium aluminate hydrates and these reactions cause destmction of concrete ... 

The second group consists of calcium aluminate expansive cements, in which calcium aluminate cement is a matrix and the source of aluminate ions. The latter one can originate also from the calcium aluminate hydrate C AH, added in the grinding process. Obviously, gypsum is the constituent of all these cements. 

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