Calcium aluminate hydrate phases

The following calcium aluminate hydrate phases may be formed in the hydration of calcium aluminates. 

As the calcium aluminate hydrate phases that are initially formed in the hydration of calcimn aluminate cement- AHjQ and C2AHg—are thermodynamically unstable, they convert over time to CjAHg, the only phase that is stable in the system C-A-H. At the same time AHj is formed as a by-product. 

At low or medium water/cement ratios the porosity and permeability of hydrated non-converted aluminous cement pastes are sufficiently low to confine the corrosive action of any external chemical agents to the surface region of the concrete structure. However, as the porosity increases in the course of conversion, the susceptibility to chemical attack of concrete based on aluminous cement increases. An effective way to prevent this from happening is to use initial water/cement ratios that are too low for complete hydration. Under these conditions the water liberated in the conversion of the hexagonal calcium aluminate hydrate phases, formed initially, reacts with the non-hydrated fraction of the cement, thus preserving a low porosity of the hardened paste. Note that the permeability is the main factor determining the resistance of aluminous cement concrete to chemical agents, and this has to be kept in mind when calcium aluminate cement is used in practice. 

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. 

Calcium aluminate chloride, phase in Portland cement clinker, 5 472t Calcium aluminate fluoride, phase in Portland cement clinker, 5 472t Calcium aluminoferrite, phase in Portland cement clinker, 5 472t Calcium aluminoferrite hydrate, 5 477t Calcium—aluminum alloys, 4 530 Calcium amalgam, 22 773 Calcium ammonium nitrate, 2 724 Calcium analysis, of water, 26 37 Calcium A zeolite, separation of hydrocarbons by, 16 823 Calcium—barium—silicon alloy, 22 519 Calcium-bearing manganese silicon,... 

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 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... 

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 main product of the pozzolanic reaction is an amorphous or nearly amorphous calciiun sihcate/aluminate hydrate phase similar to that formed in the hydration of calcium silicates constituting Portland cement. 

Eventually all calcium aluminate hydrates convert to CgAHg, the only phase that is thermodynamically stable in the system Ca0-Al203-H20, together with additional AH3. The rate of this conversion increases distinctly with increasing temperature. At 5°C it may take years until the conversion is completed, whereas above about 50°C the process is virtually immediate. At low humidities partial dehydration of the formed hydrates, rather than their conversion to C3AHg, predominates. Table 10.3 summarizes the effect of temperature and humidity on the stability of hydrated aluminous cement. 

As we know, the main hydrated phases of cement p>aste are calcium silicate hydrate (C-S-H), calcium hydroxide (CH), calcium aluminate hydrate (C-A-H) ettiingite (AFt) and mono-sulfoaliuminate (AFm). However, these three hydrated phases are not stable in the external environment containing sulfates. The following reactions can occur [ 2] ... 

In CAC, the CA reacts with water to form a series of calcium aluminate hydrates. These include CAHjq, C2AHg, C3AH6, and AH3 (an amorphous phase). The metastable hydrates, CAHjq and C2AHg, convert to C3AH6. The following scheme summarizes the eonversion reactions. 

---