Calcium aluminate cement conversion

CjAHg is the only stable ternary phase in the CaO-AUOj H,0 system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. CjAHg is formed in the conversion reaction of hydrated calcium aluminate cements (Section 10.1). 

Conversion is of only minor importance for the use of calcium aluminate cements in refractory applications and in mixes with Portland cement such as those described in Section 10.1.10. 

There are controversial opinions about the corrosion of reinforcing steel in the calcium aluminate cement concretes. This is linked with the less basic paste in comparison with the Portland cement one. However, it appeared in practice, that in the good quality concrete there was no difference related to the stability of steel reirrforcement in comparison with Portland cement. The destmction of reinforced concrete was linked with high w/c, in which the conversion of hexagonal hydrates into cubic caused the porosity and rapid carbonation increase [12]. 

The total heat of hydration of calcium aluminate cement is in the range 450-500 J/g, and is similar to that of Portland cement. However, 70-90% of it is liberated within the first 24 hours (at 20°C), making dissipation of the heat into the environment more difficult than in situations where Portland cement is employed, where the liberation of hydration heat takes place much more slowly. This may be critical, especially in the erection of massive stmctures, where a significant rise of temperature may take place shortly after mixing. Maximum temperatures of up to 80°C may be reached in the bulk of the concrete stracture. This in turn may accelerate the conversion of the calcium aluminate hydrates formed. Wet curing has to be employed to prevent superficial dehydration and dusting of the hardened concrete. 

After reaching a maximum value the strength of hardened calcium aluminate cement starts to decline, owing to the conversion of the primary formed CAHjg and C2AHg to... 

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. 

In seawater, calcium aluminate cement is more durable than Portland cement Conversion also takes place here, but it is usually veiy slow, except in the tidal zone or in warm waters (Baker and Banfill, 1992). If seawater is employed as mixing water the initial hydration may be retarded, but the final microstmcture of the hardened material is very similar to that made with fresh water. Chloroaluminates are formed in the hydration reaction (Raise and Pratt, 1986). 

In blends of calcium aluminate cement and non-hydrated or prehydrated dicalciiun siUcate the primary formed CAHj and C2AHg phases tend to convert to stratlingite (C2ASHg), rather than to CjAHg. This appears to be another way by which the loss of strength associated with the conversion to the latter hydrate phase may be prevented (Rao, 1980). 

The addition of finely ground calcium caibonate (calcite) to calcium aluminate cement results in the formation of calcium aluminocarbonate hydrate (C ACh j) as the product of hydration, instead of CAHg and CjAHg. At sufficiently high additions of the carbonate conversion of the cement may be avoided, and hence the strength loss associated with it may be prevented (Trivino, 1986). The long-term effectiveness of this measure is questionable, however. 

Bentsen, S., Selvod, S., and Sandberg, B. (1996) Effeet of mierosihea on conversion of high alumina eement, in Calcium Aluminate Cements (ed. R.J.Manghabai), E. F.N. Spot , London, pp. 294-298. 

George, C M., and Montgomeiy, R.G.J. (1992) Calcium aluminate cement concrete durability and conversion. A fresh look at an old subject. Materiales de Construccion (Madrid) 42 (228), 51-64. 

Calcium oxide is the main ingredient in conventional portland cements. Since limestone is the most abundant mineral in nature, it has been easy to produce portland cement at a low cost. The high solubility of calcium oxide makes it difficult to produce phosphate-based cements. However, calcium oxide can be converted to compounds such as silicates, aluminates, or even hydrophosphates, which then can be used in an acid-base reaction with phosphate, forming CBPCs. The cost of phosphates and conversion to the correct mineral forms add to the manufacturing cost, and hence calcium phosphate cements are more expensive than conventional cements. For this reason, their use has been largely limited to dental and other biomedical applications. Calcium phosphate cements have found application as structural materials, but only when wollastonite is used as an admixture in magnesium phosphate cements. Because calcium phosphates are also bone minerals, they are indispensable in biomaterial applications and hence form a class of useful CBPCs that cannot be substituted by any other. 

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 a result of exposure to humidity and even a slightly elevated temperature, a conversion reaction may start in hardened cement paste. It develops in the calcium aluminate hydrates where hexagonal crystals are transformed into cubic ones, which have smaller volume. This causes an increase of porosity and considerable decrease of strength. It has been proved that even a temperature over 20°C can initiate the reaction for which the remaining amount of mixing water may be sufficient. The conversion reaction may... 

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