Calcium aluminate cement hydration

At temperatures lower than 20 °C the metastable CAHj phase appears in this system, which is of special importance in case of high calcium aluminate cements (see Chap. 9.1). At temperature higher than 20 °C this phase is transformed into the C2AHg hydrate and amorphous AHj. According to Jones [84] it is the temperature range from 20 to 25 °C and even at temperature 22 °C the CAHj phase is still formed. Above this temperature the transformation mentioned above takes place. However, during the high calcium aluminate cement hydration and at low w/c ratio the CAHjq phase is formed even at 25 °C. 

George CM. Aspects of calcium aluminate cement hydration. Proceedings of the 30th Annual St. Louis Section of the American Ceramic Society Symposium on Refractories, St. Louis, MO, 1994. 

Calcium Aluminate Cements. Low purity calcium aluminate [12042-78-3] cements are obtained by sintering or fusing bauxite and lime in a rotary or shaft kiln. A high purity calcium aluminate cement, 2CaO 5AI2O2, capable of withstanding service temperatures of 1750°C can be prepared by the reaction of high purity lime with calcined or hydrated alumina (see Aluminum compounds). 

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. 

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). 

The C4A and CjA hydrates are most conveniently prepared by adding CaO or saturated CH solution to a supersaturated calcium aluminate solution obtained by shaking CA or white calcium aluminate cement (Section 10.1.1) with water. Such solutions typically contain up to about 1.2gCa01 and 1.9 g AljOj 1 , these concentrations depending on the shaking time, temperature, proportioning and particle size of the starting... 

The total quantity of heat evolved by a calcium aluminate cement on hydration is typically 325-400 kJ kg , which is somewhat less than that for a typical Portland cement, but due to the speed of the reaction, nearly all of it is produced during the first day. Calcium aluminate cements arc therefore suitable for use at low temperatures (down to —IOC), e.g. for winter construction in cold climates or repairs in cold stores. 

Calcium aluminate cements harden rapidly as soon as the massive precipitation of hydrates begins. This may be attributed to the fact that, unlike those of Portland cement, the major hydration products are crystalline. Relatively high proportions of water are taken up in the hydration reactions, the theoretical w/c ratios needed for complete hydration of CA being 1.14, 0.63 and 0.46 for the formation of CAHj, CjAHg -F AHj and CjAH -F 2AHj, respectively. For this reason, and also because of the rapid heat... 

Various methods have been used to obtain cements that set and harden rapidly. They include the use of Portland cement with admixtures and of mixtures containing both Portland and calcium aluminate cements, described in Sections 11.5 and lO.I.IO, respectively. Another approach has been the manufacture of clinkers containing either CuAy CaF, or C 4A, S. both of which hydrate rapidly under appropriate conditions with the formation of ettringite. 

Pastes inpregnated with PMMA or sulphur are still sufficiently permeable to water that expansion occurs on long exposure (F46). In polymer-impregnated (S108) and MDF (R64) cement pastes, there is evidence of interaction between Ca ions and carboxylate and possibly other groups of the polymer. In MDF pastes made with calcium aluminate cement, the polymer (PVA) was found to inhibit the normal hydration reactions of the cement, but to react with Ca and AH to give an ionically cross-linked polymer and calcium acetate. TEM showed the material to be essentially a dispersion of grains of clinker or hydration products in a continuous polymer matrix. 

The preparation of calcium aluminate cements is similar. Here, instead of calcium and silica, calcium and alumina react with water to form hydrated calcium aluminate [2] as the bonding phase. The initial strength gain for this material is faster than that for Portland cement. 

The contraction is linked with the hydration rate, therefore the rate of contraction increases with the CjA and CjS share, fineness of cement and w/c ratio. Typical data for Portland cement and calcium aluminate cement are listed in Table 5.5 [38]. 

Kurdowski, W., Duszak, S., Sorrentino, R Corrosion of Gehlenite Hydrate in strong choride solution. In Mangabhai, R.J., Glasser, RP (eds.) Calcium Aluminate Cements, p. 371. lOM Communications, London (2001)... 

Usually in the process of CA hydration the mixture of the two hexagonal hydrates together with the colloidal aluminum hydroxide are formed. It is all the more probable that calcium aluminate cement contains always some amount of C,2A2 phase, which hydrating gives at once these two hexagonal phases ... 

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 ... 

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]. 

A important experimental support of in situ hypothesis was presented by Cottin [76]. The mixture of calcium aluminate cement and gypsum is hydrating with too low water content. The change of volume calculated from the phase composition of the paste corresponds very well to the measured value. 

As aforementioned expansive cements produced industrially are based on the formation of ettringite. The technologies differ orrly with the type of calcium alu-minates and of the matrix material. The types of expansive cements are shown in Table 9.3. Two types of matrix are used Portland cement and calcium aluminate cement. The following phases are r ed as the sotrrce of aluminum ions CjA, CA (as calcium aliuninate cement), C4A3S and the calcitrm aliuninate hydrates C AH or CAHjq. The second technology is the oldest one, and was implemented by Dossier in forties XXc [60]. 

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. 

In the first decade of infrared research on the study of Portland cements, Vazquez (1%9), was a lead the way in the study by infrared spectroscopy the main present compounds in the Portland cement but also later, made some research about the carbonation processes of calcium aluminate cements. Also, the hydration of Portland cement and its constituents was... 

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