Hydrated calcium aluminate cement

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). 

Gaztanaga T. et al. (1997) Carbonation of hydrated calcium aluminate cement and/or ordinary Portland cement of vaiying alkali content, in Proceedings 10th ICCC Gdteborg, paper 4iv003. 

The hydrate phases constituting hydrated calcium aluminate cement decompose gradually upon heating, losing virtually all their combined water by 800-1000°C (see also section 10.8). This thermal decomposition is associated with a gradual loss of strength of the material. Starting at around 700-800°C the decomposition of hydrate phases is... 

One corrosive agent to which refractory concrete may be exposed during service is carbon monoxide (CO). To ensure an acceptable resistance of hydrated calcium aluminate cement to this agent, the cement must not contain iron oxide, which may undergo chemical reduction to bivalent iron imder these conditions. In situations in which hydrogen may come into contact with concrete based on calcium aluminate cement, the silica content of the binder may be critical, and thus should be kept low. 

Thermal Analysis of Hydrated Calcium Aluminate Cements... 

Figure 1. An idealized thermal analysis curve for hydrated calcium aluminate cement.1 1...

L. D. Hogue and W. A. Jackson, Nature of carbonation of hydrated calcium aluminate cements in castable refractories, Ind. Heat., 64(8) 45-49,1997. 

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. 

The dry product is ground to a powder and then a little calcium sulfate (CaS04) is added to slow down the setting rate of the cement. When water is added to the mixture, slow complex chemical changes occur, resulting in the formation of a hard interlocking mass of crystals of hydrated calcium aluminate and silicate. 

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 primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

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 sensitivity of concrete towards the action of seawater is above all due to the presence of calcium hydroxide and hydrated calcium aluminates, which are both susceptible to reactions with sulfate ions from the seawater. Expansive effects decrease, therefore, as the percentage of CjA in the cement or the content of Ca(OH)2 in the concrete diminishes. The traditional approach to avoid these reac-... 

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. 

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 ... 

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