Calcium aluminate cement

A variety of furnaces are used for the production of calcium aluminate cements [9.6], including rotary kilns [9.10]. The calcium aluminate, which may be in the form of a clinker, or a solidified melt is ground in tube mills to 94 to 98 % less than 90 pm. 

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. 

Mixtures of calcium aluminate and Portland cements, sometimes also with slaked lime (CH) or additional gypsum, can be used to make rapid setting mixtures for grouting and similar applications. The strengths obtainable (e.g. 25 MPa at 5 h) (G73) are relatively high for cements of this type. 

Used with appropriate refractory aggregates, calcium aluminate cements may be used to make refractory castables having applications in the steel and other heat-using industries. Cements made from bauxite may thus be used at hot face temperatures up to 1350 C. White calcium aluminate cements, which are low in iron and higher in alumina, can be used with appropriate aggregates to make castables similarly resistant up to 2000°C. 

Refractory castables have good resistance to thermal shock. 

Calcium aluminate cements have also been termed aluminous or high-alumina cements, but the former term is less specific and the latter is more appropriately used for the white cements high in AljO, mentioned above. The cements described here are those manufactured in France and the UK. Those made elsewhere are similar in essential respects. 

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Calcined alumina markets consume slightly less than 50% of the specialty alumina chemicals production (1—8,20,22—115). Worldwide usage is estimated to be about 50% for refractories (qv), 20% for abrasives, and 25% for ceramics (qv). Calcined aluminas are also used in the manufacture of tabular alumina and calcium aluminate cements (CAC). Quantities are estimated to be over 200,000 and 100,000 t, respectively (7). 

Refined calcined alumina is commonly used in combination with high purity limestone [1317-65-3] to produce high purity calcium aluminate cement (CAC). The manufacture, properties, and appHcations of CAC from bauxite limestone, as weU as high purity CAC, has been described (104). High purity CAC sinters readily in gas-fired rotary kiln calcinations at 1600 —1700 K. CAC reactions are considered practically complete when content of free CaO is less than 0.15% andloss on ignition is less than 0.5% at 1373 K. 

Table 3. Characteristics of Calcium Aluminate Cement (CAC) Mineral Constituents ...

Most commercial methanator catalysts contain nickel, supported on alumina, kaolin, or calcium aluminate cement. Sulfur and arsenic are poisons to the catalyst, which can also be fouled by carry-over of solvent from the CO2 removal system. 

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). 

When calcium aluminate cements are made by the fusion process, the solidified melt must be cmshed and then ground. The material is very hard to grind and power consumption is high. 

The catalysts used in the process are essentially nickel metal dispersed on a support material consisting of various oxide mixtures such as alumina, silica, lime, magnesia, and compounds such as calcium aluminate cements. When the catalyst is made, the nickel is present as nickel oxide which is reduced in the plant converter with hydrogen, usually the 3 1 H2 N2 synthesis gas ... 

Calcium alginate gels, 4 728 Calcium alloys, 4 530 Calcium aluminate, 2 345t Calcium aluminate cement, 2 415-416 5 500t, 502... 

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 methanator catalyst is nickel, supported by alumina, kaolin or calcium aluminate cement. After methanation the CO and CO2 content of the treated gas is of the order of a few ppm. A methanator typically operates in the temperature range of 300-400°C. Methanation reactions are strongly exothermic and hence the CO and CO2 concentrations at the inlet of the methanator should be carefully monitored, to avoid thermal runaway. 

Calcium dialuminate (CA2) occurs in some calcium aluminate cements. It reacts only slowly with water. The formula was at one time wrongly considered to be C3A5. CA2 is monoclinic, with a - 1.2840 nm, b =... 

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

Portland cement raw materials contain small proportions of MgO as noted in Section 1.1.2, these must be limited to avoid formation of more than a minor amount of periclase. The iron in Portland cement clinkers is normally present almost entirely as Fe, but calcium aluminate cements may contain both Fe " and Fe . ... 

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). 

DTA curves at 10 deg C min show a large endotherm at 130-150 C and a smaller one at about 290°C. Since the first of these is due to loss of molecular water, its height is affected by any preliminary drying that the specimen may have undergone. DTA has been used to determine the relative amounts of CAHk, and AHj, and thus indirectly also of CjAH in calcium aluminate cement concretes, but caution is needed because the CAHk, may have undergone partial dehydration and also because its thermal decomposition can itself yield AHj. To some extent, this caution also applies to the determination of CAH,o by QXDA. 

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... 

Ciment Fondu is normally made by complete fusion of limestone and bauxite at 1450-1600 C. In order to produce a cement with the desired rapid-hardening properties, both raw materials must be low in SiO,. The molten clinker is tapped off continuously from the furnace, solidifies and is typically crushed and ground to a fineness of about. 00 m- kg . Some iron is reduced to Fe . The colour of cements produced from bauxite can vary from yellow brown to black, but is commonly greyish black. White calcium aluminate cements are usually made by sintering calcined alumina with quicklime (calcium oxide) or high-purity limestone. 

Table 10.1 Typical compositions of calcium aluminate cements (weight percentages)...

Table 10.1 gives typical chemical compositions of commercially produced calcium aluminate cements. The essential compound in all of them, because it develops the main hydraulic activity and is consequently responsible for the strength development, is monocaicium aluminate, CA. In white calcium aluminate cements, it can occur with various combinations of the other binary phases of the CaO-Al20j system and corundum. The crystal structures of these compounds and the phase equilibria relating to their formation were considered in Chapter 2. In the sintering process by which these cements are made, the reaction conditions are of the utmost importance, as... 

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