Calcium aluminate cement properties

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. 

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. 

Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH > 4, including natural waters in which CO2 is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea s (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to have been reported. 

One very important niche application for calcium aluminate (cements) is as refractory castables. Key to the success of calcium aluminates in this application are their refractory properties that contrast with those of Portland cements. Although Portland cement maintains good strength when heated, reactive components (CaO) are liberated and can absorb moisture from the atmosphere when cooled, causing expansion and deterioration of, for example, kiln linings. CACs are not much susceptible and can be used to form monolithic castables and refractory cements [28, 29],... 

Due to the absence of Ca(OH)2 in the calcium aluminate cement paste a higher durabiUty of glass fibers was expected in fiber reinforced composites. Majumdar [26] proved that the calcium aluminate cement composites had much better mechanical properties, but only the case of alkali resistant glass. 

Ghosh et al. [129] proposed to add the calcium aluminate cement to Portland cement, together with calcium chloride and anhydrite. This mixture has the properties of expansive cement with setting time of about 15 min and strength 20 MPa after 2 h, 40 MPa after 7 h and 70 MPa after 1 day. Further strength increase is shght. 

The development of calcium aluminate cement was spurred by efforts to overcome the problems associated with sulfate attack on Portland cement based concrete used in the construction of railway tunnels in gypsiferous grounds. The first patent relating to this type of binder was filed in 1908 by Bied in France. The cement was introdueed into production in 1913, and became known as Ciment Fondu. After it was recognized that calcium aluminate cement gains strength much faster than Portland cement, the binder was used in World War I by the French military in the constraction of gun emplacements and shelters, where this property was of paramount importance. After the war, the cement became widely used in other structural applications however, its use in this area became limited, after failures of structures built with this cement were reported from different countries. Nowadays calcium aluminate cement is being used in a variety of special applications. 

Monocalcium aluminate (CA) is the main phase of calcimn aluminate cements, and the one mainly responsible for their properties. This phase is present in calcium aluminate cements in amounts from 40% upwards. It may incorporated variable, but usually small, amounts of FejOj and Si02, especially in cements with lower AI2O3 contents (Rayment and Majumdar, 1994). is also present in most commercial cements. Calcium... 

Table 10.2 summarizes the composition and main characteristics of a series of commercial calcium aluminate cements that roughly represent the spectrum of products currently available. Both their oxide composition and their phase composition vary over a wide range, affecting the properties of the product. Calcium aluminate cements with lower AljOj contents are those most widely used, whereas high-Al203 products are reserved for special applications. 

Table 10.2 Composition and properties of commercial calcium aluminate cements.

To alter the rate of hydration and the properties of the concrete mix made with calcium aluminate cement, chemical admixtures may be employed. However, many of the substances effective as chemical admixmres with Portland cement may be ineffective with calcium aluminate cement, or may affect the hydration of this cement differently. 

Barium aluminate cement exhibits significantly improved refractory properties compared with calcium aluminate cement, and may be considered for applications at temperatures of up to about 2000 °C. It has also the capacity to absorb radioactive and X-ray radiation very effectively. 

If calcium aluminate cement is combined with gypsum in amounts corresponding to 15-40 wt% SO3 he resulting binder exhibits expanding properties, and may be nsed as selfstressing cement. Ettringite and hydrous alumina are formed as products of hydration (Xueetal., 1986 Bayoux eta/., 1992a) ... 

Monford, G. E., Properties of Expansive Cement Made with Portland Cement, Gypsum and Calcium Aluminate Cement, J. PCA Res. Dev. Lab, 2 2 (1964)... 

Recent developments in low- and ultralow-cement castables/pumpables, the effects of the ultrahne particles are of great significance. The water requirements are low since the ultrafine silica fume particles (mostly used in these compositions) occupy part of the space of water. Although the silica fiune helps in reducing water requirements in the castable, it affects high-temperature properties due to the formation of anorthite and gehlinite phases at temperatures around 1250°C- 1400°C. The effects are somewhat minimized in ultralow cement castables due to the use of lesser quantity of calcium aluminate cement. 

Kopanda JE, MacZura G. Production processes, properties, and applications for calcium aluminate cements. In Hart LD, ed. Alumina Chemicals Science and Technology Handbook. Westerville, OH The American Ceramic Society, 1990 171-184. 

An important constituent of refractory castables that has yet to be considered is the calcium aluminate cement. This raw material is known to strongly affect the dispersion state and rheological behavior of castables due to the various reactions occurring in the material as soon as it comes into contact with the admixing water (43-46). Therefore, an understanding of the effect of additives on the hydration process of cement particles is an essential requirement to optimize the rheological properties and working time of cement-based refractory materials. 

Portland cement is classified as a hydrauHc cement, ie, it sets or cures in the presence of water. The term Portland comes from its inventor, Joseph Aspdin, who in 1824 obtained a patent for the combination of materials referred to today as Portland cement. He named it after a grayish colored, natural limestone quarried on the Isle of Portland, which his cured mixture resembled. Other types of hydrauHc cements based on calcium materials were known for many centuries before this, going back to Roman times. Portland cement is not an exact composition but rather a range of compositions, which obtain the desired final properties. The compounds that make up Portland cements are calcium siHcates, calcium aluminates, and calcium aluminoferrites (see ). 

Materials formed by acid-base reactions between calcium aluminate compounds and phosphate-containing solutions yield high-strength, low-permeability, C02-resistant cements when cured in hydrothermal environments. The addition of hollow aluminosilicate microspheres to the uncured matrix constituents yields slurries with densities as low as approximately 1200 kg/m, which cure to produce materials with properties meeting the criteria for well cementing. These formulations also exhibit low rates of carbona-tion. The cementing formulations are pumpable at temperatures up to 150° C. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

For pressing as well as extrusion, the solid electrolyte precursor particles (e.g., zirconia) are often mixed or reacted with an inorganic cementing substance. It is preferred that such adhesive materials also have ion permselective properties as the precursor particles. Phosphates of zirconium, titanium and zinc are examples of such cements although other materials such as calcium aluminate and calcium aluminosilicates are candidates as well [Arrance et al., 1969]. For these cementing materials to be effective, the metal oxides must be only partially hydrated so that they are reactive with the bonding compounds. 

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