Strength calcium aluminate cements

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... 

Hydraulic cements are another class of technologically important materials. Examples include Portland cement, calcium aluminate cement, and plaster of Paris. They harden at room temperature when their powder is mixed with water. The pastes formed this way set into a hard mass that has sufficient compression strength and can be used as stmctural materials. Their structure is generally noncrystalline. 

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. 

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],... 

Aluminate compositions include calcium aluminate cements, which have high chemical resistance, especially to sulfate, and is also used in refractory applications where ordinary Portland cements would be unsuitable. These same cements are used in bioceramic applications. The bioceramic applications reflect both the high mechanical strength of the calcium aluminate cements and also the biocompatibility of Ca-bearing phases, which bond well with, for example, bone. 

S.H. Oh et al., Preparation of calcium aluminate cement for hard tissue repair Effects of lithium fluoride and maleic acid on setting behavior, compressive strength, and biocompatibility. J. Biomed. Mater. Res. 62(4), 593-599 (2002). 

In Chapter 2 we mentioned cement and the reactions that occur during the setting and hardening of this material. There is a class of cements known as calcium aluminate cements (CACs) or high-alumina cements (HACs). These ceramics are not used as widely as Portland cement, but their attraction is the rapid hardening reactions. In 1 day CAC achieves the same strength as Portland cement achieves in a month. 

Fig. 9.4 Schematic strength development curves for calcium aluminate cement concrete at w/c = 0.4 and cement content of 400 kg/m. (according to [20])...

Fig. 9.5 Compressive strength of calcium aluminate cement concretes (according to [21]) 1 limestone aggregate, cement content 350 kg/m, w/c = 0.5, 2 crushed limestone aggregate, w/c=0.6, 3 siliceous aggregate with ground limestone, 4 siliceous boulders, w/c = 0.54, 5 siliceous aggregate,...

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. 

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. 

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... 

In mixes with Portland cement as the main constituent, the setting is due to a rapid formation of ettringite and the hydration of calcium aluminate cement. The hydration of the calcium silicates has little influence on the setting process, but contributes to the subsequent strength development (Gu et al., 1994). In addition to the phases formed in the hydration of pure Portland and calcium aluminate cements, stratlingite (gehlenite hydrate, C2SAHg) may also be formed in the hydration process. 

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. 

Legal restrictions, high price, and concerns about long-term performance limit the widespread use of calcium aluminate cement for constraction purposes. Its use may be attractive in applications where rapid strength development is required, as in the production of prefabricated concrete elements. In apphcations in which only a limited service time is planned, calcium aluminate cement may be used without concern. 

By combining calcium aluminate cement with 5-20% of Portland cement or Portland cement with 5-20% of calcium aluminate cement a binder may be obtained that exhibits a very short setting time and rapid strength development. Of these two alternatives the latter is more widely used, not least for economic reasons. 

Fentiman, C.H. et al. (1990) The effect of curing conditions on the hydration and strength development inFondu slag, m. Proceedings International Symposium on Calcium Aluminate Cements, London, p. 272. 

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