Setting calcium aluminate cement

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. 

Many inorganic salts affect the setting times of calcium aluminate cements and are effective in concentrations of 0.5% (L6,C53,R45). The effects on setting times differ greatly both in direction and in relative magnitudes from... 

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. 

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. 

There are, however, several important niche applications for CAC. Most notably, CACs are used as linings to sewers and mine tunnels. Calcium aluminate cements are resistant to chemical attack from sulfate-producing bacteria that thrive in sewer systems (especially in warmer climates), and sprayed concrete linings to sewers have been shown to resist degradation for periods up to 30 years. The high impact and abrasion resistance of CAC also makes it suitable as a lining material for ore tunnels in mines and because CAC sets rapidly, it can be sprayed onto tunnel walls (as shotcrete ) and even used as a tunnel lining. 

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. 

In calcium aluminate clinkers the lower or higher Cj2A, amounts are present, which causes set acceleration of cement paste. Sometimes in the sintered clinkers even the CjA phase can be present, due to the local inhomogeneity of the raw mixture, in which the larger limestone grains can occur. Calleja [16] proposed the simplified formulae for the calculation of calcium aluminate cement phase composition. 

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. 

Fig. 9.27 Effect of calcium aluminate cement content in the mixture with Portland cement containing less than 1 % CaOj curve I) and 2.7% CaO curve 2) on setting P initial setting time, K final setting time, (according to [1])...

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. 

Calcium aluminate cement based rapid-setting/hardening binder... 

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. 

Menetrier-Sorrentino, D., George, C.M., and Sorrentino, F.P. (1986) The setting and hardening characteristics of calcium aluminate cements studies of the system C3A and Cj2Ay-CA, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 4, pp. 339-343. 

Nilforoushan, M.R., and Sharp, J.H. (1995) The effect of additions of alkaline earth metal chlorides on the setting behaviom of refractory calcium aluminate cement. Cement and Concrete Research 25, 1523-1534. 

FAST-SETTING BLENDS OF PORTLAND CEMENT AND CALCIUM ALUMINATE CEMENT... 

Blends of ordinary Portland cement (OPC) and calcium aluminate cement (CAC) exhibit a very fast setting over a wide range of OPC/CAC ratios. Figure 17.1 shows a typical example of such behavior, but the exact setting time will also depend strongly on the... 

The reasons for the fast setting of OPC+CAC blends are not fully understood. It is assumed that in blends high in Portland cement the rapid setting is due to the formation of ettringite in a reaction between the CA phase of the calcium aluminate cement, calcium sulfate present in the Portland cement, and calcium hydroxide formed in the hydration of the tricalcium silicate phase present in Portland clinker ... 

In the CAC-rich region setting is due to hydration of the calcium aluminate cement, which is accelerated in the high-pH environment produced by the presence of Portland cement in the mix. 

If OPC+CAC mixes ate used as fast-setting cements, compositions in which Portland cement is the main constituent ate preferred. Ratios of 75-90% of Portland and 10-25% of calcium aluminate cement may be considered typical. 

To accelerate the process of hardening it has been suggested that carbon dioxide should be injected into the spreadable mix of wood, cement, and water, or that ammonimn, sodium or potassium carbonate should be added (Simatupang et al, 1995). Under these conditions the cement sets in a very short time, owing to the formation of calcimn carbonate. Effective acceleration of the hardening process may also be achieved by the use of a fast-setting cement produced by combining Portland cement with hmited amounts of calcium aluminate cement (see section 10.10.1). 

Other examples of set retarders are ethylenediamine tetraac-etic acid and nitrilotriacetic acid, or a combination of these compounds (11). StiU another example of a set retarder for Portland cement is a copolymer formed from a monomer of 2-acrylamido-2-methyl-l-propane sulfonic acid. Examples of set retarders for calcium aluminate cements are polymeric phosphate salts such as sodium tripol q3hosphate (12). 

Sugama, T., Carciello, N. R., and Gray, G. Alkali carbonation of calcium aluminate cements influence of set-retarding admixtures under hydrothermal conditions. J Materials Sci. 27,4909-4916, 1992. 

This chapterprovides adescription ofthe hydration and engineering behavior of special cements including magnesium oxychloride and oxy sulfate cement calcium aluminate cement portland-calcium aluminate blended cement phosphate cement regulated set cement and gypsum. 

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