High alumina cement hydration

A. N. Scian, Lopez. J. M. Porto, and E. Pereira. Mechanochemical activation of high alumina cements— hydration behavior Pt 1. Cement Concrete Res, 21(l) 51-60, January 1991. 

Monocalcium aluminate, a major component of high alumina cement, hydrates to metastable products CAHjo, C4AHj3, and C2AHg which eventually convert to the stable cubic aluminum hydrate, CjAH. In the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA results have shown that the degree of conversion of CAHjo and C2AHg phases to the cubic phase is marginally retarded. 

Fig. 6.11 Comparison of expansion for admixture containing coarse and fine grinds of hydrated high alumina cement (courtesy NRC).

High-alumina cement, Lumnite (Universal Atlas Division of U.S. Steel Corp.), is not a Portland cement at all, but consists entirely of calcium aluminate. It reacts with water to form several hydrated species and can give an exceptionally strong and refractory product. 

Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAl204 (or CA in cement chemist s shorthand, where C = CaO and A = AI2O3). The Al NMR spectra of this compound, in which the Al is exclusively in tetrahedral coordination, and a number of other calcium alu-minates have been determined (Muller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. 

Table 5.4. NMR parameters for calcium aluminates and their hydration products in Portland and high alumina cements, from Skibsted et al. (1993). Chemical shifts relative to Al(H20)e. ...

The cement should comply with EN 197 [26.32] (N.B., high alumina cement should never be used with lime). Lime, in the form of quicklime, hydrated lime, or lime putty, should eomply with EN 459 [26.18] see section 27.6.7. 

The hydration reaction of other calcium aluminates is similar to that of CA however, the mutual ratio of the individual lydrate phases formed may be altered. In the hydration of C 2 i the fraction of C2AHg formed is increased at the expense of CAHjq, whereas in the hydration of CA2 increased amounts of AH3 are formed. There are, however, distinct differences in the rate of hydration between different calcium aluminates in general, the reactivity increases with increasing C/A ratio. Among the constituents of high-alumina cement, CA2 is the phase with the slowest hydration CA hydrates faster, and Cj2A2 is even more reactive (Das and Daspodda, 1993). 

Alkaline-earth metal chlorides have been found to either accelerate or retard the hydration of high-alumina cement, depending on the amoimt added and the temperature (Nilforoushan and Sharp, 1995). 

Ding, J., and Beaudoin, J.J. (1996) Study of hydration mechanism in the high alumina cement—sodium silieate system. Cement and Concrete Research 26,799-804. 

Midgley, H.G. (1980) The relationship between cement clinker composition and strength recovery of hydrating high alumina cement during conversion, in Proceedings 7th ICCC, Paris, Vol. 3, pp. V68-70. 

Type M expansive cement is a combination of ordinary Portland cement, calcium aluminate cement, and additional gypsum or calcium hemihydrate, typically in the ratio 66 20 14 (Mikhailov, 1960 Budnikov and Kravchenko, 1968). It may be produced by mixing these constituents in a plant. Alternatively, an expansive additive consisting of high-alumina cement, gypsum or henuhydrate, and hydrated lime may be added to the fresh concrete mix during mixing. By this approach the extent of expansion may be controlled by the amount of additive added to the mix. 

A sodium metasilicate modified high-alumina cement may be used in geothermal wells at temperatures up to 300°C (Sugama and Garciello, 19%). Sodium calcium silicate hydrate and boehmite are formed as products of reaction in the hydrothermal reaction. 

A source of calcium-aliuninium carriers and adsorbents is provided by calcium mono- and di-aluminates, CaAl204 (CA) and CaAl407 (CA2), which are the principal phases in high-alumina cements. The first stage in the preparation of these carriers is hydration which, at elevated temperatures, gives mainly Ca3[Al(OH)6]2 (CaAHe) ... 

Related to the aluminas, especially the /3-aluminas, are tricalcium aluminate and its hydrated derivatives. These are major components of Portland cement and of high-alumina cement. A structural modification and the heat released on hydration are factors in the setting process of these cements. 

Hydration of high alumina cement requires more water, which results in mixes of better workability. The hydration process starts later, after mixing with water, but develops more quickly with high rate of liberated heat. Also, hardening is quick and usually within 24 hours about 80% of final resistance is obtained. The hardened cement has both lower porosity and permeability. 

The first applications of glass fibres in concrete-like composites were published by Biryukovitch et al. (1965), who carried out tests on elements with high alumina cement. The paste made with that cement is characterized by pH from 11.8 up to 12.05 with total amount of alkalis from only 0.15% to 0.20% and during hydration calcium hydroxide does not appear. In the... 

Calcium chloride also has an effect on the hydration of various other cementitious systems such as pozzolanic cements, slag cements, expansive cements, high alumina cement, g q)sum, rapid hardening cement, etc. (See, for example. Refs. 1 and 40.)... 

Ma, W., Brown, P. W., Hydration of Sodium Phosphate-Modified High Alumina Cement J. Mater. Res., 9 1291-1297 (1994)... 

Fu, Y., Xie, P., Gu, P., Beaudoin, J. J., Characteristics of Shrinkage Compensating Expansive Cement Containing a Pre-hydrated High Alumina Cement-based Expansive Additive, Cement Conor. 24 267-276 (1994)... 

Fu, Y., Ding, J., and Beaudoin, J. J., Effect of Different Caleium Alumina Hydrates on Ettringite Formation and Expansion of High Alumina Cement-Based Expansive Cement Pastes, Cement Conor. Res., 26 417—426 (1996)... 

Gu, P., Ramachandran, V. S., and Beaudoin, J. J., Study of Early Hydration of High Alumina Cement Containing Phosphonic Acid by Impedance Spectroscopy, J. Mater. Sci. Lett., 14 503-505 (1995)... 

In principle, all the concepts of surface chemistry previously described are also expected to be vahd for cement particles. However, the highly dissolving and reactive nature of such particles introduces a dynamic and complex feature to the particle-water system. This makes it difficult to interpret the data obtained from cement-containing suspensions, which is probably the main reason for the paucity of publications available in the literature that focus on the interactions between additives and high-alumina cement particles. The concepts described herein are therefore based on sffidies performed in aqueous pastes of calcium silicate (Portland) cement, which have been more extensively researched in recent years due to their considerable importance for concrete technology (47-50). Although calcium aluminate and calcium silicate cements are known to display distinct reaction rates and form different hydration products, similar interactions between the cementious phase and the chemical additives (admixtures) are expected to occur in aqueous suspensions. 

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