Ettringite formation

The reactions in the regulated-set cements containing Cjj A3CF2 (note mixed notation) as a principal phase resemble those in ordinary Portiand cements. Initial reaction rates are controlled by ettringite formation. Setting occurs with formation of the monosulfate, along with some transitory lower-limed calcium aluminate hydrates that convert to the monosulfate within a few hours. 

Type and amount of aluminates, calcium sulfate and free lime present in the mixture governs the rate at which ettringite forms. The presence of lime is reported to be essential for both the initial and subsequent stages of ettringite formation because it maintains a solution phase saturated with calcium ions [81]. Crystal habit, particle size and range of particle size in CSAs determine its rate of hydration and duration of expansion (Fig. 6.11). 

Day, R. L. 1992. The Effect of Secondary Ettringite Formation on the Durability of Concrete A Literature Analysis. Research and Development Bulletin RD108T, Portland Cement Association, Skokie, IL. 

Mehta, P. K. 1973. Mechanism of expansion associated with ettringite formation. Cement and Concrete Research, 3, 1 -6. 

Solem, J. K.. McCarthy, G. J. 1992. Hydration reactions and ettringite formation in selected cementitious coal conversion by-products. Material Research Society Symposium Proceedings, 245, 71-79. 

Solem-Tishmack, J. K., McCarthy, G. J., Docktor, B., Eylands, K. E., Thompson, J. S. Hassett, D. J. 1990. High-calcium coal conversion by-products Engineering properties, ettringite formation, and potential application in solidification and stabilization of selenium and boron. Cement and Concrete Research, 25, 658-670. 

Tishmack, J. K. 1999. Characterization of High-Calcium Fly Ash and Its Influence on Ettringite Formation in Portland Cement Pastes. PhD dissertation, School of Civil Engineering, Purdue University, West Lafayette, IN. 

Tishmack, J. K., Olek, J. Diamond, S. 1999. Characterization of high-calcium fly ashes and their potential influence on ettringite formation in cementitious systems. Cement, Concrete, and Aggregates, 21, 82-92. 

Hassett Thompson (1994) stated that ettringite formation requires the presence of constituent cations and anions and pH above 11.5. These conditions are met upon contact of high-Ca CCBs with moderate amounts of water, gen-... 

From the results the tendency of ettringite formation can be observed in those samples that do not have latex. In fact, this phase showed a significant reduction in peak intensity, possibly indicating that the latex inhibits its formation due to reduction of sulfate/aluminate ratio (Figures 6 and 7). Ettringite is a phase that does not remain in the concrete at ages over 28 days with time it gradually incorporates hydrated monosulfate [18]. 

Corrosion is not the only deterioration mechanism in reinforced concrete. Alkali-silica reactivity (ASR), sulphate attack, thurmasite attack, delayed ettringite formation, freeze thaw, thermal movement, settlement and other movement can all lead to concrete damage and their assessment must be included in the surveys. 

Bonin discussed the new results explaining the reasons of paste or concrete stiffening [37]. Apart of false set, occurring as the result of alkalis carbonation, which in turn react with calcium hydroxide with CaCOj precipitation, Bonin considers the case of cement paste stiffening, in which under moisture influence the rate of ettringite formation had changed [37]. 

The aforementioned reactions schemes of ettringite formation takes place in reality between ions, in solution ... 

According to Berman and Newman [ 156] the C3A hydration with ettringite formation is accompanied by the heat evolution on the level 816 J/g SO3 with the monosulphate formation—783 J/g respectively. Applying the Hess rule one can write ... 

The mechanism of their action has not been explained. There are many hypotheses, but primarily those attributing this effect to the liquid phase composition changes should be mentioned [27, 35], In the presence of alkalis the calcium ions concentration is reduced (Fig. 4.45) and simultaneously the OH ions concentration is increased. The increasing ions OH" concentration is accompanied probably with the higher rate of sulphates bonding by the ettringite formation [126]. This hypothesis is confirmed by the significant effect of NaOH addition on hydration acceleration, similarly of KOH, but somewhat weaker [45],... 

Spmng and Rechenberg [132] state that in the pastes of cements rich in alkalis C3A dissolves quickly because of the reduced Ca(OH)2 solubility. At low gypsum content the quick set can be observed as a result of the hexagonal calcium aluminate hydrates formation In the presence of gypsum excess the similar effect can be the consequence of ettringite formation. 

The thermal treatment of concrete can cause the detrimental effect of delayed ettringite formation, resulting in the deterioration of concrete elements. The problem of delayed ettringite formation belongs to the internal corrosion of concrete and will be discussed in the chapter relating to this corrosion. 

The delayed ettringite formation is rated among the internal corrosion of concrete, which is induced by a heat treatment of concrete, and thus occurring mainly during the precast concrete elements production. However, one carmot exclude the temperature rise up to 70 °C in the interior of massive concrete stracture, as a consequence of heat evolution process in hardened cement paste, and in this condition ettringite can be unstable. 

Fig. 6.29 Expansion of mortar from cement with high alkali content and granite aggregate vs time. Segment 3 results presumably from the delayed ettringite formation. (According to [85, 86])...

Fig. 6.37 Pre stressed concrete railway sleeper with cracks caused by the delayed ettringite formation...

The other stormy discussed problem refers to the role of anhydrite present in cement clinker, which slow dissolution is the cause of delayed ettringite formation [153]. In this case the delayed ettringite formation will be not linked with the heat treatment of concrete. 

The cause of delayed ettringite formation was elucidated based on the ettring-ite stability in concrete. As it is known, ettringite is stable at least at temperature 100 °C, in saturated water vapour [157]. However, Wieker et al. [151] found that ettringite is not stable at higher temperatures in the presence of alkalis. The results of these studies are presented in Table 6.2. 

Fig. 6.43 Famy s model of expansion meehanism, related to the delayed ettringite formation. Concrete treated at temperature > 70°C (Aeeording to [152])...

---