Concrete delayed ettringite formation

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. 

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. 6.37 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])...

Fu, Y., and Beaudoin, J.J. (1996) Microcracking as precursor to delayed ettringite formation in cement systems. Cement and Concrete Research 26,1493-1498. 

Concrete may deteriorate if adequate precautions are not exercised to protect it from adverse effects that could result from exposure to natural or artificial conditions. Several physical, chemical, and electrochemical processes are known to induce cracking of concrete. Concrete can have durability problems as a consequence of its exposure to seawater, sulfates, chlorides, freeze-thaw action, carbon dioxide, etc., or when it is attacked by artificially induced processes such as exposure to acids and salts in chemical plants or to fire. In recent years, a new type of durability problem was encountered that involved use of steam cured concrete products. The distress was caused by the formation of delayed ettringite. If the raw materials in concrete are not carefully controlled, there may be an eventual failure of concrete elements, eg., the presence of excess alkali in concrete that promotes alkali-aggregate expansion reaction, harmful impurities in the aggregates, or the presence of excess amounts of dead-burnt MgO. Thermal techniques in combination with others have been employed with success to examine the raw materials as well as the failed concrete. The knowledge gained from such work has been applied to produce more durable concrete. 

---