AFt phases
The main and most known AFt phase is ettringite, also existing as a natural mineral which was first described in 1874 by J. Lehmann,[6] for an occurrence near the Ettringer Bellerberg Volcano, Ettringen, Rheinland-Pfalz, Germany.
Ettringite is also known as the 'Candlot's salt' in honor of the pioneering work of the French chemist Édouard Candlot [fr] (1858-1922) who studied cement hydration and discovered calcium sulfo-aluminates.
[7][8] Henri Louis Le Chatelier also identified AFt phases when he investigated cement hydration products after the discovery of Candlot.
[10] Feldman et al. (1965) have shown that the hydration reaction of 3CaO·Al2O3 can be suppressed by CaCO3 additions and that this is primarily due to the formation of calcium carboaluminate onto the surface of the C3A grains.
[16] The mechanism is the same as with the sulfate ion released by the more soluble gypsum added to the cement clinker during its milling to prevent the flash setting of concrete.
If nothing is done to control and slow down the C3A hydration rate, concrete can be easily subjected to flash setting, especially if the ambient temperature is elevated during the summer.
A first mechanism can directly depend on the respective thermodynamic stability (solubility: dissolution-precipitation reactions) of each phase, as a function of temperature and pH.
Oxidation of iron(II) sulfides, such as pyrite (FeS2), or pyrrhotite (Fe(1-x)S), sometimes present in construction aggregates can also represent an additional internal source of sulfate into concrete.
[23] As a consequence, the hardened cement paste (HCP) is submitted to an important tensile stress and starts to crack because of the internal expansion of the concrete matrix.
To minimize the risk of DEF in massive concrete structures continuously immersed in water and subject to alkali leaching (bridge piles, locks, sluices, dams), a low Na2Oeq content is therefore also a desirable characteristic for the selected sulfate resisting (SR) cement.
[24][21] In ESA, or when pyrite oxidation is also sometimes involved in contaminated aggregates, beside ettringite crystallization, a lot of gypsum (CaSO4·2H2O) can also be formed in the ultimate degradation stage.
The sulfuric acid (H2SO4) produced by pyrite oxidation also dissolves carbonates present in the surrounding clay, or directly in the concrete aggregates, freeing up the two main ingredients necessary for this very deleterious pathology.
To minimize, and ideally to avoid, delayed ettringite formation (DEF; synonym: internal sulfate attack, ISA), several precautions can be taken:[19][25] For massive concrete structures whose temperature at core will likely exceed 65 °C during the cement setting and hardening and moreover will be immersed under water or exposed to alkali leaching, the only effective solution to avoid, or to minimize, delayed ettringite formation is to eliminate the tricalcium aluminate C3A phase in the cement clinker, or to drastically reduce its content.
Concomitantly, it will also make it possible to limit the quantity of gypsum added to clinker during its grinding to avoid the risk of cement flash setting due to the very exothermic hydration reaction of C3A.
However, this precaution is insufficient to completely eliminate the risk of formation of thaumasite (TSA) because this latter can still develop, although with more difficulty, in concrete made with sulfate-resistant cement.
