ABSTRACT
Delayed ettringite formation is a rare phenomenon in concrete. It occurs when the concrete temperature at the time of placing or soon afterwards exceeds about 70◦C. This suppresses the formation of ettringite in the concrete, which instead appears later, after hardening, in an amorphous form. The sulfate concentration in the pore liquid is high for an unusually long period of time in the hardened concrete. Eventually, in the presence of moisture, the sulfate reacts with calcium-and aluminium-containing phases of the cement paste and the cement paste expands. Due to this expansion empty cracks (gaps) are formed around aggregates. The cracks may remain empty or later be partly or even completely filled with ettringite. (Concrete Experts, 2006). In its early stages, crystalline ettringite is not seen, though some darkening of the paste may be observed (Grantham et al, 1999). Once exposed to moisture, significant cracking can result in the concrete with expansions of 5 mm/m or more being observed.
