Significance of Delayed Ettringite Formation and Damage Mechanisms
Ettringite is formed in hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium sulfate, both present in Portland cement. In the cement system, the presence of ettringite depends on the ratio of calcium sulfate to tri-calcium aluminate (C3A); when this ratio is low, ettringite forms during early hydration and then converts to the calcium aluminate monosulfate.
The primary ettringite formation in the initial stage of the hydration is seen as a positive effect because it enables the setting regulation, a damaging role is often attributed to delayed ettringite formation in hardened concrete. Damaged concretes often show large amounts of ettringite in structure damages even if they were not exposed to heat treatment (e.g. pavement concrete).
Delayed ettringite formation is a consequence of improper heating during the curing process. It is a normal product in concrete and is delayed when the temperature rises too high for it to form. During this period of high temperatures (above 158°F-176°F), the sulfate in cement paste starts to concentrate for a longer than usual period of time. In response, the sulfate will react with aluminum and calcium in the paste and start to expand, thus creating gaps that form fully around aggregate . This ultimately leaves an open space in the concrete for visible cracking and displacement to form.
Eventually it impairs the integrity of the structure and can start to expose inner reinforcement leading to problems like corrosion. DEF is not a common type of failure in concrete because it is easily prevented. One can limit the temperature of the concrete to 158°F while it is curing as well as making sure that no extra water or moisture is brought in from an outside source.
From the different theories on ettringite formation in hardened concrete, described in literature, possible damage mechanisms can be derived as follows:
- The primary ettringite formation during the initial hydration does not lead to damage, because this ettringite formation occurs in the plastic matrix and thus no stresses will be produced.
- If the ettringite formed primarily or delayed inside the micro-structure is micro-crystalline, than in hardened concrete it may develop an expansion pressure due to adsorption of water, which can cause damages if the tensile strength of the micro-structure is exceeded.
- Ettringite formed in the hardened concrete structure from less-sulfate containing compounds either delayed (due to higher temperatures during the initial hydration) or additionally (due to internal sulfate sources – e.g. through the affect of frost-, frost-deicing salt, through carbonation, from the clinker) may due to the crystal growth or due to the increase in volume cause stresses which exceed the tensile strength of the structure and can therefore cause damages.
- (The transformation of monosulfate into ettringite, for example, causes a 2.3-times increase in volume.)
- The recrystallization of ettringite in the hardened structure, due to moisture changes and accumulation of reactants, may lead to structure damages because of the crystallization pressure and the increase in volume.