Concrete degradation mainly caused by ettringite crystallization
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Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C3S) (alite), and dicalcium silicate (C2S) (belite).[1] Upon hydration, the main reaction products are calcium silicate hydrates (C-S-H) and calcium hydroxide Ca(OH)2, written as CH in the cement chemist notation. C-S-H is the phase playing the role of the glue in the cement hardened paste and responsible of its cohesion. Cement also contains two aluminate phases: C3A and C4AF, respectively the tricalcium aluminate and the tetracalcium aluminoferrite. C3A hydration products are AFm, calcium aluminoferrite monosulfate, and ettringite, a calcium aluminoferrite trisulfate (AFt). C4AF hydrates as hydrogarnet and ferrous ettringite.
Sulfate attack typically happens to ground floor slabs in contact with soils containing a source of sulfates.[2] Sulfates dissolved by ground moisture migrate into the concrete of the slab where they react with different mineral phases of the hardened cement paste.
The attack arises from soils containing SO2− 4 ions, such as MgSO4 or Na2SO4 soluble and hygroscopic salts. The tricalcium aluminate (C3A) hydrates first interact with sulfate ions to form ettringite (AFt). Ettringite crystallizes into small acicular needles slowly growing in the concrete pores. Once the pores are completely filled, ettringite can develop a high crystallization pressure inside the pores, exerting a considerable tensile stress in the concrete matrix causing the formation of cracks. Ultimately, Ca2+ ions in equilibrium with portlandite (Ca(OH)2) and C-S-H and dissolved in the concrete interstitial water can also react with SO2− 4 ions to precipitate CaSO4·2H2O (gypsum). A fraction of SO2− 4 ions can also be trapped, or sorbed, into the layered structure of C-S-H.[3] These successive reactions lead to the precipitation of expansive mineral phases inside the concrete porosity responsible for the concrete degradation, cracks and ultimately the failure of the structure.
^Lea, F.M.; Hewlett, P.C. (1998). Lea's chemistry of cement and concrete (4th ed.). London: Arnold. ISBN 0340565896. OCLC 38879581.
^Tian, Bing; Cohen, Menashi D (January 2000). "Does gypsum formation during sulfate attack on concrete lead to expansion?". Cement and Concrete Research. 30 (1): 117–123. doi:10.1016/S0008-8846(99)00211-2.
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