Ettringite carbonated phase

Carbon dioxide dissolves in the pore solution of cement paste, producing CO3", which reacts with Ca to produce CaCOj. The OH and Ca ions required by these reactions are obtained by the dissolution of CH and decomposition of the hydrated silicate and aluminate phases. C4AH. is quickly converted into C ACH, and ultimately into CaCOj and hydrous alumina monosulphate and ettringite yield CaCOj, hydrous alumina and gypsum. C-S-H is decalcified, initially by lowering of its Ca/Si ratio, and ultimately by conversion into a highly porous, hydrous form of silica. The... 

The destruction of concrete due to the formation of thaumasite replacing the C-S-H phase is a special case of sulphate corrosion. This process is promoted by the simultaneous intensive carbonation, or by use of limestone Portland cement, together with the low temperature. In Poland the destruction of concrete stractures due to the thaumasite formation has not been reported, however, this ean arise from the multicomponent paste composition of damaged concrete and very similar XRD pattern of thaumasite and ettringite. 

Sulfobelite cement is more sensitive to carbonation than ordinary Portland cement In addition to carbonation as it exists in ordinary Portland cement, which involves the poitlandite and the C-S-H phases, the ettringite present in the hardened paste may also react with the COj in the air, yielding calcite or aragonite, gypsum and alumina gel (Beretka et a/., 1992, 1997 Sherman et a/., 1995) ... 

The response of blended cements to sulfate attack may be distinctly improved by a caibonation of the surface of the material prior to exposure to sulfates (Sersale et al 1997). Improved resistance may also be achieved by adding finely dispersed calcium carbonate to the original mix. In its presence monocalcium caiboaluminate (C3A.CC.Hj2) is formed instead of monosulfate, and this phase does not convert to ettringite in the course of sulfate attack (Piasta et al, 1997). 

In addition to calcium carbonate, the formation of scawtite (Ca7Si502iH5 C03) and its decomposition with the evolution of CO2 at lower temperatures was reported through TG investigations It has also been reported that by applying DTA/TG techniques the calcium carbonate formed by exposure of cement paste to CO2 migrates to the surface of concrete in the form of layers. The rate of carbonation on low and high alkali cements has not been found to be different and that the C-S-H and ettringite phases are also carbonated. 

The carbonation of this matrix leads to the decomposition of ettringite into gypsum and CaCOa. It was suggested [124] that the ettringite in this system bonds together the other components of the matrix (CSFI and unreacted slag) and therefore this phase, even when present in small quantities, exerts a considerable influence on the mechanical properties. Various methods have been considered to overcome the carbonation problem in the supersulphated cement GRC, including surface compaction and the application of a protective layer [123]. 

To quantify bound water, it is recommended to remove the free water by solvent exchange and to avoid the often-used predrying at 105°C, as drying at 105°C removes water from the ettringite, C-S-H and AFm phases. The temperature range from 50 to 550 or 600°C is recommended to exclude carbonates. 

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