Ettringite expansion hypotheses

Cohen (C55) reviewed theories of the expansion mechanism. Probably a majority of workers (e.g. Refs I9,I10,O16,O24,BI33) have attributed expansion to forces exerted by the growth of the ettringite crystals. Of other theories, the most significant is that proposed by Mehta (MlOO). who attributed it to imbibition of water by the gelatinous layer of colloidal ettringite. On this hypothesis, expansion does not occur with the larger crystals formed at low CaO concentrations because these do not form... 

On the base of these EDS studies, a hypothesis was advanced that the formation of ettringite, inside this bright outer C-S-H gel, would result in expansion [170]. The crystallization of massive ettringite is a secondary process and is not related with expansiom... 

Chatteiji [51, 52] pnts forward several times the hypothesis that the snlphate expansion is cansed by monosnlphate formation. However, the results reported by Alunno Rossetti et al. [53] disprove this hypothesis the compacted C ASH and CS mixtures revealed high expansion due to the ettringite formation. 

The expansion mechanism has been the subject of numerous studies for mai r years. It is caused by the known contradiction consisting in the lowering of absolute volume of the system, due to ettringite or other hydrates formation this problem is discussed in Chap. 5. The oldest hypothesis was presented by Lafuma [60], which already in 1929 suggested that the formation of ettringite as a result of reaction in the solid state (topochemical reaction) will cause expansion, while occurring... 

Clastres and Murrat [72] foimd the correlation between the expansion and the growth of ettringite content, as determined by DTA (Fig. 9.14). On the other side the expansion is increasing after the calcinm sulphate consumptiom Therefore the authors [72] are of the opinion that this proeess is in agreement with hypothesis proposed by Mehta [73] in this hypothesis the expansion oeeuis due to the water absorption by the colloidal ettringite. 

Dron and Brivot [80] prepared the tablets of groimd ettringite pressed imder the pressure of 50 MPa, with low porosity, of about 17%. The tablets treated with water do not practically swell—swelling is on the level 0.4%, while in situ ettringite formation from calcium sirlphoaliuninate gives the volume increase of 40% and the destraction of samples. The authors [80] state that the Mehta hypothesis is not valid and the crystallization pressure is the cause of expansion. Simultaneously Dron and Brivot [80] calculated the approximate crystalhzation pressure based upon the change of free enthalpy of reaction ... 

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