Delayed Ettringite Formation: Suggestion of a Global Mechanism in Order to Link Previous Hypotheses

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Title: Delayed Ettringite Formation: Suggestion of a Global Mechanism in Order to Link Previous Hypotheses

Author(s): X. Brunetaud, L. Divet, and D. Damidot

Publication: Special Publication

Volume: 222

Issue:

Appears on pages(s): 63-76

Keywords: concrete; cracking; crystallization pressure; delayed ettringite formation; internal sulfate attack; mechanism of expansion

Date: 5/1/2004

Abstract:
Ettringite observed in macroscopic paste cracks during DEF is often held responsible for concrete deterioration. However, some authors have raised the hypothesis of an homogeneous paste expansion resulting from ettringite crystallisation in the C-S-H porosity, which does not attribute any mechanical effects to the subsequent formation of ettringite crystals in the voids. Thus the role of ettringite is still controversial. In this paper, we are tempting to link the two previous hypotheses in a more global mechanism: During heat treatment, thermal decomposition of ettringite can occur, whereas higher amount of sulphate and aluminate are trapped into C-S-H porosity. Then, at ambient temperature, ettringite forms in the porosity corresponding to the network of C-S-H layers. If the volume of ettringite reaches the limit of C-S-H porosity, the following ettringite crystallisation induces an homogeneous expansion of the paste. This expansion can generate peripheral cracks around aggregates. Then, the local shrinkage around voids resulting from the dissolution of the small ettringite crystals incorporated in C-S-H layers and the following precipitation of ettringite massive crystals in the cracks due to Ostwald rippening, outbreaks radial cracks. When the material is weaken by a multidirectional crack network, the pressure exerted by massive ettringite crystallisation can propagate existing cracks by strain localisation at the crack's tip, even if the crystallisation pressure is small in these conditions.