Some notes of ettringite formation in cementitous materials; Influence of hydration and thermodynamic constraints for durability
✍ Scribed by L.O. Höglund
- Publisher
- Elsevier Science
- Year
- 1992
- Tongue
- English
- Weight
- 695 KB
- Volume
- 22
- Category
- Article
- ISSN
- 0008-8846
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✦ Synopsis
The deteriorating action of sulphate on cementitious materials has been known since long. This has been attributed tO the formation of certain expanding minerals in the cement, e.g. ettringite and monosulphate. In this paper a simplified model is presented for prediction of the consequences of sulphate intrusion into different concretes.
In studying the mechanisms responsible for sulphate swelling in concretes used as barrier material and waste conditioning material in repositories for Low-and Intermediate Level radioactive waste, the need for a simple, but yet reliable model, has been recognised. Such a model has been developed which facilitates the estimation of the properties of cements and concretes in different states of maturity. Although the model is fairly detailed in order to predict both chemical and physical characteristics of the studied cements and concretes, it has been developed to serve as a tool for simplified performance assessment studies on the longevity of concrete barriers, and is not primarily intended as a detailed mechanistic research model.
The model accounts for different important hydration processes, which define the initial conditions in the concrete. Based on the chemical composition of the unhydrated cement, the phase assembly of the hydrated cement is estimated. For evaluation of the potential ettringite formation in the cement material in the long-ten'n, only thermodynamically stable minerals have been regarded.
The amount of water bound during the hydration is calculated in order to predict the porosity of the mature concrete. Based on the assembly of hydrated minerals, the model is used to estimate the response to additional sulphate supply, e.g. by percolation of ground water containing sulphate, or by degradation of different waste components producing st,!phate. The model calculates the volume expansion of the solid phases.
Some comments are given on different parameters which may influence the potential action of sulphate, e.g. the chemical composition of the cement, the mixing portions such as water to cement ratio and its influence on the porosity.
Comments are also given on different thermodynamic constraints which may influence the formation of ettringite and related substances, e.g. mechanical pressure, chioride content and temperature.