A thermodynamic model for predicting the stability of thaumasite

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• 作者：Juel ; I. ; Herfort ; D. ; Gollop ; R. ; Konnerup-Madsen ; J. ; Jakobsen ; H.J. ; Skibsted ; J.
• 关键词：Thaumasite ; Phase rule ; Portland cement ; Limestone
• 刊名：Cement and Concrete Composites
• 出版年：2003
• 期刊代码：11_09589465
• 类别：ms
• 出版时间：December, 2003
• 卷：25
• 期：8
• 页码：867-872
• 文件大小：350 K

摘要

A model based on the phase rule has been used to predict the hydrate phase mineralogy and phase proportions from the chemical composition of hydrated Portland cement altered by sulfate attack. The eight-component system on which the model is based consists of CaO, SiO₂, Al₂O₃, Fe₂O₃, MgO, CaSO₄, CaCO₃ and H₂O. The phases included in the model are C–S–H, portlandite, ettringite, hydroxy-AFm, monosulfate, monocarbonate, calcite, gypsum, thaumasite, brucite and the pore solution. The model predicts, among other things, that thaumasite, which forms at low temperature, is unstable in the presence of AFm phases, and can only form in systems that would otherwise form gypsum at higher temperatures. The model has been tested experimentally on cement pastes containing 15 and 30 wt.%limestone dust stored at 5 °C, and which were either mixed with different amounts of gypsum and stored in water, or stored in solutions of different MgSO₄ concentrations. The fully hydrated pastes have been analysed by XRD and ²⁹Si CP/MAS NMR, whilst the remaining solution was analysed by ICP. Thaumasite is only found in regions where it has been predicted to form as a stable phase.