Abstract
Thermodynamic equilibrium calculations for cementitious materials enable predictions of stable phases and solution composition. In the last two decades, thermodynamic modelling has been increasingly used to understand the impact of factors such as cement composition, hydration, leaching, or temperature on the phases and properties of a hydrated cementitious system. General thermodynamic modelling codes such as GEM-Selektor have versatile but complex user interfaces requiring a considerable learning and training time. Hence there is a need for a dedicated tool, easy to learn and to use, with little to no maintenance efforts. CemGEMS (https://cemgems.app) is a free-to-use web app developed to meet this need, i.e. to assist cement chemists, students and industrial engineers in easily performing and visualizing thermodynamic simulations of hydration of cementitious materials at temperatures 0-99 °C and pressures 1-100 bar. At the server side, CemGEMS runs the GEMS code (https://gems.web.psi.ch) using the PSI/Nagra and Cemdata18 chemical thermodynamic data-bases (https://www.empa.ch/cemdata).
 The present paper summarizes the concepts of CemGEMS and its template data, highlights unique features of value for cement chemists that are not available in other tools, presents several calculated examples related to hydration and durability of cementitious materials, and compares the results with thermodynamic modelling using the desktop GEM-Selektor code.
Highlights
Thermodynamic equilibrium calculations of cementitious systems have gained increased visibility in the last twenty years
This is an advanced feature of CemGEMS because heat generation curves can be directly compared with calorimetric data, and they are sensitive to hydration kinetics of clinker and supplementary cementitious materials (SCMs) constituents
General thermodynamic modelling codes such as GEMSelektor or PHREEQC are very versatile and offer many expert options, but their complexity requires an advanced level of understanding of chemical thermodynamics, and a dedicated training on efficiently using the codes
Summary
Thermodynamic equilibrium calculations of cementitious systems have gained increased visibility in the last twenty years. CemGEMS is supported by a detailed Tutorial site (https://cemgems.org) and a YouTube channel (https://www.youtube.com/channel/UCoKikRCou9npMcXjw6n hjxg) for posting training screencasts, which adds a value for education in cement chemistry It is important, to point out that, even though CemGEMS is user-friendly and its templates are based on reasonable expert assumptions on which phases can form in hydrated cements, CemGEMS (as any other thermodynamic modelling tool) should not be used blindly as a "push-the-button application". The assumptions which stable phases (e.g. thaumasite, gibbsite, goethite, siliceous hydrogarnet, etc.) might form, and whether their formation is kinetically limited, can have a major effect on the calculated hydrate- and pore solution composition In many cases, this can only be elucidated based on a careful comparison of modelling and experimental results. The present paper gives a brief description of CemGEMS web app, emphasizing its innovative concepts and features of value for cement chemists that are not available in other tools; and compares the results obtained by using CemGEMS with the results obtained by thermodynamic modelling with GEM-Selektor code, exemplified for hydration of (blended) Portland cements, carbonation of hydrated cements, and the effect of limestone addition on hydrated calcium sulfoaluminate cement
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