Thermodynamic modeling of Portland cement without mineral additives

Abstract

The paper deals with thermodynamic modeling of Portland cement hydration as a complex system, which describes the clinker dissolution, diffusion, and surface phenomena that occur on phase nuclei at different water/cement ratio and depend on the curing time and temperature of the whole process. Using Gibbs Energy Minimization Software (GEMS) and the hydration model proposed by Parrot and Killoh, thermodynamic modeling allows a longterm detection of the amount of both original clinker and nucleating phases such as cement paste, ettringite, Portlandite, and others. The paper presents results of GEMS modeling the phase composition during Portland cement hydration in terms of the Parrot and Killoh model and using the Rietveld refinement technique. Alite, belite, ferrite, and aluminate phases predominate in the cement paste. In order to get the Gibbs free energy of Portland cement in the vicinity of the cement system equilibrium, GEMS modeling considers a balance between the dissolution rate of clinker phases and the deposition of solid solutions of individual phases using thermodynamic parameters at different stages of cement hardening. At the initial stages of hydration, the aqueous solution is oversaturated relative to the amount of elements which determine the Portlandite and ettringite compositions. During the hydration process, the formation of stable complex hydrated silicates occurs, and pH value decreases. It is found that the main components of hardening Portland cement are calcium–silicate–hydrate (C–S–H) and Portlandite which are used herein as reference phases for the identification of their content in Portland cement. Major phases of Portland cement after 5-month curing are studied using the Rietveld method as a tool of the quantitative phase analysis. As a result, the following phase composition is identified: 47.72% tobermorite amorphous phase, 37.40% ettringite, 5.12% portlandite, 0.30% alite and 7.96% belite. The obtained experimental data are in good agreement with theoretical calculations. Ab initio assessments of the binding energy prove the lattice stability in detected major phases.