Physical and Chemical Aspects of the Nucleation of Cement-Based Materials

Pavel Demo,Šárka Hošková,Alexey Sveshnikov,David Ladman,Petra Tichá

doi:10.14311/1668

Abstract

A theoretical model of the nucleation of portlandite is proposed, and the critical size of a portlandite cluster and the energy barrier of nucleation are determined. The steady state nucleation rate and the time lag of the nucleation of portlandite are estimated for a pure solution of Ca(OH)2 in water. Possible connections with the corresponding properties for cement paste are discussed. A new method is developed for experimentally determining the concentration of Ca2+ ions during the initial stage of hydration of a cement paste. The time dependence of Ca2+ ions is measured for various water-to-cement ratio values. The results are discussed from the point of view of existing models of the induction period.

Highlights

Cementitious materials are one of the most widelyused types of materials in modern civil engineering
We have proposed a new method for experimentally determining the concentration of Ca2+ ions in hydrating cement paste during the first few hours after mixing cement with water
The method is based on a standard chelatometric titration method, but uses EDTA both as the retardant of the hydration process and as the volumetric reagent for calcium ions

Summary

Introduction

Cementitious materials are one of the most widelyused types of materials in modern civil engineering. After the initial stage of exothermic reactions, the cement paste “falls asleep”, and for the few hours practically no changes are observed in the state of the paste The duration of this so-called dormant (or induction) period directly determines the workability, since the strength of the paste begins to grow steadily after the dormant period finishes and the concrete begins to set. The membrane prevents the ions dissolved in the inner solution from reaching the main volume of the solution According to this hypothesis, the induction period ends when the osmotic pressure inside the membrane grows high enough to break it [1, 6]. The growing particles of the new phase have to overcome a certain energetical barrier by means of fluctuations, and this cannot happen instantly This hypothesis explains the induction period as an intrinsic time lag of the nucleation process [3, 10]. We measured the concentration of Ca2+ in a hydrating cement paste, and combined this with theoretical modelling of the nucleation of a water solution of calcium hydroxide to estimate the nucleation time lag of portlandite formation

Experimental

Conclusion