Faculté des sciences et techniques de l'ingénieur STI, Programme doctoral Sciences et Génie des matériaux, Institut des matériaux IMX (Laboratoire des matériaux de construction LMC)

Vector modelling of hydrating cement microstructure and kinetics

Bishnoi, Shashank ; Scrivener, Karen (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 2008 ; no 4093.

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    A new modelling framework, called μic, has been developed to enable simulations of complex particulate growths, in particular the microstructural evolution of hydrating cement paste. μic has been developed using the vector approach, which preserves the multi-scale nature of the cement microstructure. Support libraries built into the framework enable fast simulation of systems containing millions of particles, allowing every single particle in a system to be modelled and all the interactions to be calculated. The modelling framework has been developed using object oriented programming and its extensible and flexible architecture, due to this microstructural development mechanisms and algorithms can be easily added. The framework facilitates the otherwise complex task of modelling new systems and phenomena. The microstructures generated by μic can be used to obtain important information that can in the future be used to model the evolution of mechanical properties and durability-related phenomena. The model can also be used to study the mechanisms of microstructural development of cement. Various models of cement hydration kinetics and the reaction mechanism were tested using μic. It was observed that while the traditional approach to the nucleation and growth mechanism could be used to explain the acceleration of reaction-rates during the early hydration of cement pastes, the subsequent deceleration could not be reproduced. If a diffusion controlled mechanism is used to explain the deceleration, changes larger than an order of magnitude in the transport properties of C-S-H have to be assumed. Furthermore, the rate of change of reaction rates shows a continuous linear evolution through the reaction peak and the thickness around different particle sizes would be very different at the onset of the supposed diffusion regime. It was found that it is possible to explain the hydration kinetics during the first 24 hours using a nucleation and growth mechanism when a loosely packed C-S-H with a lower bulk density is assumed to form. It is proposed that this loosely packed C-S-H fills a large fraction of the microstructure within a few hours of hydration and that its density continues to increase due to an internal growth process within the bulk of the product. It was found that an initial density of C-S-H between 0.1 g/cc and 0.2 g/cc was required in order to fit the observed experimental behaviour. While this density is much lower than the generally accepted range of 1.7 g/cc to 2.1 g/cc, this low packing density can explain the absence of water in large capillary pores observed in NMR measurements that study cement hydration on wet samples, and the fibrous or ribbon-like nanostructure of C-S-H observed in high-resolution TEM images. The current study demonstrates the versatility of μic and how the possibility of modelling different phenomena on a multi-scale three-dimensional model can prove to be an important tool to achieve better understanding of cement hydration. It was also shown that the use of mechanistic, rather than empirical, rules can improve the predictive power of the models.