Vector modelling of hydrating cement microstructure and kinetics

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

Reactivity tests for supplementary cementitious materials: RILEM TC 267-TRM phase 1

A primary aim of RILEM TC 267-TRM: “Tests for Reactivity of Supplementary Cementitious Materials (SCMs)” is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well

Discussion of the paper "Accelerated growth of calcium silicate hydrates" by Luc Nicoleau

Some of the limitations of the model recently used by Nicoleau in a recent article [Accelerated growth of calcium silicate hydrates: experiments and simulations, Cement Concr. Res. 21 (2011) 1339 - 1348.] are discussed. (C) 2012 Elsevier Ltd. All rights reserved

μic: A new platform for modelling the hydration of cements

A new modelling platform, called mu ic has been developed to model the microstructural evolution of hydrating cement paste. The platform uses the vector approach and can be used for modelling particulate reactions including the hydration of many different cementitious systems involving millions of particles. In this paper, the ideas behind the development of mu ic and its main features are presented. The complexity of cement hydration and the gaps in our current understanding of cement played an important role in its design, so the platform has the primary objective of aiding, rather than replacing experiments. The platform is highly customisable as users can define materials, particles and reactions and choose or create external plugins to define models of microstructural development The platform can be used to test the validity of hypotheses by easily formulating them as input and comparing simulations with experimental results. This paper presents the design of mu ic and examples that demonstrate the important features of its performance and design. (C) 2008 Elsevier Ltd. All rights reserved

Modelling early age hydration kinetics of alite

The modelling platform mu ic [1] has been used to investigate the mechanisms occurring during the hydration of alite. It is shown that it is possible to obtain a good simulation of the hydration kinetics through the implementation of two mechanisms: a dissolution mechanism combined with nucleation and growth of products. The dissolution rate is varied according to the ratio beta, between the ion activity product and the equilibrium solubility product according the theory published by Juilland et al. [2]. The solution concentrations are computed directly from the amount of alite dissolved taking into account the amount of water present and the amount of products formed, with activities and complex ion formation calculated according to standard methods. Saturation index calculations are implemented to compute the time of precipitation of C-S-H and portlandite (CH) individually. For the main heat evolution peak, the rate controlling mechanism switches to a modified form of boundary nucleation and growth. C-S-H grows in a diffuse manner in which the density of packing of the C-S-H phase increases with hydration [3]. The rate of heat evolution obtained from the simulations is compared with isothermal calorimetry data and good agreement is found. (C) 2012 Elsevier Ltd. All rights reserved

Microstructural modelling of the elastic properties of tricalcium silicate pastes at early ages

This paper describes the numerical calculation of elastic properties of a simulated microstructure of cement paste from very early age, when most previous models fail to give accurate results. The development of elastic properties of tricalcium silicate pastes was calculated by discretising a numerical resolution-free 3D vector microstructure to a regular cubic mesh. Due to the connections formed in the microstructure as an artefact of the meshing procedure, the simulated elastic moduli were found to be higher than expected. Furthermore, the percolation of the solids was found to occur even before hydration started. A procedure to remove these artefacts, on the basis of the information available in the vector microstructures was developed. After this correction, a better agreement of the experimental results with calculations was obtained between 20% and 40% hydration. However, percolation threshold was found to be delayed significantly. More realistic estimates of percolation threshold were obtained if either flocculation or a densification of calcium silicate hydrate with hydration was assumed