Digitized Direct Simulation Model of the Microstructural Development of Cement Paste
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DIGITIZED DIRECT SIMULATION MODEL OF THE MICROSTRUCTURAL DEVELOPMENT OF CEMENT PASTE DALE P. BENTZ and EDWARD J. GARBOCZI National Institute of Standards and Technology Building Materials Division, 226/B348 Gaithersburg, Maryland 20899 ABSTRACT The complex microstructure of hardened cement paste is produced by hydration reactions between cement particles and the water in which they are suspended. In recent years, algorithms like the diffusion-limited aggregation (DLA) and Eden models have demonstrated that simple growth rules can result in complex aggregated structures. The model described in this paper simulates, via simplified growth rules, the microstructural development of hydrating cement paste. This model has similarities to DLA, but with the additional novel features of dissolution of solid particles, and a free-space nucleation probability. The percolation aspects and transport properties of the model's pore space are computed and discussed. INTRODUCTION The microstructure of cement paste is known to be complex. When starting with a disordered suspension of irregulary-shaped cement particles, which have a wide size distribution, and which subsequently undergo random growth due to a hydration reaction that occurs after mixing with water, one could expect no less. In the last decade, a number of growth or aggregation models have been developed. These models, which utilize very simple random growth rules, have been shown to produce complex aggregated structures, often with fractal morphology. Two examples are the diffusion-limited aggregation (DLA) [1] and the Eden models [2]. In light of these models, it is not unreasonable to suggest that the complex microstructure of cement paste could be simulated using a few, relatively simple growth rules, which are iterated many times. The model presented in this paper represents an attempt to implement this approach. Having a model that gives a simulated cement paste microstructure, one can proceed to compute its physical properties. This paper presents results of two such studies, the first being an examination of the percolation aspects of the pore space, and the second being a computation of the diffusivity of the pore space. HYDRATION PROCESS AND MICROSTRUCTURAL MODEL Hydration Process Neglecting extraneous details, the hydration process that cement particles undergo to produce cement p can be thought of in the following simple way. The solid cement particles supply ions to the surrounding water via dissolution of surface layers. These ions then react with the surfaces of cement particles to form solid products covering the cement particles, or spontaneously nucleate in the pore space to form crystals, which can then grow by aggregation. In cement paste the main surface product, calcium silicate hydrate, is denoted C-S-H, and the Mat. Res. Soc. Symp. Proc. Vol. 195. @1990 Materials Research Society
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main pore product, calcium hydoxide, is denoted CH, where the usual cement chemistry shorthand is C=CaO, S =SiO 2, and H=H20. To emphasize the generality of the m
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