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A digital image-based cement paste microstructural model has been applied to studying the microstructure of the interfacial zone in concrete in the presence of inert and pozzolanic mineral admixtures. This digital image approach to fundamental computer- based microstructure modelling [16] has proven to be a powerful asset in understanding the microstructure of cement-based materials. The model is flexible enough that new features, such as the incorporation of the pozzolanic reaction, can be easily added. Scenarios which would be difficult to investigate experimentally, such as various admixture reactivities, can be quickly simulated using the model. Although simple in form, the dissolution/diffusion/reaction steps of the model have been shown to generate quite realistic microstructures whose properties and characteristics agree well with those of real cement-based materials [30], allowing the model to be applied to real-world problems of technological importance. Based on the simulations carried out, the model results suggest the following conclusions for materials evaluated at a constant degree of hydration.
1. Inert mineral admixtures at a 10% addition rate will be generally ineffective (unless they affect mix properties such as bleeding) in improving the homogeneity of the interfacial zone microstructure when the w/s ratio is greater than 0.4, and especially so if they are both chemically and physically inert.
2. Pozzolanic mineral admixtures reduce, but do not eliminate, the capillary porosity gradient in the interfacial zone in concrete.
3. Pozzolanic mineral admixtures can greatly reduce the volume fraction of CH in both the interfacial zone and the bulk cement paste matrix in concrete.
4. Pozzolanic mineral admixtures improve the integrity of the interfacial zone by increasing the amount of the total C-S-H + cement phase present near the aggregate, relative to that present in the bulk paste.
5. Both the size and the reactivity of the pozzolanic admixtures are important in producing a uniform microstructure of cement paste throughout the concrete, with small, highly reactive admixtures, such as silica fume, exhibiting the greatest positive influence on the microstructure of the interfacial zone.