Search

This paper presents a computer simulation method for predicting the degree of hydration at the initial setting time of cement paste with particle agglomeration. Agglomeration is controlled by using a square-well potential for the interaction between two cement particles, and the degree of hydration at the initial setting time is determined using the burning algorithm. It is verified that, when the critical surface-to-surface distance for attachment is taken as 0.05 μm, the simulation predictions are in good agreement with experimental results. For cement particles in water at the initial stage of hydration, the number of clusters obeys the power law with an average Fisher exponent of 1.9. Finally, the effects of key factors affecting the degree of hydration at the initial setting time are quantitatively evaluated.

In this study, the influence of the aggregate-cement paste interfacial transition zone (ITZ) on the steady-state chloride diffusivity of mortars and concretes was examined using a semi-empirical, three-phase composite sphere model. Mortars and concretes were modelled as three-phase composites consisting of the aggregate, bulk cement paste and an inhomogeneous ITZ. The latter was divided into a series of homogenous concentric shell elements of equal thickness. The initial porosity and cement gradients at the ITZ were first estimated from the overall water/cement ratio (w0/c). The evolution of the porosity, solid hydration products and remnants of unreacted cement were then calculated from the hydration degree and local water/cement ratio (w/c) using Powers' empirical model. Based on the Laplacian equation, an element transfer matrix was derived analytically to predict the steady-state chloride diffusivity. The model was calibrated using the available experimental data and then applied to perform a sensitivity analysis to evaluate the effects of aggregate content, water/cement ratio, curing period, ITZ width, maximum aggregate size and aggregate gradation on diffusivity. Some of these variables are impractical to quantify by laboratory experimentation. Implications of the findings with regard to the role of ITZ on mass transport properties are discussed.

Determining the water permeability of cement paste is of great practical significance in view of cement paste water transport's importance in cement-based material design and durability assessment. The authors present an analytical cement paste water permeability prediction method. The cement paste model is an isotropic two-phase composite material, with gel and capillary pores included in two separate phases, since capillary pores have a considerably larger contribution to cement paste water permeability and a considerably larger scale than gel pores. An analytical solution is derived for cement paste water permeability through the introduction of a hypothetical homogeneous nonzero water permeability medium and application of general effective medium theory. The proposed method's primary advantage is each phase constitutent's physical properties and morphological characteristics, the Hashin-Shtrikman bounds, and the interaction of the two phases are all taken into account. Finally, there is verification of analytical solution validation with experimental results obtained from the literature. The conclusion that cement paste water permeability can be predicted with reasonable accuracy by the analytical solution is reached.