Foam concrete is a typical cement-based porous material; its special microstructure endows it with excellent properties, such as light weight, energy efficiency, thermal insulation, and fire resistance. Therefore, it is widely used as a thermal insulation material for buildings. The heat transfer modes of foam concrete include conduction, convection, and radiation. However, previous studies considered conduction to be the dominant mode, often neglecting the effects of convection and radiation. In this study, a stochastic numerical model of the foam concrete microstructure is established based on the statistical parameters of the pore structure. With this model, the heat transfer mechanism of foam concrete is analyzed at the mesoscopic level, and the equivalent thermal conductivity is calculated. By comparing four different working conditions, the influence of conduction, convection, and radiation on the heat transfer of foam concrete is analyzed, and the specific contribution rates of conduction, convection, and radiation are calculated. The results show that the convection effect is weak due to the pore size being smaller than 1 $\mathrm{mm}$; so, the influence of convection can be neglected in the heat transfer analysis of foam concrete. The contribution of radiation increases with the decrease in foam concrete density and the increase in temperature difference. When the temperature difference is 40 °C and the density is 300 $\mathrm{kg}/{\mathrm{m}}^3$, the contribution of radiation exceeds 20$\%$. Therefore, for low-density and high-temperature difference situations, the influence of radiation cannot be ignored. The heat transfer in foam concrete is mainly through conduction, but with the decrease in density and the increase in temperature difference, the contribution of conduction shows a downward trend. Nevertheless, the contribution of conduction is still much larger than that of radiation and convection.

In engineering, concrete often develops cracks due to various reasons, which accelerate the erosion rate of chloride ions in concrete and consequently expedite the degradation of the mechanical properties of concrete structures. This study simplifies the four-phase model into a two-phase model using homogenization methods. Based on this, numerical simulations are employed to investigate the influences of dimensionless structural parameters and material parameters of cracks on the equivalent diffusion coefficient of cracked concrete, and a theoretical model for the equivalent diffusion coefficient of cracked concrete is established according to Fick’s diffusion law. The research findings indicate that when cracks are positioned in the middle of the boundary through which chloride ions enter and exit the concrete, and the direction of the cracks is parallel to the diffusion direction of chloride ions; this scenario is the most detrimental to the durability of concrete. For n cracks (n ≥ 2), when they are parallel to the x-axis and symmetrical about the x-axis, and the spacing between cracks equals 1/n times the width of the concrete, this scenario is the most detrimental to the durability of concrete containing multiple cracks. Whether for a single crack or multiple cracks, when they are in the most unfavorable condition, the “parallel-then-series” theoretical model can accurately predict the equivalent diffusion coefficient of cracked concrete.

Three-dimensional printed concrete (3DPC) is an anisotropic heterogeneous material composed of a concrete matrix and the interfaces between layers and filaments that form during printing. The overall ion transport properties can be characterized by the equivalent diffusion coefficient. This paper first establishes a theoretical model to calculate the equivalent diffusion coefficient of 3DPC. Verification through numerical calculations shows that this theoretical model is highly precise. Based on this, the model was used to analyze the effects of dimensionless interface parameters on the equivalent diffusion coefficients in different directions of 3DPC. Finally, the dynamic ionic transport properties of 3DPC were investigated through finite element numerical simulation. The results of the dynamic study indicate that interfaces have a significant impact on the ion distribution and its evolution within 3DPC. The product of the interface diffusion coefficient and interface size can represent the ionic transport capacity of an interface. The stronger the ionic transport capacity of an interface, the higher the ion concentration at that interface. Due to the “drainage” effect of lateral interfaces, the ion concentration in the middle of 3DPC with a smaller equivalent diffusion coefficient is higher than that in 3DPC with a larger equivalent diffusion coefficient.