Pore structure and thermal conductivity of basalt fiber reinforced foam concrete under freeze-thaw cycles

Basalt Fiber Reinforced Foam Concrete (BFRFC) has garnered significant attention for its lightweight properties and excellent thermal insulation. While studies have explored the macro-mechanical properties of BFRFC, demonstrating its potential in construction applications, systematic research on its thermal conductivity remains insufficient. In particular, the effects of freeze-thaw cycles on its thermal conductivity have not been thoroughly analyzed, which has become a major bottleneck limiting its broader application.

The study first conducted freeze-thaw cycle experiments on four BFRFC samples with varying fiber contents. X-CT technology was used to obtain the microstructural characteristics of BFRFC, and Avizo software was employed for data filtering, noise reduction, and threshold segmentation to achieve a three-dimensional reconstruction of the microstructure. Following thermal conductivity testing, a three-dimensional model containing randomly distributed pores and fibers was generated using Matlab, and thermal conductivity simulations were performed using Comsol finite element analysis. Additionally, theoretical thermal conductivity models were established based on the multiphase medium Bruggeman model and fiber parallel-series models to systematically analyze the thermodynamic properties of BFRFC and its behavior under freeze-thaw degradation.

This study establishes a numerical simulation method combined with thermal conductivity experiments and theoretical modeling to investigate the thermal conductivity variations of Basalt Fiber Reinforced Foam Concrete (BFRFC) under freeze-thaw degradation. The research explores the impact of different densities and fiber contents on the pore characteristics and thermal conductivity of BFRFC, revealing internal heat flow distribution and fiber conduction paths under freeze-thaw conditions. The findings are as follows:(1) The pore size and shape factor of BFRFC approximately follow a log-normal distribution. With freeze-thaw erosion, the porosity and pore diameter of BFRFC increase, and the pore shapes become more irregular. Increasing material density and adding basalt fibers can effectively mitigate freeze-thaw damage.(2) The basalt fibers in BFRFC exhibit polar angles between 15° and 90° and azimuthal angles evenly distributed from 0° to 360°. As fiber content increases, the uniformity of dispersion decreases, requiring enhanced mixing for materials with high fiber content to improve fiber distribution within the BFRFC matrix.(3) The thermal conductivity of BFRFC is significantly influenced by material density and freeze-thaw cycles, primarily through changes in porosity. The impact of fiber content on thermal conductivity is relatively minor and occurs through the coupling effects of fiber content and porosity.(4) BFRFC's thermal conductivity ranges from 0.2 to 0.4 W/(m·K). It exhibits excellent thermal insulation performance in cold regions, with minimal impact from freeze-thaw degradation, making it suitable for thermal protection in cold region engineering.(5) The numerical simulation model based on BFRFC's microstructure, using Comsol to simulate thermal conductivity, shows good agreement with the thermal conductivity measurements obtained by the TC5000E instrument, effectively modeling the material's thermal performance.(6)The microstructural heat transfer distribution of BFRFC indicates that internal heat conduction primarily occurs through the cement matrix. Due to the low thermal conductivity of pores and fibers, isotherms tend to fluctuate at the pore and fiber interfaces.7) A comprehensive theoretical thermal conductivity model for BFRFC, incorporating the Bruggeman model for multiphase media and a parallel-series fiber model, was established. This model effectively predicts the thermal conductivity of BFRFC under varying fiber contents and porosity conditions.

Freeze-thaw cycles significantly affect the pore structure and thermal conductivity of BFRFC. As the number of freeze-thaw cycles increases, the porosity of BFRFC rises, pore shapes become more irregular, and thermal conductivity decreases. However, high-density and high-fiber content BFRFC demonstrates strong resistance to freeze-thaw cycles, with only minor changes in thermal conductivity. Therefore, the addition of basalt fibers significantly enhances the thermal insulation of BFRFC, showcasing excellent frost resistance in cold region engineering applications. The theoretical thermal conductivity predictions based on the multiphase medium Bruggeman model and fiber parallel-series model are consistent with experimental data and thermodynamic finite element model simulations, providing a crucial theoretical basis for predicting the degradation of BFRFC's thermal conductivity in cold regions.