Damage mechanism of engineered cementitious composites after exposed to elevated temperatures: Experimental and molecular dynamics study

Abstract

Replacing cement with large amounts of fly ash in engineered cementitious composites (ECC) is one of the win-win measures to enhance matrix ductility and effectively reduce carbon emissions. However, the high content of fly ash undoubtedly makes the mechanical properties of ECC lower, resulting in its performance at high temperature to be questioned. To solve this problem, this paper combines experiments and molecular dynamics study to fully explore and innovatively explain the performance of ECC exposed to different temperatures (20 °C, 200 °C, 400 °C, 600 °C and 800 °C) from the aspects of macroscopic mechanical properties, microstructural characteristics and molecular-scale interfaces. From 20 °C to 800 °C, the mechanical properties first increase and then decrease significantly. SEM shows the changes in the microscopic morphology at elevated temperatures, such as the increase of cracks and pores in the matrix, and the melting of the PVA fibers. TG/DSC and XRD illustrate the phase transformation at different temperatures. 29Si NMR demonstrates the decomposition of hydration products and the depolymerization of silicate chains at higher temperatures. Additionally, molecular dynamics study is carried out to characterize the interaction between PVA fibers and C–S–H gel at a molecular scale as a function of temperature. With the increase of temperature, the morphology of C–S–H gradually changes from layered structure to spherical structure, the water molecules between the C–S–H layers gradually diffuse to the C–S–H channel, and PVA chains are present by disordered distribution. The simulated results have a good agreement with experimental results and further explain the damage mechanism of ECC at elevated temperatures on the nano-scale level.