Pore structure characteristics and mechanical property of engineered cementitious composites (ECC) incorporating recycled sand

The mechanical properties of recycled sand (RS) modified ECC are intimately linked to its pore structure characteristics, but the internal pore size and distribution of RS modified ECC and its impact mechanism on the mechanical properties are still unclear. This study employed mercury intrusion porosimetry (MIP) and low field nuclear magnetic resonance (LF-NMR) techniques to analyze the pore structure of ECC incorporating RS. Furthermore, compression and tension mechanical experiments were conducted to investigate the correlation between the evolution of pore structure and mechanical properties of ECC incorporating RS. The findings demonstrate that the addition of RS increases the porosity of ECC. When replacement ratio of RS is 100 %, the volume fraction of pores decreases within the range of 10 nm to 100 nm, whereas it exhibits an increasing trend within a ranging from 100 nm to 1 mu m. The incorporation of RS results in a decrease of the volume fraction of gel pores and the increase of capillary pores in ECC owing to its high-water absorption and porous, as well as the reduction of calcium-silicate-hydrate (C-S-H) due to the surface attached old mortar. Therefore, addition of RS lead to a reduction in both the compressive strength and tensile strength of ECC. However, as a result of the pore structural flaw inherent in the recycled fine aggregate, the occurrence of multi-crack cracking, induced by interfacial imperfections between the recycled fine aggregate and matrix, contributing to development of cracks in ECC and thus enhancing its tensile strain capacity.